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symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. however, in most symbiotics, the origin of this burning is unclear. in symbiotic slow novae, however, it is linked to a past thermonuclear runaway. in 2015 june, the symbiotic slow nova ag peg was seen in only its second optical outburst since 1850. this recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks - like outbursts in classical symbiotic stars such as z and. we report swift x-ray and uv observations of ag peg made between 2015 june and 2016 january. the x-ray flux was markedly variable on a time-scale of days, particularly during four days near optical maximum, when the x-rays became bright and soft. this strong x-ray variability continued for another month, after which the x-rays hardened as the optical flux declined. the uv flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. given that accretion discs around white dwarfs with shell burning do not generally produce detectable x-rays (due to compton-cooling of the boundary layer), the x-rays probably originated via shocks in the ejecta. as the x-ray photoelectric absorption did not vary significantly, the x-ray variability may directly link to the properties of the shocked material. ag peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae. | swift observations of the 2015 outburst of ag peg - from slow nova to classical symbiotic outburst |
context. rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (pms) objects. understanding the formation of massive stars requires taking into account the accretion of angular momentum during their pms phase.aims: we study the pms evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. in particular, we look at the physical conditions that allow the production of massive stars by accretion.methods: we present pms models computed with a new version of the geneva stellar evolution code self-consistently including accretion and rotation according to various accretion scenarios for mass and angular momentum. we describe the internal distribution of angular momentum in pms stars accreting at high rates and we show how the various physical conditions impact their internal structures, evolutionary tracks, and rotation velocities during the pms and the early main sequence.results: we find that the smooth angular momentum accretion considered in previous studies leads to an angular momentum barrier and does not allow the formation of massive stars by accretion. a braking mechanism is needed in order to circumvent this angular momentum barrier. this mechanism has to be efficient enough to remove more than two thirds of the angular momentum from the inner accretion disc. due to the weak efficiency of angular momentum transport by shear instability and meridional circulation during the accretion phase, the internal rotation profiles of accreting stars reflect essentially the angular momentum accretion history. as a consequence, careful choice of the angular momentum accretion history allows circumvention of any limitation in mass and velocity, and production of stars of any mass and velocity compatible with structure equations. | massive star formation by accretion. ii. rotation: how to circumvent the angular momentum barrier? |
we examine the possible role of turbulence in feeding the emission of gamma-ray bursts (grbs). turbulence may develop in a grb jet as the result of hydrodynamic or current-driven instabilities. the jet carries dense radiation and the turbulence cascade can be damped by compton drag, passing kinetic fluid energy to photons through scattering. we identify two regimes of turbulence dissipation: (1) “viscous”—the turbulence cascade is compton-damped on a scale {{\ell }}damp} greater than the photon mean free path {{\ell }}\star . then turbulence energy is passed to photons via bulk comptonization by smooth shear flows on scale {{\ell }}\star < {{\ell }}damp}. (2) “collisionless”—the cascade avoids compton damping and extends to microscopic plasma scales much smaller than {{\ell }}\star . the collisionless dissipation energizes plasma particles, which radiate the received energy; how the dissipated power is partitioned between particles needs further investigation with kinetic simulations. we show that the dissipation regime switches from viscous to collisionless during the jet expansion, at a critical value of the jet optical depth, which depends on the amplitude of turbulence. turbulent grb jets are expected to emit nonthermal photospheric radiation. our analysis also suggests revisions of turbulent comptonization in black hole accretion disks discussed in previous works. | subphotospheric turbulence as a heating mechanism in gamma-ray bursts |
magneto-gravity-elliptic instability is addressed here considering an unbounded strained vortex (with constant vorticity $2\varomega$ and with ellipticity parameter $\varepsilon$ ) of a perfectly conducting fluid subjected to a uniform axial magnetic field (with alfvén velocity scaled from the basic magnetic field $b)$ and an axial stratification (with a constant brunt-väisälä frequency $n$ ). such a simple model allows us to formulate the stability problem as a system of equations for disturbances in terms of lagrangian fourier (or kelvin) modes which is universal for wavelengths of the perturbation sufficiently small with respect to the scale of variation of the basic velocity gradients. it can model localised patches of elliptic streamlines which often appear in some astrophysical flows (stars, planets and accretion discs) that are tidally deformed through gravitational interaction with other bodies. in the limit case where the flow streamlines are exactly circular ( $\varepsilon =0),$ there are fast and slow magneto-inertia-gravity waves with frequencies $\omega _{1,2}$ and $\omega _{3,4},$ respectively. under the effect of finite ellipticity, the resonant cases of these waves, $\omega _i-\omega _j=n\varomega$ $(i\ne j)$ ( $n$ being an integer), can become destabilising. the maximal growth rate of the subharmonic instability (related to the resonance of order $n=2)$ is determined by extending the asymptotic method by lebovitz & zweibel (astrophys. j., vol. 609, 2004, pp. 301-312). the domains of the $(k_0b/\varomega, n/\varomega )$ plane for which this instability operates are identified ( $1/k_0$ being a characteristic length scale). we demonstrate that the $n\rightarrow 0$ limit is, in fact, singular (discontinuous). the axial stable stratification enhances the subharmonic instability related to the resonance between two slow modes because, at large magnetic field strengths, its maximal growth rate is twice that found in the case without stratification. | magneto-gravity-elliptic instability |
context. binaries hosting a massive star and a non-accreting pulsar are powerful non-thermal emitters owing to the interaction of the pulsar and the stellar wind. the winds of massive stars are thought to be inhomogeneous, which could have an impact on the non-thermal emission.aims: we study numerically the impact of the presence of inhomogeneities or clumps in the stellar wind on the high-energy non-thermal radiation of high-mass binaries hosting a non-accreting pulsar.methods: we compute the trajectories and physical properties of the streamlines in the shocked pulsar wind without clumps, with a small clump, and with a large clump. this information is used to characterize the injection and the steady state distribution of non-thermal particles accelerated at shocks formed in the pulsar wind. the synchrotron and inverse compton emission from these non-thermal particles is calculated, accounting also for the effect of gamma-ray absorption through pair creation. a specific study is done for psr b1259-63/ls2883.results: when stellar wind clumps perturb the two-wind interaction region, the associated non-thermal radiation in the x-ray band, of synchrotron origin, and in the gev-tev band, of inverse compton origin, is affected by several equally important effects: (i) strong changes in the plasma velocity direction that result in doppler boosting factor variations; (ii) strengthening of the magnetic field that mainly enhances the synchrotron radiation; (iii) strengthening of the pulsar wind kinetic energy dissipation at the shock, potentially available for particle acceleration; and (iv) changes in the rate of adiabatic losses that affect the lower energy part of the non-thermal particle population. the radiation above 100 gev detected, presumably, during the post-periastron crossing of the be star disc in psr b1259-63/ls2883, can be roughly reproduced assuming that the crossing of the disc is modelled as the encounter with a large inhomogeneity.conclusions: because of the likely diverse nature of clumps in the stellar wind, and hydrodynamical instabilities, the non-thermal radiation of high-mass binaries with a non-accreting pulsar is expected to be boosted somewhat chaotically, and to present different superimposed variability patterns. some of the observed variability in gamma rays from psr b1259-63/ls2883 is qualitatively reproduced by our calculations. | non-thermal radiation from a pulsar wind interacting with an inhomogeneous stellar wind |
we examine the dependence of the total hydrogen mass m hi in late-type star-forming galaxies on rotation velocity v rot and optical size d 25 or radial scale length r 0 of the disk for two samples of galaxies: (i) isolated galaxies (amiga) and (ii) galaxies with edge-on disks (flat galaxies according to karachentsev et al.). m hi given in the hyperleda database for flat galaxies have turned out to be, on average, overestimated by 0.2 dex compared to isolated galaxies with similar v rot or d 25, which is apparently due to an overestimation of the self-absorption in the hi line. the hydrogen mass in the galaxies of both samples closely correlates with the total specific angular momentum of the galactic disk j, which is proportional to v rot d 25 or v rot r 0, with the low-surface-brightness galaxies lying along the common v rot r 0 sequence. we discuss the possibility of explaining the relationship between m hi and v rot d 25 by assuming that the gas mass in the disk is regulated by the marginal gravitational stability condition for the gas layer. comparison of the observed and theoretically expected dependences leads us to conclude that either the gravitational stability corresponds to higher values of the toomre parameter than is usually assumed, or the threshold stability condition formost galaxies was fulfilled only in the past, when the gasmass in the disks was a factor of 2-4 higher than that at present (except for the galaxies with an anomalously high observed hi content). the latter condition requires that for most galaxies the conversion of gas into stars be not compensated by the external accretion of gas onto the disk. | hi content in galactic disks: the role of gravitational instability |
we found that the su uma-type dwarf nova ny ser in the period gap [orbital period 0.097558(6) d] showed standstills twice in 2018. this is the first clear demonstration of a standstill occurring between superoutbursts of an su uma-type dwarf nova. there was no sign of superhumps during the standstill, and at least one superoutburst directly started from the standstill. this provides strong evidence that the 3:1 resonance was excited during standstills. this phenomenon indicates that the disk radius can grow during standstills. we also deduce that the condition close to the limit of the tidal instability caused early quenching of superoutbursts, which resulted in a substantial amount of matter left in the disk after the superoutburst. we think that substantial matter in the disk in a condition close to the limit of the tidal instability is responsible for standstills (as in the high-mass-transfer system ny ser) or multiple rebrightenings (as in the low-mass-transfer system v1006 cyg). | discovery of standstills in the su uma-type dwarf nova ny serpentis |
recent inspections of local available data suggest that the almost linear relation between the stellar mass of spheroids (msph) and the mass of the super massive black holes (bhs), residing at their centres, shows a break below msph ∼ 1010 m⊙, with a steeper, about quadratic relation at smaller masses. we investigate the physical mechanisms responsible for the change in slope of this relation, by comparing data with the results of the semi-analytic model of galaxy formation morgana, which already predicted such a break in its original formulation. we find that the change of slope is mostly induced by effective stellar feedback in star-forming bulges. the shape of the relation is instead quite insensitive to other physical mechanisms connected to bh accretion such as disc instabilities, galaxy mergers, active galactic nucleus (agn) feedback, or even the exact modelling of accretion on to the bh, direct or through a reservoir of low angular momentum gas. our results support a scenario where most stars form in the disc component of galaxies and are carried to bulges through mergers and disc instabilities, while accretion on to bhs is connected to star formation in the spheroidal component. therefore, a model of stellar feedback that produces stronger outflows in star-forming bulges than in discs will naturally produce a break in the scaling relation. our results point to a form of co-evolution especially at lower masses, below the putative break, mainly driven by stellar feedback rather than agn feedback. | interpreting the possible break in the black hole-bulge mass relation |
sgr a* is an ideal target to study low-luminosity accreting systems. it has been recently proposed that properties of the accretion flow around sgr a* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the s-cluster. when a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. with the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to kelvin-helmholtz instabilities. because the characteristic wind and stellar velocities are in ∼108 cm s-1 range, the shocked wind may produce detectable x-rays via thermal bremsstrahlung emission. the application of this model to the pericentre passage of s2, the brightest member of the s-cluster, shows that the shocked wind produces roughly a month long x-ray flare with a peak luminosity of l ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{m}_w= 10^{-7} m_{⊙} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. this peak luminosity is comparable to the quiescent x-ray emission detected from sgr a* and is within the detection capabilities of current x-ray observatories. its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole. | modelling accretion disc and stellar wind interactions: the case of sgr a* |
by performing a global magnetohydrodynamical simulation for the milky way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the galactic centre region. the radial distribution of the rotation frequency in the bulge region is not monotonic in general. the amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. the strength of the amplified magnetic field reaches ≳0.5 mg, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. in addition, the magnetic pressure-gradient force also drives radial flows in a similar manner. as a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. this is a natural extension into the central few 100 pc of the magnetic activity, which is observed as molecular loops at radii from a few 100 pc to 1 kpc. furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which we discuss from a viewpoint of the outflow from the bulge. | stochastic non-circular motion and outflows driven by magnetic activity in the galactic bulge region |
using the disc instability model for dwarf novae and soft x-ray transients, we investigate the stability of accretion discs in long-period binary systems. we simulate outbursts resulting from this thermal-viscous instability for two symbiotic systems, rs ophiuchi and z andromedae. the outburst properties deduced from our simulations suggest that, although the recurrent nova events observed in rs oph are caused by thermonuclear runaway at the white dwarf surface, these runaways are triggered by accretion disc instabilities. in quiescence, the disc builds up its mass, and it is only during the disc-instability outburst that mass is accreted on to the white dwarf at rates comparable to or larger than the mass-transfer rate. for a mass-transfer rate in the range 10-8 to 10^{-7} m_{⊙} yr-1, the accretion rate and the mass accreted are sufficient to lead to a thermonuclear runaway during one of a series of a few dwarf nova outbursts, which are barely visible in the optical but easily detectable in the x-rays. in the case of z and, persistent irradiation of the disc by the very hot white dwarf surface strongly modifies the dwarf nova outburst properties, making them significant only for very high mass-transfer rates, of the order of 10^{-6} m_{⊙} yr-1, much higher than the expected secular mean in this system. it is thus likely that the so-called `combination nova' outburst observed in the years 2000 to 2002 was triggered not by a dwarf-nova instability but by a mass-transfer enhancement from the giant companion, leading to an increase in nuclear burning at the accreting white dwarf surface. | disc instabilities and nova eruptions in symbiotic systems: rs ophiuchi and z andromedae |
asassn-14ko was identified as an abnormal periodic nuclear transient with a potential decreasing period. its outbursts in the optical and uv bands have displayed a consistent and smooth "fast rise and slow decay" pattern since its discovery, which has recently experienced an unexpected alteration in the last two epochs, as revealed by our proposed high-cadence swift observations. the new light curve profiles show a bump during the rising stages and a rebrightening during the declining stages, making them much broader and symmetrical than the previous ones. in the last two epochs, there is no significant difference in the x-ray spectral slope compared to the previous one, and its overall luminosity is lower than those of the previous epochs. the energy released in the early bump and rebrightening phases (~1050 erg) could be due to collision of the stripped stream from partial tidal disruption events with an expanded accretion disk. we also discussed other potential explanations, such as disk instability and star-disk collisions. further high-cadence multiwavelength observations of subsequent cycles are encouraged to comprehend the unique periodic source with its new intriguing features. | dissonance in harmony: the uv/optical periodic outbursts of asassn-14ko exhibit repeated bumps and rebrightenings |
