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nonlinear oscillatory and monotonic electron acoustic shock waves in dissipative magnetorotating electron-positron-ion plasmas containing cold dynamical electrons, superthermal electrons, and positrons have been analyzed in the stationary background of massive positive ions. the korteweg de-vries-burgers equation which describes the dynamics of the nonlinear shock structures is derived by using amplitude reductive perturbation technique. the quantitative analysis of impact of different physical parameters on the shock structures is presented here. the electron fluid viscosity plays a pivotal role for the transition of electron acoustic (ea) monotonic to oscillatory shocks and vice versa. it is remarkable that the shock structures are sensitive to the coriolis force, obliqueness, electron temperature, and the positrons concentration. the present work may be employed to explore and to understand the formation of electron acoustic shock structures in the space and laboratory plasmas with superthermal electrons and positrons under magnetorotating effects, especially in auroral zone, van allen radiation belts, planetary magnetospheres, pulsars, black-hole magnetospheres, etc. | effect of anisotropic pressure on electron acoustic oscillatory and monotonic shocks in superthermal magnetoplasma |
we measure the proper motion of the pulsar psr j1745-2900 relative to the galactic center massive black hole, sgr a*, using the very long baseline array (vlba). the pulsar has a transverse velocity of 236 ± 11 km s-1 at position angle 22 ± 2 deg east of north at a projected separation of 0.097 pc from sgr a*. given the unknown radial velocity, this transverse velocity measurement does not conclusively prove that the pulsar is bound to sgr a* however, the probability of chance alignment is very small. we do show that the velocity and position are consistent with a bound orbit originating in the clockwise disk of massive stars orbiting sgr a* and a natal velocity kick of <~ 500 km s-1. an origin among the isotropic stellar cluster is possible but less probable. if the pulsar remains radio-bright, multiyear astrometry of psr j1745-2900 can detect its acceleration and determine the full three-dimensional orbit. we also demonstrate that psr j1745-2900 exhibits the same angular broadening as sgr a* over a wavelength range of 3.6 cm to 0.7 cm, further confirming that the two sources share the same interstellar scattering properties. finally, we place the first limits on the presence of a wavelength-dependent shift in the position of sgr a*, i.e., the core shift, one of the expected properties of optically thick jet emission. our results for psr j1745-2900 support the hypothesis that galactic center pulsars will originate from the stellar disk and deepen the mystery regarding the small number of detected galactic center pulsars. | the proper motion of the galactic center pulsar relative to sagittarius a* |
we analysed in a systematic way the public integral observations spanning from december 2002 to september 2016, to investigate the hard x-ray properties of about 60 high mass x-ray binaries (hmxbs). we considered both persistent and transient sources, hosting either a be star (be/xrbs) or a blue supergiant companion (sghmxbs, including supergiant fast x-ray transients, sfxts), a neutron star, or a black hole. integral x-ray light curves (18-50 kev), sampled at a bin time of about 2 ks, were extracted for all hmxbs to derive the cumulative distribution of their hard x-ray luminosity, their duty cycle, and the range of variability of their hard x-ray luminosity. this allowed us to obtain an overall and quantitative characterization of the long-term hard x-ray activity of the hmxbs in our sample. putting the phenomenology observed with integral into context with other known source properties (e.g. orbital parameters, pulsar spin periods, etc.) together with observational constraints coming from softer x-rays (1-10 kev), enabled the investigation of the way the different hmxb sub-classes behave (and sometimes overlap). for given source properties, the different sub-classes of massive binaries seem to cluster in a suggestive way. however, for what concerns supergiant systems (sghmxbs versus sfxts), several sources with intermediate properties exist, suggesting a smooth transition between the two sub-classes. | an integral overview of high-mass x-ray binaries: classes or transitions? |
continuous gravitational waves from neutron stars could provide an invaluable resource to learn about their interior physics. a common search method involves matched filtering a modeled template against the noisy gravitational-wave data to find signals. this method suffers a mismatch (i.e., relative loss of the signal-to-noise ratio) if the signal deviates from the template. one possible instance in which this may occur is if the neutron star undergoes a glitch, a sudden rapid increase in the rotation frequency seen in the timing of many radio pulsars. in this work, we use a statistical characterization of the glitch rate and size in radio pulsars to estimate how often neutron star glitches would occur within the parameter space of continuous gravitational-wave searches and how much mismatch putative signals would suffer in the search due to these glitches. we find that for many previous and potential future searches continuous-wave signals have an elevated probability of undergoing one or more glitches and that these glitches will often lead to a substantial fraction of the signal-to-noise ratio being lost. this could lead to a failure to identify candidate gravitational-wave signals in the initial stages of a search and also to the false dismissal of candidates in subsequent follow-up stages. | statistical characterization of pulsar glitches and their potential impact on searches for continuous gravitational waves |
neutron stars are the densest objects in the universe, with m ~ 1.4 msolar and r ~ 12 km, and the equation of state associated to their internal composition is still unknown. the extreme conditions to which matter is subjected inside neutron stars could lead to a phase transition in their inner cores, giving rise to a hybrid compact object. the observation of 2msolar binary pulsars (psr j1614-2230, psr j0343+0432 and psr j0740+6620) strongly constraints theoretical models of the equation of state. moreover, the detection of gravitational waves emitted during the binary neutron star merger, gw170817, and its electromagnetic counterpart, grb170817a, impose additional constraints on the tidal deformability. in this work, we investigate hybrid stars with sequential phase transitions hadron-quark-quark in their cores.{we assume that both phase transitions are sharp and analyse the rapid and slow phase conversion scenarios.} for the outer core, we use modern hadronic equations of state. for the inner core we employ the constant speed of sound parametrization for quark matter. we analyze more than 3000 hybrid equations of state, taking into account the recent observational constraints from neutron stars. the effects of hadron-quark-quark phase transitions on the normal oscillation modes f and g, are studied under the cowling relativistic approximation. our results show that, in the slow conversion regime, a second quark-quark phase transition gives rise to a new g2 mode. we discuss the observational implications of our results associated to the gravitational waves detection and the possibility of detecting hints of sequential phase transitions and the associated g2 mode. | hybrid stars with sequential phase transitions: the emergence of the g2 mode |
in this work, we consider the regret minimization problem for reinforcement learning in latent markov decision processes (lmdp). in an lmdp, an mdp is randomly drawn from a set of $m$ possible mdps at the beginning of the interaction, but the identity of the chosen mdp is not revealed to the agent. we first show that a general instance of lmdps requires at least $\omega((sa)^m)$ episodes to even approximate the optimal policy. then, we consider sufficient assumptions under which learning good policies requires polynomial number of episodes. we show that the key link is a notion of separation between the mdp system dynamics. with sufficient separation, we provide an efficient algorithm with local guarantee, {\it i.e.,} providing a sublinear regret guarantee when we are given a good initialization. finally, if we are given standard statistical sufficiency assumptions common in the predictive state representation (psr) literature (e.g., boots et al.) and a reachability assumption, we show that the need for initialization can be removed. | rl for latent mdps: regret guarantees and a lower bound |
for the past couple of decades, the parkes radio telescope has been regularly observing the millisecond pulsars in 47 tucanae (47 tuc). this long-term timing programme was designed to address a wide range of scientific issues related to these pulsars and the globular cluster where they are located. in this paper, the first of a series, we address one of these objectives: the characterization of four previously known binary pulsars for which no precise orbital parameters were known, namely 47 tuc p, v, w and x (pulsars 47 tuc r and y are discussed elsewhere). we determined the previously unknown orbital parameters of 47 tuc v and x and greatly improved those of 47 tuc p and w. for pulsars w and x we obtained, for the first time, full coherent timing solutions across the whole data span, which allowed a much more detailed characterization of these systems. 47 tuc w, a well-known tight eclipsing binary pulsar, exhibits a large orbital period variability, as expected for a system of its class. 47 tuc x turns out to be in a wide, extremely circular, 10.9-d long binary orbit and its position is ∼3.8 arcmin away from the cluster centre, more than three times the distance of any other pulsar in 47 tuc. these characteristics make 47 tuc x a very different object with respect to the other pulsars of the cluster. | long-term observations of the pulsars in 47 tucanae - i. a study of four elusive binary systems |
we present broadband (3-78 kev) nustar x-ray imaging and spectroscopy of the crab nebula and pulsar. we show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power law in this energy band, spatially resolved spectroscopy of the nebula finds a break at ~9 kev in the spectral photon index of the torus structure with a steepening characterized by δγ ~ 0.25. we also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power law with break energy at ~12 kev and δγ ~ 0.27. we present spectral maps of the inner 100'' of the remnant and measure the size of the nebula as a function of energy in seven bands. these results find that the rate of shrinkage with energy of the torus size can be fitted by a power law with an index of γ = 0.094 ± 0.018, consistent with the predictions of kennel and coroniti. the change in size is more rapid in the nw direction, coinciding with the counter-jet where we find the index to be a factor of two larger. nustar observed the crab during the latter part of a γ-ray flare, but found no increase in flux in the 3-78 kev energy band. | broadband x-ray imaging and spectroscopy of the crab nebula and pulsar with nustar |
we present initial results from the low-latitude galactic plane region of the high time resolution universe pulsar survey conducted at the parkes 64-m radio telescope. we discuss the computational challenges arising from the processing of the terabyte-sized survey data. two new radio interference mitigation techniques are introduced, as well as a partially coherent segmented acceleration search algorithm which aims to increase our chances of discovering highly relativistic short-orbit binary systems, covering a parameter space including potential pulsar-black hole binaries. we show that under a constant acceleration approximation, a ratio of data length over orbital period of ≈0.1 results in the highest effectiveness for this search algorithm. from the 50 per cent of data processed thus far, we have redetected 435 previously known pulsars and discovered a further 60 pulsars, two of which are fast-spinning pulsars with periods less than 30 ms. psr j1101-6424 is a millisecond pulsar whose heavy white dwarf (wd) companion and short spin period of 5.1 ms indicate a rare example of full-recycling via case a roche lobe overflow. psr j1757-27 appears to be an isolated recycled pulsar with a relatively long spin period of 17 ms. in addition, psr j1244-6359 is a mildly recycled binary system with a heavy wd companion, psr j1755-25 has a significant orbital eccentricity of 0.09 and psr j1759-24 is likely to be a long-orbit eclipsing binary with orbital period of the order of tens of years. comparison of our newly discovered pulsar sample to the known population suggests that they belong to an older population. furthermore, we demonstrate that our current pulsar detection yield is as expected from population synthesis. | the high time resolution universe pulsar survey - xii. galactic plane acceleration search and the discovery of 60 pulsars |
multiwavelength follow-up of unidentified fermi sources has vastly expanded the number of known galactic-field “black widow” and “redback” millisecond pulsar binaries. focusing on their rotation-powered state, we interpret the radio to x-ray phenomenology in a consistent framework. we advocate the existence of two distinct modes differing in their intrabinary shock orientation, distinguished by the phase centering of the double-peaked x-ray orbital modulation originating from mildly relativistic doppler boosting. by constructing a geometric model for radio eclipses, we constrain the shock geometry as functions of binary inclination and shock standoff r 0. we develop synthetic x-ray synchrotron orbital light curves and explore the model parameter space allowed by radio eclipse constraints applied on archetypal systems b1957+20 and j1023+0038. for b1957+20, from radio eclipses the standoff is r 0 ∼ 0.15-0.3 fraction of binary separation from the companion center, depending on the orbit inclination. constructed x-ray light curves for b1957+20 using these values are qualitatively consistent with those observed, and we find occultation of the shock by the companion as a minor influence, demanding significant doppler factors to yield double peaks. for j1023+0038, radio eclipses imply r 0 ≲ 0.4, while x-ray light curves suggest 0.1 ≲ r 0 ≲ 0.3 (from the pulsar). degeneracies in the model parameter space encourage further development to include transport considerations. generically, the spatial variation along the shock of the underlying electron power-law index should yield energy dependence in the shape of light curves, motivating future x-ray phase-resolved spectroscopic studies to probe the unknown physics of pulsar winds and relativistic shock acceleration therein. | constraining relativistic bow shock properties in rotation-powered millisecond pulsar binaries |
pulsars are rotating neutron stars that emit periodic electromagnetic radiation. while pulsars generally slow down as they lose energy, some also experience glitches: spontaneous increases of their rotational frequency. according to several models, these glitches can also lead to the emission of long-duration transient gravitational waves (gws). we present detection prospects for such signals by comparing indirect energy upper limits on gw strain for known glitches with the sensitivity of current and future ground-based gw detectors. we first consider the optimistic case of generic constraints based on the glitch size and find that realistic matched-filter searches in the fourth ligo-virgo-kagra observing run (o4) could make a detection, or set constraints below these indirect upper limits, for equivalents of 36 out of 726 previously observed glitches, and 74 in the o5 run. with the third-generation einstein telescope or cosmic explorer, 35-40 per cent of glitches would be accessible. when specializing to a scenario where transient mountains produce the post-glitch gw emission, following the yim & jones model, the indirect upper limits are stricter. out of the smaller set of 119 glitches with measured healing parameter, as needed for predictions under that model, only 6 glitches would have been within reach for o4 and 14 for o5, with a similar percentage as before with third-generation detectors. we also discuss how this model matches the observed glitch population. | prospects for detecting transient quasi-monochromatic gravitational waves from glitching pulsars with current and future detectors |
the possibility of neutron-mirror neutron transition has recently been discussed both from theoretical and experimental perspectives. we point out that such a transition would imply mass loss in neutron stars leading to a significant change of the orbital period of neutron star binary systems. known limits on the period changes of such binaries therefore put strong limits on the above transition rate and hence on the n -n' mixing parameter ɛ'. these considerations exclude a large range of ɛ' even if the n -n' mass difference is δ m' as large as a few mev. our limits also exclude a large range of the values required to explain the neutron decay anomaly via n -n' mixing. | bounds on neutron-mirror neutron mixing from pulsar timing |
