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transitional millisecond pulsars (tmsps) switch, on roughly multi-year timescales, between rotation-powered radio millisecond pulsar (rmsp) and accretion-powered low-mass x-ray binary (lmxb) states. the tmsps have raised several questions related to the nature of accretion flow in their lmxb state and the mechanism that causes the state switch. the discovery of coherent x-ray pulsations from psr j1023+0038 (while in the lmxb state) provides us with the first opportunity to perform timing observations and to compare the neutron star’s spin variation during this state to the measured spin-down in the rmsp state. whereas the x-ray pulsations in the lmxb state likely indicate that some material is accreting onto the neutron star’s magnetic polar caps, radio continuum observations indicate the presence of an outflow. the fraction of the inflowing material being ejected is not clear, but it may be much larger than that reaching the neutron star’s surface. timing observations can measure the total torque on the neutron star. we have phase-connected nine xmm-newton observations of psr j1023+0038 over the last 2.5 years of the lmxb state to establish a precise measurement of spin evolution. we find that the average spin-down rate as an lmxb is 26.8 ± 0.4% faster than the rate (-2.39 × 10-15 hz s-1) determined during the rmsp state. this shows that negative angular momentum contributions (dipolar magnetic braking, and outflow) exceed positive ones (accreted material), and suggests that the pulsar wind continues to operate at a largely unmodified level. we discuss implications of this tight observational constraint in the context of possible accretion models. | timing observations of psr j1023+0038 during a low-mass x-ray binary state |
using the state-of-the-art ska precursor, the meerkat radio telescope, we explore the limits to precision pulsar timing of millisecond pulsars achievable due to pulse stochasticity (jitter). we report new jitter measurements in 15 of the 29 pulsars in our sample and find that the levels of jitter can vary dramatically between them. for some, like the 2.2 ms pulsar psr j2241-5236, we measure an implied jitter of just ~4 ns h-1, while others, like the 3.9 ms psr j0636-3044, are limited to ~100 ns h-1. while it is well known that jitter plays a central role to limiting the precision measurements of arrival times for high signal-to-noise ratio observations, its role in the measurement of dispersion measure (dm) has not been reported, particularly in broad-band observations. using the exceptional sensitivity of meerkat, we explored this on the bright millisecond pulsar psr j0437-4715 by exploring the dm of literally every pulse. we found that the derived single-pulse dms vary by typically 0.0085 cm-3 pc from the mean, and that the best dm estimate is limited by the differential pulse jitter across the band. we postulate that all millisecond pulsars will have their own limit on dm precision which can only be overcome with longer integrations. using high-time resolution filterbank data of 9 μs, we also present a statistical analysis of single-pulse phenomenology. finally, we discuss optimization strategies for the meerkat pulsar timing program and its role in the context of the international pulsar timing array. | measurements of pulse jitter and single-pulse variability in millisecond pulsars using meerkat |
in this review we describe recent observational and theoretical developments in our understanding of pulsar winds and pulsar-wind nebulae (pwne). we put special emphasis on the results from observations of well-characterized pwne of various types (e.g., torus-jet and bowshock-tail), the most recent mhd modeling efforts, and the status of the flaring crab pwn puzzle. | pulsar-wind nebulae. recent progress in observations and theory |
the correlation between the charge radii differences in mirror nuclei pairs and the neutron skin thickness has been studied with the so-called finite range simple effective interaction over a wide mass region. the so far precisely measured charge radii difference data within their experimental uncertainty ranges in the 34ar-34s, 36ca-36s, 38ca-38ar, and 54ni-54fe mirror pairs are used to ascertain an upper limit for the slope parameter of the nuclear symmetry energy l ≈100 mev. this limiting value of l is found to be consistent with the upper bound of the nicer psr j 0740 +6620 constraint at 1 σ level for the radius r1.4 of 1.4 m⊙ neutron stars. the lower bound of the nicer r1.4 data constrains the lower limit of l to ≈70 mev. within the range for l =70 -100 mev the tidal deformability λ1.4 constraint, which is extracted from the gw170817 event at 2 σ level, and the recent prex-2 and crex data on the neutron skin thickness are discussed. | correlations between charge radii differences of mirror nuclei and stellar observables |
modified expansion rates in the early universe prior to big bang nucleosynthesis are common in modified gravity theories, and can have a significant impact on the generation of dark matter, matter-antimatter asymmetry, primordial black holes and the primordial gravitational wave (pgw) spectrum. here we study the pgw spectrum in modified gravity theories, in early universe cosmology. in particular, we consider scalar-tensor and extradimensional scenarios, investigating the detection prospects in current and future gw observatories. for the scalar-tensor case, pgw could be potentially observed by laser interferometers operating in the high-frequency range, while for the extradimensional case they could be detected even at low frequencies with pulsar timing arrays. we find that data from the planned network of several gw detectors operating across various frequency ranges could be able to distinguish between various modified gravity scenarios. | primordial gravitational wave signals in modified cosmologies |
recent observations of x-ray pulsars at low luminosities allow, for the first time, the comparison of theoretical models of the emission from highly magnetized neutron star atmospheres at low mass-accretion rates (ṁ ≲ 1015 g s−1) with the broadband x-ray data. the purpose of this paper is to investigate spectral formation in the neutron star atmosphere at low ṁ and to conduct a parameter study of the physical properties of the emitting region. we obtain the structure of the static atmosphere, assuming that coulomb collisions are the dominant deceleration process. the upper part of the atmosphere is strongly heated by the braking plasma, reaching temperatures of 30-40 kev, while its denser isothermal interior is much cooler (∼2 kev). we numerically solve the polarized radiative transfer in the atmosphere with magnetic compton scattering, free-free processes, and nonthermal cyclotron emission due to possible collisional excitations of electrons. the strongly polarized emitted spectrum has a double-hump shape that is observed in low-luminosity x-ray pulsars. a low-energy "thermal" component is dominated by extraordinary photons that can leave the atmosphere from deeper layers because of their long mean free path at soft energies. we find that a high-energy component is formed because of resonant comptonization in the heated nonisothermal part of the atmosphere even in the absence of collisional excitations. however, these latter, if present, affect the ratio of the two components. a strong cyclotron line originates from the optically thin, uppermost zone. a fit of the model to nustar and swift/xrt observations of gx 304−1 provides an accurate description of the data with reasonable parameters. the model can thus reproduce the characteristic double-hump spectrum observed in low-luminosity x-ray pulsars and provides insights into spectral formation. | x-ray emission from magnetized neutron star atmospheres at low mass-accretion rates. i. phase-averaged spectrum |
we report the discovery of seven new galactic pulsars with the canadian hydrogen intensity mapping experiment's fast radio burst (chime/frb) backend. these sources were first identified via single pulses in chime/frb, then followed up with chime/pulsar. four sources appear to be rotating radio transients, pulsar-like sources with occasional single-pulse emission with an underlying periodicity. of those four sources, three have detected periods ranging from 220 ms to 2.726 s. three sources have more persistent but still intermittent emission and are likely intermittent or nulling pulsars. we have determined phase-coherent timing solutions for the latter two. these seven sources are the first discovery of previously unknown galactic sources with chime/frb and highlight the potential of fast radio burst detection instruments to search for intermittent galactic radio sources. | first discovery of new pulsars and rrats with chime/frb |
we have analyzed in this work the updated sample of neutron star masses derived from the study of a variety of 96 binary systems containing at least one neutron star using bayesian methods. after updating the multimodality of the distributions found in previous works, we determined the maximum mass implied by the sample using a robust truncation technique, with the result $m_{max} \sim 2.5-2.6 \, m_{\odot}$. we have checked that this mass is actually consistent by generating synthetic data and employing a posterior predictive check. a comparison with seven published $m_{max}$ values inferred from the remnant of the ns-ns merger gw170817 was performed and the tension between the latter and the obtained $m_{max}$ value quantified. finally, we performed a local outlier factor test and verified that the result for $m_{max}$ encompasses the highest individual mass determinations with the possible exception of psr j1748-2021b. the conclusion is that the whole distribution already points toward a high value of $m_{max}$, while several lower values derived from the ns-ns merger event are disfavored and incompatible with the higher binary system masses. a large $m_{max}$ naturally accommodates the lower mass component of the event gw190814 as a neutron star. | the maximum mass of neutron stars may be higher than expected: an inference from binary systems |
the crab pulsar is the prime example of an emitter of giant pulses. these short, very bright pulses are thought to originate near the light cylinder, at ~1600 km from the pulsar. the pulsar's location inside the crab nebula offers an unusual opportunity to resolve the emission regions, using the nebula, which scatters radio waves, as a lens. we attempt to do this using a sample of 61,998 giant pulses found in coherently combined european vlbi network observations at 18 cm. these were taken at times of relatively strong scattering and hence good effective resolution. from correlations between pulse spectra, we show that the giant pulse emission regions are indeed resolved. we infer apparent diameters of ~2000 and ~2400 km for the main and interpulse components, respectively, and show that with these sizes the correlation amplitudes and decorrelation timescales and bandwidths can be understood quantitatively, both in our observations and in previous ones. using pulse-spectra statistics and correlations between polarizations, we also show that the nebula resolves the nanoshots that comprise individual giant pulses. the implied diameters of ~1100 km far exceed light-travel-time estimates, suggesting the emitting plasma is moving relativistically, with γ ≃ 104, as inferred previously from drifting bands during the scattering tail of a giant pulse. if so, the emission happens over a region extended along the line of sight by ~107 km. we conclude that relativistic motion likely is important for producing giant pulses, and may be similarly for other sources of short, bright radio emission, such as fast radio bursts. | resolving the emission regions of the crab pulsar's giant pulses. ii. evidence for relativistic motion |
ultralight bosons are well-motivated particles from various physical and cosmological theories and can be spontaneously produced during the superradiant process, forming a dense hydrogenlike cloud around the spinning black hole. after the growth saturates, the cloud slowly depletes its mass through gravitational-wave emission. in this work we study the orbit dynamics of a circular binary system containing such a gravitational atom saturated in various spin-0, -1 and -2 superradiant states, taking into account both the effects of dynamical friction and the cloud mass depletion. we estimate the significance of mass depletion, finding that although dynamical friction could dominate the inspiral phase, it typically does not affect the outspiral phase driven by the mass depletion. focusing on the large orbit radius, we investigate the condition to observe the outspiral and the detectability of the cloud via pulsar-timing signal in the case of black hole-pulsar binary. | signatures of ultralight bosons in compact binary inspiral and outspiral |
we present the science case for the proposed daksha high energy transients mission. daksha will comprise of two satellites covering the entire sky from 1~kev to $>1$~mev. the primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study gamma ray bursts (grbs). daksha is a versatile all-sky monitor that can address a wide variety of science cases. with its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. daksha can make key strides in grb research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. daksha will provide continuous monitoring of x-ray pulsars. it will detect magnetar outbursts and high energy counterparts to fast radio bursts. using earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard x-ray sources including active galactic nuclei, x-ray binaries, and slow transients like novae. correlation studies between the two satellites can be used to probe primordial black holes through lensing. daksha will have a set of detectors continuously pointing towards the sun, providing excellent hard x-ray monitoring data. closer to home, the high sensitivity and time resolution of daksha can be leveraged for the characterization of terrestrial gamma-ray flashes. | science with the daksha high energy transients mission |
we present daksha, a proposed high energy transients mission for the study of electromagnetic counterparts of gravitational wave sources, and gamma ray bursts. daksha will comprise of two satellites in low earth equatorial orbits, on opposite sides of earth. each satellite will carry three types of detectors to cover the entire sky in an energy range from 1 kev to >1 mev. any transients detected on-board will be announced publicly within minutes of discovery. all photon data will be downloaded in ground station passes to obtain source positions, spectra, and light curves. in addition, daksha will address a wide range of science cases including monitoring x-ray pulsars, studies of magnetars, solar flares, searches for fast radio burst counterparts, routine monitoring of bright persistent high energy sources, terrestrial gamma-ray flashes, and probing primordial black hole abundances through lensing. in this paper, we discuss the technical capabilities of daksha, while the detailed science case is discussed in a separate paper. | daksha: on alert for high energy transients |
high time resolution radio surveys over the last few years have discovered a population of millisecond-duration transient bursts called fast radio bursts (frbs), which remain of unknown origin. frbs exhibit dispersion consistent with propagation through a cold plasma and dispersion measures indicative of an origin at cosmological distances. in this paper, we perform monte carlo simulations of a cosmological population of frbs, based on assumptions consistent with observations of their energy distribution, their spatial density as a function of redshift and the properties of the interstellar and intergalactic media. we examine whether the dispersion measures, fluences, derived redshifts, signal-to-noise ratios and effective widths of known frbs are consistent with a cosmological population. statistical analyses indicate that at least 50 events at parkes are required to distinguish between a constant comoving frb density, and an frb density that evolves with redshift like the cosmological star formation rate density. | are the distributions of fast radio burst properties consistent with a cosmological population? |
the present study explores the f (r ,t ) modified gravity on the basis of observational data for three different compact stars with matter profile as anisotropic fluid without electric charge. in this respect, we adopt the well-known karmarker condition and assume a specific and interesting model for grr metric potential component which is compatible with this condition. this choice further leads to a viable form of metric component gtt by utilizing the karmarkar condition. we also present the interior geometry in the reference of schwarzschild interior and kohler-chao cosmological like solutions for f (r ,t ) theory. moreover, we calculate the spacetime constants by using the masses and radii from the observational data of three different compact stars namely 4u 1538-52, lmc x-4 and psr j1614-2230. in order to explore the viability and stability of the obtained solutions, some physical parameters and properties are presented graphically for all three different compact object models. it is noticed that the parameters c and λ have some important and considerable role for these solutions. it is concluded that our obtained solutions are physically acceptable, bearing a well-behave nature in f (r ,t ) modified gravity. | realistic stellar anisotropic model satisfying karmarker condition in f(r, t) gravity |
