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we present a new solution of embedding class i describing the interior of a spherically symmetric charged anisotropic stellar configuration. the exact analytic solution has been explored by considering buchdahl type metric potential g_{rr}. using this analytic solution, we have discussed various physical aspects of a compact star. the solution is free from central singularities. the solution also satisfies wec, sec, nec and dec. the compactness parameter 2m/rb as obtained from the solution satisfies the buchdahl-andreasson condition. finally we have compared the calculated masses and radii of well-known compact star candidates like rx j1856.5-3754, xte j1739-285, psr b0943+10 and sax j1808.4-3658 with their observational values. | charged anisotropic buchdahl solution as an embedding class i spacetime |
we are presenting a new class of well-behaved solutions in embedding class-i. we proceed our calculations by assuming a new type of grr metric potential and solved for the other metric gtt using karmarkar condition. the necessary condition that any solutions of einstein's field equations to be class-i is to satisfy karmarkar condition and sufficient condition is to satisfy pandey-sharma condition i.e. r2323 ≠ 0. the solution also satisfies strong energy condition, null energy condition, dominant energy condition and weak energy condition. the obtained compactness parameter is within buchdahl limit i.e. 2 m / r ≤ 8 / 9. the solution also satisfies the causality condition and can represent stable stellar fluid system as the adiabatic index γ > 4 / 3 and the stability factor holds - 1 ≤ vt2 - vr2 ≤ 0 good. finally, we have tuned our solution for two compact stars psr j1614-2230 and 4u1608-52 which are well-behaved in all respects. | a new relativistic stellar model with anisotropic fluid in karmarkar space-time |
to assist with the commissioning (jiang et al. 2019) of the five-hundred-meter aperture spherical radio telescope (fast), we performed a pulsar search, with the primary goal of developing and testing the pulsar data acquisition and processing pipelines. we tested and used three pipelines, two (p1 and p2 hereafter) searched for the periodic signature of pulsars whereas the other one was used to search for bright single pulses (p3 hereafter). a pulsar candidate was discovered in the observation on the 22nd august, 2017, and later confirmed by the parkes radio telescope on the 10th september, 2017. this pulsar, named psr j1900-0134, was the first pulsar discovered by fast. the pulsar has a pulse period of 1.8 s and a dispersion measure (dm) of 188\,pc\,cm$^{-3}$. | the first pulsar discovered by fast |
we provide new exact solutions to the einstein-maxwell system of equations for matter configurations with anisotropy and charge. the spacetime is static and spherically symmetric. a quadratic equation of state is utilised for the matter distribution. by specifying a particular form for one of the gravitational potentials and the electric field intensity we obtain new exact solutions in isotropic coordinates. in our general class of models, an earlier model with a linear equation of state is regained. for particular choices of parameters we regain the masses of the stars psr j1614-2230, 4u 1608-52, psr j1903+0327, exo 1745-248 and sax j1808.4-3658. a comprehensive physical analysis for the star psr j1903+0327 reveals that our model is reasonable. | compact stars with quadratic equation of state |
in the summer of 2012, during a pulsar search collaboratory workshop, two high-school students discovered j1930-1852, a pulsar in a double neutron star (dns) system. most dns systems are characterized by short orbital periods, rapid spin periods, and eccentric orbits. however, j1930-1852 has the longest spin period ({{p}spin} ∼ 185 ms) and orbital period ({{p}b} ∼ 45 days) yet measured among known, recycled pulsars in dns systems, implying a shorter than average and/or inefficient recycling period before its companion went supernova. we measure the relativistic advance of periastron for j1930-1852, \dot{ω }=0.00078 (4) deg yr-1, which implies a total mass ({{m}tot}=2.59 (4) {{m}⊙ }) consistent with other dns systems. the 2σ constraints on {{m}tot} place limits on the pulsar and companion masses ({{m}p}\lt 1.32 {{m}⊙ } and {{m}c}\gt 1.30 {{m}⊙ } respectively). j1930-1852’s spin and orbital parameters challenge current dns population models and make j1930-1852 an important system for further investigation. | psr j1930-1852: a pulsar in the widest known orbit around another neutron star |
we report here the einstein@home discovery of psr j1913+1102, a 27.3 ms pulsar found in data from the ongoing arecibo palfa pulsar survey. the pulsar is in a 4.95 hr double neutron star (dns) system with an eccentricity of 0.089. from radio timing with the arecibo 305 m telescope, we measure the rate of advance of periastron to be \dot{ω }=5.632(18)° yr-1. assuming general relativity accurately models the orbital motion, this corresponds to a total system mass of m tot = 2.875(14) {m}⊙ , similar to the mass of the most massive dns known to date, b1913+16, but with a much smaller eccentricity. the small eccentricity indicates that the second-formed neutron star (ns) (the companion of psr j1913+1102) was born in a supernova with a very small associated kick and mass loss. in that case, this companion is likely, by analogy with other systems, to be a light (∼1.2 {m}⊙ ) ns; the system would then be highly asymmetric. a search for radio pulsations from the companion yielded no plausible detections, so we cannot yet confirm this mass asymmetry. by the end of 2016, timing observations should permit the detection of two additional post-keplerian parameters: the einstein delay (γ), which will enable precise mass measurements and a verification of the possible mass asymmetry of the system, and the orbital decay due to the emission of gravitational waves ({\dot{p}}b), which will allow another test of the radiative properties of gravity. the latter effect will cause the system to coalesce in ∼0.5 gyr. | einstein@home discovery of a double neutron star binary in the palfa survey |
bow-shock pulsar wind nebulae (pwne) show a variety of morphological shapes. we attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have certain influence on the morphology). we identify three basic types of bow-shock nebulae: (i) a rifle bullet (pulsar spin axis and proper velocity are aligned), (ii) a `frisbee' (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a `cart wheel' (such as frisbee but the spin axis is perpendicular to the plane of the sky). using results of the 3d magnetohydrodynamics simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock pwne, as well as variations seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing `whiskers', `tails', `filled-in', and `mushroom' shapes, as well as non-symmetric morphologies. on the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the pwn morphology. in a few cases, when we clearly identify specific morphological structures, our results do not favour alignment of the pulsar spin axis and proper velocity. our calculations of the underlying emission processes explain the low-synchrotron x-ray efficiency (in terms of the spin-down luminosity) and imply an energetically subdominant contribution of the inverse compton process. | 3d dynamics and morphology of bow-shock pulsar wind nebulae |
in more than four years of observation the large area telescope on board the fermi satellite has identified pulsed γ-ray emission from more than 80 young or middle-aged pulsars, in most cases providing light curves with high statistics. fitting the observed profiles with geometrical models can provide estimates of the magnetic obliquity α and of the line of sight angle ζ, yielding estimates of the radiation beaming factor and radiated luminosity. using different γ-ray emission geometries (polar cap, slot gap, outer gap, one pole caustic) and core plus cone geometries for the radio emission, we fit γ-ray light curves for 76 young or middle-aged pulsars and we jointly fit their γ-ray plus radio light curves when possible. we find that a joint radio plus γ-ray fit strategy is important to obtain (α,ζ) estimates that can explain simultaneously detectable radio and γ-ray emission: when the radio emission is available, the inclusion of the radio light curve in the fit leads to important changes in the (α,ζ) solutions. the most pronounced changes are observed for outer gap and one pole caustic models for which the γ-ray only fit leads to underestimated α or ζ when the solution is found to the left or to the right of the main α-ζ plane diagonal respectively. the intermediate-to-high altitude magnetosphere models, slot gap, outer gap, and one pole caustic, are favoured in explaining the observations. we find no apparent evolution of α on a time scale of 106 years. for all emission geometries our derived γ-ray beaming factors are generally less than one and do not significantly evolve with the spin-down power. a more pronounced beaming factor vs. spin-down power correlation is observed for slot gap model and radio-quiet pulsars and for the outer gap model and radio-loud pulsars. the beaming factor distributions exhibit a large dispersion that is less pronounced for the slot gap case and that decreases from radio-quiet to radio-loud solutions. for all models, the correlation between γ-ray luminosity and spin-down power is consistent with a square root dependence. the γ-ray luminosities obtained by using the beaming factors estimated in the framework of each model do not exceed the spin-down power. this suggests that assuming a beaming factor of one for all objects, as done in other studies, likely overestimates the real values. the data show a relation between the pulsar spectral characteristics and the width of the accelerator gap. the relation obtained in the case of the slot gap model is consistent with the theoretical prediction. appendices are available in electronic form at http://www.aanda.org | light-curve modelling constraints on the obliquities and aspect angles of the young fermi pulsars |
more than a dozen binary systems are now established as sources of variable, high-energy (he, 0.1- 100 gev) gamma rays. five are also established sources of very-high-energy (vhe, ⩾ 100 gev) gamma rays. the mechanisms behind gamma-ray emission in binaries are very diverse. my current understanding is that they divide up into four types of systems: gamma-ray binaries, powered by pulsar rotation; microquasars, powered by accretion onto a black hole or neutron star; novae, powered by thermonuclear runaway on a white dwarf; colliding wind binaries, powered by stellar winds from massive stars. some of these types had long been suspected to emit gamma rays (microquasars), others have taken the community by surprise (novae). my purpose here is to provide a brief review of the current status of gamma-ray emission from binaries, in the context of related objects where similar mechanisms are at work (pulsar wind nebulae, active galactic nuclei, supernova remnants). | gamma-ray emission from binaries in context |
the green bank north celestial cap (gbncc) pulsar survey will cover the entire northern sky (δ > -40°) at 350 mhz, and is one of the most uniform and sensitive all-sky pulsar surveys to date. we have created a pipeline to reanalyze gbncc survey data to take a 350 mhz census of all pulsars detected by the survey, regardless of their discovery survey. of the 1413 pulsars in the survey region, we were able to recover 670. for these we present measured signal-to-noise ratios (s/n), flux densities, pulse widths, profiles, and where appropriate, refined measurements of dispersion measures (dms) (656 out of 670) and new or improved spectral indices (339 out of 670 total, 47 new, 292 improved). we also measure the period-pulse width relation at 350 mhz to scale as w∝ p-0.27. detection scans for several hundred sources were reanalyzed in order to inspect pulsars' single pulse behavior and 223 were found to exhibit evidence of nulling. with a detailed analysis of measured and expected s/n values and the evolving radio frequency interference environment at 350 mhz, we assess the gbncc survey's sensitivity as a function of spin period, dm, and sky position. we find the sky-averaged limiting flux density of the survey to be 0.74 mjy. combining this analysis with psrpoppy pulsar population simulations, we predict 60/5 nonrecycled/msp discoveries in the survey's remaining 21,000 pointings, and we begin to place constraints on population model parameters. | the green bank north celestial cap pulsar survey. v. pulsar census and survey sensitivity |
using data from the large european array for pulsars, and the effelsberg telescope, we study the scintillation parameters of the millisecond pulsar psr j0613-0200 over a 7 yr timespan. the 'secondary spectrum' - the 2d power spectrum of scintillation - presents the scattered power as a function of time delay, and contains the relative velocities of the pulsar, observer, and scattering material. we detect a persistent parabolic scintillation arc, suggesting scattering is dominated by a thin, anisotropic region. the scattering is poorly described by a simple exponential tail, with excess power at high delays; we measure significant, detectable scattered power at times out to ${\sim}5 \, \mu {\rm s}$ , and measure the bulk scattering delay to be between 50 to 200-=ns with particularly strong scattering throughout 2013. these delays are too small to detect a change of the pulse profile shape, yet they would change the times of arrival as measured through pulsar timing. the arc curvature varies annually, and is well fitted by a one-dimensional scattering screen ${\sim}40{{\ \rm per\ cent}}$ of the way towards the pulsar, with a changing orientation during the increased scattering in 2013. effects of uncorrected scattering will introduce time delays correlated over time in individual pulsars, and may need to be considered in gravitational wave analyses. pulsar timing programmes would benefit from simultaneously recording in a way that scintillation can be resolved, in order to monitor the variable time delays caused by multipath propagation. | measuring interstellar delays of psr j0613-0200 over 7 yr, using the large european array for pulsars |