recently, two photons from the crab nebula with energy of approximately 1 pev were detected by the large high altitude air shower observatory (lhaaso), opening an ultrahigh-energy window for studying pulsar wind nebulae (pwne). remarkably, the lhaaso spectrum at the highest-energy end shows a possible hardening, which could indicate the presence of a new component. a two-component scenario with a main electron component and a secondary proton component has been proposed to explain the whole spectrum of the crab nebula, requiring a proton energy of 1046-1047 erg remaining in the present crab nebula. in this paper, we study the energy content of relativistic protons in pulsar winds using the lhaaso data of the crab nebula, considering the effect of diffusive escape of relativistic protons. depending on the extent of the escape of relativistic protons, the total energy of protons lost in the pulsar wind could be 10-100 times larger than that remaining in the nebula presently. we find that the current lhaaso data allow up to (10-50)% of the spindown energy of pulsars being converted into relativistic protons. the escaping protons from pwne could make a considerable contribution to the cosmic-ray flux of 10-100 pev. we also discuss the leptonic scenario for the possible spectral hardening at pev energies. | pev emission of the crab nebula: constraints on the proton content in pulsar wind and implications |
we investigate the effect of feeble interaction of dark matter (dm) with hadronic matter on the equation of state (eos) and structural properties of neutron stars (nss) in static conditions. for the purpose, we adopt the effective chiral model for the hadronic sector and for the first time in the context of possible existence of dm inside nss, we introduce the dm-sm interaction through light new physics mediator. moreover, the mass of dm fermion, the mediator and the coupling are adopted from the self-interaction constraint from bullet cluster and from present day relic abundance. within the considered framework, the work highlights the underlying stiffening of eos in the presence of dm fermion of mass of the order of a few gev compared to the no-dm scenario. consequently, the maximum gravitational mass of ns is obtained consistent with the bounds from the most massive pulsars that were not satisfied with the hadronic matter eos alone. the estimates of radius and tidal deformability of 1.4-m⊙ ns and the tidal deformabilities of the individual components of the binary neutron stars (bns) associated with gw170817 are all in good agreement with the individual constraints obtained from gw170817 observation of bns merger. | implications of feebly interacting dark sector on neutron star properties and constraints from gw170817 |
we study the impact of a binary companion on black hole superradiance at orbital frequencies away from the resonance bands of the superradiant cloud. a superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. below a critical binary separation, this superradiance rate becomes negative, and the boson cloud gets absorbed by the black hole. this critical binary separation leads to tight constraints on the resonant transitions of the cloud. especially, a companion with mass ratio q >10-3 invalidates all fine structure transitions, as well as almost all bohr transitions except those starting from the |ψ211⟩ state. meanwhile, the backreaction on the companion manifests itself as a torque acting on the binary, producing floating/sinking orbits that can be verified via pulsar timing. in addition, the possible termination of cloud growth may help to alleviate the current bounds on the ultralight boson mass from various null detections. | termination of superradiance from a binary companion |
we present measurements of the polarization of x-rays in the 2-8 kev band from the pulsar in the ultracompact low-mass x-ray binary 4u 1626-67 using data from the imaging x-ray polarimetry explorer (ixpe). the 7.66 s pulsations were clearly detected throughout the ixpe observations as well as in the nicer soft x-ray observations, which we used as the basis for our timing analysis and to constrain the spectral shape over the 0.4-10 kev energy band. chandra hetgs high-resolution x-ray spectra were also obtained near the times of the ixpe observations for firm spectral modeling. we found an upper limit on the pulse-averaged linear polarization of <4% (at 95% confidence). similarly, there was no significant detection of polarized flux in pulse phase intervals when subdividing the bandpass by energy. however, spectropolarimetric modeling over the full bandpass in pulse phase intervals provided a marginal detection of polarization of the power-law spectral component at the 4.8% ± 2.3% level (90% confidence). we discuss the implications concerning the accretion geometry onto the pulsar, favoring two-component models of the pulsed emission. | observations of 4u 1626-67 with the imaging x-ray polarimetry explorer |
context. several new ultrahigh-energy (uhe) γ-ray sources have recently been discovered by the large high altitude air shower observatory (lhaaso) collaboration. these represent a step forward in the search for the so-called galactic pevatrons, the enigmatic sources of the galactic cosmic rays up to pev energies. however, it has been shown that multi-tev γ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse compton scattering from electrons in a radiation-dominated environment. a clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source.aims: we aim to understand the nature of the unidentified source lhaaso j2108+5157, which is one of the few known uhe sources with no very high-energy (vhe) counterpart.methods: we observed lhaaso j2108+5157 in the x-ray band with xmm-newton in 2021 for a total of 3.8 hours and at tev energies with the large-sized telescope prototype (lst-1), yielding 49 hours of good-quality data. in addition, we analyzed 12 years of fermi-lat data, to better constrain emission of its high-energy (he) counterpart 4fgl j2108.0+5155. we used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source.results: we found an excess (3.7σ) in the lst-1 data at energies e > 3 tev. further analysis of the whole lst-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of γ = 1.6 ± 0.2 the range of 0.3 − 100 tev. we did not find any significant extended emission that could be related to a supernova remnant (snr) or pulsar wind nebula (pwn) in the xmm-newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. we revealed a new potential hard source in fermi-lat data with a significance of 4σ and a photon index of γ = 1.9 ± 0.2, which is not spatially correlated with lhaaso j2108+5157, but including it in the source model we were able to improve spectral representation of the he counterpart 4fgl j2108.0+5155.conclusions: the lst-1 and lhaaso observations can be explained as inverse compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100−30+70 tev. the low magnetic field in the source imposed by the x-ray upper limits on synchrotron emission is compatible with a hypothesis of a pwn or a tev halo. furthermore, the spectral properties of the he counterpart are consistent with a geminga-like pulsar, which would be able to power the vhe-uhe emission. nevertheless, the lack of a pulsar in the neighborhood of the uhe source is a challenge to the pwn/tev-halo scenario. the uhe γ rays can also be explained as π0 decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. indeed, the hard spectrum in the lst-1 band is compatible with protons escaping a shock around a middle-aged snr because of their high low-energy cut-off, but the origin of the he γ-ray emission remains an open question. | multiwavelength study of the galactic pevatron candidate lhaaso j2108+5157 |
superresolution (sr) optical microscopy has allowed the investigation of many biological structures below the diffraction limit; however, most of the techniques are hampered by the need for fluorescent labels. nonlinear label-free techniques such as second-harmonic generation (shg) provide structurally specific contrast without the addition of exogenous labels, allowing observation of unperturbed biological systems. we use the photonic nanojet (pnj) phenomena to achieve sr-shg. a resolution of ∼ λ / 6 with respect to the fundamental wavelength, that is, a ∼ 2.3 -fold improvement over conventional or diffraction-limited shg under the same imaging conditions is achieved. crucially we find that the polarization properties of excitation are maintained in a pnj. this is observed in experiment and simulations. this may have widespread implications to increase sensitivity by detection of polarization-resolved shg by observing anisotropy in signals. these new, to the best of our knowledge, findings allowed us to visualize biological shg-active structures such as collagen at an unprecedented and previously unresolvable spatial scale. moreover, we demonstrate that the use of an array of self-assembled high-index spheres overcomes the issue of a limited field of view for such a method, allowing pnj-assisted sr-shg to be used over a large area. dysregulation of collagen at the nanoscale occurs in many diseases and is an underlying cause in diseases such as lung fibrosis. here we demonstrate that psr-shg allows unprecedented observation of changes at the nanoscale that are invisible by conventional diffraction-limited shg imaging. the ability to nondestructively image shg-active biological structures without labels at the nanoscale with a relatively simple optical method heralds the promise of a new tool to understand biological phenomena and drive drug discovery. | superresolved polarization-enhanced second-harmonic generation for direct imaging of nanoscale changes in collagen architecture |
in this article we put forward a novel phenomenological paradigm in which particle physics beyond the standard model may be tested by radio astronomy if these are related to a first order phase transition in the early universe. for this type of dark matter models, the first order phase transition takes place at kev scales, and hence, induces the production of a stochastic gravitational waves background that can be detected from pulsar timing measurements. we demonstrate this hypothetical feasibility by studying a class of majoron dark matter models, which are related to a first order phase transition of the u(1)l or u(1) b - l symmetry and are consequently dubbed as violent majorons. these phenomena are expected to be examined by the ongoing and forthcoming radio experiments, including fast, ska and ipta. | testing dark matter models with radio telescopes in light of gravitational wave astronomy |
in this paper, we propose a new scenario in which a rapidly rotating strongly magnetized pulsar without any surrounding supernova ejecta repeatedly produces fast radio bursts (frbs) via a range of possible mechanisms; simultaneously, an ultra-relativistic electron/positron pair wind from the pulsar sweeps up its ambient dense interstellar medium, giving rise to a non-relativistic pulsar wind nebula (pwn). we show that the synchrotron radio emission from such a pwn is bright enough to account for the recently discovered persistent radio source associated with the repeating frb 121102 within reasonable ranges of the model parameters. our pwn scenario is consistent with the non-evolution of the dispersion measure inferred from all of the repeating bursts observed in four years. | radio emission from pulsar wind nebulae without surrounding supernova ejecta: application to frb 121102 |
we report the measurement of the acoustic quality factor of a gram-scale, kilohertz-frequency superfluid resonator, detected through the parametric coupling to a superconducting niobium microwave cavity. for temperatures between 400 mk and 50 mk, we observe a t^{-4} temperature dependence of the quality factor, consistent with a 3-phonon dissipation mechanism. we observe q factors up to 1.4× 10^8, consistent with the dissipation due to dilute ^3he impurities, and expect that significant further improvements are possible. these experiments are relevant to exploring quantum behavior and decoherence of massive macroscopic objects, the laboratory detection of continuous gravitational waves from pulsars, and the probing of possible limits to physical length scales. | ultra-high q acoustic resonance in superfluid ^4he |
nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. the understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. this is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (hvs) between 14 and 26 kv and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip. the temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 μm with an initial pressure of 5 × 108 pa at a temperature of 1200 k for a hv of 20 kv. this yields an initial energy in the range of a few 10-5 j that varies with the applied hv. the dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 pa and 50 k, respectively. these predictions from cavitation theory are corroborated by optical emission spectroscopy. after igniting the nanosecond plasma, the electrical power oscillates in the feed line between hv pulser and plasma chamber with a ring down time of the order of 60 ns. these reflected pulses re-ignite a plasma inside the expanding bubble periodically. broadband emission due to recombination and bremsstrahlung becomes visible within the first 30 ns. at later times, line emission dominates. stark broadening of the spectral lines of hα (656 nm) and oi (777 nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas. | nanosecond plasmas in water: ignition, cavitation and plasma parameters |
the hawc collaboration has recently reported the detection of bright and spatially extended multi-tev gamma-ray emission from geminga, monogem, and a handful of other nearby, middle-aged pulsars. the angular profile of the emission observed from these pulsars is surprising, in that it implies that cosmic-ray diffusion is significantly inhibited within ∼25 pc of these objects, compared to the expectations of standard galactic diffusion models. this raises the important question of whether the diffusion coefficient in the local interstellar medium is also low, or whether it is instead better fit by the mean galactic value. here, we utilize recent observations of the cosmic-ray electron spectrum (extending up to ∼20 tev ) by the h.e.s.s. collaboration to show that the local diffusion coefficient cannot be as low as it is in the regions surrounding geminga and monogem. instead, we conclude that cosmic rays efficiently diffuse through the bulk of the local interstellar medium. among other implications, this further supports the conclusion that pulsars significantly contribute to the observed positron excess and provides strong evidence for spatially varying diffusion coefficients throughout the milky way. | measuring the local diffusion coefficient with h.e.s.s. observations of very high-energy electrons |
pulsed emission from almost one hundred millisecond pulsars (msps) has been detected in γ-rays by the fermi large-area telescope. the global properties of this population remain relatively unconstrained despite many attempts to model their spatial and luminosity distributions. we perform here a self-consistent bayesian analysis of both the spatial distribution and luminosity function simultaneously. distance uncertainties, arising from errors in the parallax measurement or galactic electron-density model, are marginalized over. we provide a public python package (available from http://github.com/tedwards2412/mspdist) for calculating distance uncertainties to pulsars derived using the dispersion measure by accounting for the uncertainties in galactic electron-density model ymw16. finally, we use multiple parametrizations for the msp population and perform bayesian model comparison, finding that a broken power-law luminosity function with lorimer spatial profile are preferred over multiple other parametrizations used in the past. the best-fitting spatial distribution and number of γ-ray msps is consistent with results for the radio population of msps. | bayesian model comparison and analysis of the galactic disc population of gamma-ray millisecond pulsars |