the second data release from the gaia mission (gaia dr2) includes, among its billion entries, astrometric parameters for binary companions to a number of known pulsars, including white dwarf companions to millisecond pulsars (msps) and the non-degenerate components of so-called “black widow” and “redback” systems. we find 22 such counterparts in dr2, of which 12 have statistically significant measurements of parallax. these dr2 optical proper motions and parallaxes provide new measurements of the distances and transverse velocities of the associated pulsars. for the most part, the results agree with existing radio interferometric and pulsar timing-based astrometry, as well as other distance estimates based on photometry or associations, and for some pulsars they provide the best known distance and velocity estimates. in particular, two of these pulsars have no previous distance measurement: psr j1227-4853, for which gaia measures a parallax of 0.62 ± 0.16 mas, and psr j1431-4715, with a gaia parallax of 0.64 ± 0.16 mas. using the gaia distance measurements, we find that dispersion-measure-based distance estimates calculated using the cordes & lazio and yao et al. galactic electron density models are on average slightly underestimated, which may be a selection effect due to the over-representation of pulsars at high galactic latitudes in the present gaia sample. while the gaia dr2 results do not quite match the precision that can be achieved by dedicated pulsar timing or radio interferometry, taken together they constitute a small but important improvement to the pulsar distance scale, and the subset of msps with distances measured by gaia may help improve the sensitivity of pulsar timing arrays to nanohertz gravitational waves. | binary pulsar distances and velocities from gaia data release 2 |
the recent discovery of gravitational waves from merging black holes has generated interest in primordial black holes as a possible component of dark matter. in this paper, we show that pulsar timing may soon have sufficient data to constrain 1 - 1000 m⊙ primordial black holes (pbhs) via the nondetection of a third-order shapiro time delay as the black holes move around the galactic halo. we present the results of a monte carlo simulation which suggests that future data from known pulsars may be capable of constraining the pbh density more stringently than other existing methods in the mass range ∼1 - 30 m⊙ . we find that timing new pulsars discovered using the proposed square kilometre array may constrain primordial black holes in this mass range to comprise less than ∼1 %- 10 % of the dark matter. | pulsar timing can constrain primordial black holes in the ligo mass window |
massive black hole binaries (mbhbs) are expected to form at the centre of merging galaxies during the hierarchical assembly of the cosmic structure, and are expected to be the loudest sources of gravitational waves (gws) in the low-frequency domain. however, because of the dearth of energy exchanges with background stars and gas, many of these mbhbs may stall at separations that are too large for gw emission to drive them to coalescence in less than a hubble time. triple mbh systems are then bound to form after a further galaxy merger, triggering a complex and rich dynamics that can eventually lead to mbh coalescence. here, we report on the results of a large set of numerical simulations, where mbh triplets are set in spherical stellar potentials and mbh dynamics is followed through 2.5 post-newtonian orders in the equations of motion. from our full suite of simulated systems, we find that a fraction ( ≃ 20-30 per cent) of the mbh binaries that would otherwise stall is led to coalesce within a hubble time. the corresponding coalescence time-scale peaks around 300 myr, while the eccentricity close to the plunge, albeit small, is non-negligible (≲0.1). we construct and discuss marginalized probability distributions of the main parameters involved and, in a companion paper of the series, we will use the results presented here to forecast the contribution of mbh triplets to the gw signal in the nhz regime probed by pulsar timing array experiments. | post-newtonian evolution of massive black hole triplets in galactic nuclei - ii. survey of the parameter space |
pulsar timing arrays are presently the only means to search for the gravitational wave stochastic background from super massive black hole binary populations, considered to be within the grasp of current or near-future observations. the stringent upper limit from the parkes pulsar timing array has been interpreted as excluding (>90% confidence) the current paradigm of binary assembly through galaxy mergers and hardening via stellar interaction, suggesting evolution is accelerated or stalled. using bayesian hierarchical modelling we consider implications of this upper limit for a range of astrophysical scenarios, without invoking stalling, nor more exotic physical processes. all scenarios are fully consistent with the upper limit, but (weak) bounds on population parameters can be inferred. recent upward revisions of the black hole-galaxy bulge mass relation are disfavoured at 1.6σ against lighter models. once sensitivity improves by an order of magnitude, a non-detection will disfavour the most optimistic scenarios at 3.9σ. | no tension between assembly models of super massive black hole binaries and pulsar observations |
the neutron-star interior composition explorer is an x-ray astrophysics payload that will be placed on the international space station. its primary science goal is to measure with high accuracy the pulse profiles that arise from the non-uniform thermal surface emission of rotation-powered pulsars. modeling general relativistic effects on the profiles will lead to measuring the radii of these neutron stars and to constraining their equation of state. achieving this goal will depend, among other things, on accurate knowledge of the source, sky, and instrument backgrounds. we use here simple analytic estimates to quantify the level at which these backgrounds need to be known in order for the upcoming measurements to provide significant constraints on the properties of neutron stars. we show that, even in the minimal-information scenario, knowledge of the background at a few percent level for a background-to-source countrate ratio of 0.2 allows for a measurement of the neutron star compactness to better than 10% uncertainty for most of the parameter space. these constraints improve further when more realistic assumptions are made about the neutron star emission and spin, and when additional information about the source itself, such as its mass or distance, are incorporated. | measuring neutron star radii via pulse profile modeling with nicer |
we argue that the localization of the repeating fast radio bursts (frbs) at ∼1 gpc excludes a rotationally powered type of radio emission (e.g., analogs of crab’s giant pulses coming from very young energetic pulsars) as the origin of frbs. | fast radio bursts’ emission mechanism: implication from localization |
the young, fast-spinning x-ray pulsar j0537-6910 displays an extreme glitch activity, with large spin-ups interrupting its decelerating rotation every ~100 d. we present nearly 13 yr of timing data from this pulsar, obtained with the rossi x-ray timing explorer. we discovered 22 new glitches and performed a consistent analysis of all 45 glitches detected in the complete data span. our results corroborate the previously reported strong correlation between glitch spin-up size and the time to the next glitch, a relation that has not been observed so far in any other pulsar. the spin evolution is dominated by the glitches, which occur at a rate of ~3.5 per year, and the post-glitch recoveries, which prevail the entire interglitch intervals. this distinctive behaviour provides invaluable insights into the physics of glitches. the observations can be explained with a multicomponent model that accounts for the dynamics of the neutron superfluid present in the crust and core of neutron stars. we place limits on the moment of inertia of the component responsible for the spin-up and, ignoring differential rotation, the velocity difference it can sustain with the crust. contrary to its rapid decrease between glitches, the spin-down rate increased over the 13 yr, and we find the long-term braking index nl = -1.22(4), the only negative braking index seen in a young pulsar. we briefly discuss the plausible interpretations of this result, which is in stark contrast to the predictions of standard models of pulsar spin-down. | pulsar spin-down: the glitch-dominated rotation of psr j0537-6910 |
the cosmic-ray flux of positrons is measured with high precision by the space-borne particle spectrometer ams-02. the hypothesis that pulsar wind nebulae (pwne) can significantly contribute to the excess of the positron (e+) cosmic-ray flux has been consolidated after the observation of a γ-ray emission at tev energies of a few degree size around geminga and monogem pwne. in this work we undertake massive simulations of galactic pulsars populations, adopting different distributions for their position in the galaxy, intrinsic physical properties, pair emission models, in order to overcome the incompleteness of the atnf catalog. we fit the e+ ams-02 data together with a secondary component due to collisions of primary cosmic rays with the interstellar medium. we find that several mock galaxies have a pulsar population able to explain the observed e+ flux, typically by few, bright sources. we determine the physical parameters of the pulsars dominating the e+ flux, and assess the impact of different assumptions on radial distributions, spin-down properties, galactic propagation scenarios and e+ emission time. | constraining positron emission from pulsar populations with ams-02 data |
in this work, we explore the applicability of standard theoretical models of accretion to the observed properties of m51 ulx-7. the spin-up rate and observed x-ray luminosity are evidence of a neutron star with a surface magnetic field of 2-7 × 1013 g, rotating near equilibrium. analysis of the x-ray light curve of the system (swift/xrt data) reveals the presence of a ∼39 d superorbital period. we argue that the superorbital periodicity is due to disc precession, and that material is accreted on to the neutron star at a constant rate throughout it. moreover, by attributing this modulation to the free precession of the neutron star we estimate a surface magnetic field strength of 3-4 × 1013 g. the agreement of these two independent estimates provide strong constraints on the surface polar magnetic field strength of the ns. | m51 ulx-7: superorbital periodicity and constraints on the neutron star magnetic field |
gravitational waves provide a new probe of the universe which can reveal a number of cosmological and astrophysical phenomena that cannot be observed by electromagnetic waves. different frequencies of gravitational waves are detected by different means. among them, precision measurements of pulsar timing provides a natural detector for gravitational waves with light-year scale wavelengths. in this review, first a basic framework to detect a stochastic gravitational wave background using pulsar timing array is introduced, and then possible interpretations of the latest observational result of 12.5-year nanograv data are described. | implication of pulsar timing array experiments on cosmological gravitational wave detection |
in the face of increasingly severe flood disasters, urban resilience is a new path for future urban development. based on a review of relevant literature on urban resilience, we focusing on flood disaster scenes, present the perspective of "pressure - state - response" to the urban resilience process decomposition based on resilience city theories, disaster theories and ecology theories. we establish the rain flood situation of quantitative evaluation model of city resilience, combined with rough set theory to the urban economy, society and ecological integrated into three subsystems elements to investigate the spatiotemporal resilience of guangzhou city. the results indicate that the overall urban resilience of guangzhou from 2015 to 2019 is at a medium level, and shows a slight upward trend, among which the state resilience shows a sharp decline trend. the areas with high comprehensive resilience are mainly distributed in the main urban area, while the areas with weak comprehensive resilience are mainly distributed in the new urban development area. the index of vegetation coverage, regional economic spatial stability and average rainfall showed the most obvious decreasing trend. from 2050 to 2070, the future urban resilience of guangzhou under each rcps model will develop to a deteriorating trend, among which, the urban resilience under 2050 rcp2.6 scenario is the best, and the urban resilience under 2070 rcp8.5 scenario is the best. finally, we put forward a systematic urban resilience improvement strategy from three aspects of pressure resilience, state resilience and response resilience. this study is helpful to provide detailed decision-making assistance for urban resilience construction and case support for quantitative research on urban resilience. | assessment of temporal and spatial progress of urban resilience in guangzhou under rainstorm scenarios |
we report the first radio interferometric search at 843 mhz for fast transients, particularly fast radio bursts (frbs). the recently recommissioned swinburne university of technology's digital backend for the molonglo observatory synthesis telescope array (the utmost) with its large collecting area (18 000 m2) and wide instantaneous field of view (7.80 deg2) is expected to be an efficient tool to detect frbs. as an interferometer it will be capable of discerning whether the frbs are truly a celestial population. we show that utmost at full design sensitivity can detect an event approximately every few days. we report on two preliminary frb surveys at about 7 per cent and 14 per cent, respectively, of the array's final sensitivity. several pulsars have been detected via single pulses and no frbs were discovered with pulse widths (w), in the range 655.36 μs < w < 41.9 ms and dispersion measures (dms) in the range 100 < dm < 2000 pc cm-3. this non-detection sets a 2σ upper limit of the sky rate of not more than 1000 events sky-1 d-1 at 843 mhz down to a flux limit of 11 jy for 1 ms frbs. we show that this limit is consistent with previous survey limits at 1.4 ghz and 145 mhz and set a lower limit on the mean spectral index of frbs of α > -3.2. | fast radio transient searches with utmost at 843 mhz |
we investigate the equation of state for the recently developed hybrid quark-meson-nucleon model under neutron-star conditions of β -equilibrium and charge neutrality. the model has the characteristic feature that, at increasing baryon density, the chiral symmetry is restored within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. most important for this study are the nucleon (neutron, proton) and n (1535 ) states. we present different sets for two free parameters, which result in compact star mass-radius relations in accordance with modern constraints on the mass from psr j 0348 +0432 and on the compactness from gw170817. we also consider the threshold for the direct urca process for which a new relationship is given, and suggest as an additional constraint on the parameter choice of the model that this process becomes operative at best for stars with masses above the range for binary radio pulsars, m >1.4 m⊙ . | chiral symmetry restoration by parity doubling and the structure of neutron stars |
constraints on neutron star masses and radii now come from a variety of sources: theoretical and experimental nuclear physics, astrophysical observations including pulsar timing, thermal and bursting x-ray sources, and gravitational waves, and the assumptions inherent to general relativity and causality of the equation of state. these measurements and assumptions also result in restrictions on the dense matter equation of state. the two most important structural parameters of neutron stars are their typical radii, which impacts intermediate densities in the range of one to two times the nuclear saturation density, and the maximum mass, which impacts the densities beyond about three times the saturation density. especially intriguing has been the multi-messenger event gw170817, the first observed binary neutron star merger, which provided direct estimates of both stellar masses and radii as well as an upper bound to the maximum mass. | neutron star mass and radius measurements |
in outburst, neutron star x-ray binaries produce less powerful jets than black holes at a given x-ray luminosity. this has made them more difficult to study as they fade towards quiescence. to explore whether neutron stars power jets at low accretion rates (lx ≲ 1036 erg s-1), we investigate the radio and x-ray properties of three accreting millisecond x-ray pulsars (igr j17511-3057, sax j1808.4-3658 and igr j00291+5934) during their outbursts in 2015, and of the non-pulsing neutron star cen x-4 in quiescence (2015) and in outburst (1979). we did not detect the radio counterpart of igr j17511-3057 in outburst or of cen x-4 in quiescence, but did detect igr j00291+5934 and sax j1808.4-3658, showing that at least some neutron stars launch jets at low accretion rates. while the radio and x-ray emission in igr j00291+5934 seem to be tightly correlated, the relationship in sax j1808.4-3658 is more complicated. we find that sax j1808.4-3658 produces jets during the reflaring tail, and we explore a toy model to ascertain whether the radio emission could be attributed to the onset of a strong propeller. the lack of a universal radio/x-ray correlation, with different behaviours in different neutron star systems (with various radio/x-ray correlations; some being radio faint and others not), points at distinct disc-jet interactions in individual sources, while always being fainter in the radio band than black holes at the same x-ray luminosity. | disc-jet coupling in low-luminosity accreting neutron stars |