the remarkable null pulse coincident with the 2016 glitch in vela rotation indicates a dynamical event involving the crust and the magnetosphere of the neutron star. we propose that a crustal quake associated with the glitch strongly disturbed the vela magnetosphere and thus interrupted its radio emission. we present the first global numerical simulations of a neutron starquake. our code resolves the elasto-dynamics of the entire crust and follows the evolution of alfvén waves excited in the magnetosphere. we observe rayleigh surface waves propagating away from the epicenter of the quake, around the circumference of the crust—an instance of the so-called whispering gallery modes. the rayleigh waves set the initial spatial scale of the magnetospheric disturbance. once launched, the aflvén waves bounce in the closed magnetosphere, become de-phased, and generate strong electric currents, capable of igniting electric discharge. most likely, the discharge floods the magnetosphere with electron-positron plasma, quenching the radio emission. we find that the observed ∼0.2 s disturbance is consistent with the damping time of the crustal waves if the crust is magnetically coupled to the superconducting core of the neutron star. the quake is expected to produce a weak x-ray burst of short duration. | a quake quenching the vela pulsar |
previous studies have shown that the radiation emitted by a rapidly rotating magnetar embedded in a young supernova can greatly amplify its luminosity. these one-dimensional studies have also revealed the existence of an instability arising from the piling up of radiatively accelerated matter in a thin dense shell deep inside the supernova. here, we examine the problem in two dimensions and find that, while instabilities cause mixing and fracture this shell into filamentary structures that reduce the density contrast, the concentration of matter in a hollow shell persists. the extent of the mixing depends upon the relative energy input by the magnetar and the kinetic energy of the inner ejecta. the light curve and spectrum of the resulting supernova will be appreciably altered, as will the appearance of the supernova remnant, which will be shellular and filamentary. a similar pile up and mixing might characterize other events where energy is input over an extended period by a centrally concentrated source, e.g., a pulsar, radioactive decay, a neutrino-powered wind, or colliding shells. the relevance of our models to the recent luminous transient asassn-15lh is briefly discussed. | magnetar-powered supernovae in two dimensions. i. superluminous supernovae |
we report on observations of the pulsar/be star binary system psr j2032+4127/mt91 213 in the energy range between 100 {gev} and 20 {tev} with the very energetic radiation imaging telescope array and major atmospheric gamma imaging cherenkov telescope arrays. the binary orbit has a period of approximately 50 years, with the most recent periastron occurring on 2017 november 13. our observations span from 18 months prior to periastron to one month after. a new point-like gamma-ray source is detected, coincident with the location of psr j2032+4127/mt91 213. the gamma-ray light curve and spectrum are well characterized over the periastron passage. the flux is variable over at least an order of magnitude, peaking at periastron, thus providing a firm association of the tev source with the pulsar/be star system. observations prior to periastron show a cutoff in the spectrum at an energy around 0.5 {tev}. this result adds a new member to the small population of known tev binaries, and it identifies only the second source of this class in which the nature and properties of the compact object are firmly established. we compare the gamma-ray results with the light curve measured with the x-ray telescope on board the neil gehrels swift observatory and with the predictions of recent theoretical models of the system. we conclude that significant revision of the models is required to explain the details of the emission that we have observed, and we discuss the relationship between the binary system and the overlapping steady extended source, tev j2032+4130. | periastron observations of tev gamma-ray emission from a binary system with a 50-year period |
we demonstrate the blind interferometric detection and localization of two fast radio bursts (frbs) with subarcminute precision on the 400 m baseline between the canadian hydrogen intensity mapping experiment (chime) and the chime pathfinder. in the same spirit as very long baseline interferometry (vlbi), the telescopes were synchronized to separate clocks, and the channelized voltage (herein referred to as baseband) data were saved to a disk with correlation performed offline. the simultaneous wide field of view and high sensitivity required for blind frb searches implies a high data rate—6.5 terabits per second (tb/s) for chime and 0.8 tb s-1 for the pathfinder. since such high data rates cannot be continuously saved, we buffer data from both telescopes locally in memory for $\approx 40\,{\rm{s}}$ , and write to the disk upon receipt of a low-latency trigger from the chime fast radio burst instrument (chime/frb). the $\approx 200$ deg2 field of view of the two telescopes allows us to use in-field calibrators to synchronize the two telescopes without needing either separate calibrator observations or an atomic timing standard. in addition to our frb observations, we analyze bright single pulses from the pulsars b0329+54 and b0355+54 to characterize systematic localization errors. our results demonstrate the successful implementation of key software, triggering, and calibration challenges for chime/frb outriggers: cylindrical vlbi outrigger telescopes which, along with the chime telescope, will localize thousands of single frb events with sufficient precision to unambiguously associate a host galaxy with each burst. | a synoptic vlbi technique for localizing nonrepeating fast radio bursts with chime/frb |
fast reconnection operating in magnetically dominated plasmas is often invoked in models for magnetar giant flares, for magnetic dissipation in pulsar winds, or to explain the gamma-ray flares observed in the crab nebula; hence, its investigation is of paramount importance in high-energy astrophysics. here we study, by means of two-dimensional numerical simulations, the linear phase and the subsequent non-linear evolution of the tearing instability within the framework of relativistic resistive magnetohydrodynamics (mhd), as appropriate in situations where the alfvén velocity approaches the speed of light. it is found that the linear phase of the instability closely matches the analysis in classical mhd, where the growth rate scales with the lundquist number s as s-1/2, with the only exception of an enhanced inertial term due to the thermal and magnetic energy contributions. in addition, when thin current sheets of inverse aspect ratio scaling as s-1/3 are considered, the so-called ideal tearing regime is retrieved, with modes growing independently of s and extremely fast, on only a few light crossing times of the sheet length. the overall growth of fluctuations is seen to solely depend on the value of the background alfvén velocity. in the fully non-linear stage, we observe an inverse cascade towards the fundamental mode, with petschek-type supersonic jets propagating at the external alfvén speed from the x-point, and a fast reconnection rate at the predicted value {r}∼ (ln s)^{-1}. | fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited |
the structure of a light quark star is studied within a new two-flavor nambu-jona-lasinio model. by retaining the contribution from the vector term in the fierz-transformed lagrangian, a two-solar-mass pure quark star is achieved. to overcome the disadvantage of three-momentum truncation in the regularization procedure, we introduce the proper-time regularization. we also employ the newly proposed definition of vacuum pressure [j. phys. g 45, 105001 (2018), 10.1088/1361-6471/aadeb0], in which the quasi-wigner vacuum (corresponding to the quasi-wigner solution of the gap equation) is used as the reference ground state. the free parameters include only a mixing constant α which weighs the contribution from the fierz-transformed lagrangian. we constrain α to be around 0.9 by the observed mass of pulsars psr j 0348 +0432 and psr j 1614 -2230 . we find that the calculated surface energy density meets the requirement (>2.80 ×1014 g /cm3 ) [phys. rev. d 99, 043001 (2019), 10.1103/physrevd.99.043001]. besides, for a 1.4-solar-mass star, the deformability λ is calculated, which is consistent with a recent analysis on the binary neutron star merger gw170817 with λ in (0,630) for large component spins and 300-230+420 when restricting the magnitude of the component spins [phys. rev. x 9, 011001 (2019), 10.1103/physrevx.9.011001], and satisfies the constraints 200 <λ <800 of early works. | nonstrange quark stars from an njl model with proper-time regularization |
we investigate neutron star moments of inertia from bayesian posterior probability distributions of the nuclear equation of state that incorporate information from microscopic many-body theory and empirical data of finite nuclei. we focus on psr j0737-3039a and predict that for this 1.338 m⊙ neutron star the moment of inertia lies in the range 1.04 ×1045g cm2<i <1.51 ×1045g cm2 at the 95% credibility level, while the most probable value for the moment of inertia is i ∼=1.36 ×1045g cm2 . assuming a measurement of the psr j0737-3039a moment of inertia to 10% precision, we study the implications for neutron star radii and tidal deformabilities. we also determine the crustal component of the moment of inertia and find that for typical neutron star masses 1.3 m⊙<m <1.5 m⊙ the crust contributes 1 -6 % of the total moment of inertia, below what is needed to explain large pulsar glitches in the scenario of strong neutron entrainment. | predicting the moment of inertia of pulsar j0737-3039a from bayesian modeling of the nuclear equation of state |
it is generally held that >100 tev emission from astrophysical objects unambiguously demonstrates the presence of pev protons or nuclei, due to the unavoidable klein-nishina suppression of inverse compton emission from electrons. however, in the presence of inverse compton dominated cooling, hard high-energy electron spectra are possible. we show that the environmental requirements for such spectra can naturally be met in spiral arms, and in particular in regions of enhanced star formation activity, the natural locations for the most promising electron accelerators: powerful young pulsars. our scenario suggests a population of hard ultra-high energy sources is likely to be revealed in future searches, and may also provide a natural explanation for the 100 tev sources recently reported by the high-altitude water cherenkov observatory. | ultra-high energy inverse compton emission from galactic electron accelerators |
context. central compact objects (ccos) in supernova remnants are isolated thermally emitting neutron stars (nss). they are most probably characterized by a magnetic field strength that is roughly two orders of magnitude lower than that of most of the radio and accreting pulsars. the thermal emission of ccos can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition.aims: the cco in hess j1731-347 is one of the brightest objects in this class. starting from 2007, it was observed several times with different x-ray satellites. here we present our analysis of two new xmm-newton observations of the source performed in 2013 which increase the total exposure time of the data available for spectral analysis by a factor of about five compared to the analyses presented before.methods: we use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the cco. from our fits, we derive constraints on the physical parameters of the emitting star such as its mass, radius, distance, and effective temperature. we also use the new data to derive new upper limits on the source pulsations and to confirm the absence of a long-term flux and spectral variability.results: the analysis shows that atmosphere models are clearly preferred by the fit over the blackbody spectral function. under the assumption that the x-ray emission is uniformly produced by the entire star surface (supported by the lack of pulsations), hydrogen atmosphere models lead to uncomfortably large distances of the cco, above 7-8 kpc. on the other hand, the carbon atmosphere model formally excludes distances above 5-6 kpc and is compatible with the source located in the scutum-crux (~3 kpc) or norma-cygnus (~4.5 kpc) galactic spiral arm. we provide and discuss the corresponding confidence contours in the ns mass-radius plane. the measured effective temperature indicates that the ns is exceptionally hot for the estimated age of ~30 kyr. we discuss possible cooling scenarios to explain this property, as well as possible additional constraints on the star mass and radius from cooling theory. | the neutron star in hess j1731-347: central compact objects as laboratories to study the equation of state of superdense matter |
two energetic hard x-ray bursts from the rotation-powered pulsar psr j1119-6127 recently triggered the fermi and swift space observatories. we have performed in-depth spectral and temporal analyses of these two events. our extensive searches in both observatories’ data for lower luminosity bursts uncovered 10 additional events from the source. we report here on the timing and energetics of the 12 bursts from psr j1119-6127 during its burst active phase on 2016 july 26 and 28. we also found a spectral softer x-ray flux enhancement in a post-burst episode, which shows evidence of cooling. here we discuss the implications of these results on the nature of this unusual high-field radio pulsar, which firmly place it within the typical magnetar population. | magnetar-like x-ray bursts from a rotation-powered pulsar, psr j1119-6127 |