we report on a high-precision timing analysis and an astrophysical study of the binary millisecond pulsar, psr j1909-3744, motivated by the accumulation of data with well improved quality over the past decade. using 15 yr of observations with the nançay radio telescope, we achieve a timing precision of approximately 100 ns. we verify our timing results by using both broad-band and sub-band template matching methods to create the pulse time-of-arrivals. compared with previous studies, we improve the measurement precision of secular changes in orbital period and projected semimajor axis. we show that these variations are both dominated by the relative motion between the pulsar system and the solar system barycentre. additionally, we identified four possible solutions to the ascending node of the pulsar orbit, and measured a precise kinetic distance of the system. using our timing measurements and published optical observations, we investigate the binary history of this system using the stellar evolution code mesa, and discuss solutions based on detailed wd cooling at the edge of the wd age dichotomy paradigm. we determine the 3d velocity of the system and show that it has been undergoing a highly eccentric orbit around the centre of our galaxy. furthermore, we set up a constraint over dipolar gravitational radiation with the system, which is complementary to previous studies given the mass of the pulsar. we also obtain a new limit on the parametrized post-newtonian parameter, $\left| \hat\alpha_1 \right|$ < 2.1 × 10-5 at 95 per cent confidence level, which is fractionally better than previous best published value and achieved with a more concrete method. | a revisit of psr j1909-3744 with 15-yr high-precision timing |
psr j1640+2224 is a binary millisecond pulsar (bmsp) with a white dwarf (wd) companion. recent observations indicate that the wd is very likely to be a ∼0.7 m⊙ co wd. thus, the bmsp should have evolved from an intermediate-mass x-ray binary (imxb). however, previous investigations on imxb evolution predict that the orbital periods of the resultant bmsps are generally < 40 days, in contrast with the 175 day orbital period of psr j1640+2224. in this paper, we explore the influence of the mass of the neutron star (ns) and the chemical compositions of the companion star on the formation of bmsps. our results show that the final orbital period becomes longer with increasing ns mass, and the wd mass becomes larger with decreasing metallicity. in particular, to reproduce the properties of psr j1640+2224, the ns was likely born massive (>2.0 m⊙). | on the formation of psr j1640+2224: a neutron star born massive? |
we report results of the first search to date for continuous gravitational waves from unstable r-modes from the pulsar $\mathrm{psr}\ {\rm{j}}0537\mbox{--}6910$ . we use data from the first two observing runs of the advanced ligo network. we find no significant signal candidate and set upper limits on the amplitude of gravitational-wave signals, which are within an order of magnitude of the spin-down values. we highlight the importance of having timing information at the time of the gravitational-wave observations, i.e., rotation frequency and frequency-derivative values, and glitch-occurrence times, such as those that a nicer campaign could provide. | first search for r-mode gravitational waves from psr j0537-6910 |
a rotating fluid star, endowed with a magnetic field, can undergo a form of precessional motion: a sum of rigid-body free precession and a non-rigid response. on secular time-scales this motion is dissipated by bulk and shear viscous processes in the stellar interior and magnetospheric braking in the exterior, changing the inclination angle between the rotation and magnetic axes. using our recent solutions for the non-rigid precessional dynamics, and viscous dissipation integrals derived in this paper, we make the only self-consistent calculation to date of these dissipation rates. we present the first results for the full coupled evolution of spindown and inclination angle for a model of a late-stage proto-neutron star with a strong toroidal magnetic field, allowing for both electromagnetic torques and internal dissipation when evolving the inclination angle. we explore this coupled evolution for a range of initial inclination angles, rotation rates, and magnetic field strengths. for fixed initial inclination angle, our results indicate that the neutron-star population naturally evolves into two classes: near-aligned and near-orthogonal rotators - with typical pulsars falling into the latter category. millisecond magnetars can evolve into the near-aligned rotators which mature magnetars appear to be, but only for small initial inclination angle and internal toroidal fields stronger than roughly 5 × 1015 g. once any model has evolved to either an aligned or orthogonal state, there appears to be no further evolution away from this state at later times. | neutron-star spindown and magnetic inclination-angle evolution |
we report on the results of the extensive multi-wavelength campaign from optical to gev γ-rays of the 2014 periastron passage of psr b1259-63, which is a unique high-mass γ-ray emitting binary system with a young pulsar companion. observations demonstrate the stable nature of the post-periastron gev flare and prove the coincidence of the flare with the start of rapid decay of the hα equivalent width, usually interpreted as a disruption of the be stellar disc. intensive x-ray observations reveal changes in the x-ray spectral behaviour happening at the moment of the gev flare. we demonstrate that these changes can be naturally explained as a result of synchrotron cooling of monoenergetic relativistic electrons injected into the system during the gev flare. | multi-wavelength observations of the binary system psr b1259-63/ls 2883 around the 2014 periastron passage |
we perform a detailed quantitative analysis of the recent ams-02 electron and positron data. we investigate the interplay between the emission from primary astrophysical sources, namely supernova remnants and pulsar wind nebulae, and the contribution from a dark matter annihilation or decay signal. our aim is to assess the information that can be derived on dark matter properties when both dark matter and primary astrophysical sources are assumed to jointly contribute to the leptonic observables measured by the ams-02 experiment. we investigate both the possibility to set robust constraints on the dark matter annihilation/decay rate and the possibility to look for dark matter signals within realistic models that take into account the full complexity of the astrophysical background. our results show that ams-02 data enable to probe efficiently vast regions of the dark matter parameter space and, in some cases, to set constraints on the dark matter annihilation/decay rate that are comparable or even stronger than the ones derived from other indirect detection channels. | dark matter vs. astrophysics in the interpretation of ams-02 electron and positron data |
the galactic centre hosts a puzzling stellar population in its inner few parsecs, with a high abundance of surprisingly young, relatively massive stars bound within the deep potential well of the central supermassive black hole, sagittarius a* (ref. 1). previous studies suggest that the population of objects emitting soft x-rays (less than 10 kiloelectronvolts) within the surrounding hundreds of parsecs, as well as the population responsible for unresolved x-ray emission extending along the galactic plane, is dominated by accreting white dwarf systems. observations of diffuse hard-x-ray (more than 10 kiloelectronvolts) emission in the inner 10 parsecs, however, have been hampered by the limited spatial resolution of previous instruments. here we report the presence of a distinct hard-x-ray component within the central 4 × 8 parsecs, as revealed by subarcminute-resolution images in the 20-40 kiloelectronvolt range. this emission is more sharply peaked towards the galactic centre than is the surface brightness of the soft-x-ray population. this could indicate a significantly more massive population of accreting white dwarfs, large populations of low-mass x-ray binaries or millisecond pulsars, or particle outflows interacting with the surrounding radiation field, dense molecular material or magnetic fields. however, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation, and cosmic-ray production in the galactic centre. | extended hard-x-ray emission in the inner few parsecs of the galaxy |
we construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (wd) by a binary neutron star (ns) companion. nuclear reactions in the disc mid-plane burn the wd matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. a model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the bernoulli parameter of the mid-plane to a fixed value ≲0. we perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for wds of different initial compositions. for c/o wds, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. the nucleosynthesis in helium wd discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). by the end of the simulation, a substantial fraction of the wd mass is unbound in outflows at characteristic velocities of ∼109 cm s-1. the outflows from wd-ns merger discs contain 10-4-3 × 10-3 m⊙ of radioactive 56ni, resulting in fast (∼ week long) dim (∼1040 erg s-1) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ∼1043 erg s-1. the accreted mass on to the ns is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the ns to periods of ∼10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars. | time-dependent models of accretion discs with nuclear burning following the tidal disruption of a white dwarf by a neutron star |
the radiation from accreting x-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. however, phase-resolved and energy-resolved polarimetry of x-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. here we present the first results of the observations of the accreting x-ray pulsar vela x-1 performed with the imaging x-ray polarimetry explorer. vela x-1 is considered to be the archetypal example of a wind-accreting, high-mass x-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. the spectropolarimetric analysis of the phase-averaged data for vela x-1 reveals a polarization degree (pd) of 2.3% ± 0.4% at the polarization angle (pa) of -47.°3 ± 5.°4. a low pd is consistent with the results obtained for other x-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. the energy-resolved analysis shows the pd above 5 kev reaching 6%-10% and a ~90° difference in the pa compared to the data in the 2-3 kev range. the phase-resolved spectropolarimetric analysis finds a pd in the range 0%-9% with the pa varying between -80° and 40°. | ixpe observations of the quintessential wind-accreting x-ray pulsar vela x-1 |
during their most recent observing run, ligo/virgo reported the gravitational wave (gw) transient s191110af, a burst signal at a frequency of 1.78 khz that lasted for 0.104 s. while this signal was later deemed nonastrophysical, genuine detections of uncertain origin will occur in the future. here we study the potential for detecting gws from neutron star fluid oscillations, which have mode frequency and duration matching those of s191110af and which can be used to constrain the equation of state of nuclear matter. assuming that such transient oscillations can be excited to energies typical of a pulsar glitch, we use measured properties of known glitching pulsars to estimate the amplitude of gws produced by such events. we find that current gw detectors may observe nearby pulsars undergoing large events with energy similar to vela pulsar glitch energies, while next-generation detectors could observe a significant number of events. finally, we show that it is possible to distinguish between gws produced by rapidly rotating and slowly rotating pulsars from the imprint of rotation on the f -mode frequency. | gravitational waves from transient neutron star f -mode oscillations |
an intermediate-mass black hole (imbh) was recently reported to reside in the centre of the galactic globular cluster (gc) ngc 6624, based on timing observations of a millisecond pulsar (msp) located near the cluster centre in projection. we present dynamical models with multiple mass components of ngc 6624 - without an imbh - which successfully describe the surface brightness profile and proper motion kinematics from the hubble space telescope (hst) and the stellar-mass function at different distances from the cluster centre. the maximum line-of-sight acceleration at the position of the msp accommodates the inferred acceleration of the msp, as derived from its first period derivative. with discrete realizations of the models we show that the higher-order period derivatives - which were previously used to derive the imbh mass - are due to passing stars and stellar remnants, as previously shown analytically in literature. we conclude that there is no need for an imbh to explain the timing observations of this msp. | mass models of ngc 6624 without an intermediate-mass black hole |
the soft gamma-ray repeater candidate sgr 0755−2933 was discovered in 2016 by swift/bat, which detected a short (∼30 ms), powerful burst typical of magnetars. to understand the nature of the source, we present here an analysis of follow-up observations of the tentative soft-x-ray counterpart of the source obtained with swift/xrt, nustar, and chandra. from our analysis we conclude that, based on the observed counterpart position and properties, sgr 0755−2933 is not a soft gamma-ray repeater but rather a new high-mass x-ray binary. we suggest it be referred to as 2sxps j075542.5−293353. we therefore conclude that the available data do not allow us to confirm existence and identify the true soft-x-ray counterpart to the burst event. the presence of a soft counterpart is nevertheless essential to unambiguously associate the burst with a magnetar flare, and we conclude that the magnetar origin of the burst and a precise burst location remain uncertain and require further investigation. | sgr 0755−2933: a new high-mass x-ray binary with the wrong name |
we report the discovery of a highly polarized, highly variable, steep-spectrum radio source, askap j173608.2-321635, located ~4° from the galactic center in the galactic plane. the source was detected six times between 2020 january and 2020 september as part of the australian square kilometre array pathfinder variables and slow transients (askap vast) survey at 888 mhz. it exhibited a high degree (~25%) of circular polarization when it was visible. we monitored the source with the meerkat telescope from 2020 november to 2021 february on a 2-4 week cadence. the source was not detected with meerkat before 2021 february 7 when it appeared and reached a peak flux density of 5.6 mjy. the source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. the rotation measure of the source varied significantly, from -11.8 ± 0.8 rad m-2 to -64.0 ± 1.5 rad m-2 over three days. no x-ray counterpart was found in follow-up swift or chandra observations about a week after the first meerkat detection, with upper limits of ~5.0 × 1031 erg s-1 (0.3-8 kev, assuming a distance ~10 kpc). no counterpart is seen in new or archival near-infrared observations down to j = 20.8 mag. we discuss possible identifications for askap j173608.2-321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a galactic center radio transient: none of these fully explains the observations, which suggests that askap j173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys. | discovery of askap j173608.2-321635 as a highly polarized transient point source with the australian ska pathfinder |
radio-loud magnetars display a wide variety of radio-pulse phenomenology seldom seen among the population of rotation-powered pulsars. spectropolarimetry of the radio pulses from these objects has the potential to place constraints on their magnetic topology and unveil clues about the magnetar radio emission mechanism. here, we report on eight observations of the magnetar swift j1818.0-1607 taken with the parkes ultra-wideband low receiver covering a wide frequency range from 0.7 to 4 ghz over a period of 5 months. the magnetar exhibits significant temporal profile evolution over this period, including the emergence of a new profile component with an inverted spectrum, two distinct types of radio emission mode switching, detected during two separate observations, and the appearance and disappearance of multiple polarization modes. these various phenomena are likely a result of ongoing reconfiguration of the plasma content and electric currents within the magnetosphere. geometric fits to the linearly polarized position angle indicate we are viewing the magnetar at an angle of ~99○ from the spin axis, and its magnetic and rotation axes are misaligned by ~112○. while conducting these fits, we found the position angle swing had reversed direction on mjd 59062 compared to observations taken 15 d earlier and 12 d later. we speculate this phenomena may be evidence the radio emission from this magnetar originates from magnetic field lines associated with two co-located magnetic poles that are connected by a coronal loop. | the dynamic magnetosphere of swift j1818.0-1607 |
