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fast radio bursts (frbs) are an emerging class of short and bright radio transients whose sources remain enigmatic. within the galactic centre, the non-detection of pulsars within the inner ∼10 pc has created a missing pulsar problem that has intensified with time. with all reserve, we advance the notion that the two problems could be linked by a common solution: the collapse of neutron stars (ns) due to capture and sedimentation of dark matter (dm) within their cores. bramante & linden showed that certain dm properties allow for rapid ns collapse within the high dm density environments near galactic centres while permitting ns survival elsewhere. each dm-induced collapse could generate an frb as the ns magnetosphere is suddenly expelled. this scenario could explain several features of frbs: their short time scales, large energies, locally produced scattering tails, and high event rates. we predict that frbs are localized to galactic centres, and that our own galactic centre harbours a large population of ns-mass (m ∼ 1.4 m⊙) black holes. the dm-induced collapse scenario is intrinsically unlikely because it can only occur in a small region of allowable dm parameter space. however, if observed to occur, it would place tight constraints on dm properties. | dark matter-induced collapse of neutron stars: a possible link between fast radio bursts and the missing pulsar problem. |
we have performed a new search for radio pulsars in archival data of the intermediate and high galactic latitude parts of the southern high time resolution universe pulsar survey. this is the first time the entire dataset has been searched for binary pulsars, an achievement enabled by gpu-accelerated dedispersion and periodicity search codes nearly 50 times faster than the previously used pipeline. candidate selection was handled entirely by a machine learning algorithm, allowing for the assessment of 17.6 million candidates in a few person-days. we have also introduced an outlier detection algorithm for efficient radio-frequency interference (rfi) mitigation on folded data, a new approach that enabled the discovery of pulsars previously masked by rfi. we discuss implications for future searches, particularly the importance of expanding work on rfi mitigation to improve survey completeness. in total, we discovered 23 previously unknown sources, including 6 millisecond pulsars and at least 4 pulsars in binary systems. we also found an elusive but credible redback candidate that we have yet to confirm. | the high time resolution universe survey - xiv. discovery of 23 pulsars through gpu-accelerated reprocessing |
we present a spectral study of the ultraluminous be/x-ray transient pulsar swift j0243.6+6124 using neutron star interior composition explorer (nicer) observations during the system’s 2017-2018 giant outburst. the 1.2-10 kev energy spectrum of the source can be approximated with an absorbed cutoff power-law model. we detect strong, luminosity-dependent emission lines in the 6-7 kev energy range. a narrow 6.42 kev line, observed in the sub-eddington regime, is seen to evolve into a broad fe-line profile in the super-eddington regime. other features are found at 6.67 and 6.97 kev in the fe-line complex. an asymmetric broad-line profile, peaking at 6.67 kev, is possibly due to doppler effects and gravitational redshift. the 1.2-79 kev broadband spectrum from nuclear spectroscopic telescope array (nustar) and nicer observations at the outburst peak is well described by an absorbed cutoff power law plus multiple gaussian lines and a blackbody component. physical reflection models are also tested to probe the broad iron-line feature. depending on the mass accretion rate, we found emission sites that are evolving from ∼5000 km to a range closer to the surface of the neutron star. our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultrafast outflow expected at super-eddington accretion rates. we do not detect any signatures of a cyclotron absorption line in the nicer or nustar data. | an evolving broad iron line from the first galactic ultraluminous x-ray pulsar swift j0243.6+6124 |
photoelectrochemical (pec) water splitting to produce hydrogen fuel was first reported 50 years ago1, yet artificial photosynthesis has not become a widespread technology. although planar si solar cells have become a ubiquitous electrical energy source economically competitive with fossil fuels, analogous pec devices have not been realized, and standard si p-type/n-type (p-n) junctions cannot be used for water splitting because the bandgap precludes the generation of the needed photovoltage. an alternative paradigm, the particle suspension reactor (psr), forgoes the rigid design in favour of individual pec particles suspended in solution, a potentially low-cost option compared with planar systems2,3. here we report si-based psrs by synthesizing high-photovoltage multijunction si nanowires (sinws) that are co-functionalized to catalytically split water. by encoding a p-type-intrinsic-n-type (p-i-n) superlattice within single sinws, tunable photovoltages exceeding 10 v were observed under 1 sun illumination. spatioselective photoelectrodeposition of oxygen and hydrogen evolution co-catalysts enabled water splitting at infrared wavelengths up to approximately 1,050 nm, with the efficiency and spectral dependence of hydrogen generation dictated by the photonic characteristics of the sub-wavelength-diameter sinws. although initial energy conversion efficiencies are low, multijunction sinws bring the photonic advantages of a tunable, mesoscale geometry and the material advantages of si—including the small bandgap and economies of scale—to the psr design, providing a new approach for water-splitting reactors. | water splitting with silicon p-i-n superlattices suspended in solution |
nicer, swift/bat, and maxi have continued to monitor the outburst of the be/x-ray binary pulsar ls v +44 17 (also known as rx j0440.9+4431). the source peaked at a pulse-phase averaged count rate of ~1500 c/s in the nicer energy band and is now slowly decreasing in count rate. | continued x-ray monitoring of ls v +44 17 with nicer, swift/bat, and maxi |
radiation detectors based on cadmium zinc telluride (czt) compounds are becoming popular solutions thanks to their high detection efficiency, room temperature operation, and to their reliability in compact detection systems for medical, astrophysical, or industrial applications. however, despite a huge effort to improve the technological process, czt detectors' full potential has not been completely exploited when both high spatial and energy resolution are required by the application, especially at low energies (<10 kev), limiting their application in energy-resolved photon counting (erpc) systems. this gap can also be attributed to the lack of dedicated front-end electronics which can bring out the best in terms of detector spectroscopic performances. in this work, we present the latest results achieved in terms of energy resolution using sirio, a fast low-noise charge sensitive amplifier, and a linear-array pixel detector, based on boron oxide encapsulated vertical bridgman-grown b-vb czt crystals. the detector features a 0.25-mm pitch, a 1-mm thickness and is operated at a ‑700-v bias voltage. an equivalent noise charge of 39.2 el. r.m.s. (corresponding to 412 ev fwhm) was measured on the test pulser at 32 ns peaking time, leading to a raw resolution of 1.3% (782 ev fwhm) on the 59 kev line at room temperature (+20 °c) using an uncollimated 241am, largely improving the current state of the art for czt-based detection systems at such short peaking times, and achieving an optimum resolution of 0.97% (576 ev fwhm) at 1 µs peaking time. the measured energy resolution at the 122 kev line and with 1 µs peaking time of a 57co raw uncollimated spectrum is 0.96% (1.17 kev). these activities are in the framework of an italian collaboration on the development of energy-resolved x-ray scanners for material recycling, medical applications, and non-destructive testing in the food industry. | advances in high-energy-resolution cdznte linear array pixel detectors with fast and low noise readout electronics |
a new measurement of a spatially extended gamma-ray signal from the center of the andromeda galaxy (m31) has recently been published by the fermi-lat collaboration, reporting that the emission broadly resembles the so-called galactic center excess (gce) of the milky way (mw). the weight of the evidence is steadily accumulating on a millisecond pulsar (msps) origin for the gce. these elements prompt us to compare these observations with what is, perhaps, the simplest model for an msp population, which is solely obtained by rescaling of the msp luminosity function that is determined in the local mw disk via the respective stellar mass of the systems. remarkably, we find that without free fitting parameters, this model can account for both the energetics and the morphology of the gce within uncertainties. for m31, the estimated luminosity due to primordial msps is expected to only contribute about a quarter of the detected emission, although a stronger contribution cannot be excluded given the large uncertainties. if correct, the model predicts that the m31 disk emission due to msps is not far below the present upper bound. we also discuss additional refinements of this simple model. using the correlation between globular cluster gamma-ray luminosity and stellar encounter rate, we gauge the dynamical msp formation in the bulge. this component is expected to contribute to the gce only at a level of ≲5%, it could affect the signal’s morphology. we also comment on the limitations of our model and on future perspectives for improved diagnostics. | millisecond pulsar origin of the galactic center excess and extended gamma-ray emission from andromeda: a closer look |
recent ultra-intense lasers of subcritical fields and proposed x-ray polarimetry for highly magnetized neutron stars of supercritical fields have attracted attention to vacuum birefringence, a unique feature of the nonlinear vacuum under strong electromagnetic fields. we propose a formulation of the vacuum birefringence in a strong magnetic field (b ) and a weak electric field (e ), including the effect of electromagnetic wrench (g ≡-e .b ≠0 ). to do so, we derive a closed expression of the one-loop effective lagrangian for the combined magnetic and electric fields by using the formula of the one-loop effective lagrangian for an arbitrarily strong magnetic field. we then employ the expression to derive the polarization and magnetization of the vacuum, from which we obtain the permittivity and permeability for a weak probe field. specifically, we find the refractive indices and the associated polarization vectors of the probe field for the case of parallel magnetic and electric fields. the proposed formulation reproduces the known results for pure magnetic fields in the proper limit. finally, we apply the formulation to the goldreich-julian pulsar model. our formulation reveals the importance of the electromagnetic wrench in vacuum birefringence: it can reduce the difference between refractive indices and rotate polarization vectors to a significant degree. such a quantitative understanding is crucial to the x-ray polarimetry for magnetized neutron stars or magnetars, which will demonstrate the fundamental feature of the strongly-modified quantum vacuum and estimate the extreme fields surrounding those astrophysical bodies. | vacuum birefringence at one-loop in a supercritical magnetic field superposed with a weak electric field and application to pulsar magnetosphere |
recent measurements of neutron star mass from several candidates (psr j 1614 -2230 , psr j 0348 +0432 , msp j 0740 +6620 ) set the lower bound on the maximum possible mass for this class of compact objects ∼2 m⊙ . existence of stars with high mass brings the possibility of existence of exotic matter (hyperons, meson condensates) at the core region of the objects. in this work, we investigate the (anti)kaon (k- , k¯ 0 ) condensation in β -equilibrated nuclear matter within the framework of covariant density functional theory. the functionals in the kaonic sector are constrained by the experimental studies on k- atomic, kaon-nucleon scattering data fits. we find that the equation of state softens with the inclusion of (anti)kaon condensates, which lowers the maximum mass of neutron star. in one of the density-independent coupling cases, the k- condensation is through a first order phase transition type, which produces a 2 m⊙ neutron star. the first order phase transition results in mixed phase region in the inner core of the stars. while k¯0 condensation appears via second-order phase transition for all the models we consider here. | dense matter equation of state of a massive neutron star with antikaon condensation |
we present new, tight, constraints on the cosmological background of gravitational waves (gws) using the latest measurements of cmb temperature and polarization anisotropies provided by the planck, bicep2 and keck array experiments. these constraints are further improved when the gw contribution neffgw to the effective number of relativistic degrees of freedom neff is also considered. parametrizing the tensor spectrum as a power law with tensor-to-scalar ratio r , tilt nt and pivot 0.01 mpc-1, and assuming a minimum value of r =0.001 , we find r <0.089 , nt=1. 7-2.0+2.1 (95% cl, no neffgw) and r <0.082 , nt=-0.0 5-0.87+0.58 (95% cl, with neffgw). when the recently released 95 ghz data from keck array are added to the analysis, the constraints on r are improved to r <0.067 (95% cl, no neffgw), r <0.061 (95% cl, with neffgw). we discuss the limits coming from direct detection experiments such as ligo-virgo, pulsar timing (european pulsar timing array) and cmb spectral distortions (firas). finally, we show future constraints achievable from a core-like mission: if the tensor-to-scalar ratio is of order 1 0-2 and the inflationary consistency relation nt=-r /8 holds, core will be able to constrain nt with an error of 0.16 at 95% cl. in the case that lensing b -modes can be subtracted to 10% of their power, a feasible goal for core, these limits will be improved to 0.11 at (95% cl). | updated constraints and forecasts on primordial tensor modes |
gravitational wave (gw) astronomy using a pulsar timing array requires high-quality millisecond pulsars (msps), correctable interstellar propagation delays, and high-precision measurements of pulse times of arrival. here we identify noise in timing residuals that exceeds that predicted for arrival time estimation for msps observed by the north american nanohertz observatory for gravitational waves. we characterize the excess noise using variance and structure function analyses. we find that 26 out of 37 pulsars show inconsistencies with a white-noise-only model based on the short timescale analysis of each pulsar, and we demonstrate that the excess noise has a red power spectrum for 15 pulsars. we also decompose the excess noise into chromatic (radio-frequency-dependent) and achromatic components. associating the achromatic red-noise component with spin noise and including additional power-spectrum-based estimates from the literature, we estimate a scaling law in terms of spin parameters (frequency and frequency derivative) and data-span length and compare it to the scaling law of shannon & cordes. we briefly discuss our results in terms of detection of gws at nanohertz frequencies. | the nanograv nine-year data set: excess noise in millisecond pulsar arrival times |
accretion of matter onto a magnetic, rotating object can be strongly affected by the interaction with its magnetic field. this occurs in a variety of astrophysical settings involving young stellar objects, white dwarfs, and neutron stars. as matter is endowed with angular momentum, its inflow toward the star is often mediated by an accretion disc. the pressure of matter and that originating from the stellar magnetic field balance at the magnetospheric radius: at smaller distances the motion of matter is dominated by the magnetic field, and funnelling towards the magnetic poles ensues. however, if the star, and thus its magnetosphere, is fast spinning, most of the inflowing matter will be halted at the magnetospheric radius by centrifugal forces, resulting in a characteristic reduction of the accretion luminosity. the onset of this mechanism, called the propeller, has been widely adopted to interpret a distinctive knee in the decaying phase of the light curve of several transiently accreting x-ray pulsar systems. by comparing the observed luminosity at the knee for different classes of objects with the value predicted by accretion theory on the basis of the independently measured magnetic field, spin period, mass, and radius of the star, we disclose here a general relation for the onset of the propeller which spans about eight orders of magnitude in spin period and ten in magnetic moment. the parameter-dependence and normalisation constant that we determine are in agreement with basic accretion theory. | a universal relation for the propeller mechanisms in magnetic rotating stars at different scales |