close-in super-earths are the most abundant exoplanets known. it has been hypothesized that they form in the inner regions of protoplanetary discs, out of the dust that may accumulate at the boundary between the inner region susceptible to the magneto-rotational instability (mri) and an mri-dead zone further out. in paper i, we presented a model for the viscous inner disc which includes heating due to both irradiation and mri-driven accretion; thermal and non-thermal ionization; dust opacities; and dust effects on ionization. here, we examine how the inner disc structure varies with stellar, disc, and dust parameters. for high accretion rates and small dust grains, we find that: (1) the main sources of ionization are thermal ionization and thermionic and ion emission; (2) the disc features a hot, high-viscosity inner region, and a local gas pressure maximum at the outer edge of this region (in line with previous studies); and (3) an increase in the dust-to-gas ratio pushes the pressure maximum outwards. consequently, dust can accumulate in such inner discs without suppressing the mri, with the amount of accumulation depending on the viscosity in the mri-dead regions. conversely, for low accretion rates and large dust grains, there appears to be an additional steady-state solution in which: (1) stellar x-rays become the main source of ionization; (2) mri-viscosity is high throughout the disc; and (3) the pressure maximum ceases to exist. hence, if planets form in the inner disc, larger accretion rates (and thus younger discs) are favoured. | mri-active inner regions of protoplanetary discs - ii. dependence on dust, disc, and stellar parameters |
self-gravity plays an important role in the evolution of rotationally supported systems such as protoplanetary discs, accretion discs around black holes, or galactic discs, as it can both feed turbulence and lead to gravitational fragmentation. while such systems can be studied in the shearing box approximation with high local resolution, the large density contrasts that are possible in the case of fragmentation still limit the utility of eulerian codes with constant spatial resolution. in this paper, we present a novel self-gravity solver for the shearing box based on the treepm method of the moving-mesh code arepo. the spatial gravitational resolution is adaptive, which is important to make full use of the quasi-lagrangian hydrodynamical resolution of the code. we apply our new implementation to two- and three-dimensional, self-gravitating discs combined with a simple β-cooling prescription. for weak cooling we find a steady, gravito-turbulent state, while for strong cooling the formation of fragments is inevitable. to reach convergence for the critical cooling efficiency above which fragmentation occurs, we require a smoothing of the gravitational force in the two-dimensional case that mimics the stratification of the three-dimensional simulations. the critical cooling efficiency we find, β ≈ 3, as well as the box-averaged quantities characterizing the gravito-turbulent state, agrees well with various previous results in the literature. interestingly, we observe stochastic fragmentation for β > 3, which slightly decreases the cooling efficiency required to observe fragmentation over the lifetime of a protoplanetary disc. the numerical method outlined here appears well suited to study the problem of galactic discs as well as the magnetized, self-gravitating discs. | gravito-turbulence in local disc simulations with an adaptive moving mesh |
we found an active state lasting for ~200 d in the am cvn star nsv 1440 in 2022. during this state, the object reached a magnitude of 16.5, 2.0-2.5 mag above quiescence, and showed a number of superposed normal outbursts. such an active state was probably brought either by an enhanced mass-transfer from the secondary or increased quiescent viscosity of the accretion disk. these possibilities are expected to be distinguished by an observation of the interval to the next superoutburst. we also found that the brightness and the course toward the end of the event were similar to the post-superoutburst fading tail in 2021. the mechanism producing the 2022 active state and post-superoutburst fading tails in am cvn stars may be the same, and the present finding is expected to clarify the nature of these still poorly understood fading tails in am cvn stars, and potentially of the corresponding phenomenon in hydrogen-rich wz sge stars. we also note that the faint, long "superoutbursts" in long-period am cvn stars claimed in the past were not true outbursts powered by disk instability, but were more likely phenomena similar to the 2022 active state in nsv 1440. | the 2022 active state of the am cvn star nsv 1440 |
aims: we study the spectral evolution of the h1743-322 during outbursts in the rxte era. we aim to connect the variation of the spectral parameters with the accretion parameters along with the progress of the outbursts. we understand the evolution of the accretion parameters and hence the dynamics of the accretion process in light of the irradiated disc instability model.methods: we provide a comprehensive study of all the outbursts of h1743-322 between 2003 and 2011. we performed spectral modelling of all the rxte/pca observations using phenomenological models. also, we carried out spectral modelling by a hydrodynamic accretion flow model and estimated the accretion parameters. we applied the irradiated disc instability scenario in the presence of both keplerian and sub-keplerain accretion components to understand the evolution of accretion parameters. for this purpose, we propose a toy model for the time variation of the accretion rates following a powerlaw during outbursts.results: all of the outbursts show spectral state transitions in the hardness-intensity diagram. the 2003 and 2004 outbursts are long-duration outbursts and relatively softer than the other outbursts. the 2008b and 2011 outbursts provide a unique opportunity to estimate the critical accretion rate (ṁdc) for triggering an outburst in this system within a narrow range of 0.076 < ṁdc < 0.086 (in eddington units). in the absence of any dynamical measurement, we attempt to constrain a few orbital parameters of the system using an assumed mass and ṁdc in the range. | spectral and accretion evolution of h1743-322 during outbursts in rxte era |
in currently favoured hierarchical cosmologies, the formation of massive black hole binaries (mbhbs) following galaxy mergers is unavoidable. still, due the complex physics governing the (hydro)dynamics of the post-merger dense environment of stars and gas in galactic nuclei, the final fate of those mbhbs is still unclear. in gas-rich environments, it is plausible that turbulence and gravitational instabilities feed gas to the nucleus in the form of a series of cold incoherent clumps, thus providing a way to exchange energy and angular momentum between the mbhb and its surroundings. within this context, we present a suite of smoothed-particle-hydrodynamical models to study the evolution of a sequence of near-radial turbulent gas clouds as they infall towards equal-mass, circular mbhbs. we focus on the dynamical response of the binary orbit to different levels of anisotropy of the incoherent accretion events. compared to a model extrapolated from a set of individual cloud-mbhb interactions, we find that accretion increases considerably and the binary evolution is faster. this occurs because the continuous infall of clouds drags inwards circumbinary gas left behind by previous accretion events, thus promoting a more effective exchange of angular momentum between the mbhb and the gas. these results suggest that sub-parsec mbhbs efficiently evolve towards coalescence during the interaction with a sequence of individual gas pockets. | accretion of clumpy cold gas onto massive black hole binaries: a possible fast route to binary coalescence |
we study the hydrodynamical stability of the laminar flows associated with warped astrophysical discs using numerical simulations of warped shearing boxes. we recover linear growth rates reported previously due to a parametric resonance of inertial waves, and show that the non-linear saturated state can significantly reduce the laminar flows, meaning that the warp would evolve on much longer time-scales than would be concluded from the internal torques due to these laminar flows. towards larger warp amplitudes, we find first of all a reversal of angular momentum flux, indicating that the mass distribution would evolve in an antidiffusive manner, and secondly that the linear growth rates disappear, possibly because of the very strong shear in the laminar flows in this regime. for discs with small enough viscosity, a non-linear state can still be found when linear growth rates are absent by introducing a large enough perturbation, either by starting from a non-linear state obtained at smaller warp amplitude, or by starting from a state with no laminar flows. | local numerical simulations of warped discs |
the accretion phase of star formation is investigated in magnetically dominated clouds that have an initial subcritical mass-to-flux ratio. we employ non-ideal magnetohydrodynamic simulations that include ambipolar diffusion and ohmic dissipation. during the early prestellar phase, the mass-to-flux ratio rises towards the critical value for collapse, and during this time the angular momentum of the cloud core is reduced significantly by magnetic braking. once a protostar is formed in the core, the accretion phase is characterized by the presence of a small amount of angular momentum but a large amount of magnetic flux in the near-protostellar environment. the low angular momentum leads to a very small (or even non-existent) disc and weak outflow, while the large magnetic flux can lead to an interchange instability that rapidly removes flux from the central region. the effective magnetic braking in the early collapse phase can even lead to a counterrotating disc and outflow, in which the rotation direction of the disc and outflow is opposite to that of the infalling envelope. the solutions with a counterrotating disc, tiny disc, or non-existent disc (direct collapse) are unique outcomes that are realized in collapse from magnetically dominated clouds with an initial subcritical mass-to-flux ratio. | different modes of star formation - ii. gas accretion phase of initially subcritical star-forming clouds |
context. although the disc instability model is widely accepted as the explanation for dwarf nova outbursts, it is still necessary to compare its predictions to observations because many of the constraints on angular momentum transport in accretion discs are derived from the application of this model to real systems.aims: we test the predictions of the model concerning the multicolour time evolution of outbursts for two well-observed systems, ss cyg and vw hyi.methods: we calculate the multicolour evolution of dwarf nova outbursts using the disc instability model and taking into account the contribution from the irradiated secondary, the white dwarf and the hot spot.results: observations definitely show the existence of a hysteresis in the optical colour-magnitude diagram during the evolution of dwarf nova outbursts. we find that the disc instability model naturally explains the existence and the orientation of this hysteresis. for the specific cases of ss cyg and vw hyi, the colour and magnitude ranges covered during the evolution of the system are in reasonable agreement with observations. however, the observed colours are bluer than observed near the peak of the outbursts, as in steady systems, and the amplitude of the hysteresis cycle is smaller than observed. the predicted colours significantly depend on the assumptions made for calculating the disc spectrum during rise, and on the magnitude of the secondary irradiation for the decaying part of the outburst.conclusions: improvements in the spectral disc models are strongly needed if the system evolution in the uv is to be addressed. | modelling hystereses observed during dwarf nova outbursts |
massive molecular gas has been discovered in giant elliptical galaxies at the centers of galaxy clusters. to reveal its role in active galactic nucleus (agn) feedback in those galaxies, we construct a semianalytical model of gas circulation. this model especially focuses on the massive molecular gas (interstellar cold gas on a scale of ~10 kpc) and the circumnuclear disk (≲0.5 kpc). we consider the destruction of the interstellar cold gas by star formation and the gravitational instability for the circumnuclear disk. our model can reproduce the basic properties of the interstellar cold gas and the circumnuclear disk, such as their masses. we also find that the circumnuclear disk tends to stay at the boundary between stable and unstable states. this works as an "adjusting valve" that regulates mass accretion toward the supermassive black hole. on the other hand, the interstellar cold gas serves as a "fuel tank" in the agn feedback. even if the cooling of the galactic hot gas is prevented, the interstellar cold gas can sustain the agn activity for ≳0.5 gyr. we also confirm that the small entropy of hot gas (≲30 kev cm2) or the short cooling time (≲1 gyr) is a critical condition for the existence of massive amounts of molecular gas in the galaxy. the dissipation time of the interstellar cold gas may be related to the critical cooling time. the galaxy behavior is described by a simple relation among the disk stability, the cloud dissipation time, and the gas cooling rate. | massive molecular gas as a fuel tank for active galactic nuclei feedback in central cluster galaxies |
we present a series of simulations in both pure hydrodynamics (hd) and magnetohydrodynamics (mhd) exploring the degree to which alignment of disks subjected to external precessional torques (e.g., as in the “bardeen-petterson” effect) is dependent upon the disk sound speed cs . across the range of sound speeds examined, we find that the influence of the sound speed can be encapsulated in a simple “lumped-parameter” model proposed by sorathia et al. in this model, alignment fronts propagate outward at a speed ≃0.2rωprecess(r), where ωprecess is the local test-particle precession frequency. meanwhile, transonic radial motions transport angular momentum both inward and outward at a rate that can, in steady-state, be described roughly in terms of an orientation diffusion model with diffusion coefficient ≃ 2{c}s2/{{ω }}, for local orbital frequency ω. the competition between the two leads, in isothermal disks, to a stationary position for the alignment front at a radius \propto {c}s-4/5. for alignment to happen at all, the disk must either be turbulent due to the magnetorotational instability in mhd, or, in hd, it must be cool enough for the bending waves driven by disk warp to be nonlinear at their launch point. contrary to long-standing predictions, warp dynamics in mhd disks appears to be independent of the ratio cs /(αv orb), for orbital speed v orb and ratio of stress to pressure α. in purely hd disks, i.e., those with no internal stresses other than bulk viscosity, warmer disks align weakly or not at all; cooler disks align qualitatively similarly to mhd disks. | sound speed dependence of alignment in accretion disks subjected to lense-thirring torques |
we observed rz lmi, which is renowned for its extremely short (∼19 d) supercycle and is a member of a small, unusual class of cataclysmic variables called er uma-type dwarf novae, in 2013 and 2016. in 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, and 60 d for three consecutive superoutbursts. we consider that the object virtually experienced a transition to the nova-like state (permanent superhumper). this observed behavior reproduced the prediction of the thermal-tidal instability model extremely well. we detected a precursor in the 2016 superoutburst and detected growing (stage a) superhumps with a mean period of 0.0602(1) d in 2016 and in 2013. combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in wz sge-type dwarf novae, having orbital periods similar to rz lmi. by using least absolute shrinkage and selection operator (lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1) d. we estimated an orbital period of 0.05792 d, which suggests a mass ratio of 0.105(5). this relatively large mass ratio is even above that of ordinary su uma-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in rz lmi may be a result of a stripped secondary with an evolved core in a system evolving toward an am cvn-type object. | rz leonis minoris bridging between er ursae majoris-type dwarf nova and nova-like system |
buoyancy of the fossil magnetic field in the accretion disks of young stars is investigated. it is assumed that the parker instability leads to the formation of slender flux tubes of toroidal magnetic field in the regions of effective magnetic field generation. stationary solution of the induction equation is written in the form in which the buoyancy is treated as the additional mechanism of the magnetic flux escape. we calculate the fossil magnetic field intensity in the accretion disks of young t tauri stars for the cases when radius of the magnetic flux tubes amft= 0.1 h,0.5 h or 1 h, where h is the accretion disk height scale. calculations show that the buoyancy limits toroidal magnetic field growth, so that its strength is comparable with the vertical magnetic field strength for the case amft= 0.1 h. | magnetic field buoyancy in accretion disks of young stars |