we report the follow-up of 10 pulsars discovered by the five-hundred-metre aperture spherical radio-telescope (fast) during its commissioning. the pulsars were discovered at a frequency of 500-mhz using the ultrawide-band (uwb) receiver in drift-scan mode, as part of the commensal radio astronomy fast survey (crafts). we carried out the timing campaign with the 100-m effelsberg radio-telescope at l-band around 1.36 ghz. along with 11 fast pulsars previously reported, fast seems to be uncovering a population of older pulsars, bordering and/or even across the pulsar death-lines. we report here two sources with notable characteristics. psr j1951+4724 is a young and energetic pulsar with nearly 100 per cent of linearly polarized flux density and visible up to an observing frequency of 8 ghz. psr j2338+4818, a mildly recycled pulsar in a 95.2-d orbit with a carbon-oxygen white dwarf (wd) companion of $\gtrsim 1\, \rm {m}_{\odot }$, based on estimates from the mass function. this system is the widest wd binary with the most massive companion known to-date. conspicuous discrepancy was found between estimations based on ne2001 and ymw16 electron density models, which can be attributed to underrepresentation of pulsars in the sky region between galactic longitudes 70° < l < 100°. this work represents one of the early crafts results, which start to show potential to substantially enrich the pulsar sample and refine the galactic electron density model. | fast early pulsar discoveries: effelsberg follow-up |
dark matter (dm) can consist of very light bosons behaving as a classical scalar field that experiences coherent oscillations. the presence of this dm field would perturb the dynamics of celestial bodies, either because the (oscillating) dm stress tensor modifies the gravitational potentials of the galaxy or if dm is directly coupled to the constituents of the body. we study secular variations of the orbital parameters of binary systems induced by such perturbations. two classes of effects are identified. effects of the first class appear if the frequency of dm oscillations is in resonance with the orbital motion; these exist for general dm couplings including the case of purely gravitational interaction. effects of the second class arise if dm is coupled quadratically to the masses of the binary system members and do not require any resonant condition. the exquisite precision of binary pulsar timing can be used to constrain these effects. current observations are not sensitive to oscillations in the galactic gravitational field, though a discovery of pulsars in regions of high dm density may improve the situation. for dm with direct coupling to ordinary matter, the current timing data are already competitive with other existing constraints in the range of dm masses ∼10-22-10-18 ev . future observations are expected to increase the sensitivity and probe new regions of parameters. | secular effects of ultralight dark matter on binary pulsars |
we report the discovery of a new unidentified extended γ-ray source in the galactic plane named lhaaso j0341+5258 with a pretrial significance of 8.2 standard deviations above 25 tev. the best-fit position is r.a. = 55°34 ± 0°11 and decl. = 52°97 ± 0°07. the angular size of lhaaso j0341+5258 is 0°29 ± 0°06stat ± 0°02sys. the flux above 25 tev is about 20% of the flux of the crab nebula. although a power-law fit of the spectrum from 10 to 200 tev with the photon index α = 2.98 ± 0.19stat ± 0.02sys is not excluded, the lhaaso data together with the flux upper limit at 10 gev set by the fermi-lat observation, indicate a noticeable steepening of an initially hard power-law spectrum with a cutoff at ≈50 tev. we briefly discuss the origin of ultra-high-energy gamma rays. the lack of an energetic pulsar and a young supernova remnant inside or in the vicinity of lhaaso j0341+5258 challenge, but do not exclude, both the leptonic and hadronic scenarios of gamma-ray production. | discovery of a new gamma-ray source, lhaaso j0341+5258, with emission up to 200 tev |
cosmic gravitons are expected in the mhz-ghz regions that are currently unreachable by the operating wide-band interferometers and where various classes of electromechanical detectors have been proposed through the years. the minimal chirp amplitude detectable by these instruments is often set on the basis of the sensitivities reachable by the detectors currently operating in the audio band. by combining the observations of the pulsar timing arrays, the limits from wide-band detectors and the other phenomenological bounds we show that this requirement is far too generous and even misleading since the actual detection of relic gravitons well above the khz would demand chirp and spectral amplitudes that are ten or even fifteen orders of magnitude smaller than the ones currently achievable in the audio band, for the same classes of stochastic sources. we then examine more closely the potential high-frequency signals and show that the sensitivity in the chirp and spectral amplitudes must be even smaller than the ones suggested by the direct and indirect constraints on the cosmic gravitons. we finally analyze the high-frequency detectors in the framework of hanbury-brown twiss interferometry and argue that they are actually more essential than the ones operating in the audio band (i.e. between few hz and few khz) if we want to investigate the quantumness of the relic gravitons and their associated second-order correlation effects. we suggest, in particular, how the statistical properties of thermal and non-thermal gravitons can be distinguished by studying the corresponding second-order interference effects. | relic gravitons and high-frequency detectors |
the last decade has seen a leapfrog in the interest of x-ray polarimetry with a number of new polarization measurements in hard x-rays from astrosat, polar, gap, and pogo+. the measurements provide some interesting insights into various astrophysical phenomena such as coronal geometry and disk-jet connection in black hole x-ray binaries, hard x-ray emission mechanism in pulsars, and gamma-ray bursts. they also highlight an increase in polarization with energy, which makes hard x-ray polarimetry extremely appealing. there are a number of confirmed hard x-ray polarimetry experiments which along with the existing instruments (astrosat and integral) make this field further exciting. polarization experiments may also see significant progress in sensitivity with new developments in scintillator readouts, active pixel sensors, and cadmium zinc telluride detectors. in particular, the advent of hard x-ray focusing optics will enable the designing of compact focal plane polarimeters with a multifold enhancement in sensitivity. in this review, we will focus on the recent polarimetry findings and science potential of hard x-ray polarimetry along with possible improvements in the measurement techniques. | hard x-ray polarimetry—an overview of the method, science drivers, and recent findings |
we have carried out the galactic plane pulsar snapshot (gpps) survey by using the five-hundred-meter aperture spherical radio telescope (fast), the most sensitive systematic pulsar survey in the galactic plane. in addition to more than 500 pulsars already discovered through normal periodical search, we report here the discovery of 76 new transient radio sources with sporadic strong pulses, detected by using the newly developed module for a sensitive single-pulse search. their small dm values suggest that they all are galactic rotating radio transients (rrats). they show different properties in the follow-up observations. more radio pulses have been detected from 26 transient radio sources but no periods can be found due to a limited small number of pulses from all fast observations. the follow-up observations show that 16 transient sources are newly identified as being the prototypes of rrats with a period already determined from more detected sporadic pulses, and 10 sources are extremely nulling pulsars, and 24 sources are weak pulsars with sparse strong pulses. on the other hand, 48 previously known rrats have been detected by the fast, either during verification observations for the gpps survey or through targeted observations of applied normal fast projects. except for one rrat with four pulses detected in a session of 5-minute observation and four rrats with only one pulse detected in a session, sensitive fast observations reveal that 43 rrats are just generally weak pulsars with sporadic strong pulses or simply very nulling pulsars, so that the previously known rrats always have an extreme emission state together with a normal hardly detectable weak emission state. this is echoed by the two normal pulsars j1938+2213 and j1946+1449 with occasional brightening pulses. though strong pulses of rrats are very outstanding in the energy distribution, their polarization angle variations follow the polarization angle curve of the averaged normal pulse profile, suggesting that the predominant sparse pulses of rrats are emitted in the same region with the same geometry as normal weak pulsars. | the fast galactic plane pulsar snapshot survey. ii. discovery of 76 galactic rotating radio transients and the enigma of rrats |
china's high-speed socioeconomic development has been characterized by rapid urbanization and a series of ecological environment issues, especially air pollution. this paper proposed a comprehensive indicator system for the evaluation of urbanization and the air environment in wuhan, a metropolis in central china, and then investigated the relationship between urbanization and the air environment from the perspective of coupling coordination theory. furthermore, a coupling coordination degree model (ccdm) was constructed to estimate the relationship by using panel data collected from 1996–2013. the results showed the following: (1) spatial urbanization and air environment quality make the greatest contributions to the compound system compared to other first grade indicators, indicating that they are critical factors to consider when the government is making relevant decisions about urban sprawl. (2) the comprehensive level of the air environment initially decreased from 1996–1998 and then increased with fluctuations in wuhan, with several inflection points, meaning that the variation process of the air environment is unstable and repetitive, and efforts to improve the air environment must persist over a long period of time. lastly, (3) the degree of coordinated coupling between urbanization and the air environment exhibited an s-shaped curve, indicating that wuhan changed from having slightly unbalanced development to barely balanced development and then entered into a period of superiorly balanced development, with the air environment lagging. thus, the ambient urban air quality has been subject to great pressure, and the government should continue to take effective action to improve it. | research on the coupling coordination relationship between urbanization and the air environment: a case study of the area of wuhan |
in order to increase signal-to-noise ratio in optical imaging, most detectors sacrifice resolution to increase pixel size in a confined area, which impedes further development of high throughput holographic imaging. although the pixel super-resolution technique (psr) enables resolution enhancement, it suffers from the trade-off between reconstruction quality and super-resolution ratio. in this work, we report a high-fidelity psr phase retrieval method with plug-and-play optimization, termed pnp-psr. it decomposes psr reconstruction into independent sub-problems based on generalized alternating projection framework. an alternating projection operator and an enhancing neural network are employed to tackle the measurement fidelity and statistical prior regularization, respectively. pnp-psr incorporates the advantages of individual operators, achieving both high efficiency and noise robustness. extensive experiments show that pnp-psr outperforms the existing techniques in both resolution enhancement and noise suppression. | plug-and-play pixel super-resolution phase retrieval for digital holography |
we report rebrightening from the be/x-ray binary pulsar ls v +44 17 (also known as rx j0440.9+4431) detected by nicer, swift/bat and maxi/gsc. the pulsar was recently reported to be in an x-ray outburst (atel #15835, #15848). | rebrightening of 1 crab from ls v+44 17 observed by swift/bat and nicer |
pulsars have long been studied in the electromagnetic spectrum. their environments are rich in high-energy cosmic-ray electrons and positrons likely enriching the interstellar medium (ism) with such particles. in this work we use recent cosmic-ray observations from the ams-02, calet, and dampe collaborations to study the averaged properties of the local milky way pulsar population. we perform simulations of the local milky way pulsar population, for interstellar medium assumptions in agreement with a range of cosmic-ray nuclei measurements. each such simulation contains ∼104 pulsars of unique age, location, initial spin-down power, and cosmic-ray electron/positron spectra. we produce more than 7 ×103 such milky way pulsar simulations. we account for and study (i) the pulsars' birth rates and the stochastic nature of their birth, (ii) their initial spin-down power distribution, (iii) their time evolution in terms of their braking index and characteristic spin-down timescale, (iv) the fraction of spin-down power going to cosmic-ray electrons and positrons, and (v) their propagation through the interstellar medium and the heliosphere. we find that pulsars of ages ∼105- 107 yr , have a braking index that on average has to be 3 or larger. given that electromagnetic spectrum observations of young pulsars find braking indices lower than 3, our work provides strong hints that pulsars' braking index increases on average as they age, allowing them to retain some of their rotational energy. moreover, we find that pulsars have relatively uniform properties as sources of cosmic-ray electrons and positrons in terms of the spectra they produce and likely release o (10 %) of their rotational energy to cosmic rays in the ism. finally, we find at ≃12 gev positrons a spectral feature that suggests a new subpopulation of positron sources contributing at these energies. | utilizing cosmic-ray positron and electron observations to probe the averaged properties of milky way pulsars |
the five-hundred-meter aperture spherical radio telescope (fast) is by far the largest telescope of any kind ever built. fast produced its first light in september 2016 and it is now under commissioning, with normal operation to commence in late 2019. during testing and early science operation, fast has started making astronomical discoveries, particularly pulsars of various kinds, including millisecond pulsars, binaries, gamma-ray pulsars, etc. the papers in this mini-volume propose ambitious observational projects to advance our knowledge of astronomy, astrophysics and fundamental physics in many ways. although it may take fast many years to achieve all the goals explained in these papers, taken together they define a powerful strategic vision for the next decade. | preface: planning the scientific applications of the five-hundred-meter aperture spherical radio telescope |
magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in x-rays. pulsed radio emission following some x-ray outbursts have been detected, albeit its physical origin is unclear. it has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. five months after magnetar sgr 1935+2154's x-ray outburst and its associated fast radio burst (frb) 20200428, a radio pulsar phase was discovered. here we report the discovery of x-ray spectral hardening associated with the emergence of periodic radio pulsations from sgr 1935+2154 and a detailed analysis of the properties of the radio pulses. the complex radio pulse morphology, which contains both narrow-band emission and frequency drifts, has not been seen before in other magnetars, but is similar to those of repeating frbs - even though the luminosities are many orders of magnitude different. the observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed x-ray spectral hardening. | atypical radio pulsations from magnetar sgr 1935+2154 |
an ensemble of inspiraling supermassive black hole binaries should produce a stochastic background of very low frequency gravitational waves. this stochastic background is predicted to be a power law, with a gravitational-wave strain spectral index of -2/3, and it should be detectable by a network of precisely timed millisecond pulsars, widely distributed on the sky. this paper reports a new "time slicing" analysis of the 11 yr data release from the north american nanohertz observatory for gravitational waves (nanograv) using 34 millisecond pulsars. methods to flag potential "false-positive" signatures are developed, including techniques to identify responsible pulsars. mitigation strategies are then presented. we demonstrate how an incorrect noise model can lead to spurious signals, and we show how independently modeling noise across 30 fourier components, spanning nanograv's frequency range, effectively diagnoses and absorbs the excess power in gravitational-wave searches. this results in a nominal, and expected, progression of our gravitational-wave statistics. additionally, we show that the first interstellar medium event in psr j1713+0747 pollutes the common red-noise process with low spectral index noise, and we use a tailored noise model to remove these effects. | the nanograv 11 yr data set: evolution of gravitational-wave background statistics |