ngc 300 ulx1 is a newly identified ultra-luminous x-ray pulsar. the system is associated with the supernova impostor sn 2010da that was later classified as a possible supergiant be x-ray binary. in this work we report on the spin period evolution of the neutron star based on all the currently available x-ray observations of the system. we argue that the x-ray luminosity of the system has remained almost constant since 2010, at a level above ten times the eddington limit. moreover, we find evidence that the spin period of the neutron star evolved from ∼126 s down to ∼18 s within a period of about 4 years. we explain this unprecedented spin evolution in terms of the standard accretion torque theory. an intriguing consequence for ngc 300 ulx1 is that a neutron star spin reversal should have occurred a few years after the sn 2010da event. | ngc 300 ulx1: a test case for accretion torque theory |
in the canonical picture of pulsars, radio emission arises from a narrow cone centred on the star's magnetic axis but many basic details remain unclear. we use high-quality polarization data taken with the parkes radio telescope to constrain the geometry and emission heights of pulsars showing interpulse emission, and include the possibility that emission heights in the main and interpulse may be different. we show that emission heights are low in the centre of the beam, typically less than 3 per cent of the light cylinder radius. the emission beams are underfilled in longitude, with an average profile width only 60 per cent of the maximal beamwidth and there is a strong preference for the visible emission to be located on the trailing part of the beam. we show substantial evidence that the emission heights are larger at the beam edges than in the beam centre. there is some indication that a fan-like emission beam explains the data better than conal structures. finally, there is a strong correlation between handedness of circular polarization in the main and interpulse profiles, which implies that the hand of circular polarization is determined by the hemisphere of the visible emission. | on the beam properties of radio pulsars with interpulse emission |
we calculate the dispersion measure (dm) contributed by the intergalactic medium (igm) to the total measured dm for fast radio bursts (frbs). we use the marenostrum instituto de ciencias del espacio (mice) onion universe simulation to track the evolution of the dark matter particle density over a large range of redshifts. we convert this dark matter particle number density to the corresponding free electron density and then integrate it to find the dm as a function of redshift. this approach yields an intergalactic dm of {dm}}igm}(z=1)={800}-170+7000 pc cm-3, with the large errors representative of the structure in the igm. we place limits on the redshifts of the current population of observed frbs. we also use our results to estimate the host galaxy contribution to the dm for the first repeater, frb 121102, and show that the most probable host galaxy dm contribution, {dm}}host}≈ 310 pc cm-3, is consistent with the estimate made using the balmer emission lines in the spectrum of the host galaxy, {dm}}balmer}=324 pc cm-3. we also compare our predictions for the host galaxy contribution to the dm for the observations of frb 180924 and frb 190523, both of which have been localized to a host galaxy. | estimates of fast radio burst dispersion measures from cosmological simulations |
we present x-ray, ultraviolet, and optical observations of 1rxs j154439.4-112820, the most probable counterpart of the unassociated fermi-lat source 3fgl j1544.6-1125. the optical data reveal rapid variability, which is a feature of accreting systems. the x-rays exhibit large-amplitude variations in the form of fast switching (within ∼10 s) between two distinct flux levels that differ by a factor of ≈10. the detailed optical and x-ray behavior is virtually identical to that seen in the accretion-disk-dominated states of the transitional millisecond pulsar (msp) binaries psr j1023+0038 and xss j12270-4859, which are also associated with γ-ray sources. based on the available observational evidence, we conclude that 1rxs j154439.4-112820 and 3fgl j1544.6-1125 are the same object, with the x-rays arising from intermittent low-luminosity accretion onto an msp and the γ-rays originating from an accretion-driven outflow. 1rxs j154439.4-112820 is only the fourth γ-ray-emitting low-mass x-ray binary system to be identified and is likely to sporadically undergo transformations to a non-accreting rotation-powered pulsar system. | identification of the high-energy gamma-ray source 3fgl j1544.6-1125 as a transitional millisecond pulsar binary in an accreting state |
dark matter annihilations taking place in nearby subhalos could appear as gamma-ray sources without detectable counterparts at other wavelengths. in this study, we consider the collection of unassociated gamma-ray sources reported by the fermi collaboration in an effort to identify the most promising dark matter subhalo candidates. while we identify 24 bright, high-latitude, non-variable sources with spectra that are consistent with being generated by the annihilations of ~ 20-70 gev dark matter particles (assuming annihilations to bb̄), it is not possible at this time to distinguish these sources from radio-faint gamma-ray pulsars. deeper multi-wavelength observations will be essential to clarify the nature of these sources. it is notable that we do not find any such sources that are well fit by dark matter particles heavier than ~100 gev. we also study the angular distribution of the gamma-rays from this set of subhalo candidates, and find that the source 3fgl j2212.5+0703 prefers a spatially extended profile (of width ~ 0.15°) over that of a point source, with a significance of 4.2σ (3.6σ after trials factor). although not yet definitive, this bright and high-latitude gamma-ray source is well fit as a nearby subhalo of mχ ≃ 20-50 gev dark matter particles (annihilating to bb̄) and merits further multi-wavelength investigation. based on the subhalo distribution predicted by numerical simulations, we derive constraints on the dark matter annihilation cross section that are competitive to those resulting from gamma-ray observations of dwarf spheroidal galaxies, the galactic center, and the extragalactic gamma-ray background. | examining the fermi-lat third source catalog in search of dark matter subhalos |
fast-spinning strongly magnetized newborn neutron stars (nss), including nascent magnetars, are popularly implemented as the engine of luminous stellar explosions. here, we consider the scenario that they power various stripped-envelope (se) supernovae (sne), not only superluminous sne ic but also broad-line (bl) sne ibc and possibly some ordinary sne ibc. this scenario is also motivated by the hypothesis that galactic magnetars largely originate from fast-spinning nss as remnants of se sne. by consistently modeling the energy injection from magnetized wind and {}56{ni} decay, we show that proto-nss with ≳ 10 {ms} rotation and a poloidal magnetic field of {b}{{dip}}≳ 5× {10}14 {{g}} can be harbored in ordinary sne ibc. on the other hand, millisecond proto-nss can solely power bl sne ibc if they are born with {b}{{dip}}≳ 5× {10}14 {{g}} and superluminous sne ic with {b}{{dip}}≳ {10}13 {{g}}. then, we study how multi-messenger emission can be used to discriminate such pulsar-driven sn models from other competitive scenarios. first, high-energy x-ray and gamma-ray emission from embryonic pulsar wind nebulae can probe the underlying newborn pulsar. follow-up observations of se sne using nustar ∼ 50{--}100 {days} after the explosion are strongly encouraged for nearby objects. we also discuss possible effects of gravitational waves (gws) on the spin-down of proto-nss. if millisecond proto-nss with {b}{{dip}}\lt {{a}} {few}× {10}13 {{g}} emit gws through, e.g., non-axisymmetric rotation deformed by the inner toroidal fields of {b}{{t}}≳ {10}16 {{g}}, the gw signal can be detectable from ordinary sne ibc in the virgo cluster by advanced ligo, advanced virgo, and kagra. | multi-messenger tests for fast-spinning newborn pulsars embedded in stripped-envelope supernovae |
we present the first fully self-consistent three dimensional model of a neutron star's magnetic field, generated by electric currents in the star's crust via the hall effect. we find that the global-scale field converges to a dipolar hall-attractor state, as seen in recent axisymmetric models, but that small-scale features in the magnetic field survive even on much longer time scales. these small-scale features propagate toward the dipole equator, where the crustal electric currents organize themselves into a strong equatorial jet. by calculating the distribution of magnetic stresses in the crust, we predict that neutron stars with fields stronger than 1 014 g can still be subject to starquakes more than 1 05 yr after their formation. | three dimensional simulation of the magnetic stress in a neutron star crust |
we present the measurements of scatter broadening timescales ({{τ }sc}) for 124 pulsars at 327 mhz using the upgraded ooty radio telescope. these pulsars lie in the dispersion measure range of 37-503 pc cm-3 and declination (δ) range of -57°\lt δ \lt 60{}^\circ . new {{τ }sc} estimates for 58 pulsars are presented, increasing the sample of all such measurements by about 40% at 327 mhz. using all available {{τ }sc} measurements in the literature, we investigate the dependence of {{τ }sc} on dispersion measure. our measurements, together with previously reported values for {{τ }sc}, affirm that the ionized interstellar medium up to 3 kpc is consistent with the kolmogorov spectrum, while it deviates significantly beyond this distance. | scatter broadening measurements of 124 pulsars at 327 mhz |
we report on the results of a recent blind search survey for gamma-ray pulsars in fermi large area telescope (lat) data being carried out on the distributed volunteer computing system, einstein@home. the survey has searched for pulsations in 118 unidentified pulsar-like sources, requiring about 10,000 years of cpu core time. in total, this survey has resulted in the discovery of 17 new gamma-ray pulsars, of which 13 are newly reported in this work, and an accompanying paper. these pulsars are all young, isolated pulsars with characteristic ages between 12 kyr and 2 myr, and spin-down powers between 1034 and 4 × 1036 erg s-1. two of these are the slowest spinning gamma-ray pulsars yet known. one pulsar experienced a very large glitch {{δ }}f/f≈ 3.5× {10}-6 during the fermi mission. in this, the first of two associated papers, we describe the search scheme used in this survey, and estimate the sensitivity of our search to pulsations in unidentified fermi-lat sources. one such estimate results in an upper limit of 57% for the fraction of pulsed emission from the gamma-ray source associated with the cas a supernova remnant, constraining the pulsed gamma-ray photon flux that can be produced by the neutron star at its center. we also present the results of precise timing analyses for each of the newly detected pulsars. | the einstein@home gamma-ray pulsar survey. i. search methods, sensitivity, and discovery of new young gamma-ray pulsars |
we present the data analysis pipeline, commissioning observations, and initial results from the greenburst fast radio burst (frb) detection system on the robert c. byrd green bank telescope (gbt) previously described by surnis et al., which uses the 21-cm receiver observing commensally with other projects. the pipeline makes use of a state-of-the-art deep learning classifier to winnow down the very large number of false-positive single-pulse candidates that mostly result from radio frequency interference. in our observations, totalling 156.5 d so far, we have detected individual pulses from 20 known radio pulsars that provide an excellent verification of the system performance. we also demonstrate, through blind injection analyses, that our pipeline is complete down to a signal-to-noise threshold of 12. depending on the observing mode, this translates into peak flux sensitivities in the range 0.14-0.89 jy. although no frbs have been detected to date, we have used our results to update the analysis of lawrence et al. to constrain the frb all-sky rate to be $1150^{+200}_{-180}$ per day above a peak flux density of 1 jy. we also constrain the source count index α = 0.84 ± 0.06, which indicates that the source count distribution is substantially flatter than expected from a euclidean distribution of standard candles (where α = 1.5). we discuss this result in the context of the frb redshift and luminosity distributions. finally, we make predictions for detection rates with greenburst, as well as other ongoing and planned frb experiments. | initial results from a real-time frb search with the gbt |
after spending almost a decade in a radio-quiet state, the anomalous x-ray pulsar xte j1810-197 turned back on in early 2018 december. we have observed this radio magnetar at 1.5 ghz with nearly daily cadence since the first detection of radio re-activation on 2018 december 8. in this paper, we report on the current timing properties of xte j1810-197 and find that the magnitude of the spin frequency derivative has increased by a factor of 2.6 over our 48-d data set. we compare our results with the spin-down evolution reported during its previous active phase in the radio band. we also present total intensity pulse profiles at five different observing frequencies between 1.5 and 8.4 ghz, collected with the lovell and the effelsberg telescopes. the profile evolution in our data set is less erratic than what was reported during the previous active phase, and can be seen varying smoothly between observations. profiles observed immediately after the outburst show the presence of at least five cycles of a very stable ∼50 ms periodicity in the main pulse component that lasts for at least tens of days. this remarkable structure is seen across the full range of observing frequencies. | spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar xte j1810-197 |
the phenomena of the deconfinement of hadronic matter into quark matter at high density, relevant to hybrid star (hs) cores, is studied in the present work. the effective chiral model describes the pure hadronic phase while for the quark phase the mit bag model is chosen with density-dependent bag pressure. phase transition is achieved using maxwell construction. the effect of variation of the asymptotic value of the bag pressure (bas) is analyzed w.r.t. the mass and radius of the hss. the presence of hyperons in the hadronic sector also has a significant effect on the choice of the value of bas. both the hadronic composition and the choice of bas significantly affect the stability of the star. the gross structural properties of the resultant hs are calculated in a static condition and compared with the various constraints on them from different observational and empirical perspectives. the static properties, like the maximum gravitational mass of the hs obtained with bas = 80 mev fm-3, is consistent with the limits imposed from the observational analysis of psr j0348+0432 and psr j0740+6620. the estimates of r1.4 and r1.6 of hss are found to be within the range prescribed from gw170817 analysis. also the m-r solutions of the hss are in excellent agreement with the recently obtained nicer data for psr j0030+0451. the results of maximum surface redshift, obtained with a hybrid equation of state, satisfy the constraints from 1e 1207.4-5209 and rx j0720.4-3125. the work is also extended to obtain the tidal deformation properties of the hss. the obtained value of λ1.4 is consistent with the bound obtained from gw170817 data analysis. | properties of hybrid stars with a density-dependent bag model |
we present a model for atmospheric wind circulation in binary millisecond pulsar (msp) companions, showing how the optical light curve (lc) and radial velocities are sensitive to the wind flow, causing lc orbital phase shifts and asymmetries, as observed for several "spider" msps. velocity widths of spectral lines offer additional opportunities for measuring surface wind speed. as examples, we fit optical data for the black widow pulsar j1959+2048 and the redback pulsar j2215+5135; the wind heating models (wh) are statistically strongly preferred over direct heating (dh) for both objects, although the latter is even better fit with a heated spot. in general, wh effects tend to increase the inferred orbital inclination i and decrease the inferred companion center-of-mass radial velocity amplitude kc; both effects decrease the inferred neutron star mass. even with such a decrease, we find large masses for the two neutron stars: 2.18 ± 0.09 m⊙ and 2.28-0.09+0.10 m⊙, respectively (for the modest surface speeds fit from the bulk heat flow; supersonic photospheric winds can slightly change these values). these are among the highest masses known, and our improved modeling increases confidence that the results are important for understanding the dense matter equation of state. | atmospheric circulation on black widow companions |
pulsar timing array (pta) searches for a gravitational-wave background (gwb) typically include time-correlated "red" noise models intrinsic to each pulsar. using a simple simulated pta data set with an injected gwb signal we show that the details of the red noise models used, including the choice of amplitude priors and even which pulsars have red noise, have a striking impact on the gwb statistics, including both upper limits and estimates of the gwb amplitude. we find that the standard use of uniform priors on the red noise amplitude leads to 95% upper limits, as calculated from one-sided bayesian credible intervals, that are less than the injected gwb amplitude 50% of the time. in addition, amplitude estimates of the gwb are systematically lower than the injected value by 10%-40%, depending on which models and priors are chosen for the intrinsic red noise. we tally the effects of model and prior choice and demonstrate how a "dropout" model, which allows flexible use of red noise models in a bayesian approach, can improve gwb estimates throughout. | model dependence of bayesian gravitational-wave background statistics for pulsar timing arrays |