we study the structural evolution of isolated star-forming (sf) galaxies in the illustristng100-1 hydrodynamical simulation, with a focus on investigating the growth of the central core density within 2 kpc (σ*,2 kpc) in relation to total stellar mass (m*) at z < 0.5. first, we show that several observational trends in the σ*,2 kpc-m* plane are qualitatively reproduced in illustristng, including the distributions of active galactic nuclei (agn), sf galaxies, quiescent galaxies, and radial profiles of stellar age, specific star formation rate (ssfr), and metallicity. we find that galaxies with dense cores evolve parallel to the σ*,2 kpc-m* relation, while galaxies with diffuse cores evolve along shallower trajectories. we investigate possible drivers of rapid growth in σ*,2 kpc compared to m*. both the current ssfr gradient and the black hole (bh) accretion rate are indicators of past core growth, but are not predictors of future core growth. major mergers (although rare in our sample; ~10 per cent) cause steeper core growth, except for high-mass ($m_{\rm *}\gtrsim 10^{10} \, {\rm m}_{\odot }$) mergers, which are mostly dry. disc instabilities, as measured by the fraction of mass with toomre q < 2, are not predictive of rapid core growth. instead, rapid core growth results in more stable discs. the cumulative bh feedback history sets the maximum rate of core growth, preventing rapid growth in high-mass galaxies ($\gtrsim 10^{9.5} \, {\rm m}_{\odot }$). for massive galaxies, the total specific angular momentum of accreting gas is the most important predictor of future core growth. our results suggest that the angular momentum of accreting gas controls the slope, width, and zero-point evolution of the σ*,2 kpc-m* relation. | the structural evolution of isolated galaxies at low redshift in the illustristng simulation |
the magnetorotational instability (mri) is thought to play an important role in enabling accretion in sufficiently ionized astrophysical disks. the rate at which mri-driven turbulence transports angular momentum is intimately related to both the strength of the amplitudes of the fluctuations on various scales and the degree of anisotropy of the underlying turbulence. this has motivated several studies to characterize the distribution of turbulent power in spectral space. in this paper we investigate the anisotropic nature of mri-driven turbulence using a pseudo-spectral code and introduce novel ways for providing a robust characterization of the underlying turbulence. we study the growth of the mri and the subsequent transition to turbulence via parasitic instabilities, identifying their potential signature in the late linear stage. we show that the general flow properties vary in a quasi-periodic way on timescales comparable to ∼10 inverse angular frequencies, motivating the temporal analysis of its anisotropy. we introduce a 3d tensor invariant analysis to quantify and classify the evolution of the anisotropy of the turbulent flow. this analysis shows a continuous high level of anisotropy, with brief sporadic transitions toward two- and three-component isotropic turbulent flow. this temporal-dependent anisotropy renders standard shell averaging especially when used simultaneously with long temporal averages, inadequate for characterizing mri-driven turbulence. we propose an alternative way to extract spectral information from the turbulent magnetized flow, whose anisotropic character depends strongly on time. this consists of stacking 1d fourier spectra along three orthogonal directions that exhibit maximum anisotropy in fourier space. the resulting averaged spectra show that the power along each of the three independent directions differs by several orders of magnitude over most scales, except the largest ones. our results suggest that a first-principles theory to describe fully developed mri-driven turbulence will likely have to consider the anisotropic nature of the flow at a fundamental level. | on the anisotropic nature of mri-driven turbulence in astrophysical disks |
we describe a new mechanism that leads to the destabilization of non-axisymmetric waves in astrophysical discs with an imposed radial temperature gradient. this might apply, for example, to the outer parts of protoplanetary discs. we use linear density wave theory to show that non-axisymmetric perturbations generally do not conserve their angular momentum in the presence of a forced temperature gradient. this implies an exchange of angular momentum between linear perturbations and the background disc. in particular, when the disturbance is a low-frequency trailing wave and the disc temperature decreases outwards, this interaction is unstable and leads to the growth of the wave. we demonstrate this phenomenon through numerical hydrodynamic simulations of locally isothermal discs in 2d using the fargo code and in 3d with the zeus-mp and pluto codes. we consider radially structured discs with a self-gravitating region which remains stable in the absence of a temperature gradient. however, when a temperature gradient is imposed we observe exponential growth of a one-armed spiral mode (azimuthal wavenumber m = 1) with co-rotation radius outside the bulk of the spiral arm, resulting in a nearly stationary one-armed spiral pattern. the development of this one-armed spiral does not require the movement of the central star, as found in previous studies. because destabilization by a forced temperature gradient does not explicitly require disc self-gravity, we suggest this mechanism may also affect low-frequency one-armed oscillations in non-self-gravitating discs. | one-armed spirals in locally isothermal, radially structured self-gravitating discs |
the bright, erratic black hole x-ray binary grs 1915+105 has long been a target for studies of disk instabilities, radio/infrared jets, and accretion disk winds, with implications that often apply to sources that do not exhibit its exotic x-ray variability. with the launch of the neutron star interior composition explorer (nicer), we have a new opportunity to study the disk wind in grs 1915+105 and its variability on short and long timescales. here we present our analysis of 39 nicer observations of grs 1915+105 collected during five months of the mission data validation and verification phase, focusing on fe xxv and fe xxvi absorption. we report the detection of strong fe xxvi in 32 (>80%) of these observations, with another four marginal detections; fe xxv is less common, but both likely arise in the well-known disk wind. we explore how the properties of this wind depend on broad characteristics of the x-ray lightcurve: mean count rate, hardness ratio, and fractional rms variability. the trends with count rate and rms are consistent with an average wind column density that is fairly steady between observations but varies rapidly with the source on timescales of seconds. the line dependence on spectral hardness echoes the known behavior of disk winds in outbursts of galactic black holes; these results clearly indicate that nicer is a powerful tool for studying black hole winds. | a persistent disk wind in grs 1915+105 with nicer |
the low luminosity, x-ray flaring activity, of the sub-class of high-mass x-ray binaries called supergiant fast x-ray transients, has been investigated using xmm-newton public observations, taking advantage of the products made publicly available by the extras project. one of the goals of extras was to extract from the xmm-newton public archive information on the aperiodic variability of all sources observed in the soft x-ray range with epic (0.2-12 kev). adopting a bayesian block decomposition of the x-ray light curves of a sample of sfxts, we picked out 144 x-ray flares, covering a large range of soft x-ray luminosities (1032-1036 erg s-1). we measured temporal quantities, like the rise time to and the decay time from the peak of the flares, their duration and the time interval between adjacent flares. we also estimated the peak luminosity, average accretion rate, and energy release in the flares. the observed soft x-ray properties of low-luminosity flaring activity from sfxts is in qualitative agreement with what is expected by the application of the rayleigh-taylor instability model in accreting plasma near the neutron star magnetosphere. in the case of rapidly rotating neutron stars, sporadic accretion from temporary discs cannot be excluded. | supergiant fast x-ray transients uncovered by the extras project: flares reveal the development of magnetospheric instability in accreting neutron stars |
the ionization state of the gas plays a key role in the magnetohydrodynamics (mhd) of protoplanetary disks. however, the ionization state can depend on the gas dynamics, because electric fields induced by mhd turbulence can heat up plasmas and thereby affect the ionization balance. to study this nonlinear feedback, we construct an ionization model that includes plasma heating by electric fields and impact ionization by heated electrons, as well as charging of dust grains. we show that when plasma sticking onto grains is the dominant recombination process, the electron abundance in the gas decreases with increasing electric field strength. this is a natural consequence of electron-grain collisions whose frequency increases with the electron's random velocity. the decreasing electron abundance may lead to a self-regulation of mhd turbulence. in some cases, not only the electron abundance but also the electric current decreases with increasing field strength in a certain field range. the resulting n-shaped current-field relation violates the fundamental assumption of the non-relativistic mhd that the electric field is uniquely determined by the current density. at even higher field strengths, impact ionization causes an abrupt increase of the electric current as expected by previous studies. we find that this discharge current is multi-valued (i.e., the current-field relation is s-shaped) under some circumstances, and that the intermediate branch is unstable. the n/s-shaped current-field relations may yield hysteresis in the evolution of mhd turbulence in some parts of protoplanetary disks. | the nonlinear ohm's law: plasma heating by strong electric fields and its effects on the ionization balance in protoplanetary disks |
swift j1753.5-0127 (j1753) is a candidate black hole low-mass x-ray binary (bh-lmxb) that was discovered in outburst in 2005 may. it remained in outburst for ∼12 yr, exhibiting a wide range of variability on various time-scales, before entering quiescence after two short-lived, low-luminosity `mini-outbursts' in 2017 april. the unusually long outburst duration in such a short-period (porb ≈ 3.24 hr) source, and complex variability observed during this outburst period, challenges the predictions of the widely accepted disc-instability model, which has been shown to broadly reproduce the behaviour of lmxb systems well. the long-term behaviour observed in j1753 is reminiscent of the z cam class of dwarf novae, whereby variable mass transfer from the companion star drives unusual outbursts, characterized by stalled decays and abrupt changes in luminosity. using sophisticated modelling of the multiwavelength light curves and spectra of j1753, during the ∼12 yr the source was active, we investigate the hypothesis that periods of enhanced mass transfer from the companion star may have driven this unusually long outburst. our modelling suggests that j1753 is in fact a bh-lmxb analogue to z cam systems, where the variable mass transfer from the companion star is driven by the changing irradiation properties of the system, affecting both the disc and companion star. | the curious case of swift j1753.5-0127: a black hole low-mass x-ray binary analogue to z cam type dwarf novae |
previous comparisons of experimental data with nonlinear numerical simulations of density stratified taylor-couette (tc) flows revealed nonlinear interactions of strato-rotational instability (sri) modes that lead to periodic changes in the sri spirals and their axial propagation. these pattern changes are associated with low-frequency velocity modulations that are related to the dynamics of two competing spiral wave modes propagating in opposite directions. in the present paper, a parameter study of the sri is performed using direct numerical simulations to evaluate the influence of the reynolds numbers, the stratification, and of the container geometry on these sri low-frequency modulations and spiral pattern changes. the results of this parameter study show that the modulations can be considered as a secondary instability that are not observed for all sri unstable regimes. the findings are of interest when the tc model is related to star formation processes in accretion discs. this article is part of the theme issue `taylor-couette and related flows on the centennial of taylor's seminal philosophical transactions paper (part 2)'. | a parameter study of strato-rotational low-frequency modulations: impacts on momentum transfer and energy distribution |
gro j1744-28, commonly known as the `bursting pulsar', is a low-mass x-ray binary containing a neutron star and an evolved giant star. this system, together with the rapid burster (mxb 1730-33), are the only two systems that display the so-called type ii x-ray bursts. these types of bursts, which last for tens of seconds, are thought to be caused by viscous instabilities in the disc; however, the type ii bursts seen in gro j1744-28 are qualitatively very different from those seen in the archetypal type ii bursting source, the rapid burster. to understand these differences and to create a framework for future study, we perform a study of all x-ray observations of all three known outbursts of the bursting pulsar which contained type ii bursts, including a population study of all type ii x-ray bursts seen by rxte. we find that the bursts from this source are best described in four distinct phenomena or `classes' and that the characteristics of the bursts evolve in a predictable way. we compare our results with what is known for the rapid burster and put out results in the context of models that try to explain this phenomena. | the evolution of x-ray bursts in the `bursting pulsar' gro j1744-28 |
increasing observational and theoretical evidence points to binary white dwarf (wd) mergers as the origin of some, if not most, normal type ia supernovae (sne ia). in this paper, we discuss the post-merger evolution of binary wd mergers and their relevance to the double-degenerate channel of sne ia. we present 3d simulations of carbon-oxygen (c/o) wd binary systems undergoing unstable mass transfer, where we vary both the total mass and the mass ratio. we demonstrate that these systems generally give rise to a one-armed gravitational spiral instability. the spiral density modes transport mass and angular momentum in the disk even in the absence of a magnetic field and are most pronounced in systems with secondary-to-primary mass ratios larger than 0.6. we further analyze carbon burning in these systems to assess the possibility of detonation. unlike the case of a 1.1+1.0 {m}⊙c/o wd binary, we find that wd binary systems with lower mass and smaller mass ratios do not detonate as sne ia up to ∼8-22 outer dynamical times. two additional models do, however, undergo net heating, and their secular increase in temperature could possibly result in a detonation on timescales longer than those considered here. | one-armed spiral instability in double-degenerate post-merger accretion disks |
we investigated the quasi-periodic oscillation (qpo) features in the accretion-powered x-ray pulsar cen x-3 observed by insight-hxmt. for two observations in 2020 when cen x-3 was in an extremely soft state, the power density spectrum revealed the presence of obvious qpo features at ~40 mhz with an averaged fractional rms amplitude of ~9 per cent. we study the mhz qpo frequency and rms amplitude over orbital phases, and find that the qpo frequency is ~33-39 mhz at the orbital phase of 0.1-0.4, increasing to ~37-43 mhz in the orbital phase of 0.4-0.8, but has no strong dependence on x-ray intensity. we also carried out an energy-dependent qpo analysis, the rms amplitude of the mhz qpos have a decreasing trend as the energy increases from 2 to 20 kev. in addition, the qpo time-lag analysis shows that the time-delay is ~20 ms (a hard lag) in the range of ~5-10 kev, and becomes negative [time-lag of -(20-70) ms] above ~10 kev. the different qpo theoretical models are summarized and discussed. in the end, we suggest that these energy-dependent timing features as well as the origin of mhz qpos in cen x-3 may be ascribed to an instability when the accretion disc is truncated near the corotation radius. | detection of a quasi-periodic oscillation at40 mhz in cen x-3 with insight-hxmt |
based on previous studies of quasi-periodic oscillations (qpos) in neutron star (ns) low-mass x-ray binaries, mhz qpos are believed to be related to “marginally stable” burning on the ns surface. our study of phase-resolved energy spectra of these oscillations in 4u 1636-53 shows that the oscillations are not caused by variations in the blackbody temperature of the ns, but reveals a correlation between the change of the count rate during the mhz qpo pulse and the spatial extent of a region emitting blackbody emission. the maximum size of the emission area, {r}{bb}2={216.7}-86.4+93.2 km2, provides direct evidence that the oscillations originate from a variable surface area constrained on the ns and are therefore not related to instabilities in the accretion disk. the obtained lower limit on the size of the ns (11.0 km) rules out equations of state that prefer small ns radii. observations of mhz qpos in ns lmxbs with nicer and extp will reduce the statistical uncertainty in the lower limit on the ns radius, which together with better estimates of the hardening factor and distance, will allow for improved discrimination between different equations of state and compact star models. furthermore, future missions will allow us to measure the peak blackbody emission area for a single mhz qpo pulse, which will push the lower limit to larger radii. | millihertz quasi-periodic oscillations in 4u 1636-536: putting possible constraints on the neutron star size |
it is quite likely that self-gravity will play an important role in the evolution of accretion discs, in particular, those around young stars, and those around supermassive black holes. we summarise, here, our current understanding of the evolution of such discs, focussing more on discs in young stellar system, than on discs in active galactic nuclei. we consider the conditions under which such discs may fragment to form bound objects, and when they might, instead, be expected to settle into a quasi-steady, self-regulated state. we also discuss how this understanding may depend on the mass of the disc relative to the mass of the central object, and how it might depend on the presence of external irradiation. additionally, we consider whether or not fragmentation might be stochastic, where we might expect it to occur in an actual protostellar disc, and if there is any evidence for fragmentation actually playing a role in the formation of planetary-mass bodies. although there are still a number of outstanding issue, such as the convergence of simulations of self-gravitating discs, whether or not there is more than one mode of fragmentation, and quite what role self-gravitating discs may play in the planet-formation process, our general understanding of these systems seems quite robust. | the evolution of self-gravitating accretion discs |