we measure the effects of interstellar scattering on average pulse profiles from 13 radio pulsars with simple pulse shapes. we use data from the lofar high band antennas, at frequencies between 110 and 190 mhz. we apply a forward fitting technique, and simultaneously determine the intrinsic pulse shape, assuming single gaussian component profiles. we find that the constant τ, associated with scattering by a single thin screen, has a power-law dependence on frequency τ ∝ ν-α, with indices ranging from α = 1.50 to 4.0, despite simplest theoretical models predicting α = 4.0 or 4.4. modelling the screen as an isotropic or extremely anisotropic scatterer, we find anisotropic scattering fits lead to larger power-law indices, often in better agreement with theoretically expected values. we compare the scattering models based on the inferred, frequency-dependent parameters of the intrinsic pulse, and the resulting correction to the dispersion measure (dm). we highlight the cases in which fits of extreme anisotropic scattering are appealing, while stressing that the data do not strictly favour either model for any of the 13 pulsars. the pulsars show anomalous scattering properties that are consistent with finite scattering screens and/or anisotropy, but these data alone do not provide the means for an unambiguous characterization of the screens. we revisit the empirical τ versus dm relation and consider how our results support a frequency dependence of α. very long baseline interferometry, and observations of the scattering and scintillation properties of these sources at higher frequencies, will provide further evidence. | scattering analysis of lofar pulsar observations |
we present coordinated chandra x-ray observatory and karl g. jansky very large array observations of the transitional millisecond pulsar psr j1023+0038 in its low-luminosity accreting state. the unprecedented five hours of strictly simultaneous x-ray and radio continuum coverage for the first time unambiguously show a highly reproducible, anti-correlated variability pattern. the characteristic switches from the x-ray high mode into a low mode are always accompanied by a radio brightening with a duration that closely matches the x-ray low mode interval. this behavior cannot be explained by a canonical inflow/outflow accretion model where the radiated emission and the jet luminosity are powered by, and positively correlated with, the available accretion energy. we interpret this phenomenology as alternating episodes of low-level accretion onto the neutron star during the x-ray high mode that are interrupted by rapid ejections of plasma by the active rotation-powered pulsar, possibly initiated by a reconfiguration of the pulsar magnetosphere, that cause a transition to a less x-ray luminous mode. the observed anti-correlation between radio and x-ray luminosity has an additional consequence: transitional msps can make excursions into a region of the radio/x-ray luminosity plane previously thought to be occupied solely by black hole x-ray binary sources. this complicates the use of this luminosity relation for identifying candidate black holes, suggesting the need for additional discriminants when attempting to establish the true nature of the accretor. | simultaneous chandra and vla observations of the transitional millisecond pulsar psr j1023+0038: anti-correlated x-ray and radio variability |
polarized radio emission from psr j1745-2900 has already been used to investigate the strength of the magnetic field in the galactic center (gc), close to sagittarius a*. here we report how persistent radio emission from this magnetar, for over four years since its discovery, has revealed large changes in the observed faraday rotation measure (rm), by up to 3500 rad m-2 (a 5% fractional change). from simultaneous analysis of the dispersion measure, we determine that these fluctuations are dominated by variations in either the projected magnetic field or the free electron content within the gc, along the changing line of sight to the rapidly moving magnetar. from a structure function analysis of rm variations, and a recent epoch of rapid change of rm, we determine a minimum scale of magneto-ionic fluctuations of size ∼2 au at the gc distance, inferring psr j1745-2900 is just ∼0.1 pc behind an additional scattering screen. | large magneto-ionic variations toward the galactic center magnetar, psr j1745-2900 |
chaos theory is integrated with multi-gene genetic programming (mggp) engine as a new hybrid model for river flow forecasting. this is to be referred to as chaos-mggp and its performance is tested using daily historic flow time series at four gauging stations in two countries with a mix of both intermittent and perennial rivers. three models are developed: (i) local prediction model (lpm); (ii) standalone mggp; and (iii) chaos-mggp, where the first two models serve as the benchmark for comparison purposes. the phase-space reconstruction (psr) parameters of delay time and embedding dimension form the dominant input signals derived from original time series using chaos theory and these are transferred to chaos-mggp. the paper develops a procedure to identify global optimum values of the psr parameters for the construction of a regression-type prediction model to implement the chaos-mggp model. the inter-comparison of the results at the selected four gauging stations shows that the chaos-mggp model provides more accurate forecasts than those of stand-alone mggp or lpm models. | chaos-based multigene genetic programming: a new hybrid strategy for river flow forecasting |
the first pulsating ultraluminous x-ray source (pulx) to be identified is m82 x-2. after the discovery in 2014, nustar observed the m82 field 15 times throughout 2015 and 2016. in this paper, we report the results of pulsation searches in all of these data sets and find only one new detection. this new detection allows us to refine the orbital period of the source and measure an average spin-down rate between 2014 and 2016 of ∼-6 × 10-11 hz s-1, which is in contrast to the strong spin-up seen during the 2014 observations, representing the first detection of spin-down in a pulx system. thanks to the improved orbital solution allowed by this new detection, we are also able to detect pulsations in additional segments of the original 2014 data set. we find a glitch superimposed on the very strong and variable spin-up already reported - the first positive glitch identified in a pulx system. we discuss the new findings in the context of current leading models for pulxs. | all at once: transient pulsations, spin-down, and a glitch from the pulsating ultraluminous x-ray source m82 x-2 |
we study the transport of high-energy particles in pulsar wind nebulae (pwn) using three-dimensional magnetohydrodynamic (mhd) and test-particle simulations, as well as a fokker-planck particle transport model. the latter includes radiative and adiabatic losses, diffusion, and advection on the background flow of the simulated mhd nebula. by combining the models, the spatial evolution of flux and photon index of the x-ray synchrotron emission is modelled for the three nebulae g21.5-0.9, the inner regions of vela, and 3c 58, thereby allowing us to derive governing parameters: the magnetic field strength, average flow velocity, and spatial diffusion coefficient. for comparison, the nebulae are also modelled with the semi-analytic kennel & coroniti model but the porth et al. model generally yields better fits to the observational data. we find that high velocity fluctuations in the turbulent nebula (downstream of the termination shock) give rise to efficient diffusive transport of particles, with average péclet number close to unity, indicating that both advection and diffusion play an important role in particle transport. we find that the diffusive transport coefficient of the order of ∼ 2 × 1027(ls/0.42 ly) cm2 s- 1 (ls is the size of the termination shock) is independent of energy up to extreme particle lorentz factors of γp ∼ 1010. | diffusion in pulsar wind nebulae: an investigation using magnetohydrodynamic and particle transport models |
context. the nearby large magellanic cloud (lmc) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in γ-rays. the lmc was detected at 0.1-100 gev as an extended source with cgro/egret and using early observations with the fermi-lat. the emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 doradus.aims: studies of the origin and transport of cosmic rays (crs) in the milky way are frequently hampered by line-of-sight confusion and poor distance determination. the lmc offers a complementary way to address these questions by revealing whether and how the γ-ray emission is connected to specific objects, populations of objects, and structures in the galaxy.methods: we revisited the γ-ray emission from the lmc using about 73 months of fermi-lat p7rep data in the 0.2-100 gev range. we developed a complete spatial and spectral model of the lmc emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for cr-induced interstellar emission.results: in addition to identifying psr j0540-6919 through its pulsations, we find two hard sources positionally coincident with plerion n 157b and supernova remnant n 132d, which were also detected at tev energies with h.e.s.s. we detect an additional soft source that is currently unidentified. extended emission dominates the total flux from the lmc. it consists of an extended component of about the size of the galaxy and additional emission from three to four regions with degree-scale sizes. if it is interpreted as crs interacting with interstellar gas, the large-scale emission implies a large-scale population of ~1-100 gev crs with a density of ~30% of the local galactic value. on top of that, the three to four small-scale emission regions would correspond to enhancements of the cr density by factors 2 to 6 or higher, possibly more energetic and younger populations of crs compared to the large-scale population. an alternative explanation is that this is emission from an unresolved population of at least two dozen objects, such as pulsars and their nebulae or supernova remnants. this small-scale extended emission has a spatial distribution that does not clearly correlate with known components of the lmc, except for a possible relation to cavities and supergiant shells.conclusions: the fermi-lat gev observations allowed us to detect individual sources in the lmc. three of the newly discovered sources are associated with rare and extreme objects. the 30 doradus region is prominent in gev γ-rays because psr j0540-6919 and n 157b are strong emitters. the extended emission from the galaxy has an unexpected spatial distribution, and observations at higher energies and in radio may help to clarify its origin. fits file of fig. 1 is only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/586/a71 | deep view of the large magellanic cloud with six years of fermi-lat observations |
context. the positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 gev, and it was found to be a steadily increasing function of energy above ~10 gev. this behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their propagation in the interstellar medium. the observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the galaxy, the so-called weakly interacting massive particles (wimps). alternatively, it could be produced by nearby sources, such as pulsars.aims: these hypotheses are probed in light of the latest ams-02 positron fraction measurements. as regards dark matter candidates, regions in the annihilation cross section to mass plane, which best fit the most recent data, are delineated and compared to previous measurements. the explanation of the anomaly in terms of a single nearby pulsar is also explored.methods: the cosmic ray positron transport in the galaxy is described using a semi-analytic two-zone model. propagation is described with green functions as well as with bessel expansions. for consistency, the secondary and primary components of the positron flux are calculated together with the same propagation model. the above mentioned explanations of the positron anomaly are tested using χ2 fits. the numerical package micromegas is used to model the positron flux generated by dark matter species. the description of the positron fraction from conventional astrophysical sources is based on the pulsar observations included in the australia telescope national facility (atnf) catalogue.results: the masses of the favoured dark matter candidates are always larger than 500 gev, even though the results are very sensitive to the lepton flux. the fermi measurements point systematically to much heavier candidates than the recently released ams-02 observations. since the latter are more precise, they are much more constraining. a scan through the various individual annihilation channels disfavours leptons as the final state. on the contrary, the agreement is excellent for quark, gauge boson, or higgs boson pairs, with best-fit masses in the 10 to 40 tev range. the combination of annihilation channels that best matches the positron fraction is then determined at fixed wimp mass. a mixture of electron and tau lepton pairs is only acceptable around 500 gev. adding b-quark pairs significantly improves the fit up to a mass of 40 tev. alternatively, a combination of the four-lepton channels provides a good fit between 0.5 and 1 tev, with no muons in the final state. concerning the pulsar hypothesis, the region of the distance-to-age plane that best fits the positron fraction for a single source is determined.conclusions: the only dark matter species that fulfils the stringent gamma ray and cosmic microwave background bounds is a particle annihilating into four leptons through a light scalar or vector mediator, with a mixture of tau (75%) and electron (25%) channels, and a mass between 0.5 and 1 tev. the positron anomaly can also be explained by a single pulsar, and a list of five pulsars from the atnf catalogue is given. we investigate how this list could evolve when more statistics are accumulated. those results are obtained with the cosmic ray transport parameters that best fit the b/c ratio. uncertainties in the propagation parameters turn out to be very significant. in the wimp annihilation cross section to mass plane for instance, they overshadow the error contours derived from the positron data. | a new look at the cosmic ray positron fraction |
we study explosion characteristics of ultra-stripped supernovae (sne), which are candidates of sne generating binary neutron stars (nss). as a first step, we perform stellar evolutionary simulations of bare carbon-oxygen cores of mass from 1.45 to 2.0 m⊙ until the iron cores become unstable and start collapsing. we then perform axisymmetric hydrodynamics simulations with spectral neutrino transport using these stellar evolution outcomes as initial conditions. all models exhibit successful explosions driven by neutrino heating. the diagnostic explosion energy, ejecta mass, ni mass, and ns mass are typically ∼1050 erg, ∼0.1 m⊙, ∼0.01 m⊙, and ≈1.3 m⊙, which are compatible with observations of rapidly evolving and luminous transient such as sn 2005ek. we also find that the ultra-stripped sn is a candidate for producing the secondary low-mass ns in the observed compact binary nss like psr j0737-3039. | neutrino-driven explosions of ultra-stripped type ic supernovae generating binary neutron stars |
several theories exist to explain the source of the bright, millisecond duration pulses known as fast radio bursts (frbs). if the progenitors of frbs are non-cataclysmic, such as giant pulses from pulsars, pulsar-planet binaries, or magnetar flares, frb emission may be seen to repeat. we have undertaken a survey of the fields of eight known frbs from the high time resolution universe survey to search for repeating pulses. although no repeat pulses were detected the survey yielded the detection of a new frb, described in petroff et al. (2015a). from our observations we rule out periodic repeating sources with periods p ≤ 8.6 h and rule out sources with periods 8.6 < p < 21 h at the 90 per cent confidence level. at p ≥ 21 h our limits fall off as ∼1/p. dedicated and persistent observations of frb source fields are needed to rule out repetition on longer time-scales, a task well-suited to next generation wide-field transient detectors. | a survey of frb fields: limits on repeatability |