the x-ray radiation produced on the surface of accreting magnetised neutron stars is expected to be strongly polarised. a swing of the polarisation vector with the pulsar phase gives a direct measure of the source inclination and magnetic obliquity. in the case of rapidly rotating millisecond pulsars, the relativistic motion of the emission region causes additional rotation of the polarisation plane. here, we develop a relativistic rotating vector model, where we derive analytical expression for the polarisation angle as a function of the pulsar phase accounting for relativistic aberration and gravitational light bending in the schwarzschild metric. we show that in the case of fast pulsars the rotation of the polarisation plane can reach tens of degrees, strongly influencing the observed shape of the polarisation angle's phase dependence. the rotation angle grows nearly linearly with the spin rate but it is less sensitive to the neutron star radius. overall, this angle is large even for large spots. our results have implications with regard to the modelling of x-ray polarisation from accreting millisecond pulsars that are to be observed with the upcoming imaging x-ray polarimeter explorer and the enhanced x-ray timing and polarimetry mission. the x-ray polarisation may improve constraints on the neutron star mass and radius coming from the pulse profile modelling. | relativistic rotating vector model for x-ray millisecond pulsars |
context. radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (ptas) that aim to detect nanohertz gravitational waves.aims: we aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations.methods: we use the pulsar timing technique to obtain highly precise dispersion measure (dm) time series. our dataset consists of observations of 36 millisecond pulsars, which were observed for up to 7.1 yr with the low frequency array (lofar) telescope at a centre frequency of ~150 mhz. seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence.results: we achieve a median dm precision of the order of 10-5 cm-3 pc for a significant fraction of our sources. we detect significant variations of the dm in all pulsars with a median dm uncertainty of less than 2 × 10-4 cm-3 pc. the noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1-10 μs at 1.4 ghz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 μs or better that ptas aim for. we found no evidence for a dependence of dm on radio frequency for any of the sources in our sample.conclusions: the dm time series we obtained using lofar could in principle be used to correct higher-frequency data for the variations of the dispersive delay. however, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (toas) at 1.4 ghz or a high dm precision at low frequencies, but not both, due to spectral properties. combining the higher-frequency toas with those from lofar to measure the infinite-frequency toa and dm would improve the result. | dispersion measure variability for 36 millisecond pulsars at 150 mhz with lofar |
the strong interaction at low-energy scales determines the equation of state (eos) of supranuclear matters in neutron stars (nss). it is conjectured that the bulk dense matter may be composed of strangeons, which are quark clusters with nearly equal numbers of u, d, and s quarks. to characterize the strong-repulsive interaction at short distance and the non-relativistic nature of strangeons, a phenomenological lennard-jones model with two parameters is used to describe the eos of strangeon stars (sss). for the first time, we investigate the oscillation modes of non-rotating sss and obtain their frequencies for various parametrizations of the eos. we find that the properties of radial oscillations of sss are different from those of nss, especially for stars with relatively low central energy densities. moreover, we calculate the f-mode frequency of non-radial oscillations of sss within the relativistic cowling approximation. the frequencies of the f mode of sss are found to be in the range 6.7-8.7 khz. finally, we study the universal relations between the f-mode frequency and global properties of sss, such as the compactness and the tidal deformability. the results we obtained are relevant to pulsar timing and gravitational waves, and will help to probe nss' eoss and infer non-perturbative behaviours in quantum chromodynamics. | oscillation modes and gravitational waves from strangeon stars |
by applying a relativistic mean-field description of neutron star matter with density dependent couplings, we analyze the properties of two different matter compositions: nucleonic matter with δ baryons and nucleonic matter with hyperons and δ baryons. the δ -meson couplings are allowed to vary within a wide range of values obtained by experimental data, while the hyperon-meson couplings are fitted to hypernuclear properties. neutron star properties with no deconfinement phase transition are studied. it is verified that many models are excluded because the effective nucleon mass becomes zero before the maximum mass configuration is attained. hyperon-free compact stars with δ -dominated composition are possible: the deltic stars. it is found that with a convenient choice of parameters the existence of deltic stars with 80% of δ baryons at the center of the star is possible. however, the presence of hyperons lowers the δ baryon fraction to values below 20% at the center and below 30% at 2-3 saturation densities. it is discussed that, in the presence of δ baryons, the hyperon softening is not so drastic because δ 's couple more strongly to the ω meson, and the stiffness of the equation of state is determined by the ω dominance at high densities. the speed of sound reflects very well this behavior. the compactness of the pulsar rx j0720.4-3125 imposes xσ δ>xω δ>1 and favors xρ δ>1 . | δ baryons in neutron stars |
we search for transient sources in a sample of ultraluminous x-ray sources (ulxs) from the 3xmm-dr4 release of the xmm-newton serendipitous source catalogue in order to find candidate neutron star ulxs alternating between an accreting state and the propeller regime, in which the luminosity drops dramatically. by examining their fluxes and flux upper limits, we identify five ulxs that demonstrate long-term variability of over an order of magnitude. using chandra and swift data to further characterize their light curves, we find that two of these sources are detected only once and could be x-ray binaries in outburst that only briefly reach ulx luminosities. two others are consistent with being super-eddington accreting sources with high levels of inter-observation variability. one source, m51 ulx-4, demonstrates apparent bimodal flux behaviour that could indicate the propeller regime. it has a hard x-ray spectrum, but no significant pulsations in its timing data, although with an upper limit of 10 per cent of the signal pulsed at ∼1.5 hz a pulsating ulx cannot be excluded, particularly if the pulsations are transient. by simulating xmm-newton observations of a population of pulsating ulxs, we predict that there could be approximately 200 other bimodal ulxs that have not been observed sufficiently well by xmm-newton to be identified as transient. | searching for propeller-phase ulxs in the xmm-newton serendipitous source catalogue |
we have identified 2xmm j125556.57+565846.4, at a distance of 600 pc, as a binary system consisting of a normal star and a probable dormant neutron star. optical spectra exhibit a slightly evolved f-type single star, displaying periodic doppler shifts with a 2.76-d keplerian circular orbit, with no indication of light from a secondary component. optical and uv photometry reveal ellipsoidal modulation with half the orbital period, due to the tidal deformation of the f-star. the mass of the unseen companion is constrained to the range of 1.1-$2.1\, \mathrm{m}_{\odot }$ at 3σ confidence, with the median of the mass distribution at $1.4\, \mathrm{m}_{\odot }$, the typical mass of known neutron stars. a main-sequence star cannot masquerade as the dark companion. the distribution of possible companion masses still allows for the possibility of a very massive white dwarf. the companion itself could also be a close pair consisting of a white dwarf and an m star, or two white dwarfs, although the binary evolution that would lead to such a close triple system is unlikely. similar ambiguities regarding the certain identification of a dormant neutron star are bound to affect most future discoveries of this type of non-interacting system. if the system indeed contains a dormant neutron star, it will become, in the future, a bright x-ray source and afterwards might even host a millisecond pulsar. | probable dormant neutron star in a short-period binary system |
neutron stars host the strongest magnetic fields that we know of in the universe. their magnetic fields are the main means of generating their radiation, either magnetospheric or through the crust. moreover, the evolution of the magnetic field has been intimately related to explosive events of magnetars, which host strong magnetic fields, and their persistent thermal emission. the evolution of the magnetic field in the crusts of neutron stars has been described within the framework of the hall effect and ohmic dissipation. yet, this description is limited by the fact that the maxwell stresses exerted on the crusts of strongly magnetised neutron stars may lead to failure and temperature variations. in the former case, a failed crust does not completely fulfil the necessary conditions for the hall effect. in the latter, the variations of temperature are strongly related to the magnetic field evolution. finally, sharp gradients of the star's temperature may activate battery terms and alter the magnetic field structure, especially in weakly magnetised neutron stars. in this review, we discuss the recent progress made on these effects. we argue that these phenomena are likely to provide novel insight into our understanding of neutron stars and their observable properties. | magnetic field evolution in neutron star crusts: beyond the hall effect |
context. extended gamma-ray emission, interpreted as halos formed by the inverse-compton scattering of ambient photons by electron-positron pairs, is observed toward a number of middle-aged pulsars. the physical origin and actual commonness of the phenomenon in the galaxy remain unclear. the conditions of pair confinement seem extreme compared to what can be achieved in recent theoretical models.aims: we searched for scenarios minimizing as much as possible the extent and magnitude of diffusion suppression in the halos in j0633+1746 and b0656+14, and explored the implications on the local positron flux if they are applied to all nearby middle-aged pulsars.methods: we used a phenomenological static two-zone diffusion framework, and compared its predictions with fermi-lat and hawc observations of the two halos, and with the local positron flux measured with ams-02.results: while strong diffusion suppression of two to three orders of magnitude at ∼100 tev is required by the data, it is possible to find solutions with diffusion suppression extents as small as 30 pc for both objects. if all nearby middle-aged pulsars develop such halos, their combined positron flux including the contribution from geminga would saturate the ≳100 gev ams-02 measurement for injection efficiencies that are much smaller than those inferred for the canonical halos in j0633+1746 and b0656+14, and more generally with the values typical of younger pulsar wind nebulae. conversely, if positrons from other nearby pulsars are released in the interstellar medium without any confinement around the source, their total positron flux fits into the observed spectrum for the same injection efficiencies of a few tens of percent for all pulsars, from objects a few thousand years in age that power bright pulsar wind nebulae to much older objects like j0633+1746 and b0656+14.conclusions: it seems simpler to assume that most middle-aged pulsars do not develop halos, although the evidence supporting this scenario depends on the actual properties of the local pulsar population and on the uncertain physics driving the formation and evolution of halos. the occurrence rate of the phenomenon could be as low as ∼5−10%, and the local positron flux in the ∼0.1−1.0 tev range would thus be attributed to a few dozen nearby middle-aged pulsars rapidly releasing pairs into the interstellar medium, with a possible contribution over part or most of the range by j0633+1746, and at higher energies by b0656+14. | are pulsar halos rare?. modeling the halos around psrs j0633+1746 and b0656+14 in the light of fermi-lat, hawc, and ams-02 observations and extrapolating to other nearby pulsars |
the moon and mercury both have permanently shaded regions (psrs) at their poles, which are locations that do not see the sun for geologically long periods of time. the psrs of the moon and mercury have very cold temperatures (<120 k) and as a consequence act as traps for volatile materials. volatile enhancements have been detected and characterized at both planetary bodies, but the volatile concentrations at mercury's poles are significantly larger than at the moon's poles. this paper documents the study of psr volatiles at the moon and mercury that has taken place over the past 60 years. starting with speculative ideas in the 1950s and 1960s, the field of psr volatiles has emerged into a thriving subfield of planetary science that has significant implications for scientific studies of the solar system, as well as future human exploration of the solar system. while much has been learned about psrs and psr volatiles, many foundational aspects of psrs are still not understood. one of the most important unanswered questions is why the psr volatile concentrations at the moon and mercury are so different. after describing the initial predictions and measurements of psr volatiles, this paper documents a variety of psr measurements, summarizes the current understanding of psr volatiles, and then suggests what new measurements and studies are needed to answer many of the remaining open questions about psr volatiles. | a tale of two poles: toward understanding the presence, distribution, and origin of volatiles at the polar regions of the moon and mercury |
present paper provides a new model of anisotropic strange star corresponding to the exterior schwarzschild metric. the einstein field equations have been solved by utilizing the krori-barua (kb) ansatz (krori and barua in j. phys. a, math. gen. 8:508, 1975) in presence of quintessence field characterized by a parameter ωqwith . the obtained solutions are free from central singularity. our model is potentially stable. the numerical values of mass of the different strange stars saxj1808.4-3658(ss1) (radius=7.07 km), 4u1820-30 (radius=10 km), vela x-12 (radius=9.99 km), psr j 1614-2230 (radius=10.3 km) obtained from our model is very close to the observational data that confirms the validity of our proposed model. the interior solution is also matched to the exterior schwarzschild spacetime in presence of thin shell where negative surface pressure is required to hold the thin shell against collapsing. | singularity-free anisotropic strange quintessence star |
the large-scale development and utilization of coal resources have brought great challenges to the ecological environment of coal-mining areas. therefore, this paper has used scientific and effective methods to monitor and evaluate whether changes in ecological environment quality in coal-mining areas are helpful to alleviate the contradiction between human and nature and realize the sustainable development of such coal-mining areas. firstly, in order to quantify the degree of coal dust pollution in coal-mining areas, an index-based coal dust index (icdi) is proposed. secondly, based on the pressure-state-response (psr) framework, a new coal-mine ecological index (cmei) was established by using the principal component analysis (pca) method. finally, the coal-mine ecological index (cmei) was used to evaluate and detect the temporal and spatial changes of the ecological environment quality of the ningwu coalfield from 1987 to 2021. the research shows that icdi has a strong ability to extract coal dust with an overall accuracy of over 96% and a kappa coefficient of over 0.9. as a normalized difference index, icdi can better quantify the pollution degree of coal dust. the effectiveness of cmei was evaluated by four methods: sample image-based, classification-based, correlation-based, and distance-based. from 1987 to 2021, the ecological environment quality of ningwu coalfield was improved, and the mean of cmei increased by 0.1189. the percentages of improvement and degradation of ecological environment quality were 71.85% and 27.01%, respectively. the areas with obvious degradation were mainly concentrated in coal-mining areas and built-up areas. the ecological environment quality of pingshuo coal mine, shuonan coal mine, xuangang coal mine, and lanxian coal mine also showed improvement. the results of moran's index show that cmei has a strong positive spatial correlation, and its spatial distribution is clustered rather than random. coal-mining areas and built-up areas showed low–low clustering (ll), while other areas showed high–high clustering (hh). the utilization and popularization of cmei provides an important reference for decision makers to formulate ecological protection policies and implement regional coordinated development strategies. | remote-sensing evaluation and temporal and spatial change detection of ecological environment quality in coal-mining areas |