massive gas-rich galaxy discs at z ∼ 1-3 host massive star-forming clumps with typical baryonic masses in the range 107-108 m⊙ which can affect the orbital decay and concurrent growth of supermassive black hole (bh) pairs. using a set of high-resolution simulations of isolated clumpy galaxies hosting a pair of unequal-mass bhs, we study the interaction between massive clumps and a bh pair at kiloparsec scales, during the early phase of the orbital decay. we find that both the interaction with massive clumps and the heating of the cold gas layer of the disc by bh feedback tend to delay significantly the orbital decay of the secondary, which in many cases is ejected and then hovers for a whole gigayear around a separation of 1-2 kpc. in the envelope, dynamical friction is weak and there is no contribution of disc torques: these lead to the fastest decay once the orbit of the secondary bh has circularized in the disc mid-plane. in runs with larger eccentricities the delay is stronger, although there are some exceptions. we also show that, even in discs with very sporadic transient clump formation, a strong spiral pattern affects the decay time-scale for bhs on eccentric orbits. we conclude that, contrary to previous belief, a gas-rich background is not necessarily conducive to a fast bh decay and binary formation, which prompts more extensive investigations aimed at calibrating event-rate forecasts for ongoing and future gravitational-wave searches, such as with pulsar timing arrays and the future evolved laser interferometer space antenna. | supermassive black hole pairs in clumpy galaxies at high redshift: delayed binary formation and concurrent mass growth |
if the ultraluminous source (ulx) m82 x-2 sustains its measured spin-up value of dot{ν }= 10^{-10} s^{-2}, it will become a millisecond pulsar in less than 105 yr. the observed (isotropic) luminosity of 1040 erg s-1 also supports the notion that the neutron star will spin up to a millisecond period upon accreting about 0.1 m⊙ - the reported hard x-ray luminosity of this ulx, together with the spin-up value, implies torques consistent with the accretion disc extending down to the vicinity of the stellar surface, as expected for low values of the stellar dipole magnetic field (b ≲ 109 g). this suggests a new channel of millisecond pulsar formation - in high-mass x-ray binaries - and may have implications for studies of gravitational waves, and possibly for the formation of low-mass black holes through accretion-induced collapse. | an ultraluminous nascent millisecond pulsar. |
we report on the subpulse modulation properties of 1198 pulsars using the thousand-pulsar-array programme on meerkat. about 35 per cent of the analysed pulsars exhibit drifting subpulses that are more pronounced towards the death line, consistent with previous studies. we estimate that this common phenomenon is detectable in 60 per cent of the overall pulsar population if high-quality data were available for all. this large study reveals the evolution of drifting subpulses across the pulsar population in unprecedented detail. in particular, we find that the modulation period p3 follows a v-shaped evolution with respect to the characteristic age τc, such that the smallest p3 values, corresponding to the nyquist period p3 ≃ 2, are found at τc ≃ 107.5 yr. the v-shaped evolution can be interpreted and reproduced if young pulsars possess aliased fast intrinsic p3, which monotonically increase, ultimately achieving a slow unaliased p3. enhancement of irregularities in intrinsic subpulse modulation by aliasing in small-τc pulsars would explain their observed less well defined p3's and weaker spectral features. modelling these results as rotating subbeams, their circulation must slow down as the pulsar evolves. this is the opposite to that expected if circulation is driven by $\boldsymbol{e}\times \boldsymbol{b}$ drift. this can be resolved if the observed p3 periodicity is due to a beat between an $\boldsymbol{e}\times \boldsymbol{b}$ system and the pulsar period. as a by-product, we identified the correct periods and spin-down rates for 12 pulsars, for which harmonically related values were reported in the literature. | the thousand-pulsar-array programme on meerkat - viii. the subpulse modulation of 1198 pulsars |
we universally search for evidence of kinematic and spatial correlation of supernova remnant (snr) and molecular cloud (mc) associations for nearly all snrs in the coverage of the milky way imaging scroll painting co survey, i.e., 149 snrs, 170 snr candidates, and 18 pure pulsar wind nebulae in 1° < l < 230° and -5.°5 < b < 5.°5. based on high-quality and unbiased 12co/13co/c18o (j = 1-0) survey data, we apply automatic algorithms to identify broad lines and spatial correlations for molecular gas in each snr region. the 91% of snr-mc associations detected previously are identified in this paper by co line emission. overall, there could be as high as 80% of snrs associated with mcs. the proportion of snrs associated with mcs is high within the galactic longitude less than ~50°. kinematic distances of all snrs that are associated with mcs are estimated based on systemic velocities of associated mcs. the radii of snrs associated with mcs follow a lognormal distribution, which peaks at ~8.1 pc. the progenitor initial mass of these snrs follows a power-law distribution with an index of ~-2.3 that is consistent with the salpeter index of -2.35. we find that snr-mc associations are mainly distributed in a thin disk along the galactic plane, while a small number are distributed in a thick disk. with the height of these snrs from the galactic plane below ~45 pc, the distribution of the average radius relative to the height of them is roughly flat, and the average radius increases with the height when above ~45 pc. | a systematic study of associations between supernova remnants and molecular clouds |
we report on the monitoring of the final stage of the outburst from the first galactic ultraluminous x-ray pulsar swift j0243.6+6124, which reached ∼40 eddington luminosities. the main aim of the monitoring program with the swift/xrt telescope was to measure the magnetic field of the neutron star using the luminosity of transition to the `propeller' state. the visibility constraints, unfortunately, did not permit us to observe the source down to the fluxes low enough to detect such a transition. the tight upper limit on the propeller luminosity lprop < 6.8 × 1035 erg s-1 implies the dipole component of the magnetic field b < 1013 g. on the other hand, the observed evolution of the pulse profile and of the pulsed fraction with flux points to a change of the emission region geometry at the critical luminosity lcrit ∼ 3 × 1038 erg s-1 both in the rising and declining parts of the outburst. we associate the observed change with the onset of the accretion column, which allows us to get an independent estimate of the magnetic field strength close to the neutron stars surface of b > 1013 g. given the existing uncertainty in the effective magnetosphere size, we conclude that both estimates are marginally compatible with each other. | on the magnetic field of the first galactic ultraluminous x-ray pulsar swift j0243.6+6124 |
einstein's theory of general relativity has been extensively tested in weak gravitational fields, mainly with experiments in the solar system and observations of radio pulsars, and current data agree well with the theoretical predictions. nevertheless, there are a number of scenarios beyond einstein's gravity that have the same predictions for weak fields and present deviations only when gravity becomes strong. here, we try to test general relativity in the strong field regime. we fit the x-ray spectrum of the supermassive black hole in ark 564 with a disk reflection model beyond einstein's gravity, and we are able to constrain the black hole spin a* and the johannsen deformation parameters α13 and α22 separately. for α22=0 , we find a*>0.96 and -1.0 <α13<0.2 with a 99% confidence level. for α13=0 , we get a*>0.96 and -0.1 <α22<0.9 with a 99% confidence level. our measurements are, thus, consistent with the hypothesis that the supermassive compact object in ark 564 can be described by the kerr metric. | testing the kerr nature of the supermassive black hole in ark 564 |
we present results of a search for late-time radio emission and fast radio bursts (frbs) from a sample of type-i superluminous supernovae (slsne-i). we used the karl g. jansky very large array to observe 10 slsn-i more than 5 yr old at a frequency of 3 ghz. we searched fast-sampled visibilities for frbs and used the same data to perform a deep imaging search for late-time radio emission expected in models of magnetar-powered supernovae. no frbs were found. one slsn-i, ptf10hgi, is detected in deep imaging, corresponding to a luminosity of 1.2 × 1028 erg s-1. this luminosity, considered with the recent 6 ghz detection of ptf10hgi in eftekhari et al., supports the interpretation that it is powered by a young, fast-spinning (∼ms spin period) magnetar with ∼15 m ⊙ of partially ionized ejecta. broadly, our observations are most consistent with slsne-i being powered by neutron stars with fast spin periods, although most require more free-free absorption than is inferred for ptf10hgi. we predict that radio observations at higher frequencies or in the near future will detect these systems and begin constraining properties of the young pulsars and their birth environments. | a search for late-time radio emission and fast radio bursts from superluminous supernovae |
natal supernova kicks, the linear momentum compact remnants receive during their formation, are an essential part of binary population synthesis (bps) models. although these kicks are well supported by evidence, their underlying distributions and incorporation into bps models are uncertain. in this work, we investigate the nature of natal kicks using a previously proposed analytical prescription where the strength of the kick is given by $v_\text{k}=\alpha \frac{m_\text{ejecta}}{m_\text{remnant}}+\beta \, \mathrm{km\, s}^{-1}$ , for free parameters α and β. we vary the free parameters over large ranges of possible values, comparing these synthetic populations simultaneously against four constraints: the merger rate of compact binary neutron star (bns) systems, the period-eccentricity distribution of galactic bnss, the velocity distribution of single-star pulsars, and the likelihood for low ejecta mass supernovae to produce low-velocity kicks. we find that different samples of the parameter space satisfy each test, and only 1 per cent of the models satisfy all four constraints simultaneously. although we cannot identify a single best kick model, we report $\alpha =115^{+40}_{-55}\, \mathrm{km\, s}^{-1}, \beta =15^{+10}_{-15}\, \mathrm{km\, s}^{-1}$ as the centre of the region of the parameter space that fulfils all of our constraints, and expect $\beta \ge 0\, \mathrm{km\, s}^{-1}$ as a further constraint. we also suggest further observations that will enable future refinement of the kick model. a sensitive test for the kick model will be the redshift evolution of the bns merger rate since this is effectively a direct measure of the delay-time distribution for mergers. for our best-fitting values, we find that the peak of the bns merger rate is the present day. | new constraints on the bray conservation-of-momentum natal kick model from multiple distinct observations |
bow shock pulsar wind nebulae are a large class of non-thermal synchrotron sources associated to old pulsars that have emerged from their parent supernova remnant and are directly interacting with the interstellar medium. within this class a few objects show extended x-ray features, generally referred as `jets', that defies all the expectations from the canonical mhd models, being strongly misaligned respect to the pulsar direction of motion. it has been suggested that these jets might originate from high energy particles that escape from the system. here we investigate this possibility, computing particle trajectories on top of a 3d relativistic mhd model of the flow and magnetic field structure, and we show not only that beamed escape is possible, but that it can easily be asymmetric and charge separated, which as we will discuss are important aspects to explain known objects. | on the origin of jet-like features in bow shock pulsar wind nebulae |
we present timing solutions and analyses of 11 pulsars discovered by the five-hundred-meter aperture spherical radio telescope (fast). these pulsars were discovered using an ultrawide bandwidth receiver in drift-scan observations made during the commissioning phase of fast, and were then confirmed and timed using the 64-m parkes radio telescope. each pulsar has been observed over a span of at least one year. highlighted discoveries include psr j0344-0901, which displays mode-changing behaviour and may belong to the class of so-called swooshing pulsars (alongside psrs b0919+06 and b1859+07); psr j0803-0942, whose emission is almost completely linearly polarized; and psrs j1900-0134 and j1945+1211, whose well-defined polarization angle curves place stringent constraints on their emission geometry. we further discuss the detectability of these pulsars by earlier surveys, and highlight lessons learned from our work in carrying out confirmation and monitoring observations of pulsars discovered by a highly sensitive telescope, many of which may be applicable to next-generation pulsar surveys. this paper marks one of the first major releases of fast-discovered pulsars, and paves the way for future discoveries anticipated from the commensal radio astronomy fast survey. | an in-depth investigation of 11 pulsars discovered by fast |
the green bank north celestial cap survey is a 350 mhz all-sky survey for pulsars and fast radio transients using the robert c. byrd green bank telescope. to date, the survey has discovered over 190 pulsars, including 33 millisecond pulsars and 24 rotating radio transients. several exotic pulsars have been discovered in the survey, including psr j1759+5036, a binary pulsar with a 176 ms spin period in an orbit with a period of 2.04 days, an eccentricity of 0.3, and a projected semi-major axis of 6.8 light seconds. using seven years of timing data, we are able to measure one post-keplerian parameter, advance of periastron, which has allowed us to constrain the total system mass to 2.62 ± 0.03 m⊙. this constraint, along with the spin period and orbital parameters, suggests that this is a double neutron star system, although we cannot entirely rule out a pulsar-white dwarf binary. this pulsar is only detectable in roughly 45% of observations, most likely due to scintillation. however, additional observations are required to determine whether there may be other contributing effects. | the green bank northern celestial cap pulsar survey. vi. discovery and timing of psr j1759+5036: a double neutron star binary pulsar |
in this work, we investigate neutron stars (ns) in f (r ,lm) theory of gravity for the case f (r ,lm) =r +lm+σ r lm , where r is the ricci scalar and lm the lagrangian matter density. in the term σ r lm , σ represents the coupling between the gravitational and particles fields. for the first time the hydrostatic equilibrium equations in the theory are solved considering realistic equations of state and ns masses and radii obtained are subject to joint constrains from massive pulsars, the gravitational wave event gw170817 and from the psr j0030+0451 mass-radius from nasa's neutron star interior composition explorer (nicer) data. we show that in this theory of gravity, the mass-radius results can accommodate massive pulsars, while the general theory of relativity can hardly do it. the theory also can explain the observed ns within the radius region constrained by the gw170817 and psr j0030+0451 observations for masses around 1.4 m⊙. | neutron stars in f (r ,lm) gravity with realistic equations of state: joint-constrains with gw170817, massive pulsars, and the psr j0030+0451 mass-radius from nicer data |
when astrophysical jets were discovered one hundred years ago, the field of numerical simulations did not yet exit. since the arrival of programmable computers though, numerical simulations have increasingly become an indispensable tool for dealing with "tough nut" problems which involve complex dynamic and non-linear phenomena. astrophysical jets are an ideal example of such a tough nut, where multi-scale plasma physics, radiative and non-thermal processes, turbulence and relativity combine to present a formidable challenge to researchers. highlighting major achievements obtained through numerical simulations concerning the validity and nature of the blandford-znajek mechanism, the launching, collimation, acceleration and stability of jets, their interaction with the surrounding plasma, jet-galaxy feedback mechanisms etc., we trace how the field developed from its first tentative steps into the age of "maturity". we also give a brief and personal outlook on how the field may evolve in the foreseeable future. dr. porth has worked on a variety of subjects including solar physics, general relativistic accretion, jet formation- and stability and dynamics of pulsar wind nebulae. he has been maintainer and lead developer of community codes for astrophysical fluid dynamics and has led community wide code comparison efforts. oliver porth is part of the eventhorizontelescope collaboration which was awarded the breakthrough price 2020 for constructing the first image of a black hole shadow. | numerical simulations of jets |