the physical mechanism of angular momentum transport in poorly ionized regions of protoplanetary discs, the dead zones (dzs), is not understood. the presence of a dz naturally leads to conditions susceptible to the rossby wave instability (rwi), which produces vortices and spiral density waves that may revive the dz and be responsible for observed large-scale disc structures. we present a series of two-dimensional hydrodynamic simulations to investigate the role of the rwi in dzs, including its impact on the long-term evolution of the disc and its morphology. the non-linear rwi can generate reynolds stresses (effective α parameter) as large as 0.01-0.05 in the dz, helping to sustain quasi-steady accretion throughout the disc. it also produces novel disc morphologies, including azimuthal asymmetries with m = 1, 2, and atypical vortex shapes. the angular momentum transport strength and morphology are most sensitive to two parameters: the radial extent of the dz and the disc viscosity. the largest reynolds stresses are produced when the radial extent of the dz is less than its distance to the central star. such narrow dzs lead to a single vortex or two coherent antipodal vortices in the quasi-steady state. the edges of wider dzs evolve separately, resulting in two independent vortices and reduced angular momentum transport efficiency. in either case, we find that, because of the reynolds stresses generated by the non-linear rwi, gravitational instability is unlikely to play a role in angular momentum transport across the dz, unless the accretion rate is sufficiently high. | rossby wave instability and long-term evolution of dead zones in protoplanetary discs |
damping of the previously discovered resonant drag instability (rdi) of dust streaming in the protoplanetary disc is studied using the local approach to dynamics of gas-dust perturbations in the limit of the small dust fraction. turbulence in a disc is represented by the effective viscosity and diffusivity in equations of motion for gas and dust, respectively. in the standard case of the schmidt number (ratio of the effective viscosity to diffusivity) sc = 1, the reduced description of rdi in terms of the inertial wave (iw) and the streaming dust wave (sdw) falling in resonance with each other reveals that damping solution differs from the inviscid solution simply by adding the characteristic damping frequency to its growth rate. rdi is fully suppressed at the threshold viscosity, which is estimated analytically, first, for radial drift, next, for vertical settling of dust, and at last, in the case of settling combined with a radial drift of the dust. in the last case, rdi survives up to the highest threshold viscosity, with a greater excess for smaller solids. once sc ≠ 1, a new instability specific for dissipative perturbations on the dust settling background emerges. this instability of the quasi-resonant nature is referred to as settling viscous instability (svi). the mode akin to sdw (iw) becomes growing in a region of long waves provided that sc > 1 (sc < 1). svi leads to an additional increase in the threshold viscosity. | the resonant drag instability of dust streaming in turbulent protoplanetary disc |
the magnetorotational instability (mri) is thought to be a powerful source of turbulence in keplerian accretion disks. motivated by recent laboratory experiments, we study the mri driven by an azimuthal magnetic field in an electrically conducting fluid sheared between two concentric rotating cylinders. by adjusting the rotation rates of the cylinders, we approximate angular velocity profiles {ω }\propto {r}q. we perform direct numerical simulations of a steep profile close to the rayleigh line q≳ -2 and a quasi-keplerian profile q≈ -3/2 and cover wide ranges of reynolds ({{re}} ≤slant 4× {10}4) and magnetic prandtl numbers (0≤slant {{pm}}≤slant 1). in the quasi-keplerian case, the onset of instability depends on the magnetic reynolds number, with {{{rm}}}c≈ 50, and angular momentum transport scales as \sqrt{{{pm}}}{{{re}}}2 in the turbulent regime. the ratio of maxwell to reynolds stresses is set by rm. at the onset of instability both stresses have similar magnitude, whereas the reynolds stress vanishes or becomes even negative as rm increases. for the profile close to the rayleigh line, the instability shares these properties as long as {{pm}}≳ 0.1 but exhibits a markedly different character if {{pm}}\to 0, where the onset of instability is governed by the reynolds number, with {{{re}}}c ≈ 1250, and transport is via reynolds stresses and scales as re2. at intermediate pm = 0.01 we observe a continuous transition from one regime to the other, with a crossover at {{rm}}={ o }(100). our results give a comprehensive picture of angular momentum transport of the mri with an imposed azimuthal field. | transport properties of the azimuthal magnetorotational instability |
recently, two empirical correlations related to the minimum variability timescale (mts) of the light curves are discovered in gamma-ray bursts (grbs). one is the anti-correlation between mts and lorentz factor γ, and the other is the anti-correlation between the mts and gamma-ray luminosity lγ . both of the two correlations might be used to explore the activity of the central engine of grbs. in this paper, we try to understand these empirical correlations by combining two popular black hole central engine models (namely, the blandford & znajek mechanism (bz) and the neutrino-dominated accretion flow (ndaf)). by taking the mts as the timescale of viscous instability of the ndaf, we find that these correlations favor the scenario in which the jet is driven by the bz mechanism. | what can we learn about grb from the variability timescale related correlations? |
we present optical photometry of superoutbursts that occurred in 2016 of two wz sge-type dwarf novae (dne), asassn-16dt and asassn-16hg. their light curves showed a dip in brightness between the first plateau stage with no ordinary superhumps (or early superhumps) and the second plateau stage with ordinary superhumps. we find that the dip is produced by the slow evolution of the 3 : 1 resonance tidal instability and that it would likely be observed in low mass-ratio objects. an estimated mass ratio (q ≡ m2/m1) from the period of developing (stage a) superhumps [0.06420(3) d] was 0.036(2) in asassn-16dt. additionally, its superoutburst has many properties similar to those in other low-q wz sge-type dne: long-lasting stage-a superhumps, small superhump amplitudes, long delay of ordinary-superhump appearances, and a slow decline rate in the plateau stage with superhumps. its very small mass ratio and observational characteristics suggest that this system is one of the best candidates for a period bouncer—a binary accounting for the missing population of post-period minimum cataclysmic variables. although it is not clearly verified due to the lack of detection of stage-a superhumps, asassn-16hg might be a possible candidate for period bouncers on the basis of the morphology of its light curves and the small superhump amplitudes. many outburst properties of period bouncer candidates would originate from the small tidal effects of their secondary stars. | asassn-16dt and asassn-16hg: promising candidate period bouncers |
the low mass star asassn-13db experienced an exor outburst in 2013, which identified it as a young stellar object (yso). then, from 2014 to 2017 it had another outburst, longer and more luminous than the earlier. we analyze the observations of the second outburst, and compare it to eruptions of intermediate luminosity optical transients (ilots). we show that the decline of the light curve is almost identical to that of the v838 mon, a prototype of a type of ilot known as luminous red nova (lrn). this similarity becomes conspicuous when oscillations that are associated with rotation are filtered out from the light curve of asassn-13db. we suggest that the eruption was the result of accretion of a proto-planet of a few earth masses. the proto-planet was shredded by tidal forces before it was accreted onto the yso, releasing gravitational energy that powered the outburst for ≈800days, and ended in a ≈55days decline phase. when the accretion material started depleting the accretion rate lowered and the eruption light curve declined for almost two months. then it exhausted completely, creating a sharp break in the light curve. another possibility is that the mass was a result of an instability in the proto-planetary disk that lead to a large episode of accretion from an inner viscous disk. we find that the variation of the temperature of the outburst is consistent with the surface temperature expected from a depleted viscous accretion disk. the 2014–2017 outburst of asassn-13db may be the least energetic ilot to have been discovered to date, with an energy budget of only ≈1042erg. | asassn-13db 2014–2017 eruption as an intermediate luminosity optical transient |
depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. nakayama has shown that these two types of flow configurations are expected to have different stability properties. by employing the chevalier & imamura linearization method and the nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. in regions of instability, we find that preshock deceleration is always unstable to the zeroth mode with zero frequency of oscillation, but is always stable to the fundamental mode and overtones. furthermore, we also find that preshock acceleration is always unstable to the zeroth mode and that the fundamental mode and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expands above ∼12 gravitational radii at the shock radius. in regions of stability, we demonstrate the zeroth mode to be stable for the velocity profiles that exhibit preshock acceleration and deceleration. moreover, for models that are linearly unstable, our model suggests the possible existence of quasi-periodic oscillations (qpos) with ratios 2:3 and 3:5. these ratios are believed to occur in stellar and supermassive black hole candidates, for example, in grs 1915+105 and sgr a*, respectively. we expect that similar qpo ratios also exist in regions of stable shocks. | standing shock instability in advection-dominated accretion flows |
we investigate the 2d instability recently discussed by gallet et al. (2010) and ilin \& morgulis (2013) which arises when a radial crossflow is imposed on a centrifugally-stable swirling flow. by finding a simpler rectilinear example of the instability - a sheared half plane, the minimal ingredients for the instability are identified and the destabilizing/stabilizing effect of inflow/outflow boundaries clarified. the instability - christened `boundary inflow instability' here - is of critical layer type where this layer is either at the inflow wall and the growth rate is $o(\sqrt{\eta})$ (as found by ilin \& morgulis 2013), or in the interior of the flow and the growth rate is $o(\eta \log 1/\eta)$ where $\eta$ measures the (small) inflow-to-tangential-flow ratio. the instability is robust to changes in the rotation profile even to those which are very rayleigh-stable and the addition of further physics such as viscosity, 3-dimensionality and compressibility but is sensitive to the boundary condition imposed on the tangential velocity field at the inflow boundary. providing the vorticity is not fixed at the inflow boundary, the instability seems generic and operates by the inflow advecting vorticity present at the boundary across the interior shear. both the primary bifurcation to 2d states and secondary bifurcations to 3d states are found to be supercritical. assuming an accretion flow driven by molecular viscosity only so $\eta=o(re^{-1})$, the instability is not immediately relevant for accretion disks since the critical threshold is $o(re^{-2/3})$ and the inflow boundary conditions are more likely to be stress-free than non-slip. however, the analysis presented here does highlight the potential for mass entering a disk to disrupt the orbiting flow if this mass flux possesses vorticity. | instability driven by boundary inflow across shear: a way to circumvent rayleigh's stability criterion in accretion disks? |
according to rayleigh's criterion, rotating flows are linearly stable when their specific angular momentum increases radially outward. the celebrated magnetorotational instability opens a way to destabilize those flows, as long as the angular velocity is decreasing outward. using a local approximation we demonstrate that even flows with very steep positive shear can be destabilized by azimuthal magnetic fields which are current free within the fluid. we illustrate the transition of this instability to a rotationally enhanced kink-type instability in the case of a homogeneous current in the fluid, and discuss the prospects for observing it in a magnetized taylor-couette flow. | destabilization of rotating flows with positive shear by azimuthal magnetic fields |
the milky way galaxy hosts a four million solar mass black hole, sgr a*, that underwent a major accretion episode approximately 3-6 myr ago. during the episode, hundreds of young massive stars formed in a disc orbiting sgr a* in the central half parsec. the recent discovery of a hypervelocity star (hvs) s5-hvs1, ejected by sgr a* five myr ago with a velocity vector consistent with the disc, suggests that this event also produced binary star disruptions. the initial stellar disc has to be rather eccentric for this to occur. such eccentric discs can form from the tidal disruptions of molecular clouds. here, we perform simulations of such disruptions, focusing on gas clouds on rather radial initial orbits. as a result, stars formed in our simulations are on very eccentric orbits ($\bar{e}\sim 0.6$) with a lopsided configuration. for some clouds, counterrotating stars are formed. as in previous work, we find that such discs undergo a secular gravitational instability that leads to a moderate number of particles obtaining eccentricities of 0.99 or greater, sufficient for stellar binary disruption. we also reproduce the mean eccentricity of the young disc in the galactic centre, though not the observed surface density profile. we discuss missing physics and observational biases that may explain this discrepancy. we conclude that observed s-stars, hvss, and disc stars tightly constrain the initial cloud parameters, indicating a cloud mass between a few × 104 and $10^5\, {\rm m}_{\odot }$, and a velocity between ~40 and 80 km s-1 at 10 pc. | forming young and hypervelocity stars in the galactic centre via tidal disruption of a molecular cloud |
we study the hydrogen ionization instability mechanism in the context of low-mass x-ray binaries with a black hole as a central object. we make numerical calculations of the predicted outbursts' light curves and compare them to the data observed by x-ray satellites. the comparison to the data is done for five sources observed by rxte/asm (xte j1550-564, 4u 1630-472, xte j1859+226, gx 339-4, xte j1818-245) and one source observed by maxi (maxi j1659-152). the aim of this paper is to show that the hydrogen ionization instability operating in an accretion disk is responsible for the shape of outbursts observed in low-mass x-ray binaries. from the data fitting process, we put tight constraints on global source parameters such as black hole mass and disk accretion rate. the influence of chemical composition on the overall analysis is also shown. in the case of each outburst, we found the overall bolometric light curve shape that qualitatively matches the data. we were able to model the main outburst and secondary reflare often seen in the data, the latter one caused by the presence of metals in disk gas. in the case of 4u 1630-472, we analyzed two outbursts, which allowed us to put tight constraints on the black hole mass of 4 ± 0.5m⊙ and on the accretion rate of ${2}_{-0.2}^{+1.4}\times {10}^{-8}{m}_{\odot }$ yr-1. | ionization instability driven outbursts in sxts |
magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disc plane) than in long boxes (stretched in the radial direction). our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic reynolds number. to explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. as a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. in contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. as a result, different vertical sections of the tall box are permeated by large-scale non-zero azimuthal magnetic fluxes, facilitating the instability. in agreement with this picture, the cases when the dynamo is efficient are characterized by a strong intermittency of the local azimuthal magnetic fluxes. | magnetorotational dynamo action in the shearing box |
we investigate magnetohydrodynamic turbulence driven by the magnetorotational instability (mri) in keplerian disks with a nonzero net azimuthal magnetic field using shearing box simulations. as distinct from previous studies, we analyze turbulence dynamics in fourier ({\boldsymbol{k}}-) space to understand its sustenance. the linear growth of the mri with the azimuthal field has a transient character and is anisotropic in fourier space, leading to anisotropy of nonlinear processes in fourier space. as a result, the main nonlinear process appears to be a new type of angular redistribution of modes in fourier space—the nonlinear transverse cascade—rather than the usual direct/inverse cascade. we demonstrate that the turbulence is sustained by the interplay of the linear transient growth of the mri (which is the only energy supply for the turbulence) and the transverse cascade. these two processes operate at large length scales, comparable to the box size (disk scale height); the corresponding small wavenumber area, called the vital area in fourier space, is crucial for the sustenance, while outside the vital area, direct cascade dominates. the interplay of the linear and nonlinear processes in fourier space is generally too intertwined for a vivid schematization. nevertheless, we reveal the basic subcycle of the sustenance that clearly shows the synergy of these processes in the self-organization of the magnetized flow system. this synergy is quite robust and persists for the considered different aspect ratios of the simulation boxes. the spectral characteristics of the dynamical processes in these boxes are qualitatively similar, indicating the universality of the sustenance mechanism of the mri turbulence. | nonlinear transverse cascade and sustenance of mri turbulence in keplerian disks with an azimuthal magnetic field |