we present a systematic study of the unbinned, photon-by-photon likelihood technique which can be used as an alternative method to analyse phase-dependent, x-ray spectro-polarimetric observations obtained with ixpe and other photoelectric polarimeters. we apply the unbinned technique to models of the luminous x-ray pulsar hercules x-1, for which we produce simulated observations using the ixpeobssim package. we consider minimal knowledge about the actual physical process responsible for the polarized emission from the accreting pulsar and assume that the observed phase-dependent polarization angle can be described by the rotating vector model. using the unbinned technique, the detector's modulation factor, and the polarization information alone, we found that both the rotating vector model and the underlying spectro-polarimetry model can reconstruct equally well the geometric configuration angles of the accreting pulsar. however, the measured polarization fraction becomes biased with respect to the underlying model unless the energy dispersion and effective area of the detector are also taken into account. to this end, we present an energy-dispersed likelihood estimator that is proved to be unbiased. for different analyses, we obtain posterior distributions from multiple ixpeobssim realizations and show that the unbinned technique yields $\sim 10{{\ \rm per\ cent}}$ smaller error bars than the binned technique. we also discuss alternative sources, such as magnetars, in which the unbinned technique and the rotating vector model might be applied. | unbinned likelihood analysis for x-ray polarization |
we report on the spectral behavior of the first galactic ultraluminous x-ray pulsar swift j0243.6+6124 with nustar observations during its 2017-2018 outburst. at sub-eddington levels, the source spectrum is characterized by three emission components: from the accretion column, the hot spot, and a broad iron line emission region. when the source is above the eddington limit, the hot spot temperature increases and the spectrum features two more blackbody components. one blackbody component has a radius of 10-20 km and likely originate from the top of the accretion column. the other one saturates at a blackbody luminosity of (1-2) × 1038 erg s-1, coincident with the eddington limit of a neutron star. this is consistent with the scenario that super-eddington accretion onto compact objects will power optically thick outflows and indicates an accretion rate 60-80 times the critical value. this suggests that super-eddington accretion onto magnetized systems can also power massive winds. at super-eddington levels, the iron line becomes more significant and blueshifted, and is argued to be associated with the ultrafast wind in the central funnel or jets. this source, if located in external galaxies, will appear like other ultraluminous pulsars. | super-eddington accretion onto the galactic ultraluminous x-ray pulsar swift j0243.6+6124 |
in anticipation of a ligo detection of a black hole/neutron star merger, we expand on the intriguing possibility of an electromagnetic counterpart. black hole/neutron star mergers could be disappointingly dark since most black holes will be large enough to swallow a neutron star whole, without tidal disruption and without the subsequent fireworks. encouragingly, we previously found a promising source of luminosity since the black hole and the highly magnetized neutron star establish an electronic circuit—a black hole battery. in this paper, arguing against common lore, we consider the electric charge of the black hole as an overlooked source of electromagnetic radiation. relying on the well known wald mechanism by which a spinning black hole immersed in an external magnetic field acquires a stable net charge, we show that a strongly magnetized neutron star in such a binary system will give rise to a large enough charge in the black hole to allow for potentially observable effects. although the maximum charge is stable, we show there is a continuous flux of charges contributing to the luminosity. most interestingly, the spinning charged black hole then creates its own magnetic dipole to power a black hole pulsar. | black hole pulsar |
the propeller regime has been studied using axisymmetric numerical simulations, where matter accretes from a thin turbulent disk and interacts with a rotating magnetized star. a wide range of propeller strengths has been studied, from very strong (where the magnetosphere rotates much more rapidly than the inner disk) to very weak (where the magnetosphere rotates only slightly faster than the inner disk). we observed, that in all propellers, the disk-magnetosphere interaction is a strongly non-stationary process. however, the time-averaged characteristics of propeller outflows depend almost entirely on the fastness parameter, ωs, which is the ratio of the stellar angular velocity to the inner disk's keplerian velocity. the relative amount of matter ejected into the wind (the efficiency of the propeller) and its velocity, energy and angular momentum increase with ωs. in addition, qualitative differences were observed between the strong and weak propellers: in the strong propellers, matter is accelerated above the escape velocity, forming large-scale outflows that consist of conically-shaped winds and a magnetic (poynting flux) jet. in the weak propellers, matter flows into a widely-opened, conically-shaped wind with sub- or super-escape velocities that may partly fall back to the disk at some distance from the star. stars spin down and a star-disk systems lose energy and angular momentum, which flow into winds and jets. simulations were performed in dimensionless form for stars with magnetospheres of 5 - 7 stellar radii. results of the simulations can be applied to different types of magnetized stars, including classical t tauri stars (cttss), cataclysmic variables (cvs), and accreting millisecond pulsars (msps). | properties of strong and weak propellers from mhd simulations |
curvature radiation is a natural candidate for the emission mechanism of fast radio burs (frb). however, frb spectra have structure with δν/ν ∼ 0.03-0.2, inconsistent with the very smooth spectrum of curvature radiation. although this spectral structure might be attributed to chromatic scintillation or lensing, in four frb high spectral resolution data indicate scintillation decorrelation bandwidths much narrower than the observed ∼30-300 mhz spectral structure. some of the observed structures may be intrinsic to the radiation mechanism. i suggest that the observed spectral structure reflects the spatial structure of a clumpy radiating charge distribution, and that the characteristic curvature radiation frequency may be higher than the observed frequencies. in this coupled plasma-curvature radiation process the radiated spectra are the product of the spectra of the plasma wave and that of incoherent curvature radiation. the argument applies to all coherent radiation processes, including those that produce pulsar nanoshots. the implied frb `clump' charges are large, and produce electrostatic potentials that suggest electron lorentz factors ≳102. the result applies generally to coherently radiating sources. | coherent plasma-curvature radiation in frb |
in this paper we provide a comprehensive derivation of the energy density in the stochastic gravitational-wave background $\omega_\mathrm{gw}(f)$, and show how this quantity is measured in ground-based detectors such as laser interferometer gravitational-wave observatory (ligo), space-based laser interferometer space antenna (lisa), and pulsar timing arrays. by definition $\omega_\mathrm{gw}(f) \propto s_h(f)$ -- the power spectral density (psd) of the fourier modes of the gravitational-wave background. however, this is often confused with the psd of the strain signal, which we call $s_\mathrm{gw}(f)$, and is a detector-dependent quantity. this has led to confusing definitions of $\omega_\mathrm{gw}(f)$ in the literature which differ by factors of up to 5 when written in a detector-dependent way. in addition to clarifying this confusion, formulas presented in this paper facilitate easy comparison of results from different detector groups, and how to convert from one measure of the strength of the background (or an upper limit) to another. our codes are public and on github. | understanding $\\omega_\\mathrm{gw}(f)$ in gravitational wave experiments |
we report the detection of pulsed gamma-ray emission from the geminga pulsar (psr j0633+1746) between 15 gev and 75 gev. this is the first time a middle-aged pulsar has been detected up to these energies. observations were carried out with the magic telescopes between 2017 and 2019 using the low-energy threshold sum-trigger-ii system. after quality selection cuts, ∼80 h of observational data were used for this analysis. to compare with the emission at lower energies below the sensitivity range of magic, 11 years of fermi-lat data above 100 mev were also analysed. from the two pulses per rotation seen by fermi-lat, only the second one, p2, is detected in the magic energy range, with a significance of 6.3σ. the spectrum measured by magic is well-represented by a simple power law of spectral index γ = 5.62 ± 0.54, which smoothly extends the fermi-lat spectrum. a joint fit to magic and fermi-lat data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6σ significance level. the power-law tail emission detected by magic is interpreted as the transition from curvature radiation to inverse compton scattering of particles accelerated in the northern outer gap. | detection of the geminga pulsar with magic hints at a power-law tail emission beyond 15 gev |
context. the discovery of extended gamma-ray emission toward a number of middle-aged pulsars suggests the possibility of long-lived particle confinement beyond the classical pulsar wind nebula (pwn) stage. how this emerging source class can be extrapolated to a galactic population remains unclear.aims: we aim to evaluate how pulsar halos fit in existing tev observations, under the assumption that all middle-aged pulsars develop halos similar to those observed toward the j0633+1746 or b0656+14 pulsars.methods: we modeled the populations of supernova remnants, pwne, and pulsar halos in the milky way. the pwn-halo evolutionary sequence is described in a simple yet coherent framework, and both kinds of objects are assumed to share the same particle injection properties. we then assessed the contribution of the different source classes to the very-high-energy emission from the galaxy.results: the synthetic population can be made consistent with the flux distribution of all known objects, including unidentified objects, for a reasonable set of parameters. the fraction of the populations predicted to be detectable in surveys of the galactic plane with hess and hawc is then found to be in good agreement with their actual outcome, with a number of detectable halos ranging from 30 to 80% of the number of detectable pwne. prospects for cta involve the detection of 250 − 300 sources in the galactic plane survey, including 170 pwne and up to 100 halos. the extent of diffusion suppression in halos has a limited impact on such prospects but its magnitude has a strong influence. the level of diffuse emission from unresolved populations in each survey is found to be dominated by halos and comparable to large-scale interstellar radiation powered by cosmic rays above 0.1-1 tev.conclusions: pulsar halos are shown to be viable counterparts to a fraction of the currently unidentified sources if they develop around most middle-aged pulsars. yet, if the phenomenon is rare, with an occurrence rate of 5 − 10% as suggested in a previous work from the local positron flux constraint, the total number of currently known tev sources including unidentified ones cannot be accounted for in our model from young pwne only. this calls for continued efforts to model pulsar-powered emission along the full evolutionary path, including the late stages past the young nebula phase. | population synthesis of pulsar wind nebulae and pulsar halos in the milky way. predicted contributions to the very-high-energy sky |
the near-earth (within ∼100 pc) supernova explosions in the past several million years can cause the global deposition of radioactive elements (e.g., 60fe) on earth. the remnants of such supernovae are too old to be easily identified. it is therefore of great interest to search for million-year-old near-earth neutron stars or black holes, the products of supernovae. however, neutron stars and black holes are challenging to find even in our solar neighbourhood if they are not radio pulsars or x-ray/γ-ray emitters. here we report the discovery of one of the nearest (127.7 ± 0.3 pc) neutron star candidates in a detached single-lined spectroscopic binary lamost j235456.73+335625.9 (hereafter j2354). utilizing the time-resolved ground-based spectroscopy and space photometry, we find that j2354 hosts an unseen compact object with minv being 1.4-1.6 m⊙. the follow-up swift ultraviolet (uv) and x-ray observations suggest that the uv and x-ray emission is produced by the visible star rather than the compact object. hence, j2354 probably harbours a neutron star rather than a hot ultramassive white dwarf. two-hour exceptionally sensitive radio follow-up observations with five-hundred-meter aperture spherical radio telescope fail to reveal any pulsating radio signals at the 6σ flux upper limit of 12.5 µjy. therefore, the neutron star candidate in j2354 can only be revealed via our time-resolved observations. interestingly, the distance between j2354 and our earth can be as close as ∼ 50 pc around 2.5 million years (myrs) ago, as revealed by the gaia kinematics. our discovery demonstrates a promising way to unveil the hidden near-earth neutron stars in binaries by exploring the optical time domain, thereby facilitating understanding of the metal-enrichment history in our solar neighbourhood. | the nearest neutron star candidate in a binary revealed by optical time-domain surveys |
the fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the milky way discovered in ~1-100 gev data collected by the large area telescope on board the fermi gamma-ray space telescope. previous work has revealed substructure within the fermi bubbles that has been interpreted as a signature of collimated outflows from the galaxy's supermassive black hole. here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the sagittarius dwarf spheroidal galaxy (dsph). this large milky way satellite is viewed through the fermi bubbles from the position of the solar system. as a tidally and ram-pressure stripped remnant, the sagittarius dsph has no ongoing star formation, but we nevertheless demonstrate that the dwarf's millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. the measured spectrum is naturally explained by inverse compton scattering of cosmic microwave background photons by high-energy electron-positron pairs injected by millisecond pulsars belonging to the sagittarius dsph, combined with these objects' magnetospheric emission. this finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the galactic centre, the andromeda galaxy and other massive milky way dsphs. | gamma-ray emission from the sagittarius dwarf spheroidal galaxy due to millisecond pulsars |
we examine the chandrasekhar limit for white dwarfs in f (r ) gravity, with a simple polytropic equation of state describing stellar matter. we use the most popular f (r ) gravity model, namely the f (r )=r +α r2 gravity, and calculate the parameters of the stellar configurations with polytropic equation of state of the form p =k ρ1 +1 /n for various values of the parameter n . in order to simplify our analysis we use the equivalent einstein frame form of r2-gravity which is basically a scalar-tensor theory with well-known potential for the scalar field. in this description one can use simple approximations for the scalar field ϕ leaving only the potential term for it. our analysis indicates that for the nonrelativistic case with n =3 /2 , discrepancies between the r2-gravity and general relativity can appear only when the parameter α of the r2 term, takes values close to maximal limit derived from the binary pulsar data namely αmax=5 ×1015 cm2. thus, the study of low-mass white dwarfs can hardly give restrictions on the parameter α . for relativistic polytropes with n =3 we found that chandrasekhar limit can in principle change for smaller α values. the main conclusion from our calculations is the existence of white dwarfs with large masses ∼1.33 m⊙, which can impose stricter limits on the parameter α for the r2 gravity model. specifically, our estimations on the parameter α of the r2 model is α ∼1013 cm2. | maximal masses of white dwarfs for polytropes in r2 gravity and theoretical constraints |
accretion-induced collapse (aic) of massive white dwarfs (wds) has been proposed as an important way for the formation of neutron star (ns) systems. an oxygen-neon (one) wd that accretes h-rich material from a red-giant (rg) star may experience the aic process, eventually producing millisecond pulsars (msps), known as the rg donor channel. previous studies indicate that this channel can only account for msps with orbital periods $\gt 500\, \rm d$. it is worth noting that some more msps with wide orbits ($60\!-\!500\, \rm d$) have been detected by recent observations, but their origin is still highly uncertain. in this work, by employing an adiabatic power-law assumptions for the mass-transfer process, we performed a large number of complete binary evolution calculations for the formation of msps through the rg donor channel in a systematic way. we found that this channel can contribute to the observed msps with orbital periods in the range of $50\!-\!1200\, {\rm d}$, and almost all the observed msps with wide orbits can be covered by this channel in the wd companion mass versus orbital period diagram. this work indicates that the aic process provides a viable way to form msps with wide orbits. | formation of millisecond pulsars with long orbital periods by accretion-induced collapse of white dwarfs |
we follow the idea that the qcd phase diagram may be described by a crossover from a hadron resonance gas to perturbative qcd using the switch function ansatz of albright, kapusta, and young [phys. rev. c 90, 024915 (2014), 10.1103/physrevc.90.024915]. while the switch function could be calibrated at vanishing baryon chemical potential with data from lattice qcd simulations, it has been suggested recently by kapusta and welle [phys. rev. c 104, l012801 (2021), 10.1103/physrevc.104.l012801] that in the zero temperature limit, the switch function parameter μ0 could be constrained by neutron star phenomenology, in particular by massive pulsars such as psr j0740+6620 with a mass exceeding 2 msun . in this work we demonstrate that this procedure to constrain the qcd phase diagram does crucially depend on the fact that cold dense quark matter is very likely in a color superconducting state. | neutron stars with crossover to color superconducting quark matter |
m82 x-2 is the first pulsating ultraluminous x-ray source discovered. the luminosity of these extreme pulsars, if isotropic, implies an extreme mass transfer rate. an alternative is to assume a much lower mass transfer rate, but with an apparent luminosity boosted by geometrical beaming. only an independent measurement of the mass transfer rate can help discriminate between these two scenarios. in this paper, we follow the orbit of the neutron star for 7 yr, measure the decay of the orbit ( ${\dot{p}}_{\mathrm{orb}}/{p}_{\mathrm{orb}}\approx -8\cdot {10}^{-6}\,{\mathrm{yr}}^{-1}$ ), and argue that this orbital decay is driven by extreme mass transfer of more than 150 times the mass transfer limit set by the eddington luminosity. if this is true, the mass available to the accretor is more than enough to justify its luminosity, with no need for beaming. this also strongly favors models where the accretor is a highly magnetized neutron star. | orbital decay in m82 x-2 |
a new type of embedding class-i metric representing anisotropic fluid distribution is presented in this paper. the new solution satisfies tov equation and does represent a static stellar configuration. this new solution is an interesting hybrid solution where gttis of schwarzschild interior and grris of vaidya-tikekar. the new solution satisfies all the physical criteria like positive finite central pressure, density, causality condition etc. it also satisfies all the energy conditions such as sec, wec, nec and dec. using this new solution, we have presented some compact star models for her x-1, rx j1856.5-3754, cyg x-2 and psr j1614-2230 by optimizing their masses and radii. | a hybrid space-time of schwarzschild interior and vaidya-tikekar solution as an embedding class i |
the 1-70 kev persistent spectra of 15 magnetars, observed with suzaku from 2006 to 2013, were studied as a complete sample. combined with early nustar observations of four hard x-ray emitters, nine objects showed a hard power-law emission dominating at ≳ 10 kev with the 15-60 kev flux of ∼1-11× {10}-11 erg s-1 cm-2. the hard x-ray luminosity {l}{{h}}, relative to that of a soft-thermal surface radiation {l}{{s}}, tends to become higher toward younger and strongly magnetized objects. their hardness ratio, updated from a previous study and defined as ξ ={l}{{h}}/{l}{{s}}, is correlated with the measured spin-down rate \dot{p} as ξ =0.62× {(\dot{p}/{10}-11{{s}}{{{s}}}-1)}0.72, corresponding to positive and negative correlations with the dipole field strength {b}{{d}} (ξ \propto {b}{{d}}1.41) and the characteristic age {τ }{{c}} (ξ \propto {τ }{{c}}-0.68), respectively. among our sample, five transients were observed during x-ray outbursts, and the results are compared with their long-term 1-10 kev flux decays monitored with swift/xrt and rxte/pca. fading curves of three bright outbursts are approximated by an empirical formula used in the seismology, showing a ∼10-40 day plateau phase. transients show the maximum luminosities of {l}{{s}} ∼ 1035 erg s-1, which are comparable to those of persistently bright ones, and fade back to ≲1032 erg s-1. spectral properties are discussed in the framework of the magnetar hypothesis. | magnetar broadband x-ray spectra correlated with magnetic fields: suzaku archive of sgrs and axps combined with nustar, swift, and rxte |
the pulsar radio emission originates from regions below 10% of the light cylinder radius. this requires a mechanism where coherent emission is excited in relativistic pair plasma with frequency ν _{cr} which is below the plasma frequency ν_{°} i.e. ν _{cr} < ν_{°}. a possible model for the emission mechanism is charged bunches (charged solitons) moving relativistically along the curved open dipolar magnetic field lines capable of exciting coherent curvature radio emission. in this article, we review the results from high quality observations in conjunction with theoretical models to unravel the nature of coherent curvature radio emission in pulsars. | nature of coherent radio emission from pulsars |
the density-dependent relativistic hartree-fock (ddrhf) theory is extended to include δ isobars for the study of dense nuclear matter and neutron stars. to this end, we solve the rarita-schwinger equation for spin-3/2 particle. both the direct and exchange terms of the δ isobars' self-energies are evaluated in detail. in comparison with the relativistic mean field theory (hartree approximation), a weaker parameter dependence is found for ddrhf. an early appearance of δ isobars is recognized at ρb∼0.28 fm-3, comparable with that of hyperons. also, we find that the δ isobars' softening of the equation of state is mainly due to the reduced fock contributions from the coupling of the isoscalar mesons, while the pion contributions are negligibly small. we finally conclude that with typical parameter sets, neutron stars with δ isobars in their interiors could be as heavy as the two massive pulsars whose masses are precisely measured, with slightly smaller radii than normal neutron stars. | δ (1232 ) effects in density-dependent relativistic hartree-fock theory and neutron stars |
context. the relationship between the dispersion measures (dms) and redshifts of fast radio bursts (frbs) is of scientific interest. upcoming commensal surveys may detect and localise many frbs to the sub-arcsecond angular resolutions required for accurate redshift determination. meanwhile, it is important to exploit sources accumulated with more limited localisation to their maximum scientific potential.aims: we present techniques for the dm-redshift analysis of large numbers of unlocalised frbs, accounting for uncertainties due to their extragalactic dm components, redshift dependences, and progenitor scenarios.methods: we reviewed the components comprising observed frb dms. we built redshift-scalable probability distribution functions for these components, which we combined in cases of multiple progenitor scenarios. accounting for prior frb redshift distributions we inverted these models, enabling frb redshifts to be constrained.results: we illustrate the influence of frb progenitors on their observed dms, which may remain significant to redshift z ~ 3. we identify the frb sample sizes required to distinguish between multiple progenitor scenarios. we place new, physically motivated redshift constraints on all catalogued frbs to date and use these to reject potential host galaxies in the localisation area of an frb according to various models. we identify further uses for dm-redshift analysis using many frbs. we provide our code so that these techniques may be employed using increasingly realistic models as our understanding of frbs evolves. python code and examples are available at https://doi.org/10.5281/zenodo.1209920 | constraining the redshifts of unlocalised fast radio bursts |
globular clusters host a variety of lower-luminosity (lx < 1035 erg s-1) x-ray sources, including accreting neutron stars (nss) and black holes (bhs), millisecond pulsars (msps), cataclysmic variables, and chromospherically active binaries. in this paper, we provide a comprehensive catalog of more than 1100 x-ray sources in 38 galactic globular clusters (gcs) observed by the chandra x-ray observatory's chandra/acis detector. the targets are selected to complement the maveric survey's deep radio continuum maps of galactic gcs. we perform photometry and spectral analysis for each source, determine a best-fit model, and assess the possibility of it being a foreground or background source based on its spectral properties and location in the cluster. we also provide basic assessments of variability. we discuss the distribution of x-ray binaries in gcs and their x-ray luminosity function, and we carefully analyze systems with lx > 1033 erg s-1. among these moderately bright systems, we discover a new source in ngc 6539 that may be a candidate accreting stellar-mass bh or a transitional msp. we show that quiescent ns low-mass x-ray binaries in gcs may spend ∼2% of their lifetimes as transitional msps in their active (lx > 1033 erg s-1) state. finally, we identify a substantial underabundance of bright (lx > 1033 erg s-1) intermediate polars in gcs compared to the galactic field, in contrast with the literature of the past two decades. | the maveric survey: chandra/acis catalog of faint x-ray sources in 38 galactic globular clusters |
in general relativity gravitational waves propagate at the speed of light; however, in alternative theories of gravity that might not be the case. we investigate the effects of a modified speed of gravity, ct2, on the b modes of the cosmic microwave background (cmb) anisotropy in polarization. we find that a departure from the light speed value would leave a characteristic imprint on the bb spectrum part induced by tensors, manifesting as a shift in the angular scale of its peaks which allows us to constrain ct without any significant degeneracy with other cosmological parameters. we derive constraints from current data and forecast the accuracy with which ct will be measured by the next generation cmb satellites. in the former case, using the available planck and bicep2 data sets, we obtain ct2=1.30 ±0.79 and ct2<2.85 at 95% c.l. by assuming a power law primordial tensor power spectrum and ct2<2.33 at 95% c.l. if the running of the spectral index is allowed. more interestingly, in the latter case we find future cmb satellites capable of constraining ct2 at percent level, comparable with bounds from binary pulsar measurements, largely due to the absence of degeneracy with other cosmological parameters. | measuring the speed of cosmological gravitational waves |