we present new discoveries and results from long-term timing of 72 pulsars discovered in the pulsar arecibo l-band feed array (palfa) survey, including precise determination of astrometric and spin parameters, and flux density and scatter broadening measurements at 1.4 ghz. notable discoveries include two young pulsars (characteristic ages ~30 kyr) with no apparent supernova remnant associations, three mode-changing, 12 nulling and two intermittent pulsars. we detected eight glitches in five pulsars. among them is psr j1939+2609, an apparently old pulsar (characteristic age ~1 gy), and psr j1954+2529, which likely belongs to a newly emerging class of binary pulsars. the latter is the only pulsar among the 72 that is clearly not isolated: a nonrecycled neutron star with a 931 ms spin period in an eccentric (e = 0.114) wide (pb= 82.7 days) orbit with a companion of undetermined nature having a minimum mass of ~0.6 m ⊙. since operations at arecibo ceased in 2020 august, we give a final tally of palfa sky coverage, and compare its 207 pulsar discoveries to the known population. on average, they are 50% more distant than other galactic plane radio pulsars; palfa millisecond pulsars (msps) have twice the dispersion measure per unit spin period than the known population of msp in the plane. the four intermittent pulsars discovered by palfa more than double the population of such objects, which should help to improve our understanding of pulsar magnetosphere physics. the statistics for these, rotating radio transients, and nulling pulsars suggest that there are many more of these objects in the galaxy than was previously thought. | study of 72 pulsars discovered in the palfa survey: timing analysis, glitch activity, emission variability, and a pulsar in an eccentric binary |
the strong interactions at low energy scales determine the state of the supranuclear matter in the pulsar-like compact objects. it is proposed that the bulk strong matter could be composed of strangeons, which are quark clusters with a nearly equal number of three light-flavour quarks. in this work, to characterize the strong-repulsive interactions at short distances and the non-relativistic nature of the strangeons, the lennard-jones model is used to describe the equation of state (eos) of strangeon stars (sss). we investigate the static, the slowly rotating, and the tidally deformed sss in detail. the corrections resulted from the finite surface densities are considered crucially in the perturbative approaches. we also study the universal relations between the moments of inertia, the tidal deformabilities, and the quadrupole moments. those results are ready to be used for various purposes in astrophysics, and possible constraints from contemporary observations on the parameter space of the lennard-jones model are discussed. future observations of the pulsars' radio signals, the x-ray emissions from the hot spots on the surface of the stars, and the gravitational waves (gws) from binary mergers can give tighter constraints or even verify or falsify the existence of sss. | rotation and deformation of strangeon stars in the lennard-jones model |
in the present paper we propose a new model of an anisotropic strange star which admits the chaplygin equation of state. the exterior spacetime is described by a schwarzschild line element. the model is developed by assuming the finch-skea ansatz (finch and skea in class. quantum gravity 6:467, 1989. we obtain the model parameters in closed form. our model is free from a central singularity. choosing some particular values for the parameter we show that our model corroborates the observational data of the strange star psr j1614-2230 (gangopadhyay et al. in mon. not. r. astron. soc. 431:3216, 2013. | strange star admitting chaplygin equation of state in finch-skea spacetime |
since the day of its explosion, supernova (sn) 1987a has been closely monitored to study its evolution and to detect its central compact relic. in fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the sn. however, besides the detection in the atacama large millimeter/submillimeter array (alma) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (pwn), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in nustar data up to ~20 kev. we report on the simultaneous analysis of multiepoch observations of sn 1987a performed with chandra, xmm-newton, and nustar. we also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of sn 1987a. a heavily absorbed power law, consistent with the emission from a pwn embedded in the heart of sn 1987a, is needed to properly describe the high-energy part of the observed spectra. the spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. the information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the alma data. analysis of the synthetic spectra also shows that, in the near future, the main contribution to the fe k emission line will originate in the outermost shocked ejecta of sn 1987a. | additional evidence for a pulsar wind nebula in the heart of sn 1987a from multiepoch x-ray data and mhd modeling |
gamma-ray observations have shown pulsars to be efficient converters of rotational energy into gev photons and it is of wide-ranging interest to determine their contribution to the gamma-ray background. we arrive at flux predictions from both the young (<~ myr) and millisecond (~gyr) galactic pulsar populations. we find that unresolved pulsars can yield both a significant fraction of the excess gev gamma rays near the galactic center and an inverse compton flux in the inner kpc similar to that inferred by fermi. we compare models of the young pulsar population and millisecond pulsar population to constraints from gamma-ray and radio observations. overall, we find that the young pulsars should outnumber millisecond pulsars as unassociated gamma-ray point sources in this region. the number of young radio pulsars discovered near the galactic center is in agreement with our model of the young pulsar population. deeper radio observations at higher latitudes can constrain the total gamma-ray emission from both young and millisecond pulsars from the inner galaxy. while this is a step towards better understanding of pulsars, cosmic rays in the milky way, and searches for dark matter, we also discuss a few interesting puzzles that arise from the underlying physics of pulsar emission and evolution. | young and millisecond pulsar gev gamma-ray fluxes from the galactic center and beyond |
pulsar glitches provide a unique way to study neutron star microphysics because short post-glitch dynamics are directly linked to strong frictional processes on small scales. to illustrate this connection between macroscopic observables and microphysics, we review calculations of vortex interactions focusing on kelvin wave excitations and determine the corresponding mutual friction strength for realistic microscopic parameters in the inner crust. these density-dependent crustal coupling profiles are combined with a simplified treatment of the core coupling and implemented in a three-component neutron star model to construct a predictive framework for glitch rises. as a result of the density-dependent dynamics, we find the superfluid to transfer angular momentum to different parts of the crust and the core on different timescales. this can cause the spin frequency change to become non-monotonic in time, allowing for a maximum value much larger than the measured glitch size, as well as a delay in the recovery. the exact shape of the calculated glitch rise is strongly dependent on the relative strength between the crust and core mutual friction, providing the means to probe not only the crustal superfluid but also the deeper neutron star interior. to demonstrate the potential of this approach, we compare our predictive model with the first pulse-to-pulse observations recorded during the 2016 december glitch of the vela pulsar. our analysis suggests that the glitch rise behavior is relatively insensitive to the crustal mutual friction strength as long as { \mathcal b } ≳ 10-3, while being strongly dependent on the core coupling strength, which we find to be in the range 3× {10}-5≲ {{ \mathcal b }}core}≲ {10}-4. | glitch rises as a test for rapid superfluid coupling in neutron stars |
using a milky way (mw) double neutron star (dns) merger rate of 210 myr-1, as derived by the laser interferometer gravitational-wave observatory (ligo), we demonstrate that the laser interferometer space antenna (lisa) will detect on average 240 (330) dnss within the mw for a 4 yr (8 yr) mission with a signal-to-noise ratio greater than 7. even adopting a more pessimistic rate of 42 myr-1, as derived by the population of galactic dnss, we find a significant detection of 46 (65) mw dnss. these dnss can be leveraged to constrain formation scenarios. in particular, without prior information on a particular system's position and orbital period, traditional ns-discovery methods using radio telescopes alone are insensitive to dnss with porb ≲ 1 hr (merger times ≲10 myr). if a fast-merging channel exists that forms dnss at these short orbital periods, lisa affords, perhaps, the best opportunity to observationally identify and characterize these systems; we show that toy models for possible formation scenarios leave imprints on dns orbital eccentricities, which may be measured by lisa for values as small as ∼10-2. | lisa and the existence of a fast-merging double neutron star formation channel |
the gravitational wave provides a new method to examine general relativity and its alternatives in the high speed, strong field regime. alternative theories of gravity generally predict more polarizations than general relativity, so it is important to study the polarization contents of theories of gravity to reveal the nature of gravity. in this talk, we analyze the polarization contents of horndeski theory and f(r) gravity. we find out that in addition to the familiar plus and cross polarizations, a massless horndeski theory predicts an extra transverse polarization, and there is a mix of pure longitudinal and transverse breathing polarizations in the massive horndeski theory and f(r) gravity. it is possible to use pulsar timing arrays to detect the extra polarizations in these theories. we also point out that the classification of polarizations using newman–penrose variables cannot be applied to massive modes. it cannot be used to classify polarizations in einstein-æther theory or generalized tensor-vector-scalar (teves) theory, either. | the polarizations of gravitational waves† |
we have observed the relativistic binary pulsar psr j1141-6545 over a period of ∼6 yr using the parkes 64 m radio telescope, with a focus on modelling the diffractive intensity scintillations to improve the accuracy of the astrometric timing model. the long-term scintillation, which shows orbital and annual variations, allows us to measure parameters that are difficult to measure with pulsar timing alone. these include: the orbital inclination i, the longitude of the ascending node ω, and the pulsar system transverse velocity. we use the annual variations to resolve the previous ambiguity in the sense of the inclination angle. using the correct sense, and a prior probability distribution given by a constraint from pulsar timing (i = 73 ± 3°), we find ω = 24.8 ± 1.8° and we estimate the pulsar distance to be d=10^{+4}_{-3} kpc. this then gives us an estimate of this pulsar's proper motion of μαcos δ = 2.9 ± 1.0 mas yr-1 in right ascension and μδ = 1.8 ± 0.6 mas yr-1 in declination. finally, we obtain measurements of the spatial structure of the interstellar electron-density fluctuations, including: the spatial scale and anisotropy of the diffraction pattern, the distribution of scattering material along the line of sight, and spatial variation in the strength of turbulence from epoch to epoch. we find that the scattering is dominated by a thin screen at a distance of (0.724 ± 0.008)d, with an anisotropy axial ratio ar = 2.14 ± 0.11. | modelling annual and orbital variations in the scintillation of the relativistic binary psr j1141-6545 |
radio-loud neutron stars known as pulsars allow a wide range of experimental tests for fundamental physics, ranging from the study of super-dense matter to tests of general relativity (gr) and its alternatives. as a result, pulsars provide strong-field tests of gravity, they allow for the direct detection of gravitational waves in a “pulsar timing array” (pta), and they promise the future study of black hole properties. this contribution gives an overview of the on-going experiments and recent results. | pulsars as probes of gravity and fundamental physics |
we have carried out new, high-frequency, high-time-resolution observations of the crab pulsar. combining these with our previous data, we characterize bright single pulses associated with the main pulse, both the low-frequency and high-frequency interpulses, and the two high-frequency components. our data include observations at frequencies ranging from 1 to 43 ghz with time resolutions down to a fraction of a nanosecond. we find that at least two types of emission physics are operating in this pulsar. both main pulses and low-frequency interpulses, up to ∼10 ghz, are characterized by nanoshot emission—overlapping clumps of narrowband nanoshots, each with its own polarization signature. high-frequency interpulses, between 5 and 30 ghz, are characterized by spectral band emission—linearly polarized emission containing ∼30 proportionately spaced spectral bands. we cannot say whether the longer-duration high-frequency components pulses are due to a scattering process, or if they come from yet another type of emission physics. | the crab pulsar at centimeter wavelengths. ii. single pulses |
the equation of state (eos) of the neutron star (ns) matter remains an enigma. in this work we perform the bayesian parameter inference with the gravitational wave data (gw170817) and mass-radius observations of some nss (psr j 0030 +0451 , psr j 0437 -4715 , and 4u 1702-429) using the phenomenologically constructed eos models to search for a potential first-order phase transition. our phenomenological eos models take the advantages of current widely used parametrizing methods, which are flexible enough to resemble various theoretical eos models. we find that the current observation data are still not informative enough to support/rule out phase transition, due to the comparable evidences for models with and without phase transition. however, the bulk properties of the canonical 1.4 m⊙ ns and the pressure at around 2 ρsat are well constrained by the data, where ρsat is the nuclear saturation density. moreover, strong phase transition at low densities is disfavored, and the 1 σ lower bound of transition density is constrained to 1.84 ρsat. | constraint on phase transition with the multimessenger data of neutron stars |
nearly one-third of the sources listed in the third fermi large area telescope (lat) catalog (3fgl) remain unassociated. it is possible that predicted and even unanticipated gamma-ray source classes are present in these data waiting to be discovered. taking advantage of the excellent spectral capabilities achieved by the fermi lat, we use machine-learning classifiers (random forest and xgboost) to pinpoint potentially novel source classes in the unassociated 3fgl sample outside the galactic plane. here we report a total of 34 high-confidence galactic candidates at | b| ≥slant 5^\circ . the currently favored standard astrophysical interpretations for these objects are pulsars or low-luminosity globular clusters hosting millisecond pulsars (msps). yet these objects could also be interpreted as dark matter annihilation taking place in ultra-faint dwarf galaxies or dark matter subhalos. unfortunately, fermi lat spectra are not sufficient to break degeneracies between the different scenarios. careful visual inspection of archival optical images reveals no obvious evidence for low-luminosity globular clusters or ultra-faint dwarf galaxies inside the 95% error ellipses. if these are pulsars, this would bring the total number of msps at | b| ≥slant 5^\circ to 106, down to an energy flux ≈4.0 × 10-12 erg cm-2 s-1 between 100 mev and 100 gev. we find this number to be in excellent agreement with predictions from a new population synthesis of msps that predicts 100-126 high-latitude 3fgl msps depending on the choice of high-energy emission model. if, however, these are dark matter substructures, we can place upper limits on the number of galactic subhalos surviving today and on dark matter annihilation cross sections. these limits are beginning to approach the canonical thermal relic cross section for dark matter particle masses below ∼100 gev in the bottom quark (b\bar{b}) annihilation channel. | 3fgl demographics outside the galactic plane using supervised machine learning: pulsar and dark matter subhalo interpretations |