we investigate the conditions required for planet formation via gravitational instability (gi) and protoplanetary disc (ppd) fragmentation around m-dwarfs. using a suite of 64 sph simulations with 106 particles, the parameter space of disc mass, temperature, and radius is explored, bracketing reasonable values based on theory and observation. our model consists of an equilibrium, gaseous, and locally isothermal disc orbiting a central star of mass m* = m⊙/3. discs with a minimum toomre q of qmin ≲ 0.9 will fragment and form gravitationally bound clumps. some previous literature has found qmin < 1.3-1.5 to be sufficient for fragmentation. increasing disc height tends to stabilize discs, and when incorporated into q as qeff ∝ q(h/r)α for α = 0.18 is sufficient to predict fragmentation. some discrepancies in the literature regarding qcrit may be due to different methods of generating initial conditions (ics). a series of 15 simulations demonstrates that perturbing ics slightly out of equilibrium can cause discs to fragment for higher q. our method for generating ics is presented in detail. we argue that gi likely plays a role in ppds around m-dwarfs and that disc fragmentation at large radii is a plausible outcome for these discs. | fragmentation of protoplanetary discs around m-dwarfs |
we present results of the first global magnetohydrodynamic simulations of accretion disks fed by roche-lobe overflow, including vertical stratification, in order to investigate the roles of spiral shocks, magnetorotational instability (mri), and the accretion stream in disk structure and evolution. our models include a simple treatment of gas thermodynamics, with orbital mach numbers at the inner edge of the disk min of 5 and 10. we find mass accretion rates to vary considerably on all timescales, with only the mach 5 model reaching a clear quasi-stationary state. for mach 10, the model undergoes an outside-in, magnetically driven accretion event occurring on a timescale of ∼10 orbital periods of the binary. both models exhibit spiral shocks inclined with respect to the binary plane, with their position and inclination changing rapidly. however, the time-averaged location of these shocks in the equatorial plane is well fit by simple linear models. mri turbulence in the disk generates toroidal magnetic field patterns (butterfly diagrams) that are in some cases irregular, perhaps due to interaction with the spiral structure. while many of our results are in good agreement with local studies, we find some features (most notably those related to spiral shocks) can only be captured in global models such as studied here. thus, while global studies remain computationally expensive—even as idealized models—they are essential (along with more sophisticated treatment of radiation transport and disk thermodynamics) for furthering our understanding of accretion in binary systems. | stratified global mhd models of accretion disks in semidetached binaries |
dw cnc is an intermediate polar which has previously been observed in both high and low states. observations of the high state of dw cnc have previously revealed a spin period at ~38.6 min, however, observations from the 2018 to 2019 low state showed no evidence of the spin period. we present results from our analysis of 12 s cadence photometric data collected by next generation transit survey of dw cnc during the high state which began in 2019. following the previously reported suppression of the spin period signal, we identify the return of this signal during the high state, consistent with previous observations of it. we identify this as the restarting of accretion during the high state. we further identified three short outbursts lasting ~1 d in dw cnc with a mean recurrence time of ~60 d and an amplitude of ~1 mag. these are the first outbursts identified in dw cnc since 2008. due to the short nature of these events, we identify them not as a result of accretion instabilities but instead either from instabilities originating from the interaction of the magnetorotational instability in the accretion disc and the magnetic field generated by the white dwarf or the result of magnetic gating. | the return of the spin period in dw cnc and evidence of new high state outbursts |
we have investigated the outburst properties of low-mass x-ray binary transients (lmxbts) based on a comprehensive study of the outbursts observed in the past few decades. the outburst rates were estimated based on the x-ray monitoring data from swift/bat, rxte/asm, and maxi and previous reports in the literature. we found that almost all lmxbts with the orbital period below ∼12 hr showed only one outburst in these observations. there are systematic differences in the outburst rate between long-period ({p}orb}≳ 12 hr) and short-period ({p}orb}≲ 12 hr) systems. we infer that mass transfer rate is responsible for the systematic difference, since the disk instability model suggested that the mass transfer rate is a key factor affecting the quiescence time. the difference in outburst rate between long-period and short-period lmxbts is probably due to the different mass transfer mechanism at different evolutionary stages of the donors. based on the evolutionary tracks of single stars, we derived the critical orbital period for x-ray binaries that harbor a subgiant donor in various metallicities. the critical orbital period ({p}orb,{crit}}=12.4 hr) is consistent with the above orbital period boundary obtained from the statistics of outburst rates. furthermore, we found a negative correlation between the outburst rate and the orbital period in the samples for which the luminosity class of the donor star is iii/iv. the best-fitting power-law index for the black hole subsamples is roughly consistent with the theoretical prediction for those systems with a donor star evolved off the main sequence. | the relation between outburst rate and orbital period in low-mass x-ray binary transients |
we present high-resolution numerical simulations of the colliding wind system η carinae, showing accretion on to the secondary star close to periastron passage. our hydrodynamical simulations include self-gravity and radiative cooling. the smooth stellar winds collide and develop instabilities, mainly the non-linear thin shell instability, and form filaments and clumps. we find that a few days before periastron passage the dense filaments and clumps flow towards the secondary as a result of its gravitational attraction, and reach the zone where we inject the secondary wind. we run our simulations for the conventional stellar masses, m1 = 120 m⊙ and m2 = 30 m⊙, and for a high mass model, m1 = 170 m⊙ and m2 = 80 m⊙, that was proposed to better fit the history of giant eruptions. as expected, the simulations results show that the accretion processes are more pronounced for a more massive secondary star. | accretion at the periastron passage of eta carinae |
we studied dynamical balances in magnetorotational instability (mri) turbulence with a net vertical field in the shearing box model of disks. analyzing the turbulence dynamics in fourier ({\boldsymbol{k}}-)space, we identified three types of active modes that define the turbulence characteristics. these modes have lengths similar to the box size, i.e., lie in the small wavenumber region in fourier space labeled “the vital area” and are (i) the channel mode, uniform in the disk plane with the smallest vertical wavenumber; (ii) the zonal flow mode, azimuthally and vertically uniform with the smallest radial wavenumber; and (iii) the rest (parasitic) modes. the rest modes comprise those harmonics in the vital area whose energies reach more than 50% of the maximum spectral energy. the rest modes individually are not so significant compared to the channel and zonal flow modes; however, the combined action of their multitude is dominant over these two modes. these three mode types are governed by the interplay of the linear and nonlinear processes, leading to their interdependent dynamics. the linear processes consist of disk flow nonmodality modified classical mri with a net vertical field. the main nonlinear process is the transfer of modes over wavevector angles in fourier space—the transverse cascade. the channel mode exhibits episodic bursts supplied by linear mri growth, while the nonlinear processes mostly oppose this, draining the channel energy and redistributing it to the rest modes. as for the zonal flow, it does not have a linear source and is fed by nonlinear interactions of the rest modes. | active modes and dynamical balances in mri turbulence of keplerian disks with a net vertical magnetic field |
several hundred young stars lie in the innermost parsec of our galaxy. the supermassive black hole (smbh) might capture planets orbiting these stars and bring them onto nearly radial orbits. the same fate might occur to planetary embryos (pes), i.e., protoplanets born from gravitational instabilities in protoplanetary disks. in this paper, we investigate the emission properties of rogue planets and pes in the galactic center. in particular, we study the effects of photoevaporation caused by the ultraviolet background. rogue planets can hardly be detected by current or forthcoming facilities, unless they are tidally disrupted and accrete onto the smbh. in contrast, photoevaporation of pes (especially if the pe is being tidally stripped) might lead to a recombination rate as high as ≈ {10}45 s-1, corresponding to a brackett-γ luminosity {l}{br-γ }≈ {10}31 erg s-1, very similar to the observed luminosity of the dusty object g2. we critically discuss the possibility that g2 is a rogue pe, and the major uncertainties of this model. | signatures of planets and protoplanets in the galactic center: a clue to understanding the g2 cloud? |
we present results from three-dimensional, numerical relativity simulations of a tilted black hole-thick accretion disc system. the simulations are analysed using tracer particles in the disc which are advected with the flow. such tracers, which we employ in these new simulations for the first time, provide a powerful means to analyse in detail the complex dynamics of tilted black hole-torus systems. we show how its use helps to gain insight into the overall dynamics of the system, discussing the origin of the observed black hole precession and the development of a global non-axisymmetric m = 1 mode in the disc. our three-dimensional simulations show the presence of quasi-periodic oscillations (qpos) in the instantaneous accretion rate, with frequencies in a range compatible with those observed in low-mass x-ray binaries with either a black hole or a neutron star component. the frequency ratio of the dominant low-frequency peak and the first overtone is o1/f ∼ 1.9, a frequency ratio not attainable when modelling the qpos as p-mode oscillations in axisymmetric tori. | on the dynamics of tilted black hole-torus systems |
> accretion disc theory is less developed than stellar evolution theory although a similarly mature phenomenological picture is ultimately desired. while the interplay of theory and numerical simulations has amplified community awareness of the role of magnetic fields in angular momentum transport, there remains a long term challenge to incorporate the insights gained from simulations into improving practical models for comparison with observations. what has been learned from simulations that can lead to improvements beyond ss73 in practical models? here, we emphasize the need to incorporate the role of non-local transport more precisely. to show where large-scale transport would fit into the theoretical framework and how it is currently missing, we review why the wonderfully practical approach of shakura & sunyaev (astron. astrophys., vol. 24, 1973, pp. 337-355, ss73) is necessarily a mean field theory, and one which does not include large-scale transport. observations of coronae and jets, combined with the interpretation of results from shearing box simulations, of the magnetorotational instability (mri) suggest that a significant fraction of disc transport is indeed non-local. we show that the maxwell stresses in saturation are dominated by large-scale contributions and that the physics of mri transport is not fully captured by a viscosity. we also clarify the standard physical interpretation of the mri as it applies to shearing boxes. computational limitations have so far focused most attention toward local simulations, but the next generation of global simulations should help to inform improved mean field theories. mean field accretion theory and mean field dynamo theory should in fact be unified into a single theory that predicts the time evolution of spectra and luminosity from separate disc, corona and outflow contributions. finally, we note that any mean field theory, including that of ss73, has a finite predictive precision that needs to be quantified when comparing the predictions to observations. | motivation and challenge to capture both large-scale and local transport in next generation accretion theory |
iw and stars are a recently recognized group of dwarf novae which are characterized by a repeated sequence of brightening from a standstill-like phase with damping oscillations followed by a deep dip. kimura et al. (2019, pasj, submitted) recently proposed a model based on thermal-viscous disk instability in a tilted disk to reproduce the iw and-type characteristics. im eri experienced the iw and-type phase in 2018 and we recorded three cycles of the (damping) oscillation phase terminated by brightening. we identified two periods during the iw and-type state: 4-5 d small-amplitude (often damping) oscillations and a 34-43 d long cycle. this behavior is typical for an iw and-type star. the object gradually brightened within the long cycle before the next brightening, which terminated the (damping) oscillation phase. this observation agrees with the increasing disk mass during the long cycle predicted by the kimura et al. model of thermal-viscous disk instability in a tilted disk. we did not, however, succeed in detecting negative superhumps, which are considered to be the signature of a tilted disk. | iw and-type state in im eridani |
in an early work, the rossby wave instability was proposed to explain the variability thought to originate in the close vicinity of black holes but this was done in the pseudo-newtonian approach. here we present the first general relativistic (gr) hydrodynamics simulations of this instability not only proving its theorized existence in a full gr environment but also studying the effect of the strong gravity on the instability. to that end, we performed a set of simulations increasingly closer to the black hole with our new gr version of the mpi-amrvac code. this allows us to study the minute changes in the behaviour of the instability. we found that a pseudo-newtonian approach gives adequate results provided that the time-shifting induced by the black hole gravity is taken into account. hence, to view the disc as a distant observer would, a full gr ray-tracing post-treatment of the simulations is a must. | impact of the gravity of a schwarzschild black hole upon the rossby wave instability |
high-resolution sub-mm observations of some protoplanetary discs reveal non-axisymmetric features, which can often be interpreted as dust concentrations in vortices that form at the edges of gaps carved out by the embedded planets. we use recent results on the time-scale for the planet-driven vortex development in low-viscosity discs to set constraints on the mass and age of a planet producing the vortex. knowledge of the age of the central star in a vortex-bearing protoplanetary disc system allows one to set a lower limit on the planetary mass at the level of several tens of m⊕. also, an independent upper limit on the planetary mass would constrain the planetary age, although given the current direct imaging detection limits this constraint is not yet very stringent (it is also sensitively dependent on the disc scale height). these results can be extended to account for the history of planetary mass accretion if it is known. we apply our calculations to several protoplanetary discs harbouring vortex-like features as revealed by alma and set limits of (30-50) m⊕ (for disc aspect ratio of 0.1) on the minimum masses of putative planets that could be responsible for these vortices. our vortex-based method provides an independent way of constraining the properties of embedded planets, complementary to other approaches. | vortex weighing and dating of planets in protoplanetary discs |
when the accretion disc around a weakly magnetised neutron star (ns) meets the stellar surface, it should brake down to match the rotation of the ns, forming a boundary layer. as the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (sl) with negligible radial extent and structure. we perform hydrodynamical 2d spectral simulations of an sl, considering the disc as a source of matter and angular momentum. interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. for small accretion rates, we find that the sl is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. this instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the sl intermittently. at enhanced accretion rates, the sl is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. we compute artificial light curves of an sl viewed at different inclination angles. most of the simulated light curves show oscillations at frequencies close to 1 khz. we interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the sl. their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass x-ray binaries. | kilohertz quasi-periodic oscillations from neutron star spreading layers |