context. rapidly rotating neutron stars blow a relativistic, magnetized wind mainly composed of electron-positron pairs. the free expansion of the wind terminates far from the neutron star where a weakly magnetized pulsar wind nebula forms, implying efficient magnetic dissipation somewhere upstream.aims: the wind current sheet that separates the two magnetic polarities is usually considered as the most natural place for magnetic dissipation via relativistic reconnection, but its efficiency remains an open question. here, the goal of this work is to revisit this issue in light of the most recent progress in the understanding of reconnection and pulsar electrodynamics.methods: we perform large two-dimensional particle-in-cell simulations of the oblique rotator to capture the multi-scale evolution of the wind. our simulations are limited to the equatorial plane.results: we find that the current sheet breaks up into a dynamical chain of magnetic islands separated by secondary thin current sheets. the sheet thickness increases linearly with radius while the poynting flux decreases monotonically as reconnection proceeds. the radius of complete annihilation of the stripes is given by the plasma multiplicity parameter at the light cylinder. current starvation within the sheets does not occur before complete dissipation as long as there is enough charges where the sheets form. particles are efficiently heated up to a characteristic energy set by the magnetization parameter at the light cylinder. energetic pulsed synchrotron emission peaks close to the light cylinder, and presents sub-pulse variability associated with the formation of plasmoids in the sheet.conclusions: this study suggests that the striped component of the wind dissipates far before reaching the termination shock in isolated pulsars, even in very-high-multiplicity systems such as the crab pulsar. pulsars in binary systems may provide the best environments to study magnetic dissipation in the wind. | dissipation of the striped pulsar wind |
we compute the gravitational waveform from a binary system in scalar-tensor gravity at 2pn relative order. we restrict our calculation to nonspinning binary systems on quasicircular orbits and compute the spin-weighted spherical modes of the radiation. the evolution of the phase of the waveform is computed in the time and frequency domains. the emission of dipolar radiation is the lowest-order dissipative process in scalar-tensor gravity. however, stringent constraints set by current astrophysical observations indicate that this effect is subdominant to quadrupolar radiation for most prospective gravitational-wave sources. we compute the waveform for systems whose inspiral is driven by: (a) dipolar radiation (e.g., binary pulsars or spontaneously scalarized systems) and (b) quadrupolar radiation (e.g., typical sources for space-based and ground-based detectors). for case (a), we provide complete results at 2pn, whereas for case (b), we must introduce unknown terms in the 2pn flux; these unknown terms are suppressed by constraints on scalar-tensor gravity. | gravitational waveforms in scalar-tensor gravity at 2pn relative order |
the identification of the ultraluminous x-ray source (ulx) x-2 in m82 as an accreting pulsar has shed new light on the nature of a subset of ulxs, while rising new questions on the nature of the super-eddington accretion. here, by numerically solving the torque equation of the accreting pulsar within the framework of the magnetically threaded-disc scenario, we show that three classes of solutions, corresponding to different values of the magnetic field, are mathematically allowed. we argue that the highest magnetic field one, corresponding to b ∼ 1013 g, is favoured based on physical considerations and the observed properties of the source. in particular, that is the only solution which can account for the observed variations in dot{p} (over four time intervals) without requiring major changes in dot{m}, which would be at odds with the approximately constant x-ray emission of the source during the same time. for this solution, we find that the source can only accommodate a moderate amount of beaming, 0.5 ≲ b < 1. last, we show that the upper limit on the luminosity, lx < 2.5 × 1038 erg s-1 from archival observations, is consistent with a highly magnetized neutron star being in the propeller phase at that time. | nustar j095551+6940.8: a highly magnetized neutron star with super-eddington mass accretion |
a charged particle moving through a medium emits cherenkov radiation when its velocity exceeds the phase velocity of light in that medium. under the influence of a strong electromagnetic field, quantum fluctuations can become polarized, imbuing the vacuum with an effective anisotropic refractive index and allowing the possibility of cherenkov radiation from the quantum vacuum. we analyze the properties of this vacuum cherenkov radiation in strong laser pulses and the magnetic field around a pulsar, finding regimes in which it is the dominant radiation mechanism. this radiation process may be relevant to the excess signals of high energy photons in astrophysical observations. | cherenkov radiation from the quantum vacuum |
due to ionosphere absorption and the interference by natural and artificial radio emissions, ground observation of the sky at the decameter or longer is very difficult. this unexplored part of electromagnetic spectrum has the potential of great discoveries, notably in the study of cosmic dark ages and dawn, but also in heliophysics and space weather, planets, cosmic ray and neutrinos, pulsar and interstellar medium, extragalactic radio sources, and even seti. at a forum organized by the international space science institute-beijing (issi-bj), we discussed the prospect of opening up this window for astronomical observations by using a constellation of small or micro-satellites. we discussed the past experiments and the current ones such as the low frequency payload on chang'e-4 mission lander, relay satellite and the longjiang satellite, and also the future dsl mission, which is a linear array on lunar orbit which can make synthesized map of the whole sky as well as measure the global spectrum. we also discuss the synergy with other experiments, including ground global experiments such as edges, saras, sci-hi and high-z, prizm/albatros, ground imaging facillities such as lofar and mwa, and space experiments such as sunrise, dare/dapper and pratush. we also discussed some technical aspects of the dsl concept. | discovering the sky at the longest wavelengths with small satellite constellations |
numerous extended sources around galactic pulsars have shown significant γ -ray emission from gev to tev energies, revealing hundreds of tev energy electrons scattering off of the underlying photon fields through inverse compton scattering (ics). hawc tev gamma-ray observations of few-degree extended emission around the pulsars geminga and monogem, and lat gev emission around geminga, suggest that systems older than 100 kyr have multi-tev e± propagating beyond the snr-pwn system into the interstellar medium. following the discovery of few γ -ray sources by hawc at energies e >100 tev , we investigate the presence of an extended γ -ray emission in fermi-lat data around the three brightest sources detected by hawc up to 100 tev. we find an extended emission of θ68=1.0 0-0.07+0.05 deg around ehwc j1825-134 and θ68=0.71 ±0.10 deg ehwc j 1907 +063 . the analysis with ics templates on fermi-lat data point to diffusion coefficient values which are significantly lower than the average galactic one. when studied along with hawc data, the γ -ray fermi-lat data provide invaluable insight into the very high-energy electron and positron parent populations. | investigating γ -ray halos around three hawc bright sources in f e r m i -lat data |
accurate measurements of the masses of neutron stars are necessary to test binary evolution models, and to constrain the neutron star equation of state. in pulsar binaries with no measurable post-keplerian parameters, this requires an accurate estimate of the binary system's inclination and the radial velocity of the companion star by other means than pulsar timing. in this paper, we present the results of a new method for measuring this radial velocity using the binary synthesis code icarus. this method relies on constructing a model spectrum of a tidally distorted, irradiated star as viewed for a given binary configuration. this method is applied to optical spectra of the newly discovered black widow psr j1555-2908. by modeling the optical spectroscopy alongside optical photometry, we find that the radial velocity of the companion star is 397 ± 4 km s-1 (errors quoted at 95 per cent confidence interval), as well as a binary inclination of >75°. combined with γ-ray pulsation timing information, this gives a neutron star mass of 1.67$^{+0.15}_{-0.09}$ m⊙ and a companion mass of 0.060$^{+0.005}_{-0.003}$ m⊙, placing psr j1555-2908 at the observed upper limit of what is considered a black widow system. | measuring the mass of the black widow psr j1555-2908 |
psr b0950+08 is a bright nonrecycled pulsar whose single-pulse fluence variability is reportedly large. based on observations at two widely separated frequencies, 55 mhz (nenufar) and 1.4 ghz (westerbork synthesis radio telescope), we review the properties of these single pulses. we conclude that they are more similar to ordinary pulses of radio emission than to a special kind of short and bright giant pulses, observed from only a handful of pulsars. we argue that a temporal variation of the properties of the interstellar medium along the line of sight to this nearby pulsar, namely the fluctuating size of the decorrelation bandwidth of diffractive scintillation makes an important contribution to the observed single-pulse fluence variability. we further present interesting structures in the low-frequency single-pulse spectra that resemble the "sad trombones" seen in fast radio bursts (frbs); although for psr b0950+08 the upward frequency drift is also routinely present. we explain these spectral features with radius-to-frequency mapping, similar to the model developed by wang et al. (2019, apj, 876, l15) for frbs. finally, we speculate that μs-scale fluence variability of the general pulsar population remains poorly known, and that its further study may bring important clues about the nature of frbs. | dual-frequency single-pulse study of psr b0950+08 |
fast radio bursts (frbs) are extragalactic radio transients of extraordinary luminosity. studying the diverse temporal and spectral behaviour recently observed in a number of frbs may help to determine the nature of the entire class. for example, a fast spinning or highly magnetised neutron star (ns) might generate the rotation-powered acceleration required to explain the bright emission. periodic, subsecond components suggesting such rotation were recently reported in one frb, and may also exist in two more. here we report the discovery of frb 20201020a with apertif, an frb that shows five components regularly spaced by 0.411 ms. this submillisecond structure in frb 20201020a carries important clues about the progenitor of this frb specifically, and potentially about the progenitors of frbs in general. we therefore contrast its features to what is seen in other frbs and pulsars, and to the predictions of some frb models. we present a timing analysis of the frb 20201020a components carried out in order to determine the significance of the periodicity. we compare these against the timing properties of the previously reported chime frbs with subsecond quasi-periodic components, and against two apertif bursts from repeating frb 20180916b, which show complex time-frequency structure. we find the periodicity of frb 20201020a to be marginally significant at 2.4σ. its repeating subcomponents cannot be explained as pulsar rotation because the required spin rate of over 2 khz exceeds the limits set by typical ns equations of state and observations. the fast periodicity is also in conflict with a compact object merger scenario. however, these quasi-periodic components could be caused by equidistant emitting regions in the magnetosphere of a magnetar. the submillisecond spacing of the components in frb 20201020a, the smallest observed so far in a one-off frb, may rule out both a ns spin period and binary mergers as the direct source of quasi-periodic frb structure. | a fast radio burst with submillisecond quasi-periodic structure |
the detection of ∼1.5-3.2 myr old 60fe on earth indicates recent nearby core-collapse supernovae. for supernovae in multiple stars, the primary stars may become neutron stars, while former companions may become unbound and become runaway stars. we wrote software for tracing back the space motion of runaway and neutron stars to young associations of massive stars. we apply it here to the nearby young scorpius-centaurus-lupus groups, all known runaway stars possibly coming from there, and all 400 neutron stars with known transverse velocity. we find kinematic evidence that the runaway ζ oph and the radio pulsar psr b1706-16 were released by a supernova in a binary 1.78 ± 0.21 myr ago at 107 ± 4 pc distance (for pulsar radial velocity 260 ± 43 km s-1); association age and flight time determine the progenitor mass (16-18 m⊙), which can constrain supernova nucleosynthesis yields and 60fe uptake on earth. in addition, we notice that the only high-mass x-ray binary in scorpius-centaurus-lupus (1h11255-567 with μ1 and μ2 cru) may include a neutron star formed in another sn, up to ∼1.8 myr ago at 89-112 pc, i.e. also yielding 60fe detectable on earth. our scenario links 60fe found on earth to one or two individual supernovae in multiple stars. | a nearby recent supernova that ejected the runaway star ζ oph, the pulsar psr b1706-16, and 60fe found on earth |