the science cases for incorporating high time resolution capabilities into modern radio telescopes are as numerous as they are compelling. science targets range from exotic sources such as pulsars, to our sun, to recently detected possible extragalactic bursts of radio emission, the so-called fast radio bursts (frbs). originally conceived purely as an imaging telescope, the initial design of the murchison widefield array (mwa) did not include the ability to access high time and frequency resolution voltage data. however, the flexibility of the mwa's software correlator allowed an off-the-shelf solution for adding this capability. this paper describes the system that records the 100 μs and 10 khz resolution voltage data from the mwa. example science applications, where this capability is critical, are presented, as well as accompanying commissioning results from this mode to demonstrate verification. | the high time and frequency resolution capabilities of the murchison widefield array |
the discovery of pulsars as heavy as 2 solar masses has led astrophysicists to rethink the core compositions of neutron stars, ruling out many models for the nuclear equations of state (eos). we explore the hybrid stars that occur when hadronic matter is treated in a relativistic mean-field approximation and quark matter is modeled by three-flavor local and nonlocal nambu-jona-lasinio (njl) models with repulsive vector interactions. the njl models typically yield equations of state that feature a first-order transition to quark matter. assuming that the quark-hadron surface tension is high enough to disfavor mixed phases and restricting to eoss that allow stars to reach 2 solar masses, we find that the appearance of the quark-matter core either destabilizes the star immediately (this is typical for nonlocal njl models) or leads to a very short hybrid star branch in the mass-radius relation (this is typical for local njl models). using the constant-sound-speed parametrization we can see that the reason for the near absence of hybrid stars is that the transition pressure is fairly high and the transition is strongly first order. | constant-sound-speed parametrization for nambu-jona-lasinio models of quark matter in hybrid stars |
small-scale dark matter structure within the milky way is expected to affect pulsar timing. the change in gravitational potential induced by a dark matter halo passing near the line of sight to a pulsar would produce a varying delay in the light travel time of photons from the pulsar. individual transits produce an effect that would either be too rare or too weak to be detected in 30-yr pulsar observations. however, a population of dark matter subhaloes would be expected to produce a detectable effect on the measured properties of pulsars if the subhaloes constitute a significant fraction of the total halo mass. the effect is to increase the dispersion of measured period derivatives across the pulsar population. by statistical analysis of the atnf pulsar catalogue, we place an upper limit on this dispersion of log σ _{dot{p}} ≤ -17.05. we use this to place strong upper limits on the number density of ultracompact minihaloes within the milky way. these limits are completely independent of the particle nature of dark matter. | investigating dark matter substructure with pulsar timing - i. constraints on ultracompact minihaloes |
we report on the hard x-ray burst and the first ∼100 days of nicer monitoring of the soft x-ray temporal and spectral evolution of the newly discovered magnetar swift j1818.0-1607. the burst properties are typical of magnetars with a duration of t90 = 10 ± 4 ms and a temperature of kt = 8.4 ± 0.7 kev. the 2-8 kev pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. the nicer observations reveal strong timing noise with $\dot{\nu }$ varying erratically by a factor of 10, with an average long-term spin-down rate of $\dot{\nu }=(-2.48\pm 0.03)\times {10}^{-11}$ s-2, implying an equatorial surface magnetic field of 2.5 × 1014 g and a young characteristic age of ∼470 yr. we detect a large spin-up glitch at mjd 58928.56 followed by a candidate spin-down glitch at mjd 58934.81, with no accompanying flux enhancements. the persistent soft x-ray spectrum of swift j1818.0-1607 can be modeled as an absorbed blackbody with a temperature of ∼1 kev. its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the nicer observing campaign. this decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. assuming a distance of 6.5 kpc, we measure a peak x-ray luminosity of 1.9 × 1035 erg s-1, lower than its spin-down luminosity of 7.2 × 1035 erg s-1. its quiescent thermal luminosity is ≲1.7 × 1034 erg s-1, lower than those of canonical young magnetars. we conclude that swift j1818.0-1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars. | nicer observation of the temporal and spectral evolution of swift j1818.0-1607: a missing link between magnetars and rotation-powered pulsars |
aims: to estimate the compactness of the thermally-emitting isolated neutron star (ins) rx j0720.4-3125, an x-ray spin-phase-resolved spectroscopic study is conducted. in addition, to identify the genuine spin-period, an x-ray timing analysis is performed.methods: the data from all observations of rx j0720.4-3125 conducted by xmm-newton epic-pn with the same instrumental setup in 2000-2012 were reprocessed to form a homogenous dataset of solar barycenter corrected photon arrival times registered from rx j0720.4-3125. a bayesian method for the search, detection, and estimation of the parameters of an unknown-shaped periodic signal was employed. a number of single- and double-peaked complex models of light curves from pulsating neutron stars were statistically analyzed. the distribution of phases for the registered photons was calculated by folding the arrival times with the derived spin-period and the resulting distribution of phases, which was approximated with a mixed von mises distribution, and its parameters were estimated by using the expected maximization method. spin-phase-resolved spectra were extracted, a number of highly magnetized atmosphere models of an ins were used to perform simultaneous fits, and the results were verified via an markov chain monte carlo approach.results: the phase-folded light curves in different energy bands with high signal-to-noise ratio show high complexity and variations that depend on time and energy. they can be parameterized with a mixed von mises distribution, meaning with a double-peaked light curve profile that shows a dependence of the estimated parameters, such as the mean directions, concentrations, and proportion upon the energy band, indicating that radiation emerges from at least two emitting areas.conclusions: we derive a most-likely genuine spin-period of the isolated neutron star rx j0720-3125 that is twice that reported in the literature, 16.78 s instead of 8.39 s. we determine the gravitational redshift of rx j0720.4-3125 to be and estimate the compactness to be (m/m⊙)/(r/ km) = 0.105 ± 0.002. based on observations obtained with xmm-newton, an esa science mission with instruments and contributions directly funded by esa member states and the usa (nasa). | the compactness of the isolated neutron star rx j0720.4-3125 |
we report nondetections of the ∼3 × 108 yr old, slow, isolated, rotation-powered pulsar psr j2144-3933 in observations with the hubble space telescope in one optical band (f475x) and two far-ultraviolet bands (f125lp and f140lp), yielding upper bounds {f}{{f}475{{x}}}< 22.7 {njy}, {f}{{f}125{lp}}< 5.9 {njy}, and {f}{{f}140{lp}}< 19.5 {njy}, at the pivot wavelengths 4940 å, 1438 å and 1528 å, respectively. assuming a blackbody spectrum, we deduce a conservative upper bound on the surface (unredshifted) temperature of the pulsar of t< 42,000 k. this makes psr j2144-3933 the coldest known neutron star, allowing us to study thermal evolution models of old neutron stars. this temperature is consistent with models with either direct or modified urca reactions including rotochemical heating, and, considering frictional heating from the motion of neutron vortex lines, it puts an upper bound on the excess angular momentum in the neutron superfluid, j < 1044 erg s. based on observations made with the nasa/esa hubble space telescope, obtained at the space telescope science institute, which is operated by the association of universities for research in astronomy, inc., under nasa contract nas 5-26555. these observations are associated with program #13783. | hubble space telescope nondetection of psr j2144-3933: the coldest known neutron star |
the discovery and study of pulsars and fast radio bursts (frbs) in time-domain radio data is often hampered by radio frequency interference (rfi). some of this terrestrial rfi is impulsive and bright, and relatively easy to identify and remove. other anthropogenic signals, however, are weaker yet periodic, and their persistence can drown out astrophysical signals. here we show that fourier-domain excision of periodic rfi is an effective and powerful step in detecting weak cosmic signals. we find that applying the method significantly increases the signal-to-noise ratio of transient and periodic pulsar signals. in live studies we detected single pulses from pulsars and frbs that would otherwise have remained buried in background noise. we show the method has no negative effects on pulsar pulse shape, and that it enhances timing campaigns. we demonstrate the method on real-life data from a number of large radio telescopes, and conclude that fourier-domain rfi excision increases the effective sensitivity to astrophysical sources by a significant fraction, which can be even larger than an order of magnitude in the case of strong rfi. an accelerated implementation of the method runs on standard time-domain radio data formats and is publicly available. | fourier domain excision of periodic radio frequency interference |
context. high-precision pulsar timing requires accurate corrections for dispersive delays of radio waves, parametrized by the dispersion measure (dm), particularly if these delays are variable in time. in a previous paper, we studied the solar wind (sw) models used in pulsar timing to mitigate the excess of dm that is annually induced by the sw and found these to be insufficient for high-precision pulsar timing. here we analyze additional pulsar datasets to further investigate which aspects of the sw models currently used in pulsar timing can be readily improved, and at what levels of timing precision sw mitigation is possible.aims: our goals are to verify: (a) whether the data are better described by a spherical model of the sw with a time-variable amplitude, rather than a time-invariant one as suggested in literature, and (b) whether a temporal trend of such a model's amplitudes can be detected.methods: we use the pulsar timing technique on low-frequency pulsar observations to estimate the dm and quantify how this value changes as the earth moves around the sun. specifically, we monitor the dm in weekly to monthly observations of 14 pulsars taken with parts of the low-frequency array (lofar) across time spans of up to 6 years. we develop an informed algorithm to separate the interstellar variations in the dm from those caused by the sw and demonstrate the functionality of this algorithm with extensive simulations. assuming a spherically symmetric model for the sw density, we derive the amplitude of this model for each year of observations.results: we show that a spherical model with a time-variable amplitude models the observations better than a spherical model with a constant amplitude, but that both approaches leave significant sw-induced delays uncorrected in a number of pulsars in the sample. the amplitude of the spherical model is found to be variable in time, as opposed to what has been previously suggested. | the impact of solar wind variability on pulsar timing |
the major programme for observing young, non-recycled pulsars with the parkes telescope has transitioned from a narrow-band system to an ultra-wide-band system capable of observing between 704 and 4032 mhz. we report here on the initial 2 yr of observations with this receiver. results include dispersion measure (dm) and faraday rotation measure (rm) variability with time, determined with higher precision than hitherto, flux density measurements and the discovery of several nulling and mode changing pulsars. psr j1703-4851 is shown to be one of a small subclass of pulsars that has a weak and a strong mode which alternate rapidly in time. psr j1114-6100 has the fourth highest |rm| of any known pulsar despite its location far from the galactic centre. psr j1825-1446 shows variations in both dm and rm likely due to its motion behind a foreground supernova remnant. | two years of pulsar observations with the ultra-wide-band receiver on the parkes radio telescope |
slightly more than 30 years ago, whipple detection of the crab nebula was the start of very high energy gamma-ray astronomy. since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 pev. in this article we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator. | the crab pulsar and nebula as seen in gamma-rays |
it has been shown that the synergy of a scalar field coupling with both the ricci scalar and the gauss-bonnet invariant significantly affects the properties of scalarized black holes and neutron stars, including their domain of existence and the amount of scalar hair they carry. here we study the radial stability of scalarized black-hole solutions. we demonstrate that they are stable against radial perturbations for ricci couplings consistent with both a late-time cosmological attractor and the evasion of binary pulsar constraints. in addition, we investigate the effect of the ricci coupling on the hyperbolicity of the equation governing linear, radial perturbations and show that it significantly reduces the region over which hyperbolicity is lost. | stable spontaneously-scalarized black holes in generalized scalar-tensor theories |
the observable population of double neutron star (dns) systems in the milky way allow us to understand the nature of supernovae and binary stellar evolution. until now, all dns systems in wide orbits (porb > 1 d) have been found to have orbital eccentricities, e > 0.1. in this paper, we report the discovery of pulsar psr j1325-6253: a dns system in a 1.81-d orbit with a surprisingly low eccentricity of just e = 0.064. through, 1.4 yr of dedicated timing with the parkes radio telescope we have been able to measure its rate of advance of periastron, $\dot{\omega }=0{_{.}^{\circ}}138\pm 0{_{.}^{\circ}}002\, \mathrm{yr}^{-1}$. if this induced $\dot{\omega }$ is solely due to general relativity then the total mass of the system is, msys = 2.57 ± 0.06 m⊙. assuming an edge-on orbit the minimum companion mass is constrained to be mc, min > 0.98 m⊙ which implies the pulsar mass is mp, max < 1.59 m⊙. its location in the p-$\dot{p}$ diagram suggests that, like other dns systems, psr j1325-6253 is a recycled pulsar and if its mass is similar to the known examples (>1.3 m⊙), then the companion neutron star is probably less than ~1.25 m⊙ and the system is inclined at about 50°-60°. the low eccentricity along with the wide orbit of the system strongly favours a formation scenario involving an ultra-stripped supernova explosion. | the high time resolution universe pulsar survey - xvii. psr j1325-6253, a low eccentricity double neutron star system from an ultra-stripped supernova |
the radio and gamma-ray pulsar psr j2032+4127 was recently found to be in a decades-long orbit with the be star mt91 213, with the pulsar moving rapidly towards periastron. this binary shares many similar characteristics with the previously unique binary system psr b1259-63/ls 2883. here, we describe radio, x-ray, and optical monitoring of psr j2032+4127/mt91 213. our extended orbital phase coverage in radio, supplemented with fermi lat gamma-ray data, allows us to update and refine the orbital period to 45-50 yr and time of periastron passage to 2017 november. we analyse archival and recent chandra and swift observations and show that psr j2032+4127/mt91 213 is now brighter in x-rays by a factor of ∼70 since 2002 and ∼20 since 2010. while the pulsar is still far from periastron, this increase in x-rays is possibly due to collisions between pulsar and be star winds. optical observations of the hα emission line of the be star suggest that the size of its circumstellar disc may be varying by ∼2 over time-scales as short as 1-2 months. multiwavelength monitoring of psr j2032+4127/mt91 213 will continue through periastron passage, and the system should present an interesting test case and comparison to psr b1259-63/ls 2883. | multiwavelength monitoring and x-ray brightening of be x-ray binary psr j2032+4127/mt91 213 on its approach to periastron |
we present a new single-pulse pipeline for the palfa survey to efficiently identify single radio pulses from pulsars, rotating radio transients (rrats), and fast radio bursts (frbs). we conducted a sensitivity analysis of this new pipeline in which many single pulses were injected into palfa data and run through the pipeline. we find that for single pulse widths <5 ms, the sensitivity of our new pipeline is at most a factor of ∼2 less sensitive than theoretically predicted. for pulse widths >10 ms, as the dm decreases, the degradation in sensitivity gets worse and can increase up to a factor of ∼4.5. using this pipeline, we have discovered seven pulsars and two rrats, and identified three candidate rrats and one candidate frb. the confirmed pulsars and rrats have dms ranging from 133 to 386 pc cm-3 and flux densities ranging from 20 to 160 mjy. the pulsar periods range from 0.4 to 2.1 s. we report on candidate frb 141113, which is likely astrophysical and extragalactic, having dm ≃ 400 pc cm-3, which is over the galactic maximum along this line of sight by ∼100-200 pc cm-3. we consider implications for the frb population and show via simulations that if frb 141113 is real and extragalactic, the slope α of the distribution of integral source counts as a function of flux density (n(>s) ∝ s -α ) is 1.4 ± 0.5 (95% confidence range). however, this conclusion is dependent on assumptions that require verification. | palfa single-pulse pipeline: new pulsars, rotating radio transients, and a candidate fast radio burst |