searching for periodic non-accelerated signals in the presence of ideal white noise using the fully phase-coherent fast-folding algorithm (ffa) is theoretically established as a more sensitive search method than the fast fourier transform (fft) search with incoherent harmonic summing. in this paper, we present a comparison of the performance of an ffa search implementation using riptide and an fft search implementation using presto, over a range of signal parameters with white noise and with real telescope noise from the giant meterwave radio telescope (gmrt) high resolution southern sky (ghrss) survey with the upgraded gmrt (ugmrt). we find that the ffa search with appropriate de-reddening of the time series performs better than the fft search with spectral whitening for long-period pulsars under real ghrss noise conditions. we describe an ffa-search pipeline implemented for the ghrss survey looking for pulsars over a period of 0.1-100 s and up to a dispersion measure of 500 pc cm-3. we processed ghrss survey data covering ~1500 deg2 of the sky with this pipeline. we re-detected 43 known pulsars with a better signal-to-noise ratio in the ffa search than in the fft search. we also report the discovery of two new pulsars, including a long-period pulsar with a short duty cycle, using this ffa-search pipeline. a population of long-period pulsars with periods of several seconds or higher could help constrain the pulsar death line. | the gmrt high resolution southern sky survey for pulsars and transients. iii. searching for long-period pulsars |
black widows are extreme millisecond pulsar binaries where the pulsar wind ablates their low-mass companion stars. in the optical range, their light curves vary periodically due to the high irradiation and tidal distortion of the companion, which allows us to infer the binary parameters. we present simultaneous multiband observations obtained with the hipercam instrument at the 10.4-m gtc telescope for six of these systems. the combination of this five-band (us,gs, rs, is, zs) fast photometer with the world's largest optical telescope enables us to inspect the light curve range near minima. we present the first light curve for psr j1641+8049, as well as attain a significant increase in signal to noise and cadence compared with previous publications for the remaining five targets: psr j0023+0923, psr j0251+2606, psr j0636+5129, psr j0952-0607, and psr j1544+4937. we report on the results of the light-curve modelling with the icarus code for all six systems, which reveals some of the hottest and densest companion stars known. we compare the parameters derived with the limited but steadily growing black widow population for which optical modelling is available. we find some expected correlations, such as that between the companion star mean density and the orbital period of the system, which can be attributed to the high number of roche-lobe filling companions. on the other hand, the positive correlation between the orbital inclination and the irradiation temperature of the companion is puzzling. we propose such a correlation would arise if pulsars with magnetic axis orthogonal to their spin axis are capable of irradiating their companions to a higher degree. | a black widow population dissection through hipercam multiband light-curve modelling |
the possible signatures of the presence of hyperons inside neutron stars are discussed within a bayesian inference framework applied to a set of models based on a density dependent relativistic mean field description of hadronic matter. nuclear matter properties, hypernuclei properties and observational information are used to constraint the models. general properties of neutron stars such as the maximum mass, radius, tidal deformability, proton fraction, hyperon fraction and speed of sound are discussed. it is shown that the two solar mass constraint imposes that neutron stars described by equations of state that include hyperons have in average a larger radius, ≳0.5 km , and a larger tidal deformability, ≳150 , than the stars determined from a nucleonic equation of state, while the speed of sound at the center of the star is more than 25% smaller. if a 1.4 m⊙ star with a radius ≲12.5 km is measured it is quite improbable that a massive star described by the same model contains hyperons. a similar conclusion is drawn if a two solar mass star with a radius ≲11.5 km or a neutron star with a mass above 2.2 m⊙ is observed: the possible hyperon content of these stars is ruled out or very reduced. the hyperon presence inside neutron stars is compatible with the present nicer mass-radius observations of the pulsars psr j 0030 +0451 and psr j 0740 +6620 and the gravitational wave detection gw170817. it is shown that if the polytropic index γ =∂ln p /∂ln ε takes values of the order of 1.75 at not too large densities, it may indicate the onset of some kind of exotic matter, but not necessarily of deconfined quark matter. | bayesian inference of signatures of hyperons inside neutron stars |
we present the discovery of 528.6 hz pulsations in the new x-ray transient maxi j1816-195. using nicer, we observed the first recorded transient outburst from the neutron star low-mass x-ray binary maxi j1816-195 over a period of 28 days. from a timing analysis of the 528.6 hz pulsations, we find that the binary system is well described as a circular orbit with an orbital period of 4.8 hr and a projected semimajor axis of 0.26 lt-s for the pulsar, which constrains the mass of the donor star to 0.10-0.55 m ⊙. additionally, we observed 15 thermonuclear x-ray bursts showing a gradual evolution in morphology over time, and a recurrence time as short as 1.4 hr. we did not detect evidence for photospheric radius expansion, placing an upper limit on the source distance of 8.6 kpc. | the discovery of the 528.6 hz accreting millisecond x-ray pulsar maxi j1816-195 |
in the last few years, the nicer collaboration has provided mass and radius inferences, via pulse profile modeling, for two pulsars: psr j0030+0451 and psr j0740+6620. given the importance of these results for constraining the equation of state of dense nuclear matter, it is crucial to validate them and test their robustness. we therefore explore the reliability of these results and their sensitivity to analysis settings and random processes, including noise, focusing on the specific case of psr j0030+0451. we use x-ray pulse simulation and inference (x-psi), one of the two main analysis pipelines currently employed by the nicer collaboration for mass and radius inferences. with synthetic data that mimic the psr j0030+0451 nicer data set, we evaluate the recovery performances of x-psi under conditions not previously tested, including complex modeling of the thermally emitting neutron star surface. for the test cases explored, our results suggest that x-psi is capable of recovering the true mass and radius within reasonable credible intervals. this work also reveals the main vulnerabilities of the analysis: a significant dependence on noise and the presence of multimodal structure in the posterior surface. noise particularly impacts our sensitivity to the analysis settings and widths of the posterior distributions. the multimodal structure in the posterior suggests that biases could be present if the analysis is unable to exhaustively explore the parameter space. convergence testing, to ensure an adequate coverage of the parameter space and a suitable representation of the posterior distribution, is one possible solution to these challenges. | x-psi parameter recovery for temperature map configurations inspired by psr j0030+0451 |
the last five years have seen remarkable progress in our quest to determine the equation of state of neutron rich matter. recent advances across the theoretical, experimental, and observational landscape have been incorporated in a bayesian framework to refine existing covariant energy density functionals previously calibrated by the properties of finite nuclei. in particular, constraints on the maximum neutron star mass from pulsar timing, on stellar radii from the nicer mission, on tidal deformabilities from the ligo-virgo collaboration, and on the dynamics of pure neutron matter as predicted from chiral effective field theories have resulted in significant refinements to the models, particularly to those predicting a stiff symmetry energy. still, even after these improvements, we find it challenging to reproduce simultaneously the neutron skin thickness of both 208pb and 48ca recently reported by the prex/crex collaboration. | bayesian refinement of covariant energy density functionals |
<p id="par1">two classes of x-ray/gamma-ray sources, the soft gamma repeaters<index-term id="iterm1"><term>soft gamma repeater (sgr)</term></index-term> and the anomalous x-ray pulsars<index-term id="iterm2"><term>anomalous x-ray pulsar (axp)</term></index-term> have been identified with isolated, slowly spinning magnetars, neutron stars whose emission draws energy from their extremely strong magnetic field (∼1015-1016 g). magnetars are believed to form with millisecond spin period and to represent an important fraction of the whole young neutron star population. newborn magnetars can convert very quickly their rotational energy into electromagnetic and/or gravitational waves, by virtue of their magnetic field strength and fast spins. this chapter provides a brief summary of astrophysical problems and scenarios in which millisecond magnetars are believed to play a key role: these include gamma ray bursts, supernovae, gravitational wave events and fast radio bursts. | millisecond magnetars |
hard, nonthermal, persistent pulsed x-ray emission extending between 10 and ∼150 kev has been observed in nearly 10 magnetars. for inner-magnetospheric models of such emission, resonant inverse compton scattering of soft thermal photons by ultrarelativistic charges is the most efficient production mechanism. we present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. our computations employ a new formulation of the qed compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. the spectral cutoff energies are sensitive to the choices of observer viewing geometry, electron lorentz factor, and scattering kinematics. we find that electrons with energies ≲15 mev will emit most of their radiation below 250 kev, consistent with inferred turnovers for magnetar hard x-ray tails. more energetic electrons still emit mostly below 1 mev, except for viewing perspectives sampling field-line tangents. pulse profiles may be singly or doubly peaked dependent on viewing geometry, emission locale, and observed energy band. magnetic pair production and photon splitting will attenuate spectra to hard x-ray energies, suppressing signals in the fermi-lat band. the resonant compton spectra are strongly polarized, suggesting that hard x-ray polarimetry instruments such as x-calibur, or a future compton telescope, can prove central to constraining model geometry and physics. | resonant inverse compton scattering spectra from highly magnetized neutron stars |
we present the first survey results of hard x-ray point sources in the galactic center (gc) region by nustar. we have discovered 70 hard (3-79 kev) x-ray point sources in a 0.6 deg{}2 region around sgr a* with a total exposure of 1.7 ms, and 7 sources in the sgr b2 field with 300 ks. we identify clear chandra counterparts for 58 nustar sources and assign candidate counterparts for the remaining 19. the nustar survey reaches x-ray luminosities of ∼4× and ∼8 × 10{}32 erg s{}-1 at the gc (8 kpc) in the 3-10 and 10-40 kev bands, respectively. the source list includes three persistent luminous x-ray binaries (xbs) and the likely run-away pulsar called the cannonball. new source-detection significance maps reveal a cluster of hard (>10 kev) x-ray sources near the sgr a diffuse complex with no clear soft x-ray counterparts. the severe extinction observed in the chandra spectra indicates that all the nustar sources are in the central bulge or are of extragalactic origin. spectral analysis of relatively bright nustar sources suggests that magnetic cataclysmic variables constitute a large fraction (>40%-60%). both spectral analysis and logn-logs distributions of the nustar sources indicate that the x-ray spectra of the nustar sources should have kt > 20 kev on average for a single temperature thermal plasma model or an average photon index of γ = 1.5-2 for a power-law model. these findings suggest that the gc x-ray source population may contain a larger fraction of xbs with high plasma temperatures than the field population. | nustar hard x-ray survey of the galactic center region. ii. x-ray point sources |
in theories of gravity that include a scalar field, a compact object's scalar charge is a crucial quantity since it controls dipole radiation, which can be strongly constrained by pulsar timing and gravitational wave observations. however, in most such theories, computing the scalar charge requires simultaneously solving the coupled, nonlinear metric and scalar field equations of motion. in this article, we prove that in linearly coupled einstein-dilaton-gauss-bonnet gravity, a black hole's scalar charge is completely determined by the horizon surface gravity times the euler characteristic of the bifurcation surface, without solving any equations of motion. within this theory, black holes announce their horizon topology and surface gravity to the rest of the universe through the dilaton field. in our proof, a four-dimensional topological density descends to a two-dimensional topological density on the bifurcation surface of a killing horizon. we also comment on how our proof can be generalized to other topological densities on general g -bundles, and to theories where the dilaton is nonlinearly coupled to the euler density. | black hole scalar charge from a topological horizon integral in einstein-dilaton-gauss-bonnet gravity |
a neutron star was first detected as a pulsar in 1967. it is one of the most mysterious compact objects in the universe, with a radius of the order of 10 km and masses that can reach two solar masses. in fact, neutron stars are star remnants, a kind of stellar zombie (they die, but do not disappear). in the last decades, astronomical observations yielded various contraints for neutron star masses, and finally, in 2017, a gravitational wave was detected (gw170817). its source was identified as the merger of two neutron stars coming from ngc 4993, a galaxy 140 million light years away from us. the very same event was detected in γ-ray, x-ray, uv, ir, radio frequency and even in the optical region of the electromagnetic spectrum, starting the new era of multi-messenger astronomy. to understand and describe neutron stars, an appropriate equation of state that satisfies bulk nuclear matter properties is necessary. gw170817 detection contributed with extra constraints to determine it. on the other hand, magnetars are the same sort of compact object, but bearing much stronger magnetic fields that can reach up to 1015 g on the surface as compared with the usual 1012 g present in ordinary pulsars. while the description of ordinary pulsars is not completely established, describing magnetars poses extra challenges. in this paper, i give an overview on the history of neutron stars and on the development of nuclear models and show how the description of the tiny world of the nuclear physics can help the understanding of the cosmos, especially of the neutron stars. | a neutron star is born |
magnetized particle motion around black holes in conformal gravity immersed in asymptotically uniform magnetic field has been studied. we have also analyzed the behavior of magnetic fields near the horizon of the black hole in conformal gravity and shown that with the increase of conformal parameters l and n the value of angular component of magnetic field at the stellar surface decreases. the maximum value of the effective potential corresponding to circular motion of the magnetized particle increases with the increase of conformal parameters. it is shown that in all cases of neutral, charged and magnetized particle collisions in the black hole environment the center-of-mass energy decreases with the increase of conformal parameters l and n. in the case of the magnetized and negatively charged particle collisions, the innermost collision point with the maximum center-of-mass energy comes closer to the central object due to the effects of the parameters of the conformal gravity. we have applied the results to the real astrophysical scenario when a pulsar treated as a magnetized particle is orbiting the super massive black hole (smbh) sgr a* in the center of our galaxy in order to obtain the estimation of magnetized compact object's orbital parameter. the possible detection of pulsar in sgr a* close environment can provide constraints on black hole parameters. here we have shown that there is degeneracy between spin of smbh and ambient magnetic field and consequently the interaction of magnetic field ∼102 gauss with magnetic moment of magnetized neutron star can in principle mimic spin of kerr black holes up to 0.6. | magnetized particle motion around black holes in conformal gravity: can magnetic interaction mimic spin of black holes? |