the center of the galaxy is one of the prime targets in the search for a signal of annihilating (or decaying) dark matter. if such a signal were to be detected, it would shed light on one of the biggest mysteries in physics today: what is dark matter? fundamental properties of the particle nature of dark matter, such as its mass, annihilation cross section, and annihilation final states, could be measured for the first time. several experiments have searched for such a signal, and some have measured excesses that are compatible with it. a long-standing and compelling excess is observed in γ-rays by the fermi large area telescope (fermi–lat). this excess is consistent with a dark matter particle with a mass of approximately 50 (up to ∼200) gev annihilating with a velocity-averaged cross section of ∼10‑26 cm3 s‑1. although a dark matter origin of the excess remains viable, other interpretations are possible. in particular, there is some evidence that the excess is produced by a population of unresolved point sources of γ-rays—for example, millisecond pulsars. in this article, i review the current status of the observation of the fermi–lat galactic center excess, the possible interpretations of the excess, the evidence and counterevidence for each, and the prospects for resolving its origin with future measurements. | the fermi–lat galactic center excess: evidence of annihilating dark matter? |
thanks to advances in γ -ray astronomy and precise measurements of cosmic ray (cr) properties, the last decade has witnessed significant progresses in our understanding of the origin of galactic crs. in particular, gev crs have been associated with supernova remnants (snrs) with soft γ -ray spectra, while there is evidence of efficient tev cr acceleration in snrs with hard γ -ray spectra. nevertheless, no ultra-high-energy (>50 tev) γ -ray emission has been detected from shocks of snrs, implying their limited contributions to pev cr fluxes. most ultra-high-energy γ -ray sources detected so far can be associated with pulsar wind nebulae (pwne), that are known pev electron/positron accelerators. their contributions to pev crs are still a matter of debate. the galactic center region and some star clusters, on the other hand, are known sources of pev crs. detailed modelings of cr spectra and anisotropy show that the supernova that gave rise to the geminga pulsar may have significant contributions to tev cr fluxes, and pev crs should have significant contributions from source(s) in the direction of the galactic center. | the origin of galactic cosmic rays |
due to ionosphere absorption and the interference of natural and artificial radio emissions, astronomical observation from the ground becomes very difficult at the wavelengths of decametre or longer, which we shall refer to as the ultralong wavelengths. this unexplored part of the electromagnetic spectrum has the potential for great discoveries, notably in the study of cosmic dark ages and dawn, but also in heliophysics and space weather, planets and exoplanets, cosmic ray and neutrinos, pulsar and interstellar medium (ism), extragalactic radio sources, and so on. the difficulty of the ionosphere can be overcome by space observation, and the moon can shield the radio frequency interferences (rfis) from the earth. a lunar orbit array can be a practical first step to opening up the ultralong wave band. compared with a lunar surface observatory on the far side, the lunar orbit array is simpler and more economical, as it does not need to make the risky and expensive landing, can be easily powered with solar energy, and the data can be transmitted back to the earth when it is on the near-side part of the orbit. here, i describe the discovering sky at the longest wavelength (dsl) project, which will consist of a mother satellite and 6-9 daughter satellites, flying on the same circular orbit around the moon, and forming a linear interferometer array. the data are collected by the mother satellite which computes the interferometric cross-correlations (visibilities) and transmits the data back to the earth. the whole array can be deployed on the lunar orbit with a single rocket launch. the project is under intensive study in china. this article is part of a discussion meeting issue `astronomy from the moon: the next decades'. | discovering the sky at the longest wavelengths with a lunar orbit array |
the secondary component of the gw190814 event left us with a question, “whether it is a supermassive neutron star or lightest black-hole?” recently, fattoyev et al. obtained an energy density functional (edf) named as bigapple, which reproduces the mass of the neutron star is 2.60 m⊙ which is well consistent with gw190814 data. this study explores the properties of finite nuclei, nuclear matter and neutron stars by using the bigapple edf along with four well-known relativistic mean-field forces, namely nl3, g3, iopb-i and fsugarnet. the finite nuclei properties like binding energy per particle, skin thickness, charge radius, single-particle energy, and two-neutron separation energy are well predicted by the bigapple for a series of nuclei. the calculated nuclear matter quantities, such as incompressibility, symmetry energy and slope parameters at saturation density, are consistent with the empirical or experimental values where ever available. the predicted canonical tidal deformability by the bigapple parameter set is well-matched with the gw190814 data. also, the dimensionless moment of inertia lies in the range given by the analysis of psr j0737-3039a. | bigapple force and its implications to finite nuclei and astrophysical objects |
we present here a simple, systematic method for determining the correct global rotation count of a radio pulsar; an essential step for the derivation of an accurate phase-coherent ephemeris. we then build on this method by developing a new algorithm for determining the global rotational count for pulsars with sparse timing data sets. this makes it possible to obtain phase-coherent ephemerides for pulsars for which this has been impossible until now. as an example, we do this for psr j0024-7205aa, an extremely faint millisecond pulsar (msp) recently discovered in the globular cluster 47 tucanae. this algorithm has the potential to significantly reduce the number of observations and the amount of telescope time needed to follow up on new pulsar discoveries. | an algorithm for determining the rotation count of pulsars |
based on relativistic mean field (rmf) models, we study finite λ-hypernuclei and massive neutron stars. the effective n-n interactions pk1 and tm1 are adopted, while the n-λ interactions are constrained by reproducing the binding energy of λ-hyperon at 1s orbit of {}{{λ }}{}40{ca}. it is found that the λ-meson couplings follow a simple relation, indicating a fixed λ potential well for symmetric nuclear matter at saturation densities, i.e., around {v}{{λ }}=-29.786 {mev}. with those interactions, a large mass range of λ-hypernuclei can be described well. furthermore, the masses of psr j1614-2230 and psr j0348+0432 can be attained adopting the λ-meson couplings {g}{{σ }{{λ }}}/{g}{{σ }n}≳ 0.73, {g}{{ω }{{λ }}}/{g}{{ω }n}≳ 0.80 for pk1 and {g}{{σ }{{λ }}}/{g}{{σ }n}≳ 0.81, {g}{{ω }{{λ }}}/{g}{{ω }n}≳ 0.90 for tm1, respectively. this resolves the hyperon puzzle without introducing any additional degrees of freedom. supported by national natural science foundation of china (11525524, 11505157, 11375022, 11705163, 11621131001), national key basic research program of china (2013cb834400), the physics research and development program of zhengzhou university (32410017) and the office of nuclear physics in the u.s. dept. of energy. the computation for this work was supported by the hpc cluster of skltp/itp-cas and the supercomputing center, cnic, of the cas | massive neutron stars and λ-hypernuclei in relativistic mean field models |
we present charged anisotropic durgapal iv interior solutions of the general relativistic field equations in curvature coordinates. these exact solutions can be used to model stable and well-behaved compact stars. using these solutions we have presented models of well-known neutron and quark stars such as psr j1903+0327, rx-j1856.5-3754, psr b1913+16, psr j0737-3039a and cyg x-2. the equation of state (eos) corresponding to the modeled objects are studied using their compression moduli. according to our solutions it is found that the eos for cyg x-2 (neutron star) is stiffer than any other object presented and therefore more massive. furthermore, the eos for rx j1856.5-3754 (quark star) is the softest one, rendering it least massive. these solutions satisfy all the energy conditions. finally, all our presented stellar models satisfy the equilibrium condition of cooperstock and de la cruz i.e. m 2 > q 2. | some analytic models of relativistic compact stars |
we collect new and archival optical observations of nine “black-widow” millisecond pulsar binaries. new measurements include direct imaging with the keck, gemini-s, mdm, and las cumbres observatory 2 m telescopes. this is supplemented by synthesized colors from keck long-slit spectra. four black-widow optical companions are presented here for the first time. together these data provide multicolor photometry covering a large fraction of the orbital phase. we fit these light curves with a direct (photon) heating model using a version of the icarus light-curve modeling code. the fits provide distance and fill-factor estimates, inclinations, and heating powers. we compare the heating powers with the observed gev luminosities, noting that the ratio is sensitive to pulsar distance and to the gamma-ray beaming. we make a specific correction for “outer gap” model beams, but even then some sources are substantially discrepant, suggesting imperfect beaming corrections and/or errors in the fit distance. the fits prefer large metal abundance for half of the targets, a reasonable result for these wind-stripped secondaries. the companion radii indicate substantial roche-lobe filling, fc≈ 0.7-1 except for psr j0952-0607, which with fc< 0.5 has a companion density ρ ≈ 10 g cm-3, suggesting unusual evolution. we note that the direct-heating fits imply large heating powers and rather small inclinations, and we speculate that unmodeled effects can introduce such bias. | multiband optical light curves of black-widow pulsars |
the pulsar radio emission is commonly associated with the plasma outflow in the open field line tube; then a pencil beam is emitted along the pulsar magnetic axis. observations suggest that there is an additional radio emission mechanism specific for pulsars with high magnetic field at the light cylinder. these pulsars are known to be strong sources of non-thermal high-energy radiation, which could be attributed to reconnection in the current sheet separating, just beyond the light cylinder, the oppositely directed magnetic fields. pulsars with the highest magnetic field at the light cylinder (>100 kg) exhibit also radio pulses in phase with the high-energy pulses. moreover, giant radio pulses are observed in these pulsars. i argue that the reconnection process that produces high-energy emission could also be responsible for the radio emission. namely, coalescence of magnetic islands in the sheet produces magnetic perturbations that propagate away in the form of electromagnetic nano-shots. i estimate the parameters of this emission and show that they are compatible with observations. | radio emission of the crab and crab-like pulsars |
among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect “memory” signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. the north american nanohertz observatory for gravitational waves (nanograv) observes dozens of the most stable millisecond pulsars using the arecibo and green bank radio telescopes in an effort to study, among other things, gravitational wave memory. we herein present the results of a search for gravitational wave bursts with memory (bwms) using the first five years of nanograv observations. we develop original methods for dramatically speeding up searches for bwm signals. in the directions of the sky where our sensitivity to bwms is best, we would detect mergers of binaries with reduced masses of {10}9 {m}⊙out to distances of 30 mpc; such massive mergers in the virgo cluster would be marginally detectable. we find no evidence for bwms. however, with our non-detection, we set upper limits on the rate at which bwms of various amplitudes could have occurred during the time spanned by our data—e.g., bwms with amplitudes greater than 10-13 must encounter the earth at a rate less than 1.5 yr-1. | nanograv constraints on gravitational wave bursts with memory |
in the standard gravitational lensing scenario, rays from a background source are bent in the direction of a foreground lensing mass distribution. diverging lens behaviour produces deflections in the opposite sense to gravitational lensing, and is also of astrophysical interest. in fact, diverging lensing due to compact distributions of plasma has been proposed as an explanation for the extreme scattering events that produce frequency-dependent dimming of extragalactic radio sources, and may also be related to the refractive radio wave phenomena observed to affect the flux density of pulsars. in this work we study the behaviour of two families of astrophysical diverging lenses in the geometric optics limit, the power law, and the exponential plasma lenses. generally, the members of these model families show distinct behaviour in terms of image formation and magnification, however the inclusion of a finite core for certain power-law lenses can produce a caustic and critical curve morphology that is similar to the well-studied gaussian plasma lens. both model families can produce dual radial critical curves, a novel distinction from the tangential distortion usually produced by gravitational (converging) lenses. the deflection angle and magnification of a plasma lens vary with the observational frequency, producing wavelength-dependent magnifications that alter the amplitudes and the shape of the light curves. thus, multiwavelength observations can be used to physically constrain the distribution of the electron density in such lenses. | two families of astrophysical diverging lens models |