a large fraction of gamma-ray bursts (grbs) displays an x-ray plateau phase within <105 s from the prompt emission, proposed to be powered by the spin-down energy of a rapidly spinning newly born magnetar. in this work we use the properties of the galactic neutron star population to constrain the grb-magnetar scenario. we re-analyze the x-ray plateaus of all swift grbs with known redshift, between 2005 january and 2014 august. from the derived initial magnetic field distribution for the possible magnetars left behind by the grbs, we study the evolution and properties of a simulated grb-magnetar population using numerical simulations of magnetic field evolution, coupled with monte carlo simulations of pulsar population synthesis in our galaxy. we find that if the grb x-ray plateaus are powered by the rotational energy of a newly formed magnetar, the current observational properties of the galactic magnetar population are not compatible with being formed within the grb scenario (regardless of the grb type or rate at z = 0). direct consequences would be that we should allow the existence of magnetars and “super-magnetars” having different progenitors, and that type ib/c sne related to long grbs form systematically neutron stars with higher initial magnetic fields. we put an upper limit of ≤16 “super-magnetars” formed by a grb in our galaxy in the past myr (at 99% c.l.). this limit is somewhat smaller than what is roughly expected from long grb rates, although the very large uncertainties do not allow us to draw strong conclusion in this respect. | constraining the grb-magnetar model by means of the galactic pulsar population |
we present a strong hint of a connection between high-energy γ-ray emitting blazars, very high energy neutrinos, and ultrahigh-energy cosmic rays. we first identify potential hadronic sources by filtering γ-ray emitters in spatial coincidence with the high-energy neutrinos detected by icecube. the neutrino filtered γ-ray emitters are then correlated with the ultrahigh-energy cosmic rays from the pierre auger observatory and the telescope array by scanning in γ-ray flux (fγ) and angular separation (θ) between sources and cosmic rays. a maximal excess of 80 cosmic rays (42.5 expected) is found at θ ≤ 10° from the neutrino-filtered γ-ray emitters selected from the second hard fermi-lat catalogue (2fhl) and for fγ(>50 gev) ≥ 1.8 × 10-11 ph cm-2 s-1. the probability for this to happen is 2.4 × 10-5, which translates to ∼2.4 × 10-3 after compensation for all the considered trials. no excess of cosmic rays is instead observed for the complement sample of γ-ray emitters (i.e. not in spatial connection with icecube neutrinos). a likelihood ratio test comparing the connection between the neutrino-filtered and the complement source samples with the cosmic rays favours a connection between neutrino-filtered emitters and cosmic rays with a probability of ∼1.8 × 10-3 (2.9σ) after compensation for all the considered trials. the neutrino-filtered γ-ray sources that make up the cosmic rays excess are blazars of the high synchrotron peak type. more statistics is needed to further investigate these sources as candidate cosmic ray and neutrino emitters. | connecting blazars with ultrahigh-energy cosmic rays and astrophysical neutrinos |
it has long been suggested that nearby pulsars within ∼1 kpc are the leading candidate of the 10-500 gev cosmic-ray positron excess measured by pamela and other experiments. the recent measurement of the surface brightness profile of tev nebulae surrounding geminga and psr b0656+14 by the high-altitude water cherenkov observatory (hawc) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. here we argue that gev observations provide more direct constraints on the positron density in the tev nebulae in the energy range of 10-500 gev and hence on the origin of the observed positron excess. motivated by this, we search for gev emission from the tev nebulae with the fermi large area telescope (lat). no spatially extended gev emission is detected from these two tev nebulae in the framework of two-zone diffusion spatial templates, suggesting a relatively low density of gev electrons/positrons in the tev nebulae. a joint modeling of the data from hawc and fermi-lat disfavors geminga and psr b0656+14 as the dominant sources of the positron excess at ∼50-500 gev for the usual kolmogorov-type diffusion, while for an energy-independent diffusion, a dominant part of the positron excess contributed by them cannot be ruled out by the current data. | gev observations of the extended pulsar wind nebulae constrain the pulsar interpretations of the cosmic-ray positron excess |
a theoretical framework for emission originating from rapidly rotating oblate compact objects is described in detail. using a hamilton-jacobi formalism, we show that special relativistic rotational effects such as aberration of angles, doppler boosting, and time dilatation naturally emerge from the general relativistic treatment of rotating compact objects. we use the butterworth-ipser metric expanded up to the second order in rotation and hence include effects of light bending, frame-dragging, and quadrupole deviations on our geodesic calculations. we also give detailed descriptions of the numerical algorithms used and provide an open-source implementation of the numerical framework called bender. as an application, we study spectral line profiles (i.e., smearing kernels) from rapidly rotating oblate neutron stars. we find that in this metric description, the second-order quadrupole effects are not strong enough to produce narrow observable features in the spectral energy distribution for almost any physically realistic parameter combination, and hence, actually detecting them is unlikely. the full width at tenth-maximum and full width at half-maximum of the rotation smearing kernels are also reported for all viewing angles. these can then be used to quantitatively estimate the effects of rotational smearing on the observed spectra. we also calculate accurate pulse profiles and observer skymaps of emission from hot spots on rapidly rotating accreting millisecond pulsars. these allow us to quantify the strength of the pulse fractions one expects to observe from typical fast-spinning millisecond pulsars. | radiation from rapidly rotating oblate neutron stars |
modern astrophysics is undergoing a revolution. as detector technology has advanced, and astronomers have been able to study the sky with finer temporal detail, a rich diversity of sources that vary on timescales from years down to a few nanoseconds has been found. among these are fast radio bursts, which exhibit pulses of millisecond duration and anomalously high dispersion compared to galactic pulsars, first seen a decade ago. since then, a new research community has been actively working on a variety of experiments and developing models to explain this new phenomenon, and devising ways to use them as astrophysical tools. in this perspective, i describe how astronomers have reached this point, review the highlights from the first decade of research in this field, give some current breaking news, and look ahead to what might be expected in the next few years. | a decade of fast radio bursts |
it has been suggested that some classes of luminous supernovae (sne) and gamma-ray bursts (grbs) are driven by newborn magnetars. fast-rotating proto-neutron stars have also been of interest as potential sources of gravitational waves (gws). we show that for a range of rotation periods and magnetic fields, hard x-rays and gev gamma rays provide us with a promising probe of pulsar-aided sne. it is observationally known that young pulsar wind nebulae (pwne) in the milky way are very efficient lepton accelerators. we argue that, if embryonic pwne satisfy similar conditions at early stages of sne (in ∼1-10 months after the explosion), external inverse-compton emission via upscatterings of sn photons is naturally expected in the gev range as well as broadband synchrotron emission. to fully take into account the klein-nishina effect and two-photon annihilation process that are important at early times, we perform detailed calculations including electromagnetic cascades. our results suggest that hard x-ray telescopes such as nustar can observe such early pwn emission by follow-up observations in months to years. gev gamma-rays may also be detected by fermi for nearby sne, which serve as counterparts of these gw sources. detecting the signals will give us an interesting probe of particle acceleration at early times of pwne, as well as clues to driving mechanisms of luminous sne and grbs. since the bethe-heitler cross section is lower than the thomson cross section, gamma rays would allow us to study subphotospheric dissipation. we encourage searches for high-energy emission from nearby sne, especially sne ibc including super-luminous objects. | gammy-ray and hard x-ray emission from pulsar-aided supernovae as a probe of particle acceleration in embryonic pulsar wind nebulae |
we report on the first millisecond timescale radio interferometric search for the new class of transient known as fast radio bursts (frbs). we used the very large array (vla) for a 166 hr, millisecond imaging campaign to detect and precisely localize an frb. we observed at 1.4 ghz and produced visibilities with 5 ms time resolution over 256 mhz of bandwidth. dedispersed images were searched for transients with dispersion measures from 0 to 3000 pc cm-3. no transients were detected in observations of high galactic latitude fields taken from 2013 september though 2014 october. observations of a known pulsar show that images typically had a thermal-noise limited sensitivity of 120 mjy beam-1 (8σ ; stokes i) in 5 ms and could detect and localize transients over a wide field of view. our nondetection limits the frb rate to less than 7× {10}4 sky-1 day-1 (95% confidence) above a fluence limit of 1.5 jy ms. the vla rate limit is consistent with past estimates when published flux limits are recalculated with a homogeneous definition that includes effects of primary beam attenuation, dispersion, pulse width, and sky brightness. this calculation revises the frb rate downward by a factor of 2, giving the vla observations a roughly 50% chance of detecting a typical frb, assuming a pulse width of 3 ms. a 95% confidence constraint would require 600 hr of similar vla observing. our survey also limits the repetition rate of an frb to 2 times less than any known repeating millisecond radio transient. | a millisecond interferometric search for fast radio bursts with the very large array |
this paper aims to explore a class of static stellar equilibrium configuration of relativistic charged spheres made of a charged perfect fluid. for solving the einstein-maxwell field equations, we consider a particularized metric potential, buchdahl ansatz [1] and then by using a simple transformation. the study is developed by matching the interior region with riessner-nordström metric as an exterior solution. the matter content the charged sphere satisfies all the energy conditions and hydrostatic equilibrium equation, i.e. the modified tolman-oppenheimer-volkoff (tov) equation for the charged case is maintained. in addition to this, we also discuss some important properties of the charged sphere such as total electric charge, mass-radius relation, surface redshift, and the speed of sound. obtained solutions are presented by the graphical representation that provides strong evidence for a more realistic and viable stellar structure. obtained results are compared with analogue objects with similar mass and radii, such as sax j1808.4-3658, 4u 1538-52, psr j1903+327, vela x-1, and 4u1608-52. it is also noted that the buchdahl ansatz for a given transformation provides a physically viable solution only for the charged case when 0 < k < 1, where density and pressure are maximum at the center and monotonically decreasing towards the boundary. obtained results are also quite important both from theoretical and astrophysical scale to analyze other compact objects such as white dwarfs, neutron stars, boson stars, and quark stars. | relativistic charged spheres: compact stars, compactness and stable configurations |
we report on the discovery of a dramatic change in the energy spectrum of the x-ray pulsar gx 304-1 appearing at low luminosity. particularly, we found that the cut-off power-law spectrum typical for accreting pulsars, including gx 304-1 at higher luminosities of lx ∼ 1036-1037 erg s-1, transformed at lower luminosity of lx ∼ 1034 erg s-1 to a two-component spectrum peaking around 5 and 40 kev. we suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. we argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. the most plausible explanations for the high-energy component are either the cyclotron emission reprocessed by the magnetic compton scattering or the thermal radiation of deep atmospheric layers partly comptonized in the overheated upper layers. alternative scenarios are also discussed. | dramatic spectral transition of x-ray pulsar gx 304-1 in low luminous state |
we discuss the theory of pulsar-timing and astrometry probes of a stochastic gravitational-wave background with a recently developed "total-angular-momentum" (tam) formalism for cosmological perturbations. we review the formalism, emphasizing in particular the features relevant for this work and describe the observables we consider (i.e., the pulsar redshift and stellar angular displacement). using the tam approach, we calculate the angular power spectra for the observables and from them derive angular auto- and cross-correlation functions. we provide the full set of power spectra and correlation functions not only for the standard transverse-traceless propagating degrees of freedom in general relativity, but also for the four additional non-einsteinian polarizations that may arise in alternative-gravity theories. we discuss how pulsar-timing and astrometry surveys can complement and serve as cross checks to one another and comment on the importance of testing the chirality of the gravitational-wave background as a tool to understand the nature of its sources. a simple rederivation of the power spectra from the plane-wave formalism is provided in an appendix. | pulsar-timing arrays, astrometry, and gravitational waves |
context. glitches are rare spin-up events that punctuate the smooth slow-down of the rotation of pulsars. for the vela pulsar and psr j0537-6910, their large glitch sizes and the times between consecutive events have clear preferred scales (gaussian distributions), contrary to the handful of other pulsars with enough glitches for such a study. moreover, psr j0537-6910 is the only pulsar that shows a strong positive correlation between the size of each glitch and the waiting time until the following one.aims: we attempt to understand this behaviour through a detailed study of the distributions and correlations of glitch properties for the eight pulsars with at least ten detected glitches.methods: we modelled the distributions of glitch sizes and of the times between consecutive glitches for the eight pulsars with at least ten detected events. we also looked for possible correlations between these parameters and used monte carlo simulations to explore two hypotheses that could explain why the correlation so clearly seen in psr j0537-6910 is absent in other pulsars.results: we confirm the above results for vela and psr j0537-6910, and verify that the latter is the only pulsar with a strong correlation between glitch size and waiting time to the following glitch. for the remaining six pulsars, the waiting time distributions are best fitted by exponentials, and the size distributions are best fitted by either power laws, exponentials, or log-normal functions. some pulsars in the sample yield significant pearson and spearman coefficients (rp and rs) for the aforementioned correlation, confirming previous results. moreover, for all except the crab pulsar, both coefficients are positive. for each coefficient taken separately, the probability of this happening is 1/16. our simulations show that the weaker correlations in pulsars other than psr j0537-6910 cannot be due to missing glitches that are too small to be detected. we also tested the hypothesis that each pulsar may have two kinds of glitches, namely large, correlated ones and small, uncorrelated ones. the best results are obtained for the vela pulsar, which exhibits a correlation with rp = 0.68 (p-value = 0.003) if its two smallest glitches are removed. the other pulsars are harder to accommodate under this hypothesis, but their glitches are not consistent with a pure uncorrelated population either. we also find that all pulsars in our sample, except the crab pulsar, are consistent with the previously found constant ratio between glitch activity and spin-down rate, ν˙g/|ν˙| = 0.010±0.001, even though some of them have not shown any large glitches.conclusions: to explain these results, we speculate except in the case of the crab pulsar, that all glitches draw their angular momentum from a common reservoir (presumably a neutron superfluid component containing ≈1% of the star's moment of inertia). however, two different trigger mechanisms could be active, a more deterministic one for larger glitches and a more random one for smaller ones. | glitch time series and size distributions in eight prolific pulsars |