the dynamic and secondary spectra of many pulsars show evidence for long-lived, aligned images of the pulsar that are stationary on a thin scattering sheet. one explanation for this phenomenon considers the effects of wave crests along sheets in the ionized interstellar medium, such as those due to alfvén waves propagating along current sheets. if these sheets are closely aligned to our line of sight to the pulsar, high bending angles arise at the wave crests and a selection effect causes alignment of images produced at different crests, similar to grazing reflection off of a lake. using geometric optics, we develop a simple parametrized model of these corrugated sheets that can be constrained with a single observation and that makes observable predictions for variations in the scintillation of the pulsar over time and frequency. this model reveals qualitative differences between lensing from overdense and underdense corrugated sheets: only if the sheet is overdense compared to the surrounding interstellar medium can the lensed images be brighter than the line-of-sight image to the pulsar, and the faint lensed images are closer to the pulsar at higher frequencies if the sheet is underdense, but at lower frequencies if the sheet is overdense. | predicting pulsar scintillation from refractive plasma sheets |
in this work, test particle dynamics around a static regular bardeen black hole (bh) in anti-de sitter spacetime has been studied. it has been shown for neutral test particles that parameters of a regular bardeen black hole in anti-de sitter spacetime can mimic the rotation parameter of the kerr metric up to the value a≈0.9 providing the same innermost stable circular orbit (isco) radius. we have also explored the dynamics of magnetized particles with a magnetic dipole moment around a magnetically charged regular bardeen black hole in anti-de sitter spacetime. as a realistic astrophysical scenario of the study, we have treated neutron stars orbiting a supermassive black hole (smbh), in particular, the magnetar psr j1745-2900 orbiting sgr a* with the parameter β=10.2, as magnetized test particles. the magnetized particles dynamics shows that the parameter β, negative values of cosmological constant and magnetic charge parameter of the central bh cause a decrease in the isco radius. we have compared the effects of the magnetic charge of the bardeen bh with the spin of rotating kerr bh and shown that magnetic charge parameter can mimic the spin in the range a/m≃(0,0.7896) when λ=0 at the range of its values g/m≃(0,0.648). | regular bardeen black holes in anti-de sitter spacetime versus kerr black holes through particle dynamics |
we explore magnetic field configurations that lead to the formation of magnetic spots on the surface of neutron stars and the displacement of the magnetic dipole axis. we find that a toroidally dominated magnetic field is essential for the generation of a single spot with a strong magnetic field. once a spot forms, it survives for several million years, even after the total magnetic field has decayed significantly. we find that the dipole axis is not stationary with respect to the neutron star’s surface and does not in general coincide with the location of the magnetic spot. this is due to non-axisymmetric instabilities of the toroidal field that displace the poloidal dipole axis at rates that may reach 0.°4 per century. a misaligned poloidal dipole axis with the toroidal field leads to more significant displacement of the dipole axis than the fully aligned case. finally we discuss the evolution of neutron stars with such magnetic fields on the p{--}\dot{p} diagram and the observational implications. we find that neutron stars spend a very short time before they cross the death line of the p{--}\dot{p} diagram, compared to their characteristic ages. moreover, the maximum intensity of their surface magnetic field is substantially higher than the dipole component of the field. we argue that sgr 0418+5729 could be an example of this type of behavior, having a weak dipole field, yet hosting a magnetic spot responsible for its magnetar behavior. the evolution on the pulse profile and braking index of the crab pulsar, which are attributed to an increase of its obliquity, are compatible with the anticipated drift of the magnetic axis. | magnetic axis drift and magnetic spot formation in neutron stars with toroidal fields |
most of ultraluminous x-ray sources are thought to be objects accreting above their eddington limits. in the recently identified class of ultraluminous x-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small eddington limit. the accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. we propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. our calculations show that the size of the magnetosphere may be represented as the classical alfvén radius times a dimensionless factor ξ which depends only on the disc thickness. in the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. in general, increasing the disc thickness leads to a larger magnetosphere size in units of the alfvén radius. for large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. the magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders. | super-eddington accretion on to a magnetized neutron star |
the chime telescope (the canadian hydrogen intensity mapping experiment) recently built in penticton, canada, is currently being commissioned. originally designed as a cosmology experiment, it was soon recognized that chime has the potential to simultaneously serve as an incredibly useful radio telescope for pulsar science. chime operates across a wide bandwidth of 400-800 mhz and will have a collecting area and sensitivity comparable to that of the 100-m class radio telescopes. chime has a huge field of view of ~250 square degrees. it will be capable of observing 10 pulsars simultaneously, 24-hours per day, every day, while still accomplishing its missions to study baryon acoustic oscillations and fast radio bursts. it will carry out daily monitoring of roughly half of all pulsars in the northern hemisphere, including all nanograv pulsars employed in the pulsar timing array project. it will cycle through all pulsars in the northern hemisphere with a range of cadence of no more than 10 days. | pulsar science with the chime telescope |
aims: we study γ-ray emission from the shell-type supernova remnant (snr) rx j0852.0-4622 to better characterize its spectral properties and its distribution over the snr.methods: the analysis of an extended high energy spectroscopic system (h.e.s.s.) data set at very high energies (e > 100 gev) permits detailed studies, as well as spatially resolved spectroscopy, of the morphology and spectrum of the whole rx j0852.0-4622 region. the h.e.s.s. data are combined with archival data from other wavebands and interpreted in the framework of leptonic and hadronic models. the joint fermi-lat-h.e.s.s. spectrum allows the direct determination of the spectral characteristics of the parent particle population in leptonic and hadronic scenarios using only gev-tev data.results: an updated analysis of the h.e.s.s. data shows that the spectrum of the entire snr connects smoothly to the high-energy spectrum measured by fermi-lat. the increased data set makes it possible to demonstrate that the h.e.s.s. spectrum deviates significantly from a power law and is well described by both a curved power law and a power law with an exponential cutoff at an energy of ecut = (6.7 ± 1.2stat ± 1.2syst) tev. the joint fermi-lat-h.e.s.s. spectrum allows the unambiguous identification of the spectral shape as a power law with an exponential cutoff. no significant evidence is found for a variation of the spectral parameters across the snr, suggesting similar conditions of particle acceleration across the remnant. a simple modeling using one particle population to model the snr emission demonstrates that both leptonic and hadronic emission scenarios remain plausible. it is also shown that at least a part of the shell emission is likely due to the presence of a pulsar wind nebula around psr j0855-4644. a fits image of the region of interest and two text files describing the h.e.s.s. spectrum of rx j0852.0-4622 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/612/a7 | deeper h.e.s.s. observations of vela junior (rx j0852.0-4622): morphology studies and resolved spectroscopy |
we show the existence and investigate the location of the special point (sp) in which hybrid neutron star mass-radius (m-r) curves have to cross each other when they belong to a class of hybrid equation of state (eos) constructed with generic constant-speed-of-sound (css) quark matter models for which the onset deconfinement is varied. we demonstrate that for a three-parameter css model the position of the sp in the m-r diagram is largely independent of the choice of the hadronic eos, but in dependence on the stiffness of the quark matter eos it spans a region that we identify. we find that the difference between the maximum mass and the sp mass depends on the mass at the onset of deconfinement so that an upper limit of 0.19 m⊙ for this difference is obtained from which a lower limit on the radius of hybrid stars is deduced. together with a lower limit on the radius of hadronic stars, derived from a class of reasonably soft hadronic eos including hyperons, we identify a region in the m-r diagram which can be occupied only by hybrid stars. accordingly, we suggest that a nicer radius measurement on the massive pulsar psr j0740 + 6620 in the range of 8.6-11.9 km would indicate that this pulsar is a hybrid neutron star with deconfined quark matter in the inner core. | the special point on the hybrid star mass-radius diagram and its multi-messenger implications |
the galactic center excess (gce) is an extended gamma-ray source in the central region of the galaxy found in fermi large area telescope (fermi-lat) data. one of the leading explanations for the gce is an unresolved population of millisecond pulsars (msps) in the galactic bulge. due to differing star formation histories it is expected that the msps in the galactic bulge are older and therefore dimmer than those in the galactic disk. additionally, correlations between the spectral parameters of the msps and the spin-down rate of the corresponding neutron stars have been observed. this implies that the bulge msps may be spectrally different from the disk msps. we perform detailed modelling of the msps from formation until observation. although we confirm the correlations, we do not find they are sufficiently large to significantly differentiate the spectra of the bulge msps and disk msps when the uncertainties are accounted for. our results demonstrate that the population of msps that can explain the gamma-ray signal from the resolved msps in the galactic disk and the unresolved msps in the boxy bulge and nuclear bulge can consistently be described as arising from a common evolutionary trajectory for some subset of astrophysical sources common to all these different environments. we do not require that there is anything unusual about inner galaxy msps to explain the gce. additionally, we use a more accurate geometry for the distribution of bulge msps and incorporate dispersion measure estimates of the msps' distances. we find that the elongated boxy bulge morphology means that some the bulge msps are closer to us and so easier to resolve. we identify three resolved msps that have significant probabilities of belonging to the bulge population. | comparing the galactic bulge and galactic disk millisecond pulsars |
the cosmic evolution of the neutron-star merger (nsm) rate can be deduced from the observed cosmic star formation rate. this allows us to estimate the rate expected in the horizon of the gravitational wave detectors advanced virgo and advanced ligo and to compare those rates with independent predictions. in this context, the rapid neutron-capture process, or r-process, can be used as a constraint assuming nsm is the main astrophysical site for this nucleosynthetic process. we compute the early cosmic evolution of a typical r-process element, europium. eu yields from nsm are taken from recent nucleosynthesis calculations. the same approach allows us to compute the cosmic rate of core-collapse supernovae (ccsne) and the associated evolution of eu. we find that the bulk of eu observations at [fe/h] > -2.5 can be rather well fitted by either ccsn or nsm scenarios. however, at lower metallicity, the early eu cosmic evolution favours nsm as the main astrophysical site for the r-process. a comparison between our calculations and spectroscopic observations at very low metallicities allows us to constrain the coalescence time-scale in the nsm scenario to ∼0.1-0.2 gyr. these values are in agreement with the coalescence time-scales of some observed binary pulsars. finally, the cosmic evolution of eu is used to put constraints on (i) the nsm rate, (ii) the merger rate in the horizon of the gravitational wave detectors advanced virgo/ad ligo, as well as (iii) the expected rate of electromagnetic counterparts to mergers (`kilonovae') in large near-infrared surveys. | cosmic neutron-star merger rate and gravitational waves constrained by the r-process nucleosynthesis |
despite the growing number of gamma-ray sources detected by the fermi-large area telescope (lat), about one-third of the sources in each survey remains of uncertain type. we present a new deep neural network approach for the classification of unidentified or unassociated gamma-ray sources in the last release of the fermi-lat catalogue (4fgl-dr2) obtained with 10 yr of data. in contrast to previous work, our method directly uses the measurements of the photon energy spectrum and time series as input for the classification, instead of specific, human-crafted features. dense neural networks, and for the first time in the context of gamma-ray source classification recurrent neural networks, are studied in depth. we focus on the separation between extragalactic sources, i.e. active galactic nuclei, and galactic pulsars, and on the further classification of pulsars into young and millisecond pulsars. our neural network architectures provide powerful classifiers, with a performance that is comparable to previous analyses based on human-crafted features. our benchmark neural network predicts that of the sources of uncertain type in the 4fgl-dr2 catalogue, 1050 are active galactic nuclei and 78 are galactic pulsars, with both classes following the expected sky distribution and the clustering in the variability-curvature plane. we investigate the problem of sample selection bias by testing our architectures against a cross-match test data set using an older catalogue, and propose a feature selection algorithm using autoencoders. our list of high-confidence candidate sources labelled by the neural networks provides a set of targets for further multiwavelength observations addressed to identify their nature. the deep neural network architectures we develop can be easily extended to include specific features, as well as multiwavelength data on the source photon energy and time spectra coming from different instruments. | classification of fermi-lat sources with deep learning using energy and time spectra |
recent pulsar timing data reported by the nanograv collaboration indicates the existence of a stochastic gravitational wave (gw) background at a frequency $f\sim 10^{-8}~\rm hz$. we show that a dark sector consisting of a standard model (sm) gauge singlet fermion $\chi$ and a singlet scalar $\phi$, both charged under a $z_4$ symmetry, is capable of generating such a low frequency gw via strong first order phase transition (sfopt) through the modification of the standard cosmological history, where we assume faster-than-usual expansion at pre-bbn times driven by a new cosmological species $\varphi$ whose energy density red-shifts with the scale factor as $\rho_\varphi\propto a^{-\left(4+n\right)}$. depending on the choice of the fast expansion parameters, reheat temperature and effective scale of the theory, it is also possible to address correct dark matter (dm) relic abundance via freeze-in. we show that a successful first order phase transition explaining nanograv results together with planck observed dm abundance put bound on the fast expansion parameters requiring $n\lesssim 4$ to explain both. | implications of nanograv results and uv freeze-in in a fast-expanding universe |