in this paper searches for flaring astrophysical neutrino sources and sources with periodic emission with the icecube neutrino telescope are presented. in contrast to time-integrated searches, where steady emission is assumed, the analyses presented here look for a time-dependent signal of neutrinos using the information from the neutrino arrival times to enhance the discovery potential. a search was performed for correlations between neutrino arrival times and directions, as well as neutrino emission following time-dependent light curves, sporadic emission, or periodicities of candidate sources. these include active galactic nuclei, soft γ-ray repeaters, supernova remnants hosting pulsars, microquasars, and x-ray binaries. the work presented here updates and extends previously published results to a longer period that covers 4 years of data from 2008 april 5 to 2012 may 16, including the first year of operation of the completed 86 string detector. the analyses did not find any significant time-dependent point sources of neutrinos, and the results were used to set upper limits on the neutrino flux from source candidates. | searches for time-dependent neutrino sources with icecube data from 2008 to 2012 |
high brightness temperature radio transients such as pulsars and fast radio bursts (frbs) require the coherent radiation of particles. the antenna class of coherent radiation models require a large number of charged particles radiating in phase, therefore the particles must be spatially confined and have well-aligned velocities. given these necessary conditions, we look at the magnetic field induced by the currents associated with coherently emitting accelerated particles and consider the interaction between the radiating particles and the induced magnetic field. we find a maximum luminosity of coherent curvature radiation that depends on source parameters such as surface magnetic field and neutron star spin period. we find that coherent radio emission across all luminosities can be explained by coherent curvature radiation and suggest it could be universally responsible for both frbs and extreme galactic sources. using the crab pulsar as an example, we constrain the emission parameters and origin of the most extreme nanoshots to within 60 km of the pulsar surface assuming coherent curvature radiation. in agreement with recent observations, we also predict simultaneous x-ray emission from small-scale particle gyration due to the induced field. | coherent curvature radiation: maximum luminosity and high-energy emission |
psr j1713+0747 is one of the most precisely timed pulsars in the international pulsar timing array experiment. this pulsar showed an abrupt profile shape change between 2021 april 16, (mjd 59320) and 2021 april 17 (mjd 59321). in this paper, we report the results from multi-frequency observations of this pulsar carried out with the upgraded giant metrewave radio telescope (ugmrt) before and after the event. we demonstrate the profile change seen in band 5 (1260 mhz-1460 mhz) and band 3 (300 mhz-500 mhz). the timing analysis of this pulsar shows a disturbance accompanying this profile change followed by a recovery with a time-scale of ~159 days. our data suggest that a model with chromatic index as a free parameter is preferred over models with combinations of achromaticity with dm bump or scattering bump. we determine the frequency dependence to be ~ν+1.34. | evidence for profile changes in psr j1713+0747 using the ugmrt |
with strong evidence of a common-spectrum stochastic process in the most recent data sets from the nanograv collaboration, the european pulsar timing array (pta), parkes pta, and the international pta, it is crucial to assess the effects of the several astrophysical and cosmological sources that could contribute to the stochastic gravitational wave background (gwb). using the same data set creation and injection techniques as in pol et al., we assess the separability of multiple gwbs by creating single and multiple gwb source data sets. we search for these injected sources using bayesian pta analysis techniques to assess recovery and separability of multiple astrophysical and cosmological backgrounds. for a gwb due to supermassive black hole binaries and an underlying weaker background due to primordial gravitational waves with a gw energy-density ratio of ωpgw/ωsmbhb = 0.5, the bayes' factor for a second process exceeds unity at 17 yr, and increases with additional data. at 20 yr of data, we are able to constrain the spectral index and amplitude of the weaker gwb at this density ratio to a fractional uncertainty of 64% and 110%, respectively, using current pta methods and techniques. using these methods and findings, we outline a basic protocol to search for multiple backgrounds in future pta data sets. | disentangling multiple stochastic gravitational wave background sources in pta data sets |
over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. in the original `black widow', the eight-hour orbital period eclipsing pulsar psr j1959+2048 (psr b1957+20)1, high-energy emission originating from the pulsar2 is irradiating and may eventually destroy3 a low-mass companion. these systems are not only physical laboratories that reveal the interesting results of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars4, allowing for robust tests of the neutron star equation of state. here we report observations of ztf j1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate, the optical flux of which varies by a factor of more than ten. ztf j1406+1222 pushes the boundaries of evolutionary models5, falling below the 80-minute minimum orbital period of hydrogen-rich systems. the wide tertiary companion is a rare low-metallicity cool subdwarf star, and the system has a galactic halo orbit consistent with passing near the galactic centre, making it a probe of formation channels, neutron star kick physics6 and binary evolution. | a 62-minute orbital period black widow binary in a wide hierarchical triple |
the cooling mechanisms of a neutron star (hereafter referred to as ns) have the potential to reveal important features of superdense matter. the values of the surface temperatures are known for a good sample of nss aged ≤ 106 years and, with some exceptions, can be explained through standard cooling mechanisms (neutrinos and photons emissions without internal heating), as confirmed in our study. however, for older objects the surface temperatures are in some cases higher than expected, and it is necessary to consider some kind of internal heating to explain these results. with this objective, we revisit in this paper the kinetic mechanisms of heating in nss considering fermionic dark matter (dm) heating, rotochemical heating and magnetic field decay. our results indicate that nss older than ∼106 years, such as some “black widows” (bw — a subset of binary systems in which only the upper bounds of surface temperatures are known) and old pulsars, in contrast to younger nss, exhibit much higher surface temperatures than the values predicted by these three heating mechanisms. furthermore, by restricting the dm heating parameters to the current values that were fitted and/or measured for the local dm density, masses and ns radii, the models studied here also do not reproduce the upper limits of the temperatures from the surface of bws or the actual temperatures of other ancient pulsars. we conclude that if the upper limits for bws are close to real temperatures, dark heating will not represent a convincing explanation of these results, indicating that rotochemical mechanisms may be favored. | internal heating mechanisms in neutron stars |
data from the fermi large area telescope suggests that there is an extended excess of gev gamma-ray photons in the inner galaxy. identifying potential astrophysical sources that contribute to this excess is an important step in verifying whether the signal originates from annihilating dark matter. in this paper, we focus on the potential contribution of unresolved point sources, such as millisecond pulsars (msps). we propose that the statistics of the photons-in particular, the flux probability density function (pdf) of the photon counts below the point-source detection threshold-can potentially distinguish between the dark-matter and point-source interpretations. we calculate the flux pdf via the method of generating functions for these two models of the excess. working in the framework of bayesian model comparison, we then demonstrate that the flux pdf can potentially provide evidence for an unresolved msp-like point-source population. | distinguishing dark matter from unresolved point sources in the inner galaxy with photon statistics |
supermassive black hole-host galaxy relations are key to the computation of the expected gravitational wave background (gwb) in the pulsar timing array (pta) frequency band. it has been recently pointed out that standard relations adopted in gwb computations are in fact biased-high. we show that when this selection bias is taken into account, the expected gwb in the pta band is a factor of about 3 smaller than previously estimated. compared to other scaling relations recently published in the literature, the median amplitude of the signal at f = 1 yr-1 drops from 1.3 × 10-15 to 4 × 10-16. although this solves any potential tension between theoretical predictions and recent pta limits without invoking other dynamical effects (such as stalling, eccentricity or strong coupling with the galactic environment), it also makes the gwb detection more challenging. | selection bias in dynamically measured supermassive black hole samples: consequences for pulsar timing arrays |
we have modelled the near-infrared to radio images of the crab nebula with a bayesian sed model to simultaneously fit its synchrotron, interstellar (is), and supernova dust emission. we infer an is dust extinction map with an average av = 1.08 ± 0.38 mag, consistent with a small contribution ({≲ }22{{ per cent}}) to the crab's overall infrared emission. the crab's supernova dust mass is estimated to be between 0.032 and 0.049 m⊙ (for amorphous carbon grains) with an average dust temperature tdust = 41 ± 3 k, corresponding to a dust condensation efficiency of 8-12 {{ per cent}}. this revised dust mass is up to an order of magnitude lower than some previous estimates, which can be attributed to our different is dust corrections, lower spire flux densities, and higher dust temperatures than were used in previous studies. the dust within the crab is predominantly found in dense filaments south of the pulsar, with an average v-band dust extinction of av = 0.20-0.39 mag, consistent with recent optical dust extinction studies. the modelled synchrotron power-law spectrum is consistent with a radio spectral index αradio = 0.297 ± 0.009 and an infrared spectral index αir = 0.429 ± 0.021. we have identified a millimetre excess emission in the crab's central regions, and argue that it most likely results from two distinct populations of synchrotron emitting particles. we conclude that the crab's efficient dust condensation (8-12 {{ per cent}}) provides further evidence for a scenario where supernovae can provide substantial contributions to the is dust budgets in galaxies. | the dust content of the crab nebula |
this article investigates the dust ion acoustic multi-shock wave excitations in weakly relativistic multi-component plasma by assuming nonthermal, nonextensive electrons and positrons, relativistic ion fluid having kinetic viscosity and immobile dust. burgers equation is derived to investigate such excitations by applying the reductive perturbation method. the exponential functions are directly implemented to determine the novel multi-shock wave solution of burgers equation. the dust ion acoustic (dia) multi-shock wave excitations are investigated systematically to reveal the effects of parameters, namely, viscosity coefficient of ions, positron to electron density ratio, immobile dust to electron density ratio, ion to electron temperature ratio, electron to positron temperature ratio, and relativistic streaming factor of ions in the presence of nonthermal, nonextensive, and concurrently acting nonthermal and nonextensive electrons as well as positrons. it is found that the amplitudes and widths of not only single, but also multi-shock wave compressive and rarefactive electrostatic potential structures are changed with the influence of all plasma parameters. the obtained results may be useful to analyze the nature of dia multi-shock wave phenomena in various astrophysical as well as space environments (particularly, in pulsar relativistic winds with supernova ejecta) and future studies in plasma laboratory. | dust ion acoustic multi-shock wave excitations in the weakly relativistic plasmas with nonthermal nonextensive electrons and positrons |
we present a timing analysis of multiple xmm-newton and nustar observations of the ultra-luminous pulsar ngc 7793 p13 spread over its 65 d variability period. we use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with porb = 63.9+0.5-0.6 d, and find an eccentricity of e ≤ 0.15. the orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. we also analyze data from dense monitoring of the source with swift and find an optical photometric period of 63.9 ± 0.5 d and an x-ray flux period of 66.8 ± 0.4 d. the optical period is consistent with the orbital period, while the x-ray flux period is significantly longer. we discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. we put the orbital period of p13 into context with the orbital periods implied for two other ultra-luminous pulsars, m82 x-2 and ngc 5907 ulx, and discuss possible implications for the system parameters. | a tale of two periods: determination of the orbital ephemeris of the super-eddington pulsar ngc 7793 p13 |