the large european array for pulsars (leap) is an experiment that harvests the collective power of europe's largest radio telescopes in order to increase the sensitivity of high-precision pulsar timing. as part of the ongoing effort of the european pulsar timing array, leap aims to go beyond the sensitivity threshold needed to deliver the first direct detection of gravitational waves. the five telescopes presently included in leap are the effelsberg telescope, the lovell telescope at jodrell bank, the nançay radio telescope, the sardinia radio telescope and the westerbork synthesis radio telescope. dual polarization, nyquist-sampled time series of the incoming radio waves are recorded and processed offline to form the coherent sum, resulting in a tied-array telescope with an effective aperture equivalent to a 195-m diameter circular dish. all observations are performed using a bandwidth of 128 mhz centred at a frequency of 1396 mhz. in this paper, we present the design of the leap experiment, the instrumentation, the storage and transfer of data and the processing hardware and software. in particular, we present the software pipeline that was designed to process the nyquist-sampled time series, measure the phase and time delays between each individual telescope and a reference telescope and apply these delays to form the tied-array coherent addition. the pipeline includes polarization calibration and interference mitigation. we also present the first results from leap and demonstrate the resulting increase in sensitivity, which leads to an improvement in the pulse arrival times. | leap: the large european array for pulsars |
we propose a dark-matter (dm) admixed density-dependent equation of state where the fermionic dm interacts with the nucleons via higgs portal. presence of dm can hardly influence the particle distribution inside neutron star (ns) but can significantly affect the structure as well as equation of state (eos) of ns. introduction of dm inside ns softens the equation of state. we explored the effect of variation of dm mass and dm fermi momentum on the ns eos. moreover, dm-higgs coupling is constrained using dark matter direct detection experiments. then, we studied cooling of normal nss using apr and dd2 eoss and dm admixed nss using dark-matter modified dd2 with varying dm mass and fermi momentum. we have done our analysis by considering different ns masses. also dm mass and dm fermi momentum are varied for fixed ns mass and dm-higgs coupling. we calculated the variations of luminosity and temperature of ns with time for all eoss considered in our work and then compared our calculations with the observed astronomical cooling data of pulsars namely cas a, rx j0822-43, 1e 1207-52, rx j0002+62, xmmu j17328, psr b1706-44, vela, psr b2334+61, psr b0656+14, geminga, psr b1055-52 and rx j0720.4-3125. it is found that apr eos agrees well with the pulsar data for lighter and medium mass nss but cooling is very fast for heavier ns. for dm admixed dd2 eos, it is found that for all considered ns masses, all chosen dm masses and fermi momenta agree well with the observational data of psr b0656+14, geminga, vela, psr b1706-44 and psr b2334+61. cooling becomes faster as compared to normal nss in case of increasing dm mass and fermi momenta. it is infered from the calculations that if low mass super cold nss are observed in future that may support the fact that heavier wimp can be present inside neutron stars. | cooling of dark-matter admixed neutron stars with density-dependent equation of state |
we analyse the late time evolution of 12 supernovae (sne) occurring over the last ~41 yr, including nine type iip/l, two iib, and one ib/c, using ubvr optical data from the large binocular telescope (lbt) and difference imaging. we see late time (5-42 yr) emission from nine of the eleven type ii sne (eight type iip/l, one iib). we consider radioactive decay, circumstellar medium (csm) interactions, pulsar/engine driven emission, dust echoes, and shock perturbed binary companions as possible sources of emission. the observed emission is most naturally explained as csm interactions with the normal stellar winds of red supergiants with mass-loss rates in the range -7.9 ≲ log10(m⊙ yr-1) ≲ -4.8. we also place constraints on the presence of any shock heated binary companion to the type ib/c sn 2012fh and provide progenitor photometry for the type iib sn 2011dh, the only one of the six sne with pre-explosion lbt observations where the sn has faded sufficiently to allow the measurement. the results are consistent with measurements from pre-explosion hubble space telescope images. | the late time optical evolution of twelve core-collapse supernovae: detection of normal stellar winds |
in this work, we present the results of 1 yr of upgraded giant metrewave radio telescope timing measurements of psr j0514-4002a, a 4.99-ms pulsar in a 18.8-d eccentric (e = 0.89) orbit with a massive companion located in the globular cluster ngc 1851. combining these data with earlier green bank telescope data, we greatly improve the precision of the rate of advance of periastron, \dot{ω} = 0.0129592(16) \deg yr^{-1} which, assuming the validity of general relativity, results in a much refined measurement of the total mass of the binary, m_tot = 2.4730(6) m_{\odot }. additionally, we measure the einstein delay parameter, γ, something that has never been done for any binary system with an orbital period larger than {∼}10 h. the measured value, γ = 0.0216(9) s, is by far the largest for any binary pulsar. furthermore, we measure the proper motion of the system (μ _{α } = 5.19(22) and μ _{δ } = -0.56(25) mas yr^{-1}), which is not only important for analysing its motion in the cluster, but is also essential for a proper interpretation of γ, given the latter parameter's correlation with the variation of the projected semimajor axis. the measurements of γ and the proper motion enable a separation of the system component masses: we obtain a pulsar mass of m_p = 1.25^{+0.05}_{-0.06} m_{\odot } and a companion mass of m_c = 1.22^{+0.06}_{-0.05} m_{\odot }. this raises the possibility that the companion is also a neutron star. searches for radio pulsations from the companion have thus far been unsuccessful; hence, we cannot confirm the latter hypothesis. the low mass of this millisecond pulsar - one of the lowest ever measured for such objects - clearly indicates that the recycling process can be achieved with a relatively small amount of mass transfer. | upgraded giant metrewave radio telescope timing of ngc 1851a: a possible millisecond pulsar - neutron star system |
recent observations with the meerkat radio telescope reveal a unique population of faint nonthermal filaments pervading the central molecular zone, a region rich in molecular gas near the galactic center. some of those filaments are organized into groups of almost parallel filaments, seemingly sorted by their length, so that their morphology resembles a harp with radio-emitting "strings." we argue that the synchrotron-emitting gev electrons of these radio harps have been consecutively injected by the same source (a massive star or pulsar) into spatially intermittent magnetic fiber bundles within a magnetic flux tube or via time-dependent injection events. after escaping from this source, the propagation of cosmic-ray (cr) electrons inside a flux tube is governed by the theory of cr transport. we propose to use observations of radio harp filaments to gain insight into the specifics of cr propagation along magnetic fields of which there are two principle modes: crs could either stream with self-excited magnetohydrodynamic waves or diffuse along the magnetic field. to disentangle these possibilities, we conduct hydrodynamical simulations of either purely diffusing or streaming cr electrons and compare the resulting brightness distributions to the observed synchrotron profiles of the radio harps. we find compelling evidence that cr streaming is the dominant propagation mode for gev crs in one of the radio harps. observations at higher angular resolution should detect more radio harps and may help to disentangle projection effects of the possibly three-dimensional flux-tube structure of the other radio harps. | probing cosmic-ray transport with radio synchrotron harps in the galactic center |
a class of relativistic astrophysical compact objects is analyzed in the modified finch-skea geometry described by the mit bag model equation of state of interior matter, $ p=\dfrac{1}{3}\left(\rho-4b\right) $ , where b is known as the bag constant. b plays an important role in determining the physical features and structure of strange stars. we consider the finite mass of the strange quark ( $ m_{s} \neq 0 $ ) and study its effects on the stability of quark matter inside a star. we note that the inclusion of strange quark mass affects the gross properties of the stellar configuration, such as maximum mass, surface red-shift, and the radius of strange quark stars. to apply our model physically, we consider three compact objects, namely, (i) vela x-1, (ii) 4u 1820-30, and (iii) psr j 1903+327, which are thought to be strange stars. the range of b is restricted from 57.55 to $b_{\rm stable}$ ( $\rm mev/fm^{3}$ ), for which strange matter might be stable relative to iron ( $^{56}{\rm fe}$ ). however, we also observe that metastable and unstable strange matter depend on b and $ m_{s} $ . all energy conditions hold well in this approach. stability in terms of the lagrangian perturbation of radial pressure is studied in this paper. *bd and kbg thank the csir for providing the fellowship vide no 09/1219(0005)/2019-emr-i and 09/1219(0004)/2019-emr-i, respectively. | anisotropic strange quark star in finch-skea geometry and its maximum mass for non-zero strange quark mass (ms≠ 0) |
the purpose of this paper consists in presenting models of compact stars described by a new class of exact solutions to the field equations, in the context of general relativity, for a fluid configuration which is locally anisotropic in the pressure. with current sensitivities, we considered a non-linear form of modified van der waals equation of state viz., pr=α ρ2+β/ρ 1 +γ ρ , as well as a gravitational potential z(x) as a generating function by exploiting an anisotropic source of matter which served as a basis for generating the confined compact stars. the exact solutions are formed by correlating an interior space-time geometry to an exterior schwarzschild vacuum. then, we analyze the physical viability of the model generated and compare it with observational data of some heavy pulsars coming from the neutron star interior composition explorer. the model satisfies all the required pivotal physical and mathematical properties in the compact structures study, offering empirical evidence in support of the evolution of realistic stellar configurations. it is shown to be regular, viable, and stable under the influence generated by the parameters coming from the theory namely, α , β , γ , δ , everywhere within the astral fluid in the investigated high-density regime that supports the existence of realistic heavy pulsars such as psr j0348+0432, psr j0740+6620 and psr j0030+0451. | self-gravitating anisotropic model in general relativity under modified van der waals equation of state: a stable configuration |
identifying as many gamma-ray pulsars as possible in the fermi large area telescope (lat) data helps test pulsar emission models by comparing predicted and observed properties for a large, varied sample with as little selection bias as possible. it also improves extrapolations from the observed population to estimate the contribution of unresolved pulsars to the diffuse gamma-ray emission. we use a recently developed method to determine the probability that a given gamma-ray photon comes from a known position in the sky, convolving the photon’s energy with the lat’s energy-dependent point-spread function, without the need for an accurate spatial and spectral model of the gamma-ray sky around the pulsar. the method is simple and fast and, importantly, provides probabilities, or weights, for gamma-rays from pulsars too faint for phase-integrated detection. we applied the method to over a thousand pulsars for which we obtained rotation ephemerides from radio observations, and discovered gamma-ray pulsations from 16 pulsars, 12 young and 4 recycled. psr j2208+4056 has spindown power \dot{e}=8× {10}32 erg s-1, about three times lower than the previous observed gamma-ray emission “deathline.” psrs j2208+4056 and j1816-0755 have radio interpulses, constraining their geometry and perhaps enhancing their gamma-ray luminosity. we discuss whether the deathline is an artifact of selection bias due to the pulsar distance. | searching a thousand radio pulsars for gamma-ray emission |
radio pulsars are unique laboratories for a wide range of physics and astrophysics. understanding how they are created, how they evolve and where we find them in the galaxy, with or without binary companions, is highly constraining of theories of stellar and binary evolution. pulsars' relationship with a recently discovered variety of apparently different classes of neutron stars is an interesting modern astrophysical puzzle which we consider in part i of this review. radio pulsars are also famous for allowing us to probe the laws of nature at a fundamental level. they act as precise cosmic clocks and, when in a binary system with a companion star, provide indispensable venues for precision tests of gravity. the different applications of radio pulsars for fundamental physics will be discussed in part ii. we finish by making mention of the newly discovered class of astrophysical objects, the fast radio bursts, which may or may not be related to radio pulsars or neutron stars, but which were discovered in observations of the latter. | radio pulsars: the neutron star population & fundamental physics |
it has recently been demonstrated that self-consistent particle-in-cell simulations of low-obliquity pulsar magnetospheres in flat spacetime show weak particle acceleration and no pair production near the poles. we investigate the validity of this conclusion in a more realistic spacetime geometry via general-relativistic particle-in-cell simulations of the aligned pulsar magnetosphere with pair formation. we find that the addition of the frame-dragging effect makes the local current density along the magnetic field larger than the goldreich-julian value, which leads to unscreened parallel electric fields and the ignition of a pair cascade. when pair production is active, we observe field oscillations in the open field bundle, which could be related to pulsar radio emission. we conclude that general-relativistic effects are essential for the existence of the pulsar mechanism in low-obliquity rotators. | ab initio pulsar magnetosphere: the role of general relativity |
we construct equilibrium configurations of uniformly rotating neutron stars for selected relativistic mean-field nuclear matter equations of state (eos). we compute, in particular, the gravitational mass (m ), equatorial (req) and polar (rpol) radii, eccentricity, angular momentum (j ), moment of inertia (i ) and quadrupole moment (m2) of neutron stars stable against mass shedding and secular axisymmetric instability. by constructing the constant frequency sequence f =716 hz of the fastest observed pulsar, psr j1748-2446ad, and constraining it to be within the stability region, we obtain a lower mass bound for the pulsar, mmin=[1.2 - 1.4 ]m⊙ , for the eos employed. moreover, we give a fitting formula relating the baryonic mass (mb) and gravitational mass of nonrotating neutron stars, mb/m⊙=m /m⊙+(13 /200 )(m /m⊙)2 [or m /m⊙=mb/m⊙-(1 /20 )(mb/m⊙)2], which is independent of the eos. we also obtain a fitting formula, although not eos independent, relating the gravitational mass and the angular momentum of neutron stars along the secular axisymmetric instability line for each eos. we compute the maximum value of the dimensionless angular momentum, a /m ≡c j /(g m2) (or "kerr parameter"), (a /m )max≈0.7 , found to be also independent of the eos. we then compare and contrast the quadrupole moment of rotating neutron stars with the one predicted by the kerr exterior solution for the same values of mass and angular momentum. finally, we show that, although the mass quadrupole moment of realistic neutron stars never reaches the kerr value, the latter is closely approached from above at the maximum mass value, as physically expected from the no-hair theorem. in particular, the stiffer the eos, the closer the mass quadrupole moment approaches the value of the kerr solution. | fast rotating neutron stars with realistic nuclear matter equation of state |
the variable star ar scorpii (ar sco) was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. the system is composed of a cool, low-mass star in a tight, 3.55-hour orbit with a more massive white dwarf. here we report new optical observations of ar sco that show strong linear polarization (up to 40%) that varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low-level (up to a few per cent) circular polarization. these observations support the notion that, similar to neutron-star pulsars, the pulsed luminosity of ar sco is powered by the spin-down of the rapidly rotating white dwarf that is highly magnetized (up to 500 mg). the morphology of the modulated linear polarization is similar to that seen in the crab pulsar, albeit with a more complex waveform owing to the presence of two periodic signals of similar frequency. magnetic interactions between the two component stars, coupled with synchrotron radiation from the white dwarf, power the observed polarized and non-polarized emission. ar sco is therefore the first example of a white dwarf pulsar. | polarimetric evidence of a white dwarf pulsar in the binary system ar scorpii |