a new window is opening in high-energy astronomy: x-ray polarimetry. with many missions currently under development and scheduled to launch as early as 2021, observations of the x-ray polarization of accreting x-ray pulsars will soon be available. as polarization is particularly sensitive to the geometry of the emission region, the upcoming polarimeters will shed new light on the emission mechanism of these objects, provided that we have sound theoretical models that agree with current spectroscopic and timing observation and that can make predictions of the polarization parameters of the emission. we here present a new model for the polarized emission of accreting x-ray pulsars in the accretion column scenario that for the first time takes into account the macroscopic structure and dynamics of the accretion region and the propagation of the radiation towards the observer, including relativistic beaming, gravitational lensing, and quantum electrodynamics. in this paper, we present all the details of the model, while in a companion paper, we apply our model to predict the polarization parameters of the bright x-ray pulsar hercules x-1. | polarization of accreting x-ray pulsars. i. a new model |
the maximum mass of a neutron star has important implications across multiple research fields, including astrophysics, nuclear physics and gravitational wave astronomy. compact binary millisecond pulsars (with orbital periods shorter than about a day) are a rapidly-growing pulsar population, and provide a good opportunity to search for the most massive neutron stars. applying a new method to measure the velocity of both sides of the companion star, we previously found that the compact binary millisecond pulsar psr j2215+5135 hosts one of the most massive neutron stars known to date, with a mass of 2.27$\pm$0.16 m$_\odot$ (linares, shahbaz & casares, 2018). we reexamine the properties of the 0.33 m$_\odot$ companion star, heated by the pulsar, and argue that irradiation in this "redback" binary is extreme yet stable, symmetric and not necessarily produced by an extended source. we also review the neutron star mass distribution in light of this and more recent discoveries. we compile a list of all (nine) systems with published evidence for super-massive neutron stars, with masses above 2 m$_\odot$. we find that four of them are compact binary millisecond pulsars (one black widow, two redbacks and one redback candidate). this shows that compact binary millisecond pulsars are key to constraining the maximum mass of a neutron star. | super-massive neutron stars and compact binary millisecond pulsars |
large-area sky surveys show that massive galaxies undergo at least one major merger in a hubble time. ongoing pulsar timing array (pta) experiments are aimed at measuring the gravitational-wave (gw) emission from binary supermassive black holes (smbhs) at the centres of galaxy merger remnants. in this paper, using the latest observational estimates for a range of galaxy properties and scaling relations, we predict the amplitude of the gw background generated by the binary smbh population. we also predict the numbers of individual binary smbh gw sources. we predict the characteristic strain amplitude of the gw background to lie in the range 5.1 × 10-16 < ayr < 2.4 × 10-15 at a frequency of (1 yr)-1, with 95 per cent confidence. higher values within this range, which correspond to the more commonly preferred choice of galaxy merger time-scale, will fall within the expected sensitivity ranges of existing pta projects in the next few years. in contrast, we find that a pta consisting of at least 100 pulsars observed with next-generation radio telescopes will be required to detect continuous-wave gws from binary smbhs. we further suggest that gw memory bursts from coalescing smbh pairs are not viable sources for ptas. both the gw background and individual gw source counts are dominated by binaries formed in mergers between early-type galaxies of masses ≳5 × 1010 m⊙ at redshifts ≲1.5. uncertainties in the galaxy merger time-scale and the smbh mass-galaxy bulge mass relation dominate the uncertainty in our predictions. | prospects for gravitational-wave detection and supermassive black hole astrophysics with pulsar timing arrays |
using the nine-year radio-pulsar timing data set from the north american nanohertz observatory for gravitational waves (nanograv), collected at arecibo observatory and the green bank telescope, we have measured the positions, proper motions, and parallaxes for 37 millisecond pulsars. we report twelve significant parallax measurements and distance measurements, and eighteen lower limits on distance. we compare these measurements to distances predicted by the ne2001 interstellar electron density model and find them to be in general agreement. we use measured orbital-decay rates and spin-down rates to confirm two of the parallax distances and to place distance upper limits on other sources; these distance limits agree with the parallax distances with one exception, psr j1024-0719, which we discuss at length. using the proper motions of the 37 nanograv pulsars in combination with other published measurements, we calculate the velocity dispersion of the millisecond pulsar population in galactocentric coordinates. we find the radial, azimuthal, and perpendicular dispersions to be 46, 40, and 24 {km} {{{s}}}-1, respectively, in a model that allows for high-velocity outliers; or 81, 58, and 62 {km} {{{s}}}-1 for the full population. these velocity dispersions are far smaller than those of the canonical pulsar population, and are similar to older galactic disk populations. this suggests that millisecond pulsar velocities are largely attributable to their being an old population rather than being artifacts of their birth and evolution as neutron star binary systems. the components of these velocity dispersions follow similar proportions to other galactic populations, suggesting that our results are not biased by selection effects. | the nanograv nine-year data set: astrometric measurements of 37 millisecond pulsars |
pulsars may either be spun up or down by hydrodynamic instabilities during the supernova explosion of massive stars. besides rapidly rotating cases related to bipolar explosions, stellar rotation may affect the explosion of massive stars in the more common situations where the centrifugal force is minor. using 2d simulations of a simplified set-up in cylindrical geometry, we examine the impact of rotation on the standing accretion shock instability (sasi) and the corotation instability, also known as low-t/|w|. the influence of rotation on the saturation amplitude of these instabilities depends on the specific angular momentum in the accretion flow and the ratio of the shock to the neutron star radii. the spiral mode of sasi becomes more vigorous with faster rotation only if this ratio is large enough. a corotation instability develops at large rotation rates and impacts the dynamics more dramatically, leading to a strong one-armed spiral wave. non-axisymmetric instabilities are able to redistribute angular momentum radially and affect the pulsar spin at birth. a systematic study of the relationship between the core rotation period of the progenitor and the initial pulsar spin is performed. stellar rotation rates for which pulsars are spun up or down by sasi are estimated. rapidly spinning progenitors are modestly spun down by spiral modes, less than ∼30 per cent, when a corotation instability develops. given the observational constraints on pulsar spin periods at birth, this suggests that rapid rotation might not play a significant hydrodynamic role in most core-collapse supernovae. | are pulsars spun up or down by sasi spiral modes? |
the phenomenon of subpulse drifting may hold the key to understanding the pulsar emission mechanism. here, we report on new observations of psr j0034-0721 (b0031-07) carried out with the murchison widefield array at 185 {mhz}. we observe three distinct drift modes whose “vertical” drift band separations (p 3) and relative abundances are consistent with previous studies at similar and higher frequencies. the drift bands, however, are observed to change their slopes over the course of individual drift modes, which can be interpreted as a continuously changing drift rate. the implied acceleration of the intrinsic carousel rotation cannot easily be explained by plasma models based on {\boldsymbol{e}}× {\boldsymbol{b}} drift. furthermore, we find that methods of classifying the drift modes by means of p 3 measurements can sometimes produce erroneous identifications in the presence of a changing drift rate. the “horizontal” separation between drift bands (p 2) is found to be larger at later rotation phases within the pulse window, which is inconsistent with the established effects of retardation, aberration, and the motion of the visible point. longer observations spanning at least ∼10,000 pulses are required to determine how the carousel rotation parameters change from one drift sequence to the next. | low-frequency observations of the subpulse drifter psr j0034-0721 with the murchison widefield array |
vortex-mediated mutual friction governs the coupling between the superfluid and normal components in neutron star interiors. by, for example, comparing precise timing observations of pulsar glitches with theoretical predictions it is possible to constrain the physics in the interior of the star, but to do so an accurate model of the mutual friction coupling in general relativity is needed. we derive such a model directly from carter's multifluid formalism, and study the vortex structure and coupling time-scale between the components in a relativistic star. we calculate how general relativity modifies the shape and the density of the quantized vortices and show that, in the quasi-schwarzschild coordinates, they can be approximated as straight lines for realistic neutron star configurations. finally, we present a simple universal formula (given as a function of the stellar compactness alone) for the relativistic correction to the glitch rise-time, which is valid under the assumption that the superfluid reservoir is in a thin shell in the crust or in the outer core. this universal relation can be easily employed to correct, a posteriori, any newtonian estimate for the coupling time-scale, without any additional computational expense. | a universal formula for the relativistic correction to the mutual friction coupling time-scale in neutron stars |
as radio telescopes become more sensitive, radio frequency interference (rfi) is becoming more important for interesting signals of radio astronomy. there is a demand for developing an automatic, accurate and efficient rfi mitigation method. therefore, we have investigated an rfi detection algorithm. first, we introduce an asymmetrically reweighted penalized least squares (arpls) method to estimate the baseline more accurately. after removing the estimated baseline, several novel strategies were proposed based on the sumthreshold algorithm for detecting different types of rfi. the threshold parameter in sumthreshold can be determined automatically and adaptively. the adaptiveness is essential for reducing human intervention and for the online rfi processing pipeline. applications to data from the five-hundred-meter aperture spherical telescope (fast) show that the proposed scheme based on arpls and sumthreshold is superior to some typically available methods for rfi detection with respect to efficiency and performance. | radio frequency interference mitigation based on the asymmetrically reweighted penalized least squares and sumthreshold method |
pulsar magnetospheres admit nonstationary vacuum gaps that are characterized by nonvanishing e .b . the vacuum gaps play an important role in plasma production and electromagnetic wave emission. we show that these gaps generate axions whose energy is set by the gap oscillation frequency. the density of axions produced in a gap can be several orders of magnitude greater than the ambient dark matter density. in the strong pulsar magnetic field, a fraction of these axions may convert to photons, giving rise to broadband radio signals. we show that dedicated observations of nearby pulsars with radio telescopes (fast) and interferometers (ska) can probe axion-photon couplings that are a few orders of magnitude lower than current astrophysical bounds. | axion production in pulsar magnetosphere gaps |
magnetars are young, rotating neutron stars that possess larger magnetic fields ($b$ $\approx$ $10^{13}$-$10^{15}$ g) and longer rotational periods ($p$ $\approx$ 1-12 s) than ordinary pulsars. in contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. they display highly variable radio and x-ray emission, but the processes responsible for this behavior remain a mystery. we report the discovery of bright, persistent individual x-ray pulses from xte j1810-197, a transient radio magnetar, using the neutron star interior composition explorer (nicer) following its recent radio reactivation. similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. however, the x-ray pulses presented here were detected outside of a flaring state. they are less energetic and display temporal structure that differs from the impulsive x-ray events previously observed from the magnetar class, such as giant flares and short x-ray bursts. our high frequency radio observations of the magnetar, carried out simultaneously with the x-ray observations, demonstrate that the relative alignment between the x-ray and radio pulses varies on rotational timescales. no correlation was found between the amplitudes or temporal structure of the x-ray and radio pulses. the magnetar's 8.3 ghz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 ghz. many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. we find that the radio pulses from xte j1810-197 share similar characteristics to radio bursts detected from fast radio burst (frb) sources, some of which are now thought to be produced by active magnetars. | bright x-ray and radio pulses from a recently reactivated magnetar |
we present the detection of 107 pulsars with interstellar scintillation arcs at 856-1712 mhz, observed with the meerkat thousand pulsar array programme. scintillation arcs appear to be ubiquitous in clean, high s/n observations, their detection mainly limited by short observing durations and coarse frequency channel resolution. this led the survey to be sensitive to nearby, lightly scattered pulsars with high effective velocity - from a large proper motion, a screen nearby the pulsar, or a screen near the earth. we measure the arc curvatures in all of our sources, which can be used to give an estimate of screen distances in pulsars with known proper motion, or an estimate of the proper motion. the short scintillation time-scale in j1731-4744 implies a scattering screen within 12 pc of the source, strongly suggesting the association between this pulsar and the supernova remnant rcw 114. we measure multiple parabolic arcs of five pulsars, all of which are weakly scintillating with high proper motion. additionally, several sources show hints of inverted arclets suggesting scattering from anisotropic screens. building on this work, further targeted meerkat observations of many of these pulsars will improve understanding of our local scattering environment and the origins of scintillation; annual scintillation curves would lead to robust screen distance measurements, and the evolution of arclets in time and frequency can constrain models of scintillation. | the thousand pulsar array programme on meerkat - x. scintillation arcs of 107 pulsars |
very faint x-ray transients (vfxts) are x-ray transients with peak x-ray luminosities (lx) of l$_x \lesssim 10^{36}$ erg s-1, which are not well understood. we carried out a survey of 16 deg2 of the galactic bulge with the swift observatory, using short (60 s) exposures, and returning every 2 weeks for 19 epochs in 2017-18 (with a gap from 2017 november to 2018 february, when the bulge was in sun-constraint). our main goal was to detect and study vfxt behaviour in the galactic bulge across various classes of x-ray sources. in this work, we explain the observing strategy of the survey, compare our results with the expected number of source detections per class, and discuss the constraints from our survey on the galactic vfxt population. we detected 91 x-ray sources, 25 of which have clearly varied by a factor of at least 10. in total, 45 of these x-ray sources have known counterparts: 17 chromospherically active stars, 12 x-ray binaries, 5 cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio pulsars, 1 active galactic nuclei, and a young star cluster. the other 46 are of previously undetermined nature. we utilize x-ray hardness ratios, searches for optical/infrared counterparts in published catalogues, and flux ratios from quiescence to outburst to constrain the nature of the unknown sources. of these 46, 7 are newly discovered hard transients, which are likely vfxt x-ray binaries. furthermore, we find strong new evidence for a symbiotic nature of four sources in our full sample, and new evidence for accretion power in six x-ray sources with optical counterparts. our findings indicate that a large subset of vxfts is likely made up of symbiotic systems. | the swift bulge survey: motivation, strategy, and first x-ray results |
sgr j1935+2154, the host magnetar of the galactic frb200428 (atel #13681, #13684), was reported to be radio-active again on 2020 oct 8th by chime/frb team (atel #14074, #14080). | fast detection of radio bursts and pulsed emission from sgr j1935+2154 |
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