context. the disc instability model (dim) successfully explains why many accreting compact binary systems exhibit outbursts during which their luminosity increases by orders of magnitude. the dim correctly predicts which systems should be transient and works regardless of whether the accretor is a black hole, a neutron star, or a white dwarf. however, it has been known for some time that the outbursts of x-ray binaries, which contain neutron-star or black-hole accretors, exhibit hysteresis in the x-ray hardness-intensity diagram (hid). more recently, it has been shown that the outbursts of accreting white dwarfs also show hysteresis, but in a diagram combining optical, euv, and x-ray fluxes.aims: we examine the nature of the hysteresis observed in cataclysmic variables and low-mass x-ray binaries.methods: we used our disc evolution code for modelling dwarf nova outbursts, and constructed the hardness intensity diagram as predicted by the disc instability model.results: we show explicitly that the standard dim, modified only to account for disc truncation, can explain the hysteresis observed in accreting white dwarfs, but cannot explain that observed in x-ray binaries.conclusions: the spectral evidence for the existence of different accretion regimes or components (disc, corona, jets, etc.) should only be based on wavebands that are specific to the innermost parts of the discs, i.e. euv and x-rays; this task is difficult because of interstellar absorption. the existing data, however, indicate that a hysteresis is in the euv - x-ray domain is present in ss cyg. | hystereses in dwarf nova outbursts and low-mass x-ray binaries |
massive quiescent compact galaxies have been discovered at high redshifts, associated with rapid compaction and cessation of star formation (sf). in this work, we set out to quantify the time-scales in which sf is quenched in compact galaxies at intermediate redshifts. for this, we select a sample of green valley galaxies within the cosmos field in the midst of quenching their sf at 0.5 < z < 1.0 that exhibit varying degrees of compactness. based on the hδ absorption line and the 4000 å break of coadded zcosmos spectra for subsamples of normal-sized and compact galaxies we determine quenching time-scales as a function of compactness. we find that the sf quenching time-scales in green valley compact galaxies are much shorter than in normal-sized ones. in an effort to understand this trend, we use the illustris simulation to trace the evolution of the sf history, the growth rate of the central super massive black hole (smbh), and the agn feedback in compact and normal-sized galaxies. we find that the difference in sf quenching time-scales is due to the mode of the agn feedback. in the compact galaxies the kinematic-mode is dominant, being highly efficient at quenching the sf by depleting the internal gas. for normal-sized galaxies, the prevailing thermal-mode injects energy into the circumgalactic gas, preventing cold gas accretion and quenching sf via the slower strangulation mechanism. these results are consistent with the violent disc instability and gas-rich mergers scenarios, followed by strong agn and stellar feedback. although this kind of event is most expected to occur at z = 2-3, we find evidences that the formation of compact quiescent galaxies can occur at z < 1. | compact galaxies at intermediate redshifts quench faster than normal-sized galaxies |
based on early kepler data, østensen et al. found that kic 9202990 showed a 4-h and a two-week photometric period. they suggested the 4-h period was a signature of an orbital period; the longer period was possibly due to precession of an accretion disc and kic 9202990 was a cataclysmic variable with an accretion disc which is always in a bright state (a nova-like system). using the full kepler data set on kic 9202990 which covers 1421 d (quarter 2-17), and includes 1-min cadence data from the whole of quarters 5 and 16, we find that the 4-h period is stable and therefore a signature of the binary orbital period. in contrast, the 10-12 d period is not stable and shows an amplitude between 20 and 50 per cent. this longer period modulation is similar to those nova-like systems which show `stunted' outbursts. we discuss the problems that a precessing disc model has in explaining the observed characteristics and indicate why we favour a stunted outburst model. although such stunted events are considered to be related to the standard disc instability mechanism, their origin is not well understood. kic 9202990 shows the lowest amplitude and shortest period of continuous stunted outburst systems, making it an ideal target to better understand stunted outbursts and accretion instabilities in general. | continuous `stunted' outbursts detected from the cataclysmic variable kic 9202990 using kepler data |
context. fo aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. this requires explanation since in a low state, the mass-transfer rate is in principle too low for the disc to be fully ionised and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae.aims: we investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of two magnitudes in the optical band without showing outbursts.methods: we use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot).results: we show that although it is marginally possible for the accretion disc in the low state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 1019 g s-1 or even larger. this would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. we show that observations of fo aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of vy scl stars: during the decay, the magnetospheric radius exceeds the circularisation radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts.conclusions: our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still requires investigation. | the disappearance and reformation of the accretion disc during a low state of fo aquarii |
recent submillimeter observations show nonaxisymmetric brightness distributions with a horseshoe-like morphology for more than a dozen transition disks. the most-accepted explanation for the observed asymmetries is the accumulation of dust in large-scale vortices. protoplanetary disks’ vortices can form by the excitation of rossby wave instability in the vicinity of a steep pressure gradient, which can develop at the edges of a giant planet-carved gap or at the edges of an accretionally inactive zone. we studied the formation and evolution of vortices formed in these two distinct scenarios by means of two-dimensional locally isothermal hydrodynamic simulations. we found that the vortex formed at the edge of a planetary gap is short-lived, unless the disk is nearly inviscid. in contrast, the vortex formed at the outer edge of a dead zone is long-lived. the vortex morphology can be significantly different in the two scenarios: the vortex radial and azimuthal extensions are ∼1.5 and ∼3.5 times larger for the dead-zone edge compared to gap models. in some particular cases, the vortex aspect ratios can be similar in the two scenarios; however, the vortex azimuthal extensions can be used to distinguish the vortex formation mechanisms. we calculated predictions for vortex observability in the submillimeter continuum with alma. we found that the azimuthal and radial extent of the brightness asymmetry correlates with the vortex formation process within the limitations of α-viscosity prescription. | interpreting brightness asymmetries in transition disks: vortex at dead zone or planet-carved gap edges? |
context. some intermediate polars (ips) show outbursts that are much shorter than those observed in normal dwarf novae, and their origin remains unclear.aims: we examine the case of v1223 sgr, an intermediate polar that showed a short outburst in 1984, and compare the outburst characteristics with the predictions of the magnetospheric gating model.methods: we extracted outburst profiles from the american association of variable star observers (aavso) archival data. we used our code to compute the time-dependent evolution of an accretion disc truncated by the white dwarf magnetic field, using a simple description of the interaction between the disc and the magnetic field, as in d'angelo & spruit (2010, mnras, 406, 1208).results: we find that v1223 sgr underwent a series of short outbursts, with a rise lasting for typically two to three hours, and a slightly longer decay. when applied to ips, the model by d'angelo & spruit (2010, mnras, 406, 1208) accounts well for the observed outburst duration and intensity. however, we confirm that the model outcome depends sensitively on the assumptions of the rather poorly constrained model. we also searched the aavso database for short outbursts in other ips, identifying individual short outbursts in fo aqr, tv col, ny lup, and ei uma, but no series such as those observed in v1223 sgr. we also found a superoutburst, followed by a reflare in ctcv j2056−3014.conclusions: although the magnetic-gating accretion instability model is clearly responsible for the series of v1223 sgr short outbursts and most probably for similar events in other ips, the model describing this process needs improvement, in particular concerning the interaction between the magnetic field of the white dwarf and the accretion disc. this difficult task might benefit from further comparison of the model outcome with additional observations that show good time coverage and time resolution. | magnetically gated accretion model: application to short bursts in the intermediate polar v1223 sgr |
the redistribution of angular momentum is a long standing problem in our understanding of protoplanetary disc (ppd) evolution. the magnetorotational instability (mri) is considered a likely mechanism. we present the results of a study involving multifluid global simulations including ohmic dissipation, ambipolar diffusion and the hall effect in a dynamic, self-consistent way. we focus on the turbulence resulting from the non-linear development of the mri in radially stratified ppds and compare with ideal magnetohydrodynamics simulations. in the multifluid simulations, the disc is initially set up to transition from a weak hall-dominated regime, where the hall effect is the dominant non-ideal effect but approximately the same as or weaker than the inductive term, to a strong hall-dominated regime, where the hall effect dominates the inductive term. as the simulations progress, a substantial portion of the disc develops into a weak hall-dominated disc. we find a transition from turbulent to laminar flow in the inner regions of the disc, but without any corresponding overall density feature. we introduce a dimensionless parameter, αrm, to characterize accretion with αrm ≳ 0.1 corresponding to turbulent transport. we calculate the eddy turnover time, teddy, and compared this with an effective recombination time-scale, trcb, to determine whether the presence of turbulence necessitates non-equilibrium ionization calculations. we find that trcb is typically around three orders of magnitude smaller than teddy. also, the ionization fraction does not vary appreciably. these two results suggest that these multifluid simulations should be comparable to single-fluid non-ideal simulations. | global multifluid simulations of the magnetorotational instability in radially stratified protoplanetary discs |
the dynamical response of edge waves under the influence of self-gravity is examined in an idealised two-dimensional model of a proto-stellar disc, characterised in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius ?. the fluid in basic state is prescribed to rotate with a keplerian profile ? modified by some additional azimuthal sheared flow. a linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabiliser irrespective of the stratification of the system. the propagation properties of the waves are discussed in detail in the language of vorticity edge waves. the roles of both boussinesq and non-boussinesq effects upon the stability and propagation of these waves with and without the inclusion of self-gravity are then quantified. the dynamics involved with self-gravity non-boussinesq effect is shown to be a source of vorticity production where there is a jump in the basic state density in addition, self-gravity also alters the dynamics via the radial main pressure gradient, which is a boussinesq effect. further applications of these mechanical insights are presented in the conclusion including the ways in which multiple density jumps or gaps may or may not be stable. | on the mechanism of self gravitating rossby interfacial waves in proto-stellar accretion discs |
negative superhumps are believed to arise in cataclysmic variable systems when the accretion disk is tilted with respect to the orbital plane. slow retrograde precession of the line-of-nodes results in a signal—the negative superhump—with a period slightly less than the orbital period. previous studies have shown that tilted disks exhibit negative superhumps, but a consensus on how a disk initially tilts has not been reached. analytical work by lai (1999, apj, 524, 1030) suggests that a magnetic field on the primary can lead to a tilt instability in a disk when the dipole moment is offset in angle from the spin axis of the primary and when the primary’s spin axis is, itself, not aligned with the angular momentum axis of the binary orbit. however, lai did not apply his work to the formation of negative superhumps. in this paper, we add lai’s model to an existing smoothed particle hydrodynamics code. using this code, we demonstrate the emergence of negative superhumps in the “light curve” for a range of magnetic dipole moments. we show that the period deficits calculated from these negative superhumps match those in simulations using manually tilted disks. when positive superhumps appear (q≲ 0.33), we show that the period excesses calculated from these signals are also consistent with previous results. using examples, we show that the disks are tilted, though the tilt varies periodically, and that they precess in the retrograde direction. the magnetic fields found to lead to the emergence of negative superhumps lie in the kilogauss regime. | the emergence of negative superhumps in cataclysmic variables: smoothed particle hydrodynamics simulations |
we study the stability of density stratified flow between corotating vertical cylinders with rotation rates ωo<ωi and radius ratio ri/ro=0.877 , where subscripts o and i refer to the outer and inner cylinders. just as in stellar and planetary accretion disks, the flow has rotation, anticyclonic shear, and a stabilizing density gradient parallel to the rotation axis. the primary instability of the laminar state leads not to axisymmetric taylor vortex flow but to a nonaxisymmetric stratorotational instability (sri). the present work extends the range of reynolds numbers and buoyancy frequencies [n =√{(-g /ρ )(∂ ρ /∂ z ) }] examined in previous experiments. we present the first experimental results for the axial wavelength λ of the instability as a function of the internal froude number, fr=ωi/n ; λ increases by nearly an order of magnitude over the range of fr examined. for small outer cylinder reynolds number, the sri occurs for inner inner reynolds number larger than for the axisymmetric taylor vortex flow (i.e., the sri is more stable). for somewhat larger outer reynolds numbers the sri occurs for smaller inner reynolds numbers than taylor vortex flow and even below the rayleigh stability line for an inviscid fluid. shalybkov and rüdiger [astron. astrophys. 438, 411 (2005), 10.1051/0004-6361:20042492] proposed that the laminar state of a stably stratified rotating shear flow should be stable for ωo/ωi>ri/ro , but we find that this stability criterion is violated for n sufficiently large. at large reynolds number the primary instability is not the sri but a previously unreported nonperiodic state that mixes the fluid. | observations of the stratorotational instability in rotating concentric cylinders |
how a black hole accretes matter and how this process is regulated are fundamental but unsolved questions in astrophysics. in transient black-hole binaries, a lot of mass stored in an accretion disk is suddenly drained to the central black hole because of thermal-viscous instability. this phenomenon is called an outburst and is observable at various wavelengths (frank et al., 2002). during the outburst, the accretion structure in the vicinity of a black hole shows dramatical transitions from a geometrically-thick hot accretion flow to a geometrically-thin disk, and the transition is observed at x-ray wavelengths (remillard, mcclintock, 2006; done et al., 2007). however, how that x-ray transition occurs remains a major unsolved problem (dunn et al., 2008). here we report extensive optical photometry during the 2018 outburst of asassn-18ey (maxi j1820$+$070), a black-hole binary at a distance of 3.06 kpc (tucker et al., 2018; torres et al., 2019) containing a black hole and a donor star of less than one solar mass. we found optical large-amplitude periodic variations similar to superhumps which are well observed in a subclass of white-dwarf binaries (kato et al., 2009). in addition, the start of the stage transition of the optical variations was observed 5 days earlier than the x-ray transition. this is naturally explained on the basis of our knowledge regarding white dwarf binaries as follows: propagation of the eccentricity inward in the disk makes an increase of the accretion rate in the outer disk, resulting in huge mass accretion to the black hole. moreover, we provide the dynamical estimate of the binary mass ratio by using the optical periodic variations for the first time in transient black-hole binaries. this paper opens a new window to measure black-hole masses accurately by systematic optical time-series observations which can be performed even by amateur observers. | optical variability correlated with x-ray spectral transition in the black-hole transient asassn-18ey = maxi j1820+070 |
neutron star high-mass x-ray binaries with superorbital modulations in luminosity host warped inner accretion disks that occult the neutron star during precession. in smc x-1, the instability in the warped disk geometry causes superorbital period "excursions": times of instability when the superorbital period decreases from its typical value of 55 to ~40 days. disk instability makes smc x-1 an ideal system in which to investigate the effects of variable disk geometry on the inner accretion flow. using the high-resolution spectral and timing capabilities of the neutron star interior composition explorer, we examined the high state of four different superorbital cycles of smc x-1 to search for changes in spectral shape and connections to the unstable disk geometry. we performed pulse phase-averaged and phase-resolved spectroscopy to closely compare the changes in spectral shape and any cycle-to-cycle variations. while some parameters, including the photon index and absorbing column density, show slight variations with superorbital phase, these changes are most evident during the intermediate state of the superorbital cycle. few spectral changes are observed within the high state of the superorbital cycle, possibly indicating the disk instability does not significantly change smc x-1's accretion process. | constraining the evolution of the unstable accretion disk in smc x-1 with nicer |