lorentz symmetry violations can be parametrized by an effective field theory framework that contains both general relativity and the standard model of particle physics called the standard-model extension (sme). we present new constraints on pure gravity sme coefficients obtained by analyzing lunar laser ranging (llr) observations. we use a new numerical lunar ephemeris computed in the sme framework and we perform a llr data analysis using a set of 20 721 normal points covering the period of august, 1969 to december, 2013. we emphasize that linear combination of sme coefficients to which llr data are sensitive and not the same as those fitted in previous postfit residuals analysis using llr observations and based on theoretical grounds. we found no evidence for lorentz violation at the level of 10-8 for s¯t x, 10-12 for s¯x y and s¯x z, 10-11 for s¯x x-s¯y y and s¯x x+s¯y y-2 s¯z z-4.5 s¯y z, and 10-9 for s¯t y+0.43 s¯t z. we improve previous constraints on sme coefficient by a factor up to 5 and 800 compared to postfit residuals analysis of respectively binary pulsars and llr observations. | testing lorentz symmetry with lunar laser ranging |
in this work, we assess the sensitivity reach of pulsar timing array (pta) measurements to probe pointlike primordial black holes (pbhs), with an extended mass distribution, which originate from collapsed fermi balls that are formed through the aggregation of asymmetric u(1) dark fermions trapped within false vacuum bubbles during a dark first order phase transition (fopt). the pbh formation scenario is mainly characterized by the dark asymmetry, strength of the fopt, rate of fopt, and the percolation temperature. meanwhile, for pbh masses of interest lying within 10−10m⊙ − 102m⊙, the relevant signal for pta measurements is the doppler phase shift in the timing signal, due to the velocity change induced by transiting pbhs on pulsars. taking the dark asymmetry parameter to be 10−4 and 10−5, we find that percolation temperatures within the 0.1 − 10 kev range, fopt rates above 103 times the hubble parameter at percolation, and fopt strengths within 10−6 − 0.1 can give rise to pbhs that can be probed by an ska-like pta observation. on the other hand, the accompanying gravitational wave (gw) signal from the fopt can be used as a complementary probe, assuming that the peak frequency lies within the 𝒪(10−10) − 𝒪(10−7) hz range, and the peak gw abundance is above the peak-integrated sensitivity curves associated with pulsar timing observations that search for stochastic gws. at the fundamental level, a quartic effective potential for a dark scalar field can trigger the fopt. by performing a parameter scan, we obtained the class of effective potentials that lead to fopt scenarios that can be probed by ska through pulsar timing and gw observations. | probing primordial black holes from a first order phase transition through pulsar timing and gravitational wave signals |
pulsar timing-array correlation measurements offer an exciting opportunity to test the nature of gravity in the cosmologically novel nanohertz gravitational wave regime. the stochastic gravitational wave background is assumed gaussian and random, while there are limited pulsar pairs in the sky. this brings theoretical uncertainties to the correlation measurements, namely the pulsar variance due to pulsar samplings and the cosmic variance due to gaussian signals. we demonstrate a straightforward calculation of the mean and the variances on the hellings-downs correlation relying on a power spectrum formalism. we keep arbitrary pulsar distances and consider gravitational wave modes beyond einstein gravity as well as off the light cone throughout, thereby presenting the most general and, most importantly, numerically efficient calculation of the variances. | pulsar and cosmic variances of pulsar timing-array correlation measurements of the stochastic gravitational wave background |
our paper reviews the planned space-based gamma-ray telescope gamma-400 and evaluates in details its opportunities in the field of dark matter (dm) indirect searches. we estimated the gamma-400 mean sensitivity to the diphoton dm annihilation cross section in the galactic center for dm particle masses in the range of 1-500 gev. we obtained the sensitivity gain at least by 1.2-1.5 times (depending on dm particle mass) with respect to the expected constraints from 12 years of observations by fermi-lat for the case of einasto dm density profile. the joint analysis of the data from both telescopes may yield the gain up to 1.8-2.3 times. thus the sensitivity reaches the level of annihilation cross section langle σ v rangleγγ(mχ = 100 gev) ≈ 10-28 cm3/s. this will allow us to test the hypothesized narrow lines predicted by specific dm models, particularly the recently proposed pseudo-goldstone boson dm model. we also considered the decaying dm—in this case the joint analysis may yield the sensitivity gain up to 1.1-2.0 times reaching the level of dm lifetime τγν(mχ = 100 gev) ≈ 2 ṡ 1029 s. we estimated the gamma-400 sensitivity to axion-like particle (alp) parameters by a potential observation of the supernova explosion in the local group. this is very sensitive probe of alps reaching the level of alp-photon coupling constant gaγ ~ 10-13 gev-1 for alp masses ma lesssim 1 nev. we also calculated the sensitivity to alps by constraining the modulations in the spectra of the galactic gamma-ray pulsars due to possible alp-photon conversion. gamma-400 is expected to be more sensitive than the cast helioscope for alp masses ma ≈ (1--10) nev reaching gaγmin ≈ 2 ṡ 10-11 gev-1. other potentially interesting targets and candidates are briefly considered too. | dark matter searches by the planned gamma-ray telescope gamma-400 |
fast radio bursts (frbs) are millisecond transients of unknown origin(s) occurring at cosmological distances. here we, for the first time, show time-integrated-luminosity functions and volumetric occurrence rates of non-repeating and repeating frbs against redshift. the time-integrated-luminosity functions of non-repeating frbs do not show any significant redshift evolution. the volumetric occurrence rates are almost constant during the past ~10 gyr. the nearly-constant rate is consistent with a flat trend of cosmic stellar-mass density traced by old stellar populations. our findings indicate that the occurrence rate of non-repeating frbs follows the stellar-mass evolution of long-living objects with ~gyr time-scales, favouring e.g. white dwarfs, neutron stars, and black holes, as likely progenitors of non-repeating frbs. in contrast, the occurrence rates of repeating frbs may increase towards higher redshifts in a similar way to the cosmic star formation rate density or black hole accretion rate density if the slope of their luminosity function does not evolve with redshift. short-living objects with ≲myr time-scales associated with young stellar populations (or their remnants, e.g. supernova remnants, young pulsars, and magnetars) or active galactic nuclei might be favoured as progenitor candidates of repeating frbs. | no redshift evolution of non-repeating fast radio burst rates |
we perform a sub-threshold follow-up search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants (snrs) vela jr., cassiopeia a, and snr g347.3-0.5. across the three targets, we investigate the most promising ≈10,000 combinations of gravitational-wave frequency and frequency derivative values, based on the results from an einstein@home search of the ligo o1 observing run data, dedicated to these objects. the selection threshold is set so that a signal could be confirmed using the newly released o2 run ligo data. in order to achieve best sensitivity we perform two separate follow-up searches, on two distinct stretches of the o2 data. only one candidate survives the first o2 follow-up investigation, associated with the central compact object in snr g347.3-0.5, but it is not conclusively confirmed. in order to assess a possible astrophysical origin we use archival x-ray observations and search for amplitude modulations of a pulsed signal at the putative rotation frequency of the neutron star and its harmonics. this is the first extensive electromagnetic follow-up of a continuous gravitational-wave candidate performed to date. no significant associated signal is identified. new x-ray observations contemporaneous with the ligo o3 run will enable a more sensitive search for an electromagnetic counterpart. a focused gravitational-wave search in o3 data based on the parameters provided here should be easily able to shed light on the nature of this outlier. noise investigations on the ligo instruments could also reveal the presence of a coherent contamination. | search for continuous gravitational waves from the central compact objects in supernova remnants cassiopeia a, vela jr., and g347.3-0.5 |
a pulsar timing array (pta) refers to a program of regular, high-precision timing observations of a widely distributed array of millisecond pulsars. here we review the status of the three primary pta projects and the joint international pulsar timing array project. we discuss current results related to ultra-low-frequency gravitational wave searches and highlight opportunities for the near future. | gravitational wave research using pulsar timing arrays |
a sensitive test for whether a black hole is a wormhole, using astronomical observations, would be to look for perturbations in the orbit of a pulsar around the black hole, caused by a perturbing object on the other side of the wormhole. by observing a pulsar in an orbit like that of s2 around the supermassive black hole at sgr a* at the center of our galaxy, the attainable mass limit on the perturber would be approximately 104 times better than derived from current observations of s2. for a nominal stellar-mass black hole-pulsar binary, observing for 1 year could set a mass limit on a perturber more than 6 orders of magnitude better than for a pulsar orbiting sgr a*. observations of a star in a stellar-mass binary containing a black hole could set limits similar to the case of a pulsar orbiting sgr a*. | sensitive searches for wormholes |
we present pulse width measurements for a sample of radio pulsars observed with the meerkat telescope as part of the thousand-pulsar-array (tpa) programme in the meertime project. for a centre frequency of 1284 mhz, we obtain 762 w10 measurements across the total bandwidth of 775 mhz, where w10 is the width at the 10 per cent level of the pulse peak. we also measure about 400 w10 values in each of the four or eight frequency sub-bands. assuming, the width is a function of the rotation period p, this relationship can be described with a power law with power law index μ = -0.29 ± 0.03. however, using orthogonal distance regression, we determine a steeper power law with μ = -0.63 ± 0.06. a density plot of the period-width data reveals such a fit to align well with the contours of highest density. building on a previous population synthesis model, we obtain population-based estimates of the obliquity of the magnetic axis with respect to the rotation axis for our pulsars. investigating the width changes over frequency, we unambiguously identify a group of pulsars that have width broadening at higher frequencies. the measured width changes show a monotonic behaviour with frequency for the whole tpa pulsar population, whether the pulses are becoming narrower or broader with increasing frequency. we exclude a sensitivity bias, scattering and noticeable differences in the pulse component numbers as explanations for these width changes, and attempt an explanation using a qualitative model of five contributing gaussian pulse components with flux density spectra that depend on their rotational phase. | the thousand-pulsar-array programme on meerkat - vi. pulse widths of a large and diverse sample of radio pulsars |
common envelope evolution (cee) physics plays a fundamental role in the formation of binary systems, such as merging stellar gravitational wave sources, pulsar binaries, and type ia supernovae. a precisely constrained cee has become more important in the age of large surveys and gravitational wave detectors. we use an adiabatic mass-loss model to explore how the total energy of the donor changes as a function of the remnant mass. this provides a more self-consistent way to calculate the binding energy of the donor. for comparison, we also calculate the binding energy through integrating the total energy from the core to the surface. the outcome of cee is constrained by total energy conservation at the point at which both components' radii shrink back within their roche lobes. we apply our results to 142 hot subdwarf binaries. for shorter orbital period hot subdwarf b stars (sdbs), the binding energy is highly consistent. for longer orbital period sdbs in our samples, the binding energy can differ by up to a factor of 2. the common envelope (ce) efficiency parameter β ce becomes smaller than α ce for the final orbital period ${\mathrm{log}}_{10}{p}_{\mathrm{orb}}/\mathrm{days}\gt -0.5$ . we also find the mass ratios ${\mathrm{log}}_{10}q$ and ce efficiency parameters ${\mathrm{log}}_{10}{\alpha }_{\mathrm{ce}}$ and ${\mathrm{log}}_{10}{\beta }_{\mathrm{ce}}$ linearly correlate in sdbs, similarly to the findings of de marco et al. for post-agb binaries. | the common envelope evolution outcome-a case study on hot subdwarf b stars |
wave dark matter (dm) represents a class of the most representative dm candidates. due to its periodic perturbation to spacetime, the wave dm can be detected with a galactic interferometer - pulsar timing array (pta). we perform in this letter a first analysis of applying the $\gamma$-ray pta to detect the wave dm, with the data of fermi large area telescope (fermi-lat). despite the limitation in statistics, the $\gamma$-pta demonstrates a promising sensitivity potential for a mass $\sim 10^{-23}-10^{-22}$ ev. we show that the upper limits not far from those of the dedicated radio-pta projects can be achieved. particularly, we have fulfilled an analysis to cross-correlate the pulsar data, which has been essentially missing so far in real data analysis but is known to be crucial for identifying the nature of potential signals, with the fermi-lat data of two pulsars. | detecting stochastic wave dark matter with fermi-lat $\\gamma$-ray pulsar timing array |
detection of continuous gravitational waves from rapidly spinning neutron stars opens up the possibility of examining their internal physics. we develop a framework that leverages a future continuous gravitational wave detection to infer a neutron star's moment of inertia, equatorial ellipticity, and the component of the magnetic dipole moment perpendicular to its rotation axis. we assume that the neutron star loses rotational kinetic energy through both gravitational wave and electromagnetic radiation, and that the distance to the neutron star can be measured, but do not assume electromagnetic pulsations are observable or a particular neutron star equation of state. we use the fisher information matrix and monte carlo simulations to estimate errors in the inferred parameters, assuming a population of gravitational-wave-emitting neutron stars consistent with the typical parameter domains of continuous gravitational wave searches. after an observation time of 1 yr, the inferred errors for many neutron stars are limited chiefly by the error in the distance to the star. the techniques developed here will be useful if continuous gravitational waves are detected from a radio, x-ray, or gamma-ray pulsar, or else from a compact object with known distance, such as a supernova remnant. | inferring neutron star properties with continuous gravitational waves |
fast radio bursts (frbs) must be powered by uniquely energetic emission mechanisms. this requirement has eliminated a number of possible source types, but several remain. identifying the physical nature of frb emitters arguably requires good localisation of more detections, as well as broad-band studies enabled by real-time alerting. in this paper, we present the apertif radio transient system (arts), a supercomputing radio-telescope instrument that performs real-time frb detection and localisation on the westerbork synthesis radio telescope (wsrt) interferometer. it reaches coherent-addition sensitivity over the entire field of the view of the primary-dish beam. after commissioning results verified that the system performed as planned, we initiated the apertif frb survey (alert). over the first 5 weeks we observed at design sensitivity in 2019, we detected five new frbs, and interferometrically localised each of them to 0.4-10 sq. arcmin. all detections are broad band, very narrow, of the order of 1 ms in duration, and unscattered. dispersion measures are generally high. only through the very high time and frequency resolution of arts are these hard-to-find frbs detected, producing an unbiased view of the intrinsic population properties. most localisation regions are small enough to rule out the presence of associated persistent radio sources. three frbs cut through the halos of m31 and m33. we demonstrate that apertif can localise one-off frbs with an accuracy that maps magneto-ionic material along well-defined lines of sight. the rate of one every ~7 days ensures a considerable number of new sources are detected for such a study. the combination of the detection rate and localisation accuracy exemplified by the first five arts frbs thus marks a new phase in which a growing number of bursts can be used to probe our universe. | the apertif radio transient system (arts): design, commissioning, data release, and detection of the first five fast radio bursts |