we provide a bird's-eye view of neutron-star seismology, which aims to probe the extreme physics associated with these objects, in the context of gravitational-wave astronomy. focussing on the fundamental mode of oscillation, which is an efficient gravitational-wave emitter, we consider the seismology aspects of a number of astrophysically relevant scenarios, ranging from transients (like pulsar glitches and magnetar flares), to the dynamics of tides in inspiralling compact binaries and the eventual merged object and instabilities acting in isolated, rapidly rotating, neutron stars. the aim is not to provide a thorough review, but rather to introduce (some of) the key ideas and highlight issues that need further attention. | a gravitational-wave perspective on neutron-star seismology |
the core rotation rates of massive stars have a substantial impact on the nature of core-collapse (cc) supernovae and their compact remnants. we demonstrate that internal gravity waves (igws), excited via envelope convection during a red supergiant phase or during vigorous late time burning phases, can have a significant impact on the rotation rate of the pre-sn core. in typical (10 {m}⊙ ≲ m≲ 20 {m}⊙ ) supernova progenitors, igws may substantially spin down the core, leading to iron core rotation periods {p}{min,{fe}}≳ 30 {{s}}. angular momentum (am) conservation during the supernova would entail minimum ns rotation periods of {p}{min,{ns}}≳ 3 {ms}. in most cases, the combined effects of magnetic torques and igw am transport likely lead to substantially longer rotation periods. however, the stochastic influx of am delivered by igws during shell burning phases inevitably spin up a slowly rotating stellar core, leading to a maximum possible core rotation period. we estimate maximum iron core rotation periods of {p}{max,{fe}}≲ 5× {10}3 {{s}} in typical cc supernova progenitors, and a corresponding spin period of {p}{max,{ns}}≲ 500 {ms} for newborn neutron stars (nss). this is comparable to the typical birth spin periods of most radio pulsars. stochastic spin-up via igws during shell o/si burning may thus determine the initial rotation rate of most nss. for a given progenitor, this theory predicts a maxwellian distribution in pre-collapse core rotation frequency that is uncorrelated with the spin of the overlying envelope. | the spin rate of pre-collapse stellar cores: wave-driven angular momentum transport in massive stars |
we present the results from a monitoring campaign made with the neil gehrels swift observatory of the m51 galaxies, which contain several variable ultraluminous x-ray sources (ulxs). the ongoing campaign started in 2018 may, and we report here on ∼1.5 yr of observations. the campaign, which consists of 106 observations, has a typical cadence of 3-6 days, and has the goal of determining the long-term x-ray variability of the ulxs. two of the most variable sources were ulx7 and ulx8, both of which are known to be powered by neutron stars that are exceeding their isotropic eddington luminosities by factors of up to 100. this is further evidence that neutron-star-powered ulxs are the most variable. our two main results are, first, that ulx7 exhibits a periodic flux modulation with a period of 38 days varying over a magnitude and a half in flux from peak to trough. since the orbital period of the system is known to be 2 days, the modulation is superorbital, which is a near-ubiquitous property of ulx pulsars. second, we identify a new transient ulx, m51 xt-1, the onset of which occurred during our campaign, reaching a peak luminosity of ∼1040 erg s-1, before gradually fading over the next ∼200 days until it slipped below the detection limit of our observations. combined with the high-quality swift/x-ray telescope lightcurve of the transient, serendipitous observations made with chandra and xmm-newton provide insights into the onset and evolution of a likely super-eddington event. | swift monitoring of m51: a 38 day superorbital period for the pulsar ulx7 and a new transient ultraluminous x-ray source |
studying how the black hole (bh) - (galaxy) bulge mass relation evolves with redshift provides valuable insights into the co-evolution of supermassive black holes and their host galaxies. however, obtaining accurate measurement of bh masses is challenging due to the bias towards the most massive and luminous galaxies. we use an analytical astrophysical model with galaxy stellar mass function, pair fraction, merger timescale and bh-bulge mass relation extended to include redshift evolution. the model can predict the intensity of the gravitational wave background produced by a population of supermassive black hole binary (smbhb) as a function of the frequency. we focus on the bh-bulge mass relation and its variation with redshift using the eagle, illustris, tng100, tng300, horizon-agn and simba large-scale cosmological simulations. by understanding the processes and relationships concerning the formation and co-evolution of galaxies and their central bhs we can make theoretical and analytical expressions in order to refine current astrophysical models. this allows us to compare the predictions of this model with the constraints of pulsar timing array observations. here, we employ bayesian analysis for the parameter inference. by fitting the bh-bulge mass parameters to the illustris and simba simulations we analyze the changes in the constraints on the other astrophysical parameters. furthermore, we also examine the variation in smbhb merger rate with mass and redshift between these large-scale simulations. | mass-redshift dependency of supermassive black hole binaries for the gravitational wave background |
at the linear level, the gravitational wave (gw) spectrum predicted by inflation, and many of its alternatives, can have arbitrarily small amplitude and consequently an unconstrained tilt. however, at second order, tensor fluctuations are sourced by scalar fluctuations that have been measured in the cosmic microwave background (cmb). these second order fluctuations generically produce a minimum amount of tensor perturbations corresponding to a tensor-to-scalar ratio of r~ 10-6. inverting this relationship yields a bound on the tensor tilt sourced by scalar fluctuations. since this induced gw spectrum depends on the scalar spectrum, we derive a new indirect bound that involves all scales of the scalar spectrum based on cmb observations. this bound comes from the constraint on the number of effective relativistic degrees of freedom, neff. we estimate the bound using current data, and the improvements expected by future cmb experiment. the bound forces the running and running of running to conform to standard slow-roll predictions of α,β lesssim (ns-1)2 where α≡ d ns/d ln k and β ≡ d ns2/d ln k2, improving on current cmb measurements by an order of magnitude. this bound has further implications for the possibility of primordial black holes as dark matter candidates. performing a likelihood analysis including this new constraint, we find that positive α and/or β are disfavored at least at 1σ. even using conservative analysis, β + 0.074 α>8.6× 10-4 are ruled out at 3σ. finally, using bounds on the fractional energy density of gravitational waves today obtained by ligo and the pulsar timing array, we obtain a bound on the primordial scalar spectrum on these scales and give forecast for future measurements. | constraints on scalar and tensor spectra from neff |
millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. however, the relative contributions of two separate emission processes - curvature radiation from millisecond pulsar magnetospheres versus inverse compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars - have long been unclear. to address this, we search for evidence of inverse compton emission in 8-yr fermi-lat data from the directions of 157 milky way globular clusters. we find a mildly statistically significant (3.8σ) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. however, this may also be explained by a hidden correlation between the photon field densities and the stellar encounter rates of globular clusters. analysed in toto, we demonstrate that the gamma-ray emission of globular clusters can be resolved spectrally into two components: (i) an exponentially cut-off power law and (ii) a pure power law. the latter component - which we uncover at a significance of 8.2σ - has a power index of 2.79 ± 0.25. it is most naturally interpreted as inverse compton emission by cosmic-ray electrons and positrons injected by millisecond pulsars. we find the luminosity of this power-law component is comparable to, or slightly smaller than, the luminosity of the curved component, suggesting the fraction of millisecond pulsar spin-down luminosity into relativistic leptons is similar to the fraction of the spin-down luminosity into prompt magnetospheric radiation. | evidence for a high-energy tail in the gamma-ray spectra of globular clusters |
to make best use of multifaceted astronomical and nuclear data sets, probability distributions of neutron star models that can be used to propagate errors consistently from one domain to another are required. we take steps toward a consistent model for this purpose, highlight where model inconsistencies occur, and assess the resulting model uncertainty. using two distributions of nuclear symmetry energy parameters-one uniform, the other based on pure neutron matter theory-we prepare two ensembles of neutron star inner crust models. we use an extended skyrme energy density functional within a compressible liquid drop model (cldm). we fit the surface parameters of the cldm to quantum 3d hartree-fock calculations of crustal nuclei. all models predict that more than 50% of the crust by mass and 15% of the crust by thickness comprises pasta with medians of around 62% and 30%, respectively. we also present 68% and 95% ranges for the crust composition as a function of density. we examine the relationships between crust-core boundary and pasta transition properties, the thickness of the pasta layers, the symmetry energy at saturation and subsaturation densities, and the neutron skins of 208pb and 48ca. we quantify the correlations using the maximal information coefficient, which can effectively characterize nonlinear relationships. future measurements of neutron skins, information from nuclear masses and giant resonances, and theoretical constraints on pnm will be able to place constraints on the location of the pasta and crust-core boundaries and the amount of pasta in the crust. | prior probability distributions of neutron star crust models |
we made a detailed study of the timing and spectral properties of the x-ray pulsar 1a 0535+262 during the recent giant outburst in 2020 november and december. the flux of the pulsar reached a record value of ~12.5 crab as observed by swift/bat (15-50 kev) and the corresponding mass accretion rate was ~6.67 × 1017 g s-1 near the peak of the outburst. there was a transition from the subcritical to the supercritical accretion regime which allows exploring different properties of the source in the supercritical regime. a q-like feature was detected in the hardness-intensity diagram during the outburst. we observed high variability and strong energy dependence of pulse profiles during the outburst. cyclotron resonant scattering feature (crsf) was detected at ~44 kev from the nustar energy spectrum in the subcritical regime and the corresponding magnetic field was b ≃ 4.9 × 1012 g. the energy of the crsf was shifted towards lower energy in the supercritical regime. the luminosity dependence of the crsf was studied and during the supercritical regime, a negative correlation was observed between the line energy and luminosity. the critical luminosity was ~6 × 1037erg s-1 above which a state transition occurred. a reversal of correlation between the photon index and luminosity was observed near the critical luminosity. the nustar spectra can be described by a composite model with two continuum components, a blackbody emission, cut-off power law, and a discrete component to account for the iron emission line at 6.4 kev. an additional cyclotron absorption feature was included in the model. | study of timing and spectral properties of the x-ray pulsar 1a 0535+262 during the giant outburst in 2020 november-december |
in this work, we present polarization profiles for 23 millisecond pulsars observed at 820 and 1500 mhz with the green bank telescope as part of the nanograv pulsar timing array. we calibrate the data using mueller matrix solutions calculated from observations of psrs b1929+10 and j1022+1001. we discuss the polarization profiles, which can be used to constrain pulsar emission geometry, and present both the first published radio polarization profiles for nine pulsars and the discovery of very low-intensity average profile components ("microcomponents") in four pulsars. we obtain the faraday rotation measures for each pulsar and use them to calculate the galactic magnetic field parallel to the line of sight for different lines of sight through the interstellar medium. we fit for linear and sinusoidal trends in time in the dispersion measure and galactic magnetic field and detect magnetic field variations with a period of 1 yr in some pulsars, but overall find that the variations in these parameters are more consistent with a stochastic origin. | the nanograv 12.5 yr data set: polarimetry and faraday rotation measures from observations of millisecond pulsars with the green bank telescope |
we present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. we used the second data release of the 144 mhz lofar two-metre sky survey to produce circularly polarised maps with a median noise of 140 µjy beam−1 and resolution of 20″ for ≈27% of the northern sky (5634 deg2). the leakage of total intensity into circular polarisation is measured to be ≈0.06%, and our survey is complete at flux densities ≥1 mjy. a detection is considered reliable when the circularly polarised fraction exceeds 1%. we find the population of circularly polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. the stellar systems can be further separated into chromospherically active stars, m dwarfs, and brown dwarfs. based on the circularly polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. by the completion of this survey of the northern sky, we expect to detect 300±100 circularly polarised sources. v-lotss catalogue is only available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/670/a124 | v-lotss: the circularly polarised lofar two-metre sky survey |
the latest pulsar timing array data reveal evidence of nanohertz gravitational waves (gws), which have been explained by both cosmological and astrophysical sources. however, current observations lack the precision needed to differentiate between different models from the spectral index. we find that the cosmological gw sources, including bubble collisions, sound waves, domain walls, condensate fragmentations, and primordial curvature perturbations, induce large energy density perturbations so that most dark matter will exist in gravitationally self-bound subhalos. then, the observation of such substructures of dark matter can serve as a novel independent method to confirm or exclude the cosmological gw sources. | distinguishing the nanohertz gravitational-wave sources by the observations of compact dark matter subhalos |
pulsar-timing arrays (ptas) are in the near future expected to detect a stochastic gravitational-wave background (sgwb) produced by a population of inspiralling supermassive black hole binaries. in this work, we consider a background that can be anisotropic and circularly polarized. we use the expansion of the intensity and the circular polarization in terms of spherical harmonics and the overlap reduction functions for each term in this expansion. we propose an unbiased real-space estimator that can separate the intensity and circular-polarization contributions of the sgwb to pulsar-timing-residual correlations and then validate the estimator on simulated data. we compute the signal-to-noise ratio of a circular-polarization component that has a dipole pattern under different assumptions about the pta. we find that a nearly maximal circular-polarization dipole may be detectable, which can aid in determining whether or not the background is dominated by a handful of bright sources. | pulsar-timing measurement of the circular polarization of the stochastic gravitational-wave background |
the latest fermi-lat gamma-ray catalogue, 4fgl-dr3, presents a large fraction of sources without clear association to known counterparts, i.e. unidentified sources (unids). in this paper, we aim to classify them using machine learning algorithms, which are trained with the spectral characteristics of associated sources to predict the class of the unid population. with the state-of-the-art catboost algorithm, based on gradient boosting decision trees, we are able to reach a 67 per cent accuracy on a 23-class data set. removing a single of these classes - blazars of uncertain type - increases the accuracy to 81 per cent. if interested only in a binary agn/pulsar distinction, the model accuracy is boosted up to 99 per cent. additionally, we perform an unsupervised search among both known and unid population, and try to predict the number of clusters of similar sources, without prior knowledge of their classes. the full code used to perform all calculations is provided as an interactive python notebook. | classification of fermi-lat unidentified gamma-ray sources using catboost gradient boosting decision trees |