circumplanetary discs can be linearly unstable to the growth of disc tilt in the tidal potential of the star-planet system. we use 3d hydrodynamical simulations to characterize the disc conditions needed for instability, together with its long-term evolution. tilt growth occurs for disc aspect ratios, evaluated near the disc outer edge, of h/r ≳ 0.05, with a weak dependence on viscosity in the wave-like regime of warp propagation. lower mass giant planets are more likely to have circumplanetary discs that satisfy the conditions for instability. we show that the tilt instability can excite the inclination to above the threshold where the circumplanetary disc becomes unstable to kozai-lidov (kl) oscillations. dissipation in the kl unstable regime caps further tilt growth, but the disc experiences large oscillations in both inclination and eccentricity. planetary accretion occurs in episodic accretion events. we discuss implications of the joint tilt-kl instability for the detectability of circumplanetary discs, for the obliquity evolution of forming giant planets, and for the formation of satellite systems. | kozai-lidov oscillations triggered by a tilt instability of detached circumplanetary discs |
we study the central mass accretion rate and its variation with time during the outburst of x-ray transients using disk irradiation effect once the outer boundary of the disk is defined by the binary parameters. we show that the decay of the light curve and its characteristics time are well linked to the disk aspect ratio (h / r) and the binary parameters. for that we parametrize viscosity (α) parameter with h / r , assuming that there is a vertical height dependence on the energy dissipated in the disk. we derive the central accretion rate to check the sensitivity on h / r . mass accretion rate changes notably with disk height, orbital period (p) and with binary mass function (q). for the consistency check, we apply the solution to a low mass x-ray binary (lmxrb) 4u 1543-47, whose binary parameters are dynamically measured. finally, we estimate the unknown binary parameters of maxi j1543-564. using this solution for the first time we propose some predictability on p for the unknown system. the estimated value of p varies in a range from ∼10.5 to 15.5 h, when the companion mass is ∼ 0.66m⊙ . our estimated decay time scale is ∼ 40d with α ∼ 0.18. | estimating disk parameters of black hole x-ray binary maxi j1543-564: effect of disk irradiation |
a new accretion picture based on a small disk surrounding a black hole is developed for the wind-fed source cyg x-1. the hard and soft spectral states of cyg x-1 are interpreted in terms of co-spatial two-component flows for the innermost region of an accretion disk. the state transitions result from the outward expansion and inward recession of this inner disk for the hard-to-soft and soft-to-hard transition, respectively. the theoretical framework for state transitions in black hole x-ray binaries with high-mass companions involving a change in the inner disk size, thus it differs from systems with low-mass companions involving the change in the outer disk size. this fundamental difference stems from the fact that matter captured and supplied to the black hole in wind-fed systems has low specific angular momentum and is hot essentially heated in the bow and spiral shocks, whereas it has high specific angular momentum and is cool in roche lobe overflow systems. the existence of a weak cool disk around the isco region in the hard state allows for the presence of a relativistically broadened fe k line. the small disk fed by gas condensation forms without an extensive outer disk, precluding thermal instabilities and large outbursts, resulting in the lack of large amplitude outbursts and hysteresis effects in the light curve of high-mass black hole x-ray binaries. their relatively persistent x-ray emission is attributed to their wind-fed nature. | a model for spectral states and their transition in cyg x-1 |
context. gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened, rotating accretion flow surrounding the new born star or star cluster. gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high (e.g., over 10%).aims: this paper takes the ob cluster-forming region g10.6-0.4 as an example. we introduce the enclosed gas mass around its central ultra compact (uc) hii region, address the gravitational stability of the accreting gas, and outline the observed potential signatures of gravitational instability.methods: the dense gas accretion flow around the central uc hii region in g10.6-0.4 is geometrically flattened, and is in an approximately edge-on projection. the position-velocity (pv) diagrams of various molecular gas tracers on g10.6-0.4 consistently show asymmetry in the spatial- and the velocity domain. we deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures, and by directly de-projecting the pv diagrams.results: we find that within the 0.3 pc radius, an infall velocity of 1-2 km s-1 may be required to explain the observed pv diagrams. in addition, the velocity distribution traced in the pv diagrams can be interpreted by spiral arm-like structures, which may be connected with exterior infalling gas filaments. we propose that the morphology of dense gas structures appears very similar to the spatially resolved gas structures around the ob cluster-forming region g33.92+0.11 with similar gas mass and size, which is likely, however, to be in an approximately face-on projection.conclusions: the dense gas accretion flow around g10.6-0.4 appears to be toomre-unstable, which is consistent with the existence of large-scale spiral arm-like structures, and the formation of localized gas condensations. the proposed approaches for data analyses may be applied to the observations of class 0/i low-mass protostars, in diagnosis of disk gravitational instability. | interpreting observations of edge-on gravitationally unstable accretion flows. the case of g10.6-0.4 |
we observed the 2018 november outburst of cs ind and confirmed that it was a genuine superoutburst with a very long [0.12471(1) d on average] superhump period. the superoutburst was preceded by a long precursor, which was recorded for the first time in su uma-type dwarf novae. our interpretation is that the combination of a sufficient amount of mass in the disk before the ignition of the outburst and the slow development of tidal instability near the borderline of the 3 : 1 resonance caused a cooling front to start before the full development of tidal instability. this finding provides more support to the recent interpretation of slow development of the tidal instability causing various phenomena similar to wz sge-type dwarf novae in su uma-type dwarf novae with very long orbital periods. | cs indi: su uma-type dwarf nova with long precursor outburst |
an attractive scenario for producing type ia supernovae (sne ia) is a double detonation, where detonation of an accreted helium layer triggers ignition of a c/o core. whether or not such a mechanism can explain some or most sne ia depends on the properties of the helium burning, which in turn is set by the composition of the surface material. using a combination of semi-analytic and simple numerical models, i explore when turbulent mixing due to hydrodynamic instabilities during the accretion process can mix c/o core material up into the accreted helium. mixing is strongest at high accretion rates, large white dwarf (wd) masses, and slow spin rates. the mixing would result in subsequent helium burning that better matches the observed properties of sne ia. in some cases, there is considerable mixing that can lead to more than 50% c/o in the accreted layer at the time of ignition. these results will hopefully motivate future theoretical studies of such strongly mixed conditions. mixing also has implications for other types of wd surface explosions, including the so-called .ia supernovae, the calcium-rich transients (if they arise from accreting wds), and metal-enriched classical novae. | turbulent mixing on helium-accreting white dwarfs |
using an advanced semi-analytical model (sam) for galaxy formation, we investigated the statistical effects of assuming two different mechanisms for triggering agn activity on the properties of agn host galaxies. we considered a first accretion mode where agn activity is triggered by disk instabilities (di) in isolated galaxies, and a second feeding mode where galaxy mergers and fly-by events (interactions, it) are responsible for producing a sudden destabilization of large quantities of gas, causing the mass inflow onto the central supermassive black hole. the effects of including it and di modes in our sam were studied and compared with observations separately to single out the regimes in which they might be responsible for triggering agn activity. we obtained the following results: i) the predictions of our model concerning the stellar mass functions of agn hosts point out that both di and it modes are able to account for the observed abundance of agn host galaxies with m∗ ≲ 1011m⊙; for more massive hosts, the di scenario predicts a much lower space density than the it model in every redshift bin, lying below the observational estimates for redshift z > 0.8. ii) the analysis of the colour-magnitude diagram of agn hosts for redshift z < 1.5 can provide a good observational test to effectively distinguish between di and it mode, since dis are expected to yield agn host galaxy colours skewed towards bluer colours, while in the it scenario the majority of hosts are expected to reside in the red sequence. iii) while both it and di scenarios can account for agn triggered in main sequence or starburst galaxies, dis fail in triggering agn activity in passive galaxies. the lack of di agn in passive hosts is rather insensitive to changes in the model describing the di mass inflow, and it is mainly caused by the criterion for the onset of disk instabilities included in our sam. iv) the two modes are characterized by a different duration of the agn phase, with dis lasting even on time scales of ~gyr, much longer than in the it scenario, where the galaxy interaction sets the duration of the burst phase around ~ 107 - 108 yr. v) the scatter of the star formation rate sfr - lbol relation could represent another crucial diagnostics to distinguish between the two agn triggering modes, since the di scenario predicts an appreciably lower scatter (especially at low-to-intermediate agn luminosities) of the relation than the it scenario. vi) disk instabilities are unable to account for the observed fraction of agn in groups for z ≲ 1 and clusters for z ≲ 0.7, while the it scenario matches observational data well. | physical properties of agn host galaxies as a probe of supermassive black hole feeding mechanisms |
context. disk accretion onto weakly magnetized stars leads to the formation of a boundary layer (bl) where the gas loses its excess kinetic energy and settles onto the star. there are still many open questions concerning the bl, for instance the transport of angular momentum (am) or the vertical structure.aims: it is the aim of this work to investigate the am transport in the bl where the magneto-rotational instability (mri) is not operating owing to the increasing angular velocity ω(r) with radius. we will therefore search for an appropriate mechanism and examine its efficiency and implications.methods: we perform 2d numerical hydrodynamical simulations in a cylindrical coordinate system (r,ϕ) for a thin, vertically integrated accretion disk around a young star. we employ a realistic equation of state and include both cooling from the disk surfaces and radiation transport in radial and azimuthal direction. the viscosity in the disk is treated by the α-model; in the bl there is no viscosity term included.results: we find that our setup is unstable to the sonic instability which sets in shortly after the simulations have been started. acoustic waves are generated and traverse the domain, developing weak shocks in the vicinity of the bl. furthermore, the system undergoes recurrent outbursts where the activity in the disk increases strongly. the instability and the waves do not die out for over 2000 orbits.conclusions: there is indeed a purely hydrodynamical mechanism that enables am transport in the bl. it is efficient and wave mediated; however, this renders it a non-local transport method, which means that models of a effective local viscosity like the α-viscosity are probably not applicable in the bl. a variety of further implications of the non-local am transport are discussed. | wave mediated angular momentum transport in astrophysical boundary layers |
the electromagnetic signature of a point explosion near a kerr black hole (bh) is evaluated. the first repetitions produced by gravitational lensing are not periodic in time; periodicity emerges only as the result of multiple circuits of the prograde and retrograde light rings and is accompanied by exponential dimming. gravitational focusing creates a sequence of concentrated caustic features and biases the detection of a repeating source toward alignment of the bh spin with the plane of the sky. we consider the polarization pattern in the case of emission by the lorentz upboosting and reflection of a magnetic field near the explosion site. then the polarized fraction of the detected pulse approaches unity, and rays propagating near the equatorial plane maintain a consistent polarization direction. near a slowly accreting supermassive bh (smbh), additional repetitions are caused by reflection off annular fragments of an orbiting disk that has passed through an ionization instability. these results are applied to the repeating fast radio burst (frb) source 121102, giving a concrete and predictive example of how frb detectability may be biased by lensing. a gravitational lensing delay of 10-30 s, and reflection delay up to ∼104 s, are found for emission near the innermost stable circular orbit of a 3 × 105 m ⊙ smbh; these effects combine to produce interesting correlations between delay time and burst fluence. a similar repetitive pulse envelope could be seen in the gravitational wave signal produced by a collision between compact stars near a smbh. | impulsive electromagnetic emission near a black hole |
we present optical photometry of a wz sge-type dwarf nova (dn), asassn-15jd. its light curve showed a small dip in the middle of the superoutburst in 2015 for the first time among wz sge-type dne. the unusual light curve implies a delay in the growth of the 3 : 1 resonance tidal instability. also, the light curve is similar to those of two other wz sge-type stars, sss j122221.7-311523 and ot j184228.1+483742, which are believed to be the best candidates for period bouncers on the basis of their small values of the mass ratio (q ≡ m2/m1). additionally, the small mean superhump amplitude (<0.1 mag) and the long duration of no ordinary superhumps at the early stage of its superoutburst are common to the best candidates for period bouncers. its average superhump period was psh = 0.0649810(78) d and no early superhumps were detected. although we could not estimate a mass ratio of asassn-15jd with high accuracy, this object is expected to be a candidate for a period bouncer-a binary accounting for the missing population of post-period minimum cataclysmic variables-based on the above characteristics. | asassn-15jd: wz sge-type star with intermediate superoutburst between single and double ones |
the current superactive state of the recurrent nova t coronae borealis (t crb) has been observed with unprecedented detail. previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. nustar and swift observed t crb in 2015 september, roughly halfway through the rise to optical maximum. in our analysis of these data, we have found that: (i) the uv emission, as observed with swift/uvot in 2015, was already as bright as it became in 2017, after the optical peak; (ii) the soft x-ray emission (e ≲ 0.6 kev) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; (iii) the hard x-ray emitting plasma (e ≳ 2 kev) had the same temperature and about half the flux of that observed during quiescence in 2006. this phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer timescale. | dissecting a disk-instability outburst in a symbiotic star: nustar and swift observations of t coronae borealis during the rise to the “superactive” state |
the magnetorotational instability (mri) is a shear instability and thus its sensitivity to the shear parameter q = -d ln ω/d ln r is of interest to investigate. motivated by astrophysical discs, most (but not all) previous mri studies have focused on the keplerian value of q = 1.5. using simulation with eight vertical density scaleheights, we contribute to the subset of studies addressing the effect of varying q in stratified numerical simulations. we discuss why shearing boxes cannot easily be used to study q > 2 and thus focus on q < 2. as per previous simulations, which were either unstratified or stratified with a smaller vertical domain, we find that the q dependence of stress for the stratified case is not linear, contrary to the shakura-sunyaev model. we find that the scaling agrees with abramowicz, brandenburg & lasota who found it to be proportional to the shear to vorticity ratio q/(2 - q). we also find however that the shape of the magnetic and kinetic energy spectra are relatively insensitive to q and that the ratio of maxwell stress to magnetic energy ratio also remains nearly independent of q. this is consistent with a theoretical argument in which the rate of amplification of the azimuthal field depends linearly on q and the turbulent correlation time τ depends inversely on q. as such, we measure the correlation time of the turbulence and find that indeed it is inversely proportional to q. | sensitivity of the magnetorotational instability to the shear parameter in stratified simulations |