we develop a full four-dimensional numerical code to study scalar gravitational radiation emitted from binary systems and probe the vainshtein mechanism in situations that break the static and spherical symmetry, relevant for binary pulsars as well as black holes and neutron stars binaries. the present study focuses on the cubic galileon which arises as the decoupling limit of massive theories of gravity. limitations associated with the numerical methods prevent us from reaching a physically realistic hierarchy of scales; nevertheless, within this context we observe the same power law scaling of the radiated power as previous analytic estimates, and confirm a strong suppression of the power emitted in the monopole and dipole as compared with quadrupole radiation. following the trend to more physically realistic parameters, we confirm the suppression of the power emitted in scalar gravitational radiation and the recovery of general relativity with good accuracy. this paves the way for future numerical work, probing more generic, physically relevant situations and sets of interactions that may exhibit the vainshtein mechanism. | scalar gravitational radiation from binaries: vainshtein mechanism in time-dependent systems |
we compare the parallaxes of stars from vlbi astrometry in the literature to those in the gaia dr2 catalog. our full sample contains young stellar objects, evolved agb stars, pulsars, and other radio stars. excluding agb stars, which show significant discrepancies between gaia and vlbi parallaxes, and stars in binary systems, we obtain an average, systematic, parallax offset of -75 ± 29 μas for gaia dr2, consistent with their estimate of a parallax zero-point between -100 and 0 μas. | comparison of gaia dr2 parallaxes of stars with vlbi astrometry |
pulsar halos are extended gamma-ray structures generated by electrons and positrons escaping from pulsar wind nebulae (pwne), considered a new class of gamma-ray sources. they are ideal indicators for cosmic-ray propagation in localized regions of the galaxy and particle escape process from pwne. the cosmic-ray diffusion coefficient inferred from pulsar halos is more than two orders of magnitude smaller than the average value in the galaxy, which has been arousing extensive discussion. we review the recent advances in the study of pulsar halos, including the characteristics of this class of sources, the known pulsar halos, the possible mechanisms of the extremely slow diffusion, the critical roles of pulsar halos in the studies of cosmic-ray propagation and electron injection from pwne, and the implications on the problems of the cosmic positron excess and the diffuse tev gamma-ray excess. finally, we give prospects for the study in this direction based on the expectation of a larger sample of pulsar halos and deeper observations for bright sources. | gamma-ray pulsar halos in the galaxy |
with the development of ultra-intense laser technology, mev ions can be obtained from laser-foil interactions in the laboratory. these energetic ion beams can be applied in fast ignition for inertial confinement fusion, medical therapy, and proton imaging. however, these ions are mainly accelerated in the laser propagation direction. ion acceleration in an azimuthal orientation was scarcely studied. in this research, a doughnut laguerre-gaussian (lg) laser is used for the first time to examine laser-plasma interaction in the relativistic intensity regime in three-dimensional particle-in-cell simulations. studies have shown that a novel rotation of the plasma is produced from the hollow screw-like drill of an mode laser. the angular momentum of particles in the longitudinal direction produced by the lg laser is enhanced compared with that produced by the usual laser pulses, such as linearly and circularly polarized gaussian pulses. moreover, the particles (including electrons and ions) can be trapped and uniformly compressed in the dark central minimum of the doughnut lg pulse. the hollow-structured lg laser has potential applications in the generation of x-rays with orbital angular momentum, plasma accelerators, fast ignition for inertial confinement fusion, and pulsars in the astrophysical environment. | hollow screw-like drill in plasma using an intense laguerre-gaussian laser |
we present a broadband integrated photonic polarization splitter and rotator (psr) using adiabatically tapered coupled waveguides with subwavelength grating (swg) claddings. the psr adiabatically rotates and splits the fundamental transverse-magnetic (tm0) input to the fundamental transverse-electric (te0) mode in the coupler waveguide, while passing the te0 input through the same waveguide. the swgs work as an anisotropic metamaterial and facilitate modal conversions, making the psr efficient and broadband. we rigorously present our design approaches in each section and show the swg effect by comparing with and without the swg claddings. the coupling coefficients in each segment explicitly show a stronger coupling effect when the swgs are included, confirmed by the coupled-mode theory simulations. the full numerical simulation shows that the swg-psr operates at 1500-1750 nm (≈250 nm) wavelengths with an extinction ratio larger than 20 db, confirmed by the experiment for the 1490-1590 nm range. the insertion losses are below 1.3 db. since our psr is designed based on adiabatical mode evolution, the proposed psr is expected to be tolerant to fabrication variations and should be broadly applicable to polarization management in photonic integrated circuits. | broadband integrated polarization splitter and rotator using subwavelength grating claddings |
the self-excitation of plasma series resonance (psr) oscillations plays an important role in the electron heating dynamics in capacitively coupled radio-frequency (ccrf) plasmas. in a combined approach of pic/mcc simulations and a theoretical model based on an equivalent circuit, we investigate the self-excitation of psr oscillations and their effect on the electron heating in geometrically symmetric ccrf plasmas driven by multiple consecutive harmonics. the discharge symmetry is controlled via the electrical asymmetry effect (eae), i.e. by varying the total number of harmonics and tuning the phase shifts between them. it is demonstrated that psr oscillations will be self-excited under both symmetric and asymmetric conditions, if (i) the charge-voltage relation of the plasma sheaths deviates from a simple quadratic behavior and (ii) the inductance of the plasma bulk exhibits a temporal modulation. these two effects have been neglected up to now, but we show that they must be included in the model in order to properly describe the nonlinear series resonance circuit and reproduce the self-excitation of psr oscillations, which are observed in the electron current density resulting from simulations of geometrically symmetric ccrf plasmas. furthermore, the effect of psr self-excitation on the discharge current and the plasma properties, such as the potential profile, is illustrated by applying fourier analysis. high-frequency oscillations in the entire spectrum between the applied frequencies and the local electron plasma frequency are observed. as a consequence, the electron heating is strongly enhanced by the presence of psr oscillations. a complex electron heating dynamics is found during the expansion phase of the sheath, which is fully collapsed, when the psr is initially self-excited. the nonlinear electron resonance heating (nerh) associated with the psr oscillations causes a spatial asymmetry in the electron heating. by discussing the resulting ionization profile in the nonlocal regime of low-pressure ccrf plasmas, we examine why the ion flux at both electrodes remains approximately constant, independently of the phase shifts. | electron heating via self-excited plasma series resonance in geometrically symmetric multi-frequency capacitive plasmas |
the high-density behavior of stellar matter composed of nucleons and leptons under β equilibrium and charge neutrality conditions is studied with the skyrme parametrizations shown to be consistent (consistent skyrme parametrizations, cskp) with nuclear matter, pure neutron matter, symmetry energy and its derivatives in a set of 11 constraints [dutra et al., phys. rev. c 85, 035201 (2012)]. the predictions of these parametrizations on the tidal deformabilities related to the gw170817 event are also examined. the cskp that produce massive neutron stars give a range of 11.86 km⩽r1.4⩽12.55 km for the canonical star radius, in agreement with other theoretical predictions. it is shown that the cskp are compatible with the region of masses and radii obtained from the analysis of recent data from ligo and virgo collaboration (lvc). a correlation between dimensionless tidal deformability and radius of the canonical star is found, namely, λ1.4≈3.16 ×10-6r1.47.35 , with results for the cskp compatible with the recent range of λ1.4=190-120+390 from lvc. an analysis of the λ1×λ2 graph shows that all the cskp are compatible with the recent bounds obtained by lvc. finally, the universal correlation between the moment of inertia and the deformability of a neutron star, named the i-love relation, is verified for the cskp; it is also shown to be consistent with the prediction for the moment of inertia of the psr j0737-3039 primary component pulsar. | consistent skyrme parametrizations constrained by gw170817 |
a large sample of pulsars was observed as part of the meterwavelength single-pulse polarimetric emission survey. we carried out a detailed fluctuation spectral analysis that revealed periodic features in 46% of pulsars, including 22 pulsars where drifting characteristics were reported for the first time. the pulsar population can be categorized into three distinct groups: pulsars that show systematic drift motion within the pulse window, pulsars showing no systematic drift but periodic amplitude fluctuation, and pulsars with no periodic variations. we discovered the dependence of the drifting phenomenon on the spin-down energy loss (\dot{e}), with the three categories occupying distinctly different regions along the \dot{e} axis. the estimation of the drift periodicity (p3) from the peak frequency in the fluctuation spectra is ambiguous due to the aliasing effect. however, using basic physical arguments we were able to determine p3 in pulsars showing systematic drift motion. the estimated p3 values in these pulsars were anti-correlated with \dot{e}, which favored the partially screened gap model of the inner acceleration region in pulsars. | meterwavelength single-pulse polarimetric emission survey. ii. the phenomenon of drifting subpulses |
in this study, we propose a classification scheme for the phenomenon of subpulse drifting in pulsars. we have assembled an exhaustive list of pulsars that exhibit subpulse drifting from previously published results as well as recent observations using the giant meterwave radio telescope. we have estimated detailed phase variations corresponding to the drifting features. based on phase behaviour, the drifting population was classified into four groups: coherent phase-modulated drifting, switching phase-modulated drifting, diffuse phase-modulated drifting, and low-mixed phase-modulated drifting. we have re-established the previous assertion that the subpulse drifting is primarily associated with the conal components in pulsar profile. the core components generally do not show the drifting phenomenon. however, in core emission of certain pulsars longer periodic fluctuations are seen, which are similar to periodic nulling, and likely arise due to a different physical phenomenon. in general, the nature of the phase variations of the drifting features across the pulsar profile appears to be associated with specific pulsar profile classes, but we also find several examples that show departures from this trend. it has also been claimed in previous works that the spin-down energy loss is anticorrelated with the drifting periodicity. we have verified this dependence using a larger sample of drifting measurements. | classification of subpulse drifting in pulsars |
a fundamental question regarding the galactic center excess (gce) is whether the underlying structure is pointlike or smooth, often framed in terms of a millisecond pulsar or annihilating dark matter (dm) origin for the emission. we show that bayesian neural networks (nns) have the potential to resolve this debate. in simulated data, the method is able to predict the flux fractions from inner galaxy emission components to on average ∼0.5 %. when applied to the fermi photon-count map, the nn identifies a smooth gce in the data, suggestive of the presence of dm, with the estimates for the background templates being consistent with existing results. | galactic center excess in a new light: disentangling the γ -ray sky with bayesian graph convolutional neural networks |
pulsar wind nebulae (pwne) represent the largest class of sources that upcoming γ-ray surveys will detect. therefore, accurate modelling of their global emission properties is one of the most urgent problems in high-energy astrophysics. correctly characterizing these dominant objects is a needed step to allow γ-ray surveys to detect fainter sources, investigate the signatures of cosmic ray propagation, and estimate the diffuse emission in the galaxy. in this paper, we present an observationally motivated construction of the galactic pwne population. we made use of a modified one-zone model to evolve for a long period of time the entire population. the model provides, for every source, at any age, a simplified description of the dynamical and spectral evolution. the long-term effects of the reverberation phase on the spectral evolution are described, for the first time, based on physically motivated prescriptions for the evolution of the nebular radius supported by numerical studies. this effort tries to solve one of the most critical aspects of one-zone modelling, namely the typical overcompression of the nebula during the reverberation phase, resulting in a strong modification of its spectral properties at all frequencies. we compare the emission properties of our synthetic pwne population with the most updated catalogues of tev galactic sources. we find that the firmly identified and candidate pwne sum up to about 50 per cent of the expected objects in this class above threshold for detection. finally, we estimate that cherenkov telescope array will increase the number of tev-detected pwne by a factor of ≳3. | modelling the γ-ray pulsar wind nebulae population in our galaxy |
recently, it has been discovered that the transition of x-ray pulsars to the low luminosity state ( $l\lesssim 10^{35}\, {\rm erg\ \rm s^{-1}}$ ) is accompanied by a dramatic spectral change. that is, the typical power-law-like spectrum with high-energy cut-off transforms into a two-component structure with a possible cyclotron absorption feature on top of it. it was proposed that these spectral characteristics can be explained qualitatively by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper landau levels and further comptonization of the photons by electron gas. the electron gas are expected to be overheated in a thin top layer of the atmosphere. in this paper, we perform monte carlo simulations of the radiative transfer in the atmosphere of an accreting neutron star while accounting for a resonant scattering of polarized x-ray photons by thermally distributed electrons. the spectral shape is shown to be strongly polarization-dependent in soft x-rays ( $\lesssim 10\, {\rm kev}$ ) and near the cyclotron scattering feature. the results of our numerical simulations are tested against the observational data of the x-ray pulsar a 0535+262 in the low-luminosity state. we show that the spectral shape of the pulsar can be reproduced by the proposed theoretical model. we discuss applications of the discovery to the observational studies of accreting neutron stars. | spectrum formation in x-ray pulsars at very low mass accretion rate: monte carlo approach |
we present a comprehensive census of x-ray millisecond pulsars (msps) in 29 galactic globular clusters (gcs), including 68 msps with confirmed x-ray luminosities and 107 msps with x-ray upper limits. we compile previous x-ray studies of gc msps, and add new analyses of six msps (psrs j1326-4728a, j1326-4728b, j1518+0204c, j1717+4308a, j1737-0314a, and j1807-2459a) discovered in five gcs. their x-ray spectra are well described by a single blackbody model, a single power-law model, or a combination of them, with x-ray luminosities ranging from 1.9 × 1030to 8.3 × 1031 erg s-1. we find that most detected x-ray msps have luminosities between ~1030 and 3 × 1031 erg s-1. redback pulsars are a relatively bright msp population with x-ray luminosities of ~2 × 1031-3 × 1032 erg s-1. black widows show a bimodal distribution in x-ray luminosities, with eclipsing black widows between ~7 × 1030 and 2 × 1031 erg s-1, while the two confirmed non-eclipsing black widows are much fainter, with lx of 1.5-3 × 1030 erg s-1, suggesting an intrinsic difference in the populations. we estimate the total number of msps in 36 gcs by considering the correlation between the number of msps and stellar encounter rate in gcs, and suggest that between 600 and 1500 msps exist in these 36 gcs. finally, we estimate the number of x-ray-detectable msps in the galactic bulge, finding that 1-86 msps with lx > 1033 erg s-1, and 20-900 msps with lx > 1032 erg s-1 should be detectable there. | a census of x-ray millisecond pulsars in globular clusters |