we present a spectral analysis of nustar and nicer observations of the luminous, persistently accreting neutron star (ns) low-mass x-ray binary cygnus x-2. the data were divided into different branches that the source traces out on the z-track of the x-ray color-color diagram; namely, the horizontal branch, the normal branch, and the vertex between the two. the x-ray continuum spectrum was modeled in two different ways that produced comparable quality fits. the spectra showed clear evidence of a reflection component in the form of a broadened fe k line, as well as a lower-energy emission feature near 1 kev likely due to an ionized plasma located far from the innermost accretion disk. we account for the reflection spectrum with two independent models (relxillns and rdblur*rfxconv). the inferred inclination is in agreement with earlier estimates from optical observations of ellipsoidal lightcurve modeling (relxillns: i = 67° ± 4°; rdblur*rfxconv: i = 60° ± 10°). the inner disk radius remains close to the ns (r in ≤ 1.15 r isco) regardless of the source position along the z-track or how the 1 kev feature is modeled. given the optically determined ns mass of 1.71 ± 0.21 m ⊙, this corresponds to a conservative upper limit of r in ≤ 19.5 km for m = 1.92 m ⊙ or r in ≤ 15.3 km for m = 1.5 m ⊙. we compare these radius constraints to those obtained from ns gravitational wave merger events and recent nicer pulsar lightcurve modeling measurements. | radius constraints from reflection modeling of cygnus x-2 with nustar and nicer |
we show that a small but measurable shift in the eclipse midpoint time of eclipsing binary (ebs) stars of ~0.1 s over a decade baseline can be used to directly measure the galactic acceleration of stars in the milky way at ~kiloparsec distances from the sun. we consider contributions to the period drift rate from dynamical mechanisms other than the galaxy's gravitational field and show that the galactic acceleration can be reliably measured using a sample of kepler ebs with orbital and stellar parameters from the literature. the contribution from tidal decay we estimate here is an upper limit assuming the stars are not tidally synchronized. we find there are about 200 detached ebs that have estimated timing precision better than 0.5 s, and for which other dynamical effects are subdominant to the galactic signal. we illustrate the method with a prototypical, precisely timed eb using an archival kepler light curve and a modern synthetic hst light curve (which provides a decade baseline). this novel method establishes a realistic possibility to constrain dark matter substructure and the galactic potential using eclipse timing to measure galactic accelerations, along with other emerging new methods, including pulsar timing and extreme-precision radial velocity observations. this acceleration signal grows quadratically with time. therefore, given baselines established in the near future for distant ebs, we can expect to measure the period drift in the future with space missions like jwst and the roman space telescope. | eclipse timing the milky way's gravitational potential |
a deep survey of the large magellanic cloud at ~0.1-100tev photon energies with the cherenkov telescope array is planned. we assess the detection prospects based on a model for the emission of the galaxy, comprising the four known tev emitters, mock populations of sources, and interstellar emission on galactic scales. we also assess the detectability of 30 doradus and sn 1987a, and the constraints that can be derived on the nature of dark matter. the survey will allow for fine spectral studies of n157b, n132d, lmc p3, and 30 doradus c, and half a dozen other sources should be revealed, mainly pulsar-powered objects. the remnant from sn 1987a could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3-2.4 pending a flux increase by a factor >3-4 over ~2015-2035. large-scale interstellar emission remains mostly out of reach of the survey if its >10gev spectrum has a soft photon index ~2.7, but degree-scale 0.1-10tev pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100gev. the 30 doradus star-forming region is detectable if acceleration efficiency is on the order of 1-10% of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100pc. finally, the survey could probe the canonical velocity-averaged cross section for self-annihilation of weakly interacting massive particles for cuspy navarro-frenk-white profiles. | sensitivity of the cherenkov telescope array to tev photon emission from the large magellanic cloud |
we show via both analytical calculation and numerical simulation that the optimal cross-correlation statistic (os) for stochastic gravitational-wave-background (gwb) searches using data from pulsar timing arrays follows a generalized chi-squared (gx2) distribution—i.e., a linear combination of chi-squared distributions with coefficients given by the eigenvalues of the quadratic form defining the statistic. this observation is particularly important for calculating the frequentist statistical significance of a possible gwb detection, which depends on the exact form of the distribution of the os signal-to-noise ratio ρ ^≡a^gw2/σ0 in the absence of gw-induced cross correlations (i.e., the null distribution). previous discussions of the os have incorrectly assumed that the analytic null distribution of ρ ^ is well approximated by a zero-mean unit-variance gaussian distribution. empirical calculations show that the null distribution of ρ ^ has "tails" which differ significantly from those for a gaussian distribution but which follow (exactly) a gx2 distribution. thus, a correct analytical assessment of the statistical significance of a potential detection requires the use of a gx2 distribution. | analytic distribution of the optimal cross-correlation statistic for stochastic gravitational-wave-background searches using pulsar timing arrays |
pulsar timing provides a sensitive probe of small-scale structure. gravitational perturbations arising from an inhomogeneous environment could manifest as detectable perturbations in the pulsation phase. consequently, pulsar timing arrays have been proposed as a probe of dark matter substructure on mass scales as small as 10-11 m⊙ . since the small-scale mass distribution is connected to early-universe physics, pulsar timing can therefore constrain the thermal history prior to big bang nucleosynthesis (bbn), a period that remains largely unprobed. we explore here the prospects for pulsar timing arrays to detect the dark substructure imprinted by a period of early matter domination (emd) prior to bbn. emd amplifies density variations, leading to a population of highly dense sub-earth-mass dark matter microhalos. we use recently developed semianalytic models to characterize the distribution of emd-induced microhalos, and we evaluate the extent to which the pulsar timing distortions caused by these microhalos can be detected. broadly, we find that sub-0.1 -μ s timing noise residuals are necessary to probe emd. however, with 10-ns residuals, a pulsar timing array with just 70 pulsars could detect the evidence of an emd epoch with 20 years of observation time if the reheat temperature is of order 10 mev. with 40 years of observation time, pulsar timing arrays could probe emd reheat temperatures as high as 150 mev. | dark matter microhalos in the solar neighborhood: pulsar timing signatures of early matter domination |
decades long monitoring of millisecond pulsars, which exhibit highly stable rotational periods in pulsar timing array experiments is on the threshold of discovering nanohertz stochastic gravitational wave background. this paper describes the indian pulsar timing array (inpta) experiment, which employs the upgraded giant metrewave radio telescope (ugmrt) for timing an ensemble of millisecond pulsars for this purpose. we highlight inpta's observation strategies and analysis methods, which are relevant for a future pta experiment with the more sensitive square kilometer array (ska) telescope. we show that the unique multi-sub-array multi-band wide-bandwidth frequency coverage of the inpta, provides dispersion measure estimates with unprecedented precision for pta pulsars, e.g., ∼2 ×10-5 pc cm-3 for psr j1909-3744. configuring the ska-low and ska-mid as two and four sub-arrays, respectively, it is shown that comparable precision is achievable, using observation strategies similar to those pursued by the inpta, for a larger sample of 62 pulsars, requiring about 26 and 7 h per epoch for the ska-mid and the ska-low telescopes, respectively. we also review the ongoing efforts to develop pta-relevant general relativistic constructs that will be required to search for nanohertz gravitational waves from isolated super-massive black hole binary systems like blazar oj 287. these efforts should be relevant to pursue persistent multi-messenger gravitational wave astronomy during the forthcoming era of the ska telescope, the thirty meter telescope, and the next-generation event horizon telescope. | nanohertz gravitational wave astronomy during ska era: an inpta perspective |
we extract new classes of anisotropic solutions in the framework of mimetic gravity, by applying the tolman-finch-skea metric and a specific anisotropy not directly depending on it, and by matching smoothly the interior anisotropic solution to the schwarzschild exterior one. then, in order to provide a transparent picture we use the data from the 4u 1608-52 pulsar. we study the profile of the energy density, as well as the radial and tangential pressures, and we show that they are all positive and decrease toward the center of the star. furthermore, we investigate the anisotropy parameter and the anisotropic force that are both increasing functions of the radius, which implies that the latter is repulsive. additionally, by examining the radial and tangential equation-of-state parameters, we show that they are monotonically increasing, not corresponding to exotic matter. concerning the metric potentials, we find that they have no singularity, either at the center of the star or at the boundary. furthermore, we verify that all energy conditions are satisfied, we show that the radial and tangential sound speed squares are positive and sub-luminal, and we find that the surface redshift satisfies the theoretical requirement. finally, in order to investigate the stability we apply the tolman-oppenheimer-volkoff equation, we perform the adiabatic index analysis, and we examine the static case, showing that in all cases the star is stable. | new anisotropic star solutions in mimetic gravity |
periodic modulations are seen in normal pulsars (p > 0.1 s) over timescales ranging from a few seconds to several minutes. such modulations have usually been associated with the phenomenon of subpulse drifting. a number of recent studies have shown subpulse drifting to exhibit very specific physical characteristics: (i) drifting is seen only in conal components of the pulse profile and is absent in central core emission; (ii) drifting pulsars are distributed over a narrow range of spin-down energy loss ( $\dot{e}$ ), where pulsars with $\dot{e}\,\lt $ 2 × 1032 erg s-1 show this behavior; and (iii) drifting periodicity (p3) is anti-correlated with $\dot{e}$ , such that pulsars with lower values of $\dot{e}$ tend to have longer p3. these detailed characterizations of drifting behavior, on the other hand, also revealed the presence of other distinct periodic modulations, which can be broadly categorized into two types, periodic nulling, and periodic amplitude modulation. in contrast to drifting, these periodic phenomena are seen across the entire profile in both the core and conal components simultaneously and are not restricted to any specific $\dot{e}$ range. in this work we have assembled an exhaustive list of around 70 pulsars that show such periodic modulations, 22 of which were newly detected using observations from the giant meterwave radio telescope and the remaining compiled from past publications. the presence of such a significant group in the pulsar population suggests periodic modulations to be newly emergent phenomena in pulsars, with a physical origin that is distinct from that of subpulse drifting. | periodic modulation: newly emergent emission behavior in pulsars |
we present statistical analysis of a fluence-limited sample of over 1100 giant pulses from the crab pulsar, with fluence > 130 jy ms at ∼1330 mhz. these were detected in ∼260 h of observation with the national centre for radio astrophysics (ncra) 15 m radio telescope. we find that the pulse-energy distribution follows a power law with index α ≈ -3 at least up to a fluence of ∼5 jy s. the power-law index agrees well with that found for lower-energy pulses in the range 3-30 jy ms. the fluence distribution of the crab pulsar hence appears to follow a single power law over ∼3 orders of magnitude in fluence. we do not see any evidence for the flattening at high fluences reported by earlier studies. we also find that, at these fluence levels, the rate of giant-pulse emission varies by as much as a factor of ∼5 on time-scales of a few days, although the power-law index of the pulse-energy distribution remains unchanged. the slope of the fluence distribution for crab giant pulses is similar to that recently determined for the repeating frb 121102. we also find an anti-correlation between the pulse fluence and the pulse width, so that more energetic pulses are preferentially shorter. | super-giant pulses from the crab pulsar: energy distribution and occurrence rate |
in a recent work, we emphasized that an excess in trilepton events plus missing energy observed by the atlas experiment at the lhc could be interpreted as a signal of low-energy supersymmetry. in such a scenario the lightest neutralino mass is approximately mχ≃60 gev and the direct dark matter detection cross section is naturally below the current bound. in this work we present simple extensions of this scenario that lead to an explanation of the gamma-ray excess at the center of the galaxy observed by fermi-lat, as well as the antiproton excess observed by ams-02. these extensions include the addition of a small c p -violating phase in the neutralino sector or the addition of a light c p -odd higgs scalar. our study is of special relevance in view of a recent analysis that casts doubt on the previously accepted preference for millisecond pulsars as the origin of the galactic center excess. | return of the wimp: missing energy signals and the galactic center excess |
cosmological models with a dynamical dark energy field typically lead to a modified propagation of gravitational waves via an effectively time-varying gravitational coupling g (t ). the local variation of this coupling between the time of emission and detection can be probed with standard sirens. here we discuss the role that lunar laser ranging (llr) and binary pulsar constraints play in the prospects of constraining g (t ) with standard sirens. in particular, we argue that llr constrains the matter-matter gravitational coupling gn(t ), whereas binary pulsars and standard sirens constrain the quadratic kinetic gravity self-interaction gg w(t ). generically, these two couplings could be different in alternative cosmological models, in which case llr constraints are irrelevant for standard sirens. we use the hulse-taylor pulsar data and show that observations are highly insensitive to time variations of gg w(t ) yet highly sensitive to gn(t ). we thus conclude that future gravitational waves data will become the best probe to test gg w(t ), and will hence provide novel constraints on dynamical dark energy models. | standard sirens as a novel probe of dark energy |
although high-sensitivity surveys have revealed a number of highly dispersed pulsars in the inner galaxy, none have so far been found in the galactic centre (gc) region, which we define to be within a projected distance of 1 pc from sgr a*. this null result is surprising given that several independent lines of evidence predict a sizable population of neutron stars in the region. here, we present a detailed analysis of both the canonical and millisecond pulsar populations in the gc and consider free-free absorption and multipath scattering to be the two main sources of flux density mitigation. we demonstrate that the sensitivity limits of previous surveys are not sufficient to detect gc pulsar population, and investigate the optimum observing frequency for future surveys. depending on the degree of scattering and free-free absorption in the gc, current surveys constrain the size of the potentially observable population (i.e. those beaming towards us) to be up to 52 canonical pulsars and 10 000 millisecond pulsars. we find that the optimum frequency for future surveys is in the range of 9-13 ghz. we also predict that future deeper surveys with the square kilometre array will probe a significant portion of the existing radio pulsar population in the gc. | detecting pulsars in the galactic centre |
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