recent observations indicate that the mass of supermassive black holes (smbhs) correlate differently with different galaxy stellar components. comparing such observations with the results of "ab initio" galaxy formation models can provide insight on the mechanisms leading to the growth of smbhs. here we use a state-of-the-art semi-analytic model of galaxy formation to investigate the correlation of the different galaxy stellar components with the mass of the central smbh. the stellar mass in the disc, in the bulge, and in the pseudo-bulge of galaxies is related to quiescent star formation, to galaxy interactions, and to the loss of angular momentum following disc instabilities, respectively. consistently with recent findings, we find that while the predicted bulge masses are tightly correlated with the smbh masses, the correlation between the latter and the galactic discs shows a much larger scatter, in particular when bulgeless galaxies are considered. in addition, we obtain that the predicted masses of pseudo-bulges shows little or no-correlation with the masses of smbhs. we track the histories of merging, star formation, and smbh accretion to investigate the physical processes at the origin of such findings within the context of cosmological models of galaxy formation. finally, we discuss the effects of variations of our assumed fiducial model on the results. | relative growth of black holes and the stellar components of galaxies |
magnetic fields of strongly magnetized stars can trap conducting matter due to frozen-in condition. in the force-free regime, the motion of the matter along the field lines may be considered in the 'bead on a wire' approximation. such a motion, if gravity and centrifugal forces are taken into account, has equilibrium points, some of which are stable. in most cases, stability is possible in about several per cent of the possible locations. corresponding oscillation frequencies span the range from zero to $\sqrt{3}$ of the spin frequency. we suggest that this variability mode may be excited in some x-ray pulsars during the outbursts and create the peaked broad-band noise component near the break frequency in the power density spectrum, as well as produce some of the quasi-periodic oscillation features in this frequency range. existence of this variability does not require any changes in mass accretion rate and involves only a small amount of matter infiltrating from the disc and magnetic flow due to interchange instabilities. | inertial oscillation modes of an inclined dipolar magnetosphere as a source of band-limited noise in x-ray pulsars |
the polluted white dwarf (wd) system sdss j122859.93+104032.9 (sdss j1228) shows variable emission features interpreted as originating from a solid core fragment held together against tidal forces by its own internal strength, orbiting within its surrounding debris disk. estimating the size of this orbiting solid body requires modeling the accretion rate of the polluting material that is observed mixing into the wd surface. that material is supplied via sublimation from the surface of the orbiting solid body. the sublimation rate can be estimated as a simple function of the surface area of the solid body and the incident flux from the nearby hot wd. on the other hand, estimating the accretion rate requires detailed modeling of the surface structure and mixing in the accreting wd. in this work, we present mesa wd models for sdss j1228 that account for the thermohaline instability and mixing in addition to heavy element sedimentation to constrain accurately the sublimation and accretion rate necessary to supply the observed pollution. we derive a total accretion rate of ${\dot{m}}_{\mathrm{acc}}=1.8\times {10}^{11}\,{\rm{g}}\,{{\rm{s}}}^{-1}$ , several orders of magnitude higher than the ${\dot{m}}_{\mathrm{acc}}=5.6\times {10}^{8}\,{\rm{g}}\,{{\rm{s}}}^{-1}$ estimate obtained in earlier efforts. the larger mass accretion rate implies that the minimum estimated radius of the orbiting solid body is ${r}_{\min }$ = 72 km, which, although significantly larger than prior estimates, still lies within the upper bounds (a few hundred kilometers) for which the internal strength could no longer withstand the tidal forces from the gravity of the wd. | reinterpreting the polluted white dwarf sdss j122859.93+104032.9 in light of thermohaline mixing models: more polluting material from a larger orbiting solid body |
we present unstratified 3d magnetohydrodynamical (mhd) simulations of an accretion disc with a boundary layer (bl) that have a duration of ∼1000 orbital periods at the inner radius of the accretion disc. we find the surprising result that angular momentum piles up in the boundary layer, which results in a rapidly rotating belt of accreted material at the surface of the star. the angular momentum stored in this belt increases monotonically in time, which implies that angular momentum transport mechanisms in the bl are inefficient and do not couple the accretion disc to the star. this is in spite of the fact that magnetic fields are advected into the bl from the disc and supersonic shear instabilities in the bl excite acoustic waves. in our simulations, these waves carry only a small fraction (∼ 10 per cent) of the angular momentum required for steady state accretion. using analytical theory and 2d viscous simulations in the r-φ plane, we derive an analytical criterion for belt formation to occur in the bl in terms of the ratio of the viscosity in the accretion disc to the viscosity in the bl. our mhd simulations have a dimensionless viscosity (α) in the bl that is at least a factor of ∼100 smaller than that in the disc. we discuss the implications of these results for bl dynamics and emission. | inefficient angular momentum transport in accretion disc boundary layers: angular momentum belt in the boundary layer |
the availability of multi-wavelength observations and parallaxes from the space missions and very comprehensive models of agb evolution that include the accretion of matter from the circumbinary disc have strongly impacted our understanding of these enigmatic objects. the important developments made in the recent times are summarized here. the revised estimates of luminosities (derived from better-defined spectral energy distributions (seds) and new distances from gaia dr2) further support the opinion that rv tauri stars contain a mixture of post-agb stars and post-rgb stars. their locations in hr diagram also indicate that the instability strip (is) of rv tauri stars have a broader extension in the cooler edge than that of classical cepheids. a new p-l relation has been calibrated for the galactic cepheids which have a steeper slope than that derived for the population ii cepheids and rv tauri stars in magellanic clouds . the most significant chemical peculiarity exhibited by rv tauri stars and other post-agb stars is the selective depletion of refractory elements that correlates with their condensation temperatures. a large range in the size of depletion as well as in the shapes of the depletion curves has been observed. earlier models to explain this effect were mostly qualitative. recent investigators model these depletions using evolutionary codes (e.g. mesa) to evolve stars in the post-agb phase, while including accretion of metal-poor gas from circumbinary disc. these authors model the accretion rate onto a the binary post-agb star from a viscously evolving disc for a range of initial accretion rates and disc masses. it is reported that large initial accretion rates and disc masses are required to explain the large depletion and saturated depletion curve that could extend the evolution time of post-agb star. it is also proposed that the unsaturated depletion curve (with a plateau) are likely to be caused by post-rgb stars. | recent advances in rv tauri stars |
swift j1357.2-0933 underwent an episodic accretion in 2011 and provided very regular temporal and spectral evolution, making it an ideal source for exploring the nature of very faint x-ray transients. in this work, we present a detailed analysis on both x-ray and near-ultraviolet (nuv) light curves. the fluxes at all wavelengths display a near-exponential decays in the early phase and transit to a faster decay at late times. the e-folding decay time-scales monotonically decrease with photon energies, and the derived viscous time-scale is τ_dot{m} ∼ 60 d. the time-scale in the late faster decay stage is about a few days. the high ratio of nuv luminosity to x-ray luminosity indicates that the irradiation is unimportant in this outburst, while the near-exponential decay profile and the long decay time-scales conflict with the disc thermal-viscous instability model. we thus suggest that the disc is thermally stable during the observations. adopting the truncated disc model, we obtain a lower limit of peak accretion rate of 0.03 dot{m}_edd and the x-ray radiative efficiency η < 5 × 10-4, which decreases as the luminosity declines. the low x-ray radiative efficiency is caused by the combined action of advection and outflows, and naturally explains that the x-ray reprocessing is overwhelmed by the viscous radiation of the outer standard disc in the nuv regime. we also propose a possibility that the outer standard disc recedes from the central black hole, resulting in the faster decay at late times. | multiwavelength light-curve evolution of swift j1357.2-0933 during its 2011 outburst |
bondi accretion assumes that there is a sink of mass at the centre - which in the case of a black hole (bh) corresponds to the advection of matter across the event horizon. other stars, such as a neutron star (ns), have surfaces and hence the infalling matter has to slow down at the surface. we study the initial value problem in which the matter distribution is uniform and at rest at t = 0. we consider different inner boundary conditions for bhs and nss: outflow boundary condition (mimicking mass sink at the centre) valid for bhs; and reflective and steady-shock (allowing gas to cross the inner boundary at subsonic speeds) boundary conditions for nss. we also obtain a similarity solution for cold accretion on to bhs and nss. 1d simulations show the formation of an outward-propagating and a standing shock in nss for reflective and steady-shock boundary conditions, respectively. entropy is the highest at the bottom of the subsonic region for reflective boundary conditions. in 2d this profile is convectively unstable. using steady-shock inner boundary conditions, the flow is unstable to the standing accretion shock instability in 2d, which leads to global shock oscillations and may be responsible for quasi-periodic oscillations seen in the light curves of accreting systems. for steady accretion in the quiescent state, spherical accretion rate on to an ns can be suppressed by orders of magnitude compared to that on to a bh. | spherical accretion: the influence of inner boundary and quasi-periodic oscillations |
we explore the formation process of a black hole (bh) through the pair-instability collapse of a rotating population iii very massive star in axisymmetric numerical relativity. as the initial condition, we employ a progenitor star that is obtained by evolving a rapidly rotating zero-age main-sequence star with mass 320 m ⊙ until it reaches a pair-instability region. we find that for such a rapidly rotating model, a fraction of the mass, ∼10 m ⊙, forms a torus surrounding the remnant bh of mass ∼130 m ⊙, and an outflow is driven by a hydrodynamical effect. we also perform simulations, artificially reducing the initial angular velocity of the progenitor star, and find that only a small or no torus is formed and no outflow is driven. we discuss the possible evolution scenario of the remnant torus for the rapidly rotating model by considering the viscous and recombination effects and show that if an energy of ∼1052 erg is injected from the torus to the envelope, the luminosity and timescale of the explosion could be of the orders of 1043 erg s-1 and years, respectively. we also point out the possibility for observing gravitational waves associated with the bh formation for the rapidly rotating model by ground-based gravitational-wave detectors. | black hole formation and explosion from rapidly rotating very massive stars |
we report the analysis of 22 b-band light curves of the dwarf nova v4140 sgr obtained with soar optical imager (soi)/soar during two nights along the decline of a superoutburst in 2006 september 12-24 and in quiescence over 50 d following the superoutburst. 3d eclipse mapping of the outburst light curves indicates that the accretion disc is elliptical (eccentricity e = 0.13) and that superhump maximum occurs when the mass donor star is aligned with the bulge of the elliptical disc. the accretion disc is geometrically thin both in outburst and in quiescence; it fills the primary roche lobe in the outburst and shrinks to about half this size in quiescence. the stability of the eclipse shape, width and depth along quiescence and the derived disc surface brightness distribution indicate that the quiescent accretion disc is in a high-viscosity, steady-state. flickering mapping of the quiescent data reveals that the low-frequency flickering arises from an azimuthally extended stream-disc impact region at disc rim and from the innermost disc region, whereas the high-frequency flickering originates in the accretion disc. assuming the disc-related flickering to be caused by fluctuations in the energy dissipation rate induced by magnetohydrodynamic turbulence (geertsema & achterberg), we find that the quiescent disc viscosity parameter is large, α ≃ 0.2-0.4, at all radii. the high-viscosity quiescent disc and the inferred low disc temperatures in superoutburst are inconsistent with expectations of the disc-instability model, and lead to the conclusion that the outbursts of v4140 sgr are powered by mass transfer bursts from its donor star. | soar observations of the high-viscosity accretion disc of the dwarf nova v4140 sagitarii in quiescence and in outburst |
we report on two superoutbursts of the am cvn-type object cr boo in 2014 april-march and 2015 may-june. a precursor outburst accompanied both of these superoutbursts. during the rising branch of the main superoutburst in 2014, we detected growing superhumps (stage a superhumps) whose period was 0.017669(24) d. assuming that this period reflects the dynamical precession rate at the radius of the 3:1 resonance, we could estimate the mass ratio (q = m2/m1) of 0.101(4) by using the stage a superhump period and the orbital period of 0.0170290(6) d. this mass ratio is consistent with that expected from the theoretical evolutionary model of am cvn-type objects. the detection of precursor outbursts and stage a superhumps is the second case in am cvn-type objects. there are two interpretations of the outbursts of am cvn-type objects. one is a dwarf nova (dn) outbursts analogy, which suggets that the outbursts are caused by thermal and tidal instabilities. another is the vy scl-type variation, which suggests that the outbursts are caused by the variation of the mass-transfer rate of the secondary.this detection of the superhump variations strongly supports the former interpretation. | superoutburst of cr bootis: estimation of mass ratio of a typical am cvn star by stage a superhumps |
in this article, we proceed to study convection as a possible factor of episodic accretion in protoplanetary disks. within the model presented in article i, the accretion history is analyzed at different rates and areas of matter inflow from the envelope onto the disk. it is shown that the burst-like regime occurs in a wide range of parameters. the long-term evolution of the disk is also modeled, including the decreasing-with-time matter inflow from the envelope. it is demonstrated that the disk becomes convectively unstable and maintains burst-like accretion onto the star for several million years. meanwhile, the instability expands to an area of several tens of astronomical units and gradually decreases with time. it is also shown that at early stages in the disk evolution, conditions arise for gravitational instability in the outer parts of the disk and for dust evaporation in the convectively unstable inner regions. the general conclusion of the study is that convection can serve as one of the mechanisms of episodic accretion in protostellar disks, but this conclusion needs to be verified using more consistent hydrodynamic models. | evolution of a viscous protoplanetary disk with convectively unstable regions. ii. accretion regimes and long-term dynamics |
it has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of three-body h2 formation rate. the uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of population iii star formation. here, we investigate how the amount of fragmentation in primordial gas depends on the adopted three-body rate. we present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihaloes. our results on the effect of three-body rate on the evolution until the first protostar forms agree well with previous studies. however, our modified version of gadget-2 smoothed particle hydrodynamics also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted three-body h2 formation rate. we find that the fragmentation behaviour of the gas is only marginally affected by the choice of three-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars. | the role of three-body h2 formation in the fragmentation of primordial gas |
we analysed the four-decades-long x-ray light curve of the low-luminosity active galactic nucleus (llagn) ngc 7213 and discovered a fast-rise-exponential-decay (fred) pattern, i.e. the x-ray luminosity increased by a factor of ≈4 within 200 d, and then decreased exponentially with an e-folding time ≈8116 d (≈22.2 yr). for the theoretical understanding of the observations, we examined three variability models proposed in the literature: the thermal-viscous disc instability model, the radiation pressure instability model, and the tde model. we find that a delayed tidal disruption of a main-sequence star is most favourable; either the thermal-viscous disc instability model or radiation pressure instability model fails to explain some key properties observed, thus we argue them unlikely. | a decades-long fast-rise-exponential-decay flare in low-luminosity agn ngc 7213 |
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