we report detections of scintillation arcs for pulsars in globular clusters m5, m13 and m15 for the first time using the five-hundred-meter aperture spherical radio telescope (fast). from observations of these arcs at multiple epochs, we infer that screen-like scattering medium exists at distances 4.1−0.3+0.2, 6.7−0.2+0.2 and 1.3−1.0+0.7 kpc from earth in the directions of m5, m13 and m15, respectively. this means m5's and m13's scattering screens are located at 3.0−0.2+0.1 and 4.4−0.1+0.1 kpc above the galactic plane, whereas, m15's is at 0.6−0.5+0.3 kpc below the plane. we estimate the scintillation timescale and decorrelation bandwidth for each pulsar at each epoch using the one-dimensional auto-correlation in frequency and time of the dynamic spectra. we found that the boundary of the local bubble may have caused the scattering of m15, and detected the most distant off-plane scattering screens to date through pulsar scintillation, which provides evidence for understanding the medium circulation in the milky way. | galactic interstellar scintillation observed from four globular cluster pulsars by fast |
a linear transformer driver (ltd) generator capable of delivering up to 0.9 ma current pulses with 160 ns rise time has been assembled and commissioned at university of california san diego. the machine is an upgrade of the ltd-iii pulser from sandia national laboratories, consisting of 40 capacitors and 20 spark gap switches, arranged in a 20-brick configuration. the driver was modified with the addition of a new trigger system, active premagnetization of the inductive cores, a vacuum chamber with multiple diagnostic ports, and a vacuum power feed to couple the driver to plasma loads. the new machine is called compact experimental system for z -pinch and ablation research (ceszar). the driver has a maximum bipolar charge voltage of ±100 kv , but for reliability and testing, and to reduce the risk of damage to components, the machine was operated at ±60 kv , producing 550 ka peak currents with a rise time of 170 ns on a 3.5 nh short circuit. while the peak current is scaled down due to the reduced charge voltage, the pulse shape and circuit parameters are close to the results of the cavity and power feed models but suggest a slightly higher inductance than predicted. the machine was then used to drive wire array z -pinch and gas puff z -pinch experiments as initial dynamic plasma loads. the evolution of the wire array z pinch is consistent with the general knowledge of this kind of experiment, featuring wire ablation and stagnation of the precursor plasma on axis. the gas puff z pinches were configured as a single, hollow argon gas shell, in preparation for more structured gas puff targets such as multispecies, multishell implosions. the implosion dynamics agree generally with 1d magnetohydrodynamics simulation results, but large zippering and magneto-rayleigh-taylor instabilities are observed. the ceszar load region was designed to accommodate many load types to be driven by the machine, which makes it a versatile platform for studying z -pinch plasmas. the completion of the ceszar driver allows plasma experiments on energy coupling from ltd machines to plasma loads, instability mitigation techniques and magnetic field distributions in z pinches, and interface dynamics in multispecies implosions. | ma-class linear transformer driver for z -pinch research |
the imaging quality of in-line digital holography is challenged by the twin-image and aliasing effects because sensors only respond to intensity and pixels are of finite size. as a result, phase retrieval and pixel super-resolution techniques serve as two essential ingredients for high-fidelity and high-resolution holographic imaging. in this work, we combine the two as a unified optimization problem and propose a generalized algorithmic framework for pixel-super-resolved phase retrieval. in particular, we introduce the iterative projection algorithms and gradient descent algorithms for solving this problem. the basic building blocks, namely the projection operator and the wirtinger gradient, are derived and analyzed. as an example, the wirtinger gradient descent algorithm for pixel-super-resolved phase retrieval, termed as wirtinger-psr, is proposed and compared with the classical error-reduction algorithm. the wirtinger-psr algorithm is verified with both simulated and experimental data. the proposed framework generalizes well to various physical settings and helps bridging the gap between empirical studies and theoretical analyses. | generalized optimization framework for pixel super-resolution imaging in digital holography |
in the present article, the solution of the einstein-maxwell field equations in the presence of a massive scalar field under the brans-dicke (bd) gravity is obtained via embedding approach, which describes a charged anisotropic strange star model. the interior spacetime is described by a spherically symmetric static metric of embedding class i. this reduces the problem to a single-generating function of the metric potential which is chosen by appealing to physics based on regularity at each interior point of the stellar interior. the resulting model is subjected to rigorous physical checks based on stability, causality and regularity for particular object psr j1903+327. we also show that our solutions describe compact objects such as psr j1903+327; cen x-3; exo 1785-248 and lmc x-4 to an excellent approximation. novel results of our investigation reveal that the scalar field leads to higher surface charge densities which in turn affects the compactness and upper and lower values imposed by the modified buchdahl limit for charged stars. our results also show that the electric field and scalar field which originate from entirely different sources couple to alter physical characteristics such as mass-radius relation and surface redshift of compact objects. this superposition of the electric and scalar fields is enhanced by an increase in the bd coupling constant, ωbd. | anisotropic stars via embedding approach in brans-dicke gravity |
braking index measurements offer the opportunity to explore the processes affecting the long-term spin evolution of pulsars and possible evolutionary connections between the various pulsar populations. in most cases, such measurements are difficult because of the presence of short-term phenomena, such as glitches and timing noise, which obscure the long-term trends. in particular, recoveries from large glitches are the main obstacle to measuring the braking indices of young pulsars like the vela and crab pulsars. we present a new method to overcome this problem and report on braking index measurements for the vela-like pulsars, psr b1800-21 and psr b1823-13, together with an updated measurement for vela. additionally, the use of the method is extended to six more young glitching pulsars observed at jodrell bank observatory and we are able to estimate four new braking indices. values of braking indices describe the long-term evolution of the pulsars across the p-dot{p} diagram. despite some measurements being affected by considerable uncertainties, there is evidence for a common trend among young glitching pulsars, characterized by low braking indices n ≤ 2. such values introduce a new variant in the evolution of young pulsars, and their relationship with other populations in the p-dot{p} diagram, and imply that these pulsars could be a few times older than indicated by standard formulae. in this context, we analyse the case of psr b1757-24 and conclude that the pulsar could be old enough to be related to the supernova remnant g5.4-1.2. between glitches, the short-term evolution of vela-like pulsars is characterized by large interglitch braking indices nig > 10. we interpret both short- and long-term trends as signatures of the large glitch activity, and speculate that they are driven by short-term post-glitch re-coupling and a cumulative long-term decoupling of superfluid to the rotation of the star. | new long-term braking index measurements for glitching pulsars using a glitch-template method |
fast radio bursts (frbs) are millisecond-time-scale radio transients, the origins of which are predominantly extragalactic and likely involve highly magnetized compact objects. frbs undergo multipath propagation, or scattering, from electron density fluctuations on sub-parsec scales in ionized gas along the line of sight. scattering observations have located plasma structures within frb host galaxies, probed galactic and extragalactic turbulence, and constrained frb redshifts. scattering also inhibits frb detection and biases the observed frb population. we report the detection of scattering times from the repeating frb 20190520b that vary by up to a factor of 2 or more on minutes to days-long time-scales. in one notable case, the scattering time varied from 7.9 ± 0.4 ms to less than 3.1 ms ($95{{\ \rm per\ cent}}$ confidence) over 2.9 min at 1.45 ghz. the scattering times appear to be uncorrelated between bursts or with dispersion and rotation measure variations. scattering variations are attributable to dynamic, inhomogeneous plasma in the circumsource medium, and analogous variations have been observed from the crab pulsar. under such circumstances, the frequency dependence of scattering can deviate from the typical power law used to measure scattering. similar variations may therefore be detectable from other frbs, even those with inconspicuous scattering, providing a unique probe of small-scale processes within frb environments. | scattering variability detected from the circumsource medium of frb 20190520b |
the low-frequency linearly polarised radio source population is largely unexplored. however, a renaissance in low-frequency polarimetry has been enabled by pathfinder and precursor instruments for the square kilometre array. in this second paper from the polarised galactic and extragalactic all-sky mwa survey-the polarised gleam survey, or pogs-we present the results from our all-sky mwa phase i faraday rotation measure survey. our survey covers nearly the entire southern sky in the declination range $-82^\circ$ to $+30^\circ$ at a resolution between around three and seven arcminutes (depending on declination) using data in the frequency range 169−231 mhz. we have performed two targeted searches: the first covering 25 489 square degrees of sky, searching for extragalactic polarised sources; the second covering the entire sky south of declination $+30^\circ$, searching for known pulsars. we detect a total of 517 sources with 200 mhz linearly polarised flux densities between 9.9 mjy and 1.7 jy, of which 33 are known radio pulsars. all sources in our catalogues have faraday rotation measures in the range $-328.07$ to $+279.62$ rad m−2. the faraday rotation measures are broadly consistent with results from higher-frequency surveys, but with typically more than an order of magnitude improvement in the precision, highlighting the power of low-frequency polarisation surveys to accurately study galactic and extragalactic magnetic fields. we discuss the properties of our extragalactic and known-pulsar source population, how the sky distribution relates to galactic features, and identify a handful of new pulsar candidates among our nominally extragalactic source population. | the polarised gleam survey (pogs) ii: results from an all-sky rotation measure synthesis survey at long wavelengths |
we study inflation driven by a dilaton and an axion, both of which are coupled to a su(2) gauge field. we find that the inflation driven by the dilaton occurs in the early stage of inflation during which the gauge field grows due to the gauge-kinetic function. when the energy density of magnetic fields catches up with that of electric fields, chromonatural inflation takes over in the late stage of inflation, which we call delayed chromonatural inflation. thus, the delayed chromonatural inflation driven by the axion and the gauge field is induced by the dilaton. the interesting outcome of the model is the generation of chiral primordial gravitational waves on small scales. since the gauge field is inert in the early stage of inflation, it is viable in contrast to the conventional chromonatural inflation. we find the parameter region where chiral gravitational waves are generated in a frequency range higher than nhz, which are potentially detectable in future gravitational wave interferometers and pulsar-timing arrays such as deci-hertz interferometer gravitational wave observatory (decigo), evolved laser interferometer space antenna (elisa), and square kilometer array (ska). | chiral primordial gravitational waves from dilaton induced delayed chromonatural inflation |
this reference work gathers all of the latest research in the supernova field areas to create a definitive source book on supernovae, their remnants and related topics. it includes each distinct subdiscipline, including stellar types, progenitors, stellar evolution, nucleosynthesis of elements, supernova types, neutron stars and pulsars, black holes, swept up interstellar matter, cosmic rays, neutrinos from supernovae, supernova observations in different wavelengths, interstellar molecules and dust. while there is a great deal of primary and specialist literature on supernovae, with a great many scientific groups around the world focusing on the phenomenon and related subdisciplines, nothing else presents an overall survey. this handbook closes that gap at last. as a comprehensive and balanced collection that presents the current state of knowledge in the broad field of supernovae, this is to be used as a basis for further work and study by graduate students, astronomers and astrophysicists working in close/related disciplines, and established groups. editorial board editors-in-chief athem w. alsabti university college london observatory, university college london, london, uk sections: supernovae and supernova remnants supernovae and the environment of the solar system paul murdin institute of astronomy, university of cambridge, cambridge, uk section: supernovae and supernova remnants section editors david arnett steward observatory,university of arizona, tucson, az, usa section: nucleosynthesis in supernovae phil charles university of southampton, school of physics and astronomy, southampton, uk section: stellar remnants - neutron stars and black holes robert a. fesen department of physics and astronomy, dartmouth college, hanover, nh, usa section: evolution of supernovae and the interstellar medium david a. green cavendish laboratory, university of cambridge, cambridge, uk section: historical supernovae mario hamuy astronomy department, university of chile, santiago, chile; millennium institute of astrophysics, santiago, chile section: cosmology from supernovae peter hoeflich department of physics, florida state university, tallahassee, fl, usa section: explosion mechanisms of supernovae ken'ichi nomoto kavli institute for the physics and mathematics of the universe (wpi), the university of tokyo, kashiwa, chiba, japan section: supernovae and stellar evolution stephen smart astrophysics research centre, queen's university, belfast; northern ireland, uk section: light curves and spectra of supernovae mark sullivan school of physics and astronomy, university of southampton, highfield, southampton, uk section: types of supernovae friedrich-karl thielemann department of physics, university of basel, basel, switzerland sections: neutrinos, gravitational waves and cosmic rays nucleosynthesis in supernovae chengmin m. zhang national astronomical observatories, chinese academy of sciences, beijing, china; key laboratory of radio astronomy, cas, beijing, china; school of physical science, university of chinese academy of sciences, beijing, china section: stellar remnants - neutron stars and black holes. | handbook of supernovae |
accretion-induced collapse (aic) from oxygen/neon/magnesium composition white dwarf (onemg wd) + stripped helium (he) star binaries is one promising channel to form peculiar neutron star objects. it has been discussed that the wd's magnetic field may alter the accretion phase in the wd binary evolution. by considering non-magnetic and sufficiently magnetized wds, we investigate the evolution of onemg wd + he star binaries with detailed stellar evolution and binary population synthesis simulations. the role of the magnetically confined accretion in the possible formation pathway for like millisecond pulsars (msps) and magnetars is also studied. comparing with the case of spherically symmetric accretion, the mass accumulation efficiency of the wds is enhanced at low mass-transfer rate under the magnetic confinement model. the initial parameter space of the potential aic progenitor systems moves toward shorter orbital period and lower donor mass (but not so significantly) due to the effect of the magnetic confinement. this also allows final msps to have lower mass wd companions and shorter orbital periods. there is no significant difference between the galactic birth rates of the aic derived with and without the magnetic confinement, which implies that the magnetic field of the wd does not dramatically change the number of onemg wd + he star binaries which can produce aic. it is worth noting that these conclusions can be applied for the carbon/oxgen (co) wd + he star binaries as progenitors of type ia supernovae, because the accretion phases of onemg wds and co wds are similar. the galactic rate of magnetars possibly formed via aic of highly magnetized wds is $0.34\times 10^{-4}\, {\rm yr}^{-1}$. | the magnetized white dwarf + helium star binary evolution with accretion-induced collapse |
aims: the aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate.methods: for this purpose, we monitored the be-transient x-ray pulsar gro j1008-57 throughout a full orbital cycle. the current observational campaign was performed with the swift/xrt telescope in the soft x-ray band (0.5-10 kev) between two subsequent type i outbursts in january and september 2016.results: the expected transition to the propeller regime was not observed. however, transitions between different regimes of accretion were detected. in particular, after an outburst, the source entered a stable accretion state characterised by an accretion rate of 1014-1015 g s-1. we associate this state with accretion from a cold (low-ionised) disc of temperature below 6500 k. we argue that a transition to this accretion regime should be observed in all x-ray pulsars that have a certain combination of the rotation frequency and magnetic field strength. the proposed model of accretion from a cold disc is able to explain several puzzling observational properties of x-ray pulsars. | stable accretion from a cold disc in highly magnetized neutron stars |
we present a radio continuum and linear polarization study of the galactic supernova remnant (snr) g57.2+0.8, which may host the recently discovered magnetar sgr 1935+2154. the radio snr shows the typical radio continuum spectrum of a mature supernova remnant with a spectral index of α =-0.55+/- 0.02 and moderate polarized intensity. magnetic field vectors indicate a tangential magnetic field, expected for an evolved snr, in one part of the snr, and a radial magnetic field in the other. the latter can be explained by an overlapping arc-like feature, perhaps a pulsar wind nebula, emanating from the magnetar. the presence of a pulsar wind nebula is supported by the low average braking index of 1.2, which we extrapolated for the magnetar, and the detection of diffuse x-ray emission around it. we found a distance of 12.5 kpc for the snr, which identifies g57.2+0.8 as a resident of the outer spiral arm of the milky way. the snr has a radius of about 20 pc and could be as old as 41,000 yr. the snr has already entered the radiative or pressure-driven snowplow phase of its evolution. we compare independently determined characteristics like age and distance for both the snr and the soft gamma repeater sgr 1935+2154, and conclude that they are physically related. | a radio continuum and polarization study of snr g57.2+0.8 associated with magnetar sgr 1935+2154 |
we report here on initial results from the thousand-pulsar-array (tpa) programme, part of the large survey project 'meertime' on the meerkat telescope. the interferometer is used in the tied-array mode in the band from 856 to 1712 mhz, and the wide band coupled with the large collecting area and low receiver temperature make it an excellent telescope for the study of radio pulsars. the tpa is a 5 year project, which aims at to observing (a) more than 1000 pulsars to obtain high-fidelity pulse profiles, (b) some 500 of these pulsars over multiple epochs, and (c) long sequences of single-pulse trains from several hundred pulsars. the scientific outcomes from the programme will include the determination of pulsar geometries, the location of the radio emission within the pulsar magnetosphere, the connection between the magnetosphere and the crust and core of the star, tighter constraints on the nature of the radio emission itself, as well as interstellar medium studies. first, results presented here include updated dispersion measures, 26 pulsars with faraday rotation measures derived for the first time, and a description of interesting emission phenomena observed thus far. | the thousand-pulsar-array programme on meerkat - i. science objectives and first results |
we consider an uncharged anisotropic stellar model with two distinct equations of state in general relativity. the core layer has a quark matter distribution with a linear equation of state. the envelope layer has a matter distribution which is quadratic. the interfaces between the core, envelope and the vacuum exterior regions are smoothly matched. we find radii, masses and compactifications for five different compact objects which are consistent with other investigations. in particular, the properties of the pulsar object psr j1614-2230 are studied. the metric functions and the matter distribution are regular throughout the star. in particular, it is shown that the radii associated with the core and the envelope can change for different parameter values. | compact relativistic star with quadratic envelope |
the high altitude water cherenkov (hawc) telescope recently observed extended emission around the geminga and psr b0656+14 pulsar wind nebulae (pwne). these observations have been used to estimate cosmic-ray (cr) diffusion coefficients near the pwne that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in crs. two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (sdz) is confined to a small region around the pwn. such models are shown to successfully reproduce the hawc observations of the geminga pwn while retaining consistency with other cr data. it is found that the size of the sdz influences the predicted positron flux and the spectral shape of the extended γ-ray emission at lower energies that can be observed with the fermi large area telescope. if the two observed pwne are not unique, then it is likely that there are similar pockets of slow diffusion around many cr sources elsewhere in the milky way. the consequences of such a picture for galactic cr propagation is explored. | cosmic-ray propagation in light of the recent observation of geminga |
rotating neutron stars, or pulsars and magnetars, are plausibly the source of power behind many astrophysical systems, such as gamma-ray bursts, supernovae, pulsar wind nebulae, and supernova remnants. in the past several years, three-dimensional (3d) numerical simulations made it possible to compute pulsar spin-down luminosity from first principles and revealed that oblique pulsar winds are more powerful than aligned ones. however, what causes this enhanced power output of oblique pulsars is not understood. in this work, using time-dependent 3d magnetohydrodynamic and force-free simulations, we show that, contrary to the standard paradigm, the open magnetic flux, which carries the energy away from the pulsar, is laterally non-uniform. we argue that this non-uniformity is the primary reason for the increased luminosity of oblique pulsars. to demonstrate this, we construct simple analytic descriptions of aligned and orthogonal pulsar winds and combine them to obtain an accurate 3d description of the pulsar wind for any obliquity. our approach describes both the warped magnetospheric current sheet and the smooth variation of pulsar wind properties outside of it. we find that the jump in magnetic field components across the current sheet decreases with increasing obliquity, which could be a mechanism that reduces dissipation in near-orthogonal pulsars. our analytical description of the pulsar wind can be used for constructing models of pulsar gamma-ray emission, pulsar wind nebulae, neutron star powered ultra-luminous x-ray sources, and magnetar-powered core-collapse gamma-ray bursts and supernovae. | three-dimensional analytical description of magnetized winds from oblique pulsars |
in the beginning of 2023, the be transient x-ray pulsar rx j0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of february and reaching peak luminosity of ≈4.3 × 1037 erg s-1. here, we present the results of a detailed spectral and temporal study of the source based on nustar, swift, integral, and nicer observations performed during this period and covering wide range of energies and luminosities. we find that both the pulse profile shape and spectral hardness change abruptly around ≈2.8 × 1037 erg s-1, which we associate with a transition to supercritical accretion regime and erection of the accretion column. the observed pulsed fraction decreases gradually with energy up to 20 kev (with a local minimum around fluorescence iron line), which is unusual for an x-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to ≈120 kev. the broad-band energy spectra of rx j0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10-20 and 50-70 kev previously suggested for other pulsars without additional features. in particular, an absorption feature around 30 kev previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around b ~ 1013 g. | rx j0440.9+4431: another supercritical x-ray pulsar |
scalar-tensor theories of gravity are extensions of general relativity (gr) including an extra, nonminimally coupled scalar degree of freedom. a wide class of these theories, albeit indistinguishable from gr in the weak field regime, predicts a radically different phenomenology for neutron stars, due to a nonperturbative, strong-field effect referred to as spontaneous scalarization. this effect is known to occur in theories where the effective linear coupling β0 between the scalar and matter fields is sufficiently negative, i.e. β0≲-4.35 , and has been strongly constrained by pulsar timing observations. in the test-field approximation, spontaneous scalarization manifests itself as a tachyonic-like instability. recently, it was argued that, in theories where β0>0 , a similar instability would be triggered by sufficiently compact neutron stars obeying realistic equations of state. in this work we investigate the end state of this instability for some representative coupling functions with β0>0 . this is done both through an energy balance analysis of the existing equilibrium configurations, and by numerically determining the nonlinear cauchy development of unstable initial data. we find that, contrary to the β0<0 case, the final state of the instability is highly sensitive to the details of the coupling function, varying from gravitational collapse to spontaneous scalarization. in particular, we show, for the first time, that spontaneous scalarization can happen in theories with β0>0 , which could give rise to novel astrophysical tests of the theory of gravity. | highly compact neutron stars in scalar-tensor theories of gravity: spontaneous scalarization versus gravitational collapse |
pulsar wind nebulae (pwne) are the main gamma-ray emitters in the galactic plane. they are diffuse nebulae that emit nonthermal radiation. pulsar winds, relativistic magnetized outflows from the central star, shocked in the ambient medium produce a multiwavelength emission from the radio through gamma-rays. although the leptonic scenario is able to explain most pwne emission, a hadronic contribution cannot be excluded. a possible hadronic contribution to the high-energy gamma-ray emission inevitably leads to the production of neutrinos. using 9.5 yr of all-sky icecube data, we report results from a stacking analysis to search for neutrino emission from 35 pwne that are high-energy gamma-ray emitters. in the absence of any significant correlation, we set upper limits on the total neutrino emission from those pwne and constraints on hadronic spectral components. | icecube search for high-energy neutrino emission from tev pulsar wind nebulae |
the five-hundred-meter aperture spherical radio telescope (fast) is the largest single-dish radio telescope in the world. in this paper, we make forecast on the fast h i large-scale structure survey by mock observations. we consider a drift scan survey with the l-band 19 beam receiver, which may be commensal with the pulsar search and galactic h i survey. we also consider surveys at lower frequency, using either the current single feed wide-band receiver or a future multibeam phased array feed (paf) in the uhf band. we estimate the number density of detected h i galaxies and the measurement error in positions and the precision of the surveys are evaluated using both fisher matrix and simulated observations. the measurement error in the h i galaxy power spectrum is estimated, and we find that the error is relatively large even at moderate redshifts, as the number of positively detected galaxies drops drastically with increasing redshift. however, good cosmological measurement could be obtained with the intensity mapping technique where the large scale h i distribution is measured without resolving individual galaxies. the figure of merit for the dark energy equation of state with different observation times is estimated, and we find that with the existing l-band multibeam receiver, a good measurement of low redshift large-scale structure can be obtained, which complements the existing optical surveys. with a paf in the uhf band, the constraint can be much stronger, reaching the level of a dark energy task force stage iv experiment. | forecast for fast: from galaxies survey to intensity mapping |
the two leading hypotheses for the galactic center excess (gce) in the fermi data are an unresolved population of faint millisecond pulsars (msps) and dark-matter (dm) annihilation. the dichotomy between these explanations is typically reflected by modeling them as two separate emission components. however, point sources (pss) such as msps become statistically degenerate with smooth poisson emission in the ultrafaint limit (formally where each source is expected to contribute much less than one photon on average), leading to an ambiguity that can render questions such as whether the emission is ps-like or poissonian in nature ill defined. we present a conceptually new approach that describes the ps and poisson emission in a unified manner and only afterwards derives constraints on the poissonian component from the so obtained results. for the implementation of this approach, we leverage deep learning techniques, centered around a neural network-based method for histogram regression that expresses uncertainties in terms of quantiles. we demonstrate that our method is robust against a number of systematics that have plagued previous approaches, in particular dm/ps misattribution. in the fermi data, we find a faint gce described by a median source-count distribution (scd) peaked at a flux of ∼4 ×10-11 counts cm-2 s-1 (corresponding to ∼3 - 4 expected counts per ps), which would require n ∼o (104) sources to explain the entire excess (median value n =29 ,300 across the sky). although faint, this scd allows us to derive the constraint ηp≤66 % for the poissonian fraction of the gce flux ηp at 95% confidence, suggesting that a substantial amount of the gce flux is due to pss. | extracting the galactic center excess' source-count distribution with neural nets |
we report the first discovery of a fast radio burst (frb), frb 20200125a, by the green bank northern celestial cap (gbncc) pulsar survey conducted with the green bank telescope at 350 mhz. frb 20200125a was detected at a galactic latitude of 58°43 with a dispersion measure of 179 pc cm3, while electron density models predict a maximum galactic contribution of 25 pc cm3 along this line of sight. moreover, no apparent galactic foreground sources of ionized gas that could account for the excess dm are visible in multiwavelength surveys of this region. this argues that the source is extragalactic. the maximum redshift for the host galaxy is zmax = 0.17, corresponding to a maximum comoving distance of approximately 750 mpc. the measured peak flux density for frb 20200125a is 0.37 jy, and we measure a pulse width of 3.7 ms, consistent with the distribution of frb widths observed at higher frequencies. based on this detection and assuming a euclidean flux density distribution of frbs, we calculate an all-sky rate at 350 mhz of ${3.4}_{-3.3}^{+15.4}\times {10}^{3}$ frbs sky-1 day-1 above a peak flux density of 0.42 jy for an unscattered pulse having an intrinsic width of 5 ms, consistent with rates reported at higher frequencies, albeit with large uncertainties. given the recent improvements in our single-pulse search pipeline, we also revisit the gbncc survey sensitivity to various burst properties. finally, we find no evidence of strong interstellar scattering in frb 20200125a, adding to the growing evidence that some frbs have circumburst environments where free-free absorption and scattering are not significant. | first discovery of a fast radio burst at 350 mhz by the gbncc survey |
since the day of its explosion, sn 1987a (sn87a) was closely monitored with the aim to study its evolution and to detect its central compact relic. the detection of neutrinos from the supernova strongly supports the formation of a neutron star (ns). however, the constant and fruitless search for this object has led to different hypotheses on its nature. to date, the detection in the atacama large millimeter/submillimeter array data of a feature that is somehow compatible with the emission arising from a proto-pulsar wind nebula (pwn) is the only hint of the existence of such elusive compact object. here we tackle this 33 yr old issue by analyzing archived observations of sn87a performed by chandra and nustar in different years. we firmly detect nonthermal emission in the 10-20 kev energy band, due to synchrotron radiation. the possible physical mechanism powering such emission is twofold: diffusive shock acceleration (dsa) or emission arising from an absorbed pwn. by relating a state-of-the-art magnetohydrodynamic simulation of sn87a to the actual data, we reconstruct the absorption pattern of the pwn embedded in the remnant and surrounded by cold ejecta. we found that, even though the dsa scenario cannot be firmly excluded, the most likely scenario that well explains the data is that of pwn emission. | indication of a pulsar wind nebula in the hard x-ray emission from sn 1987a |
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