a construction and analysis of an exact solution to einstein's field equations with the source of matter being an anisotropic fluid that allows to model compact stars with a rate of compactness u ≤0 .2061897680 of which the orders of magnitude of pressure and density match those of neutron stars. the model is physically acceptable due to its regular geometry, the density and radial pressure are monotonically decreasing as function of the radial distance, and the speeds of the radial and tangential sounds do not break the condition of causality and as such according to the concept of "cracking" the solution is potentially stable. in a particular manner for the observational data of the star psr j0437-4715 with a mass of 1 .44 m⊙ and radius r = 13.1 km a description of its internal behaviour and range of values for the hydrostatic variables is presented. their compactness is u = 0.162299, while their maximum central density ρc=5 .949601 ×1017 kg/m3 and the maximum density on the surface ρb=1 .958845 ×1017 kg/m3, which is consistent with the expected values for the neutron stars. | a possible representation for the neutron star psr j0437-4715 |
context. high-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (dm), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. correcting for dm variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. this may, however, give incorrect measurements if the probed electron content (and therefore the dm) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies.aims: we study small-scale density variations in the ionised interstellar medium. these structures may lead to frequency-dependent dms in pulsar signals. such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data.methods: we used high-cadence, low-frequency observations with three stations from the german long-wavelength (glow) consortium, which are part of the low-frequency array (lofar). specifically, 3.5 yr of weekly observations of psr j2219+4754 are presented.results: we present the first detection of frequency-dependent dms towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured dms. the observed dm variability is significant and may be caused by extreme scattering events. potential causes for frequency-dependent dms are quantified and evaluated.conclusions: we conclude that frequency dependence of dms has been reliably detected and is indeed caused by small-scale (up to tens of aus) but steep density variations in the interstellar electron content. we find that long-term trends in dm variability equally affect dms measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited. | first detection of frequency-dependent, time-variable dispersion measures |
the parkes pulsar timing array (ppta) project monitors two dozen millisecond pulsars (msps) in order to undertake a variety of fundamental physics experiments using the parkes 64-m radio telescope. since 2017 june, we have been undertaking commensal searches for fast radio bursts (frbs) during the msp observations. here, we report the discovery of four frbs (171209, 180309, 180311, and 180714). the detected events include an frb with the highest signal-to-noise ratio ever detected at the parkes observatory, which exhibits unusual spectral properties. all four frbs are highly polarized. we discuss the future of commensal searches for frbs at parkes. | commensal discovery of four fast radio bursts during parkes pulsar timing array observations |
at high-energy γ-rays (>100 mev), the large area telescope (lat) on the fermi satellite already detected more than 145 rotation-powered pulsars (rpps), while the number of pulsars seen at soft γ-rays (20 kev-30 mev) remained small. we present a catalogue of 18 non-recycled rpps from which presently non-thermal pulsed emission has been securely detected at soft γ-rays above 20 kev, and characterize their pulse profiles and energy spectra. for 14 of them, we report new results, (re)analysing mainly data from rxte, integral, xmm-newton and chandra. the soft γ-pulsars are all fast rotators and on average ∼9.3 times younger and ∼43 times more energetic than the fermi lat sample. the majority (11 members) exhibits broad, structured single pulse profiles, and only six have double (or even multiple, vela) pulses. 15 soft γ-ray pulsar show hard power-law spectra in the hard x-ray band and reach maximum luminosities typically in the mev range. for only 7 of the 18 soft γ-ray pulsars, pulsed emission has also been detected by the lat, but 12 have a pulsar wind nebula (pwn) detected at tev energies. for six pulsars with pwne, we present also the spectra of the total emissions at hard x-rays, and for igr j18490-0000, associated with hess j1849-000 and psr j1849-0001, we used our chandra data to resolve and characterize the contributions from the point source and pwn. finally, we also discuss a sample of 15 pulsars which are candidates for future detection of pulsed soft γ-rays, given their characteristics at other wavelengths. | the soft γ-ray pulsar population: a high-energy overview |
recent studies of low redshift type ia supernovae (sn ia) indicate that half explode from less than chandrasekhar mass white dwarfs, implying ignition must proceed from something besides the canonical criticality of chandrasekhar mass sn ia progenitors. we show that 1-100 pev mass asymmetric dark matter, with imminently detectable nucleon scattering interactions, can accumulate to the point of self-gravitation in a white dwarf and collapse, shedding gravitational potential energy by scattering off nuclei, thereby heating the white dwarf and igniting the flame front that precedes sn ia. we combine data on sn ia masses with data on the ages of sn ia-adjacent stars. this combination reveals a 2.8 σ inverse correlation between sn ia masses and ignition ages, which could result from increased capture of dark matter in 1.4 vs 1.1 solar mass white dwarfs. future studies of sn ia in galactic centers will provide additional tests of dark-matter-induced type ia ignition. remarkably, both bosonic and fermionic sn ia-igniting dark matter also resolve the missing pulsar problem by forming black holes in ≳10 myr old pulsars at the center of the milky way. | dark matter ignition of type ia supernovae |
we outline a model of the crab pulsar wind nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) component-i due to fermi-i acceleration at the equatorial portion of the termination shock, with particle spectral index pi ≈ 2.2 above the injection break corresponding to γwindσwind ∼ 105, peaking in the ultraviolet (uv, γwind ∼ 102 is the bulk lorentz factor of the wind, σwind ∼ 103 is wind magnetization); and (ii) component-ii due to acceleration at reconnection layers in the bulk of the turbulent nebula, with particle index pii ≈ 1.6. the model requires relatively slow but highly magnetized wind. for both components, the overall cooling break is in the infrared at ∼0.01 ev, so that the component-i is in the fast cooling regime (cooling frequency below the peak frequency). in the optical band, component-i produces emission with the cooling spectral index of αo ≈ 0.5, softening towards the edges due to radiative losses. above the cooling break, in the optical, uv, and x-rays, component-i mostly overwhelms component-ii. we hypothesize that acceleration at large-scale current sheets in the turbulent nebula (component-ii) extends to the synchrotron burn-off limit of ɛs ∼ 100 mev. thus in our model acceleration in turbulent reconnection (component-ii) can produce both hard radio spectra and occasional gamma-ray flares. this model may be applicable to a broader class of high-energy astrophysical objects, like active galactic nuclei and gamma-ray burst jets, where often radio electrons form a different population from the high-energy electrons. | interpreting crab nebula's synchrotron spectrum: two acceleration mechanisms |
nearby sources may contribute to cosmic-ray electron (cre) structures at high energies. recently, the first dampe results on the cre flux hinted at a narrow excess at energy ∼1.4 tev . we show that in general a spectral structure with a narrow width appears in two scenarios. the first is spectrum broadening for the continuous sources with a δ -function-like injection spectrum. in this scenario, a finite width can develop after propagation through the galaxy, which can reveal the distance of the source. well-motivated sources include minispikes and subhalos formed by dark matter (dm) particles χs which annihilate directly into e+e- pairs. the second is phase-space shrinking for burstlike sources with a power-law-like injection spectrum. the spectrum after propagation can shrink at a cooling-related cutoff energy and form a sharp spectral peak. the peak can be more prominent due to the energy-dependent diffusion. in this scenario, the width of the excess constrains both the power index and the distance of the source. possible such sources are pulsar wind nebulae (pwne) and supernova remnants (snrs). we analysis the dampe excess and find that the continuous dm sources should be fairly close within ∼0.3 kpc , and the annihilation cross sections are close to the thermal value. for the burstlike source, the narrow width of the excess suggests that the injection spectrum must be hard with power index significantly less than two, the distance is within ∼(3 - 4 ) kpc , and the age of the source is ∼0.16 myr . in both scenarios, large anisotropies in the cre flux are predicted. we identify possible candidates of minispike and pwn sources in the current fermi-lat 3fgl and atnf catalog, respectively. the diffuse γ -rays from these sources can be well below the galactic diffuse γ -ray backgrounds and less constrained by the fermi-lat data, if they are located at the low galactic latitude regions. | origins of sharp cosmic-ray electron structures and the dampe excess |
context. the existence of 2 m⊙ pulsars puts very strong constraints on the equation of state (eos) of neutron stars (nss) with hyperon cores, which can be satisfied only by special models of hadronic matter. the radius-mass relation for these models is sufficiently specific that it could be subjected to an observational test with future x-ray observatories.aims: we want to study the impact of the presence of hyperon cores on the radius-mass relation for ns. we aim to find out how, and for which particular stellar mass range, a specific relation r(m), where m is the gravitational mass, and r is the circumferential radius, is associated with the presence of a hyperon core.methods: we consider a set of 14 theoretical eos of dense matter, based on the relativistic mean-field approximation, allowing for the presence of hyperons in nss. we also discuss a recent eos based on non-relativistic g-matrix theory yielding nss with hyperonic cores and m> 2m⊙. we seek correlations between r(m) and the stiffness of the eos below the hyperon threshold needed to pass the 2 m⊙ test.results: for ns masses 1.0 <m/m⊙< 1.6, we get r> 13 km, because of a very stiff pre-hyperon segment of the eos. at nuclear density (n0 = 0.16 fm-3), the pressure is significantly higher than a robust upper bound obtained recently using chiral effective field theory.conclusions: if massive nss do have a sizable hyperon core, then according to current models the radii for m = 1.0 - 1.6 m⊙ are necessarily >13 km. if, on the contrary, a ns with a radius r(obs)< 12 km is observed in this mass domain, then sizable hyperon cores in nss, as we model them now, are ruled out. future x-ray missions with <5% precision for a simultaneous m and r measurement will have the potential to solve the problem with observations of nss. irrespective of this observational test, present eos allowing for hyperons that fulfill condition mmax> 2 m⊙ yield a pressure at nuclear density that is too high relative to up-to-date microscopic calculations of this quantity. | neutron stars with hyperon cores: stellar radii and equation of state near nuclear density |
we report on an effort to extract and monitor interstellar scintillation parameters in regular timing observations collected for the north american nanohertz observatory for gravitational waves pulsar timing array. scattering delays are measured by creating dynamic spectra for each pulsar and observing epoch of wide-band observations centered near 1500 mhz and carried out at the green bank telescope and the arecibo observatory. the ∼800 mhz wide frequency bands imply dramatic changes in scintillation bandwidth across the bandpass, and a stretching routine has been included to account for this scaling. for most of the 10 pulsars for which the scaling has been measured, the bandwidths scale with frequency less steeply than expected for a kolmogorov medium. we find estimated scattering delay values that vary with time by up to an order of magnitude. the mean measured scattering delays are similar to previously published values and are slightly higher than predicted by interstellar medium models. we investigate the possibility of increasing the timing precision by mitigating timing errors introduced by the scattering delays. for most of the pulsars, the uncertainty in the time of arrival of a single timing point is much larger than the maximum variation of the scattering delay, suggesting that diffractive scintillation remains as only a negligible part of their noise budget. | the nanograv nine-year data set: monitoring interstellar scattering delays |
we present an extended grid of mean three-dimensional (3d) spectra for low-mass, pure-hydrogen atmosphere da white dwarfs (wds). we use co5bold radiation-hydrodynamics 3d simulations covering teff = 6000-11,500 k and log g = 5-6.5 (g in cm s-2) to derive analytical functions to convert spectroscopically determined 1d temperatures and surface gravities to 3d atmospheric parameters. along with the previously published 3d models, the 1d to 3d corrections are now available for essentially all known convective da wds (i.e., log g = 5-9). for low-mass wds, the correction in temperature is relatively small (a few percent at the most), but the surface gravities measured from the 3d models are lower by as much as 0.35 dex. we revisit the spectroscopic analysis of the extremely low-mass (elm) wds, and demonstrate that the 3d models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool elm wds in eclipsing double wd and wd + millisecond pulsar binary systems. we also use the 3d corrections to revise the boundaries of the zz ceti instability strip, including the recently found elm pulsators. | 3d model atmospheres for extremely low-mass white dwarfs |
we present initial pulsar results from the first station of the long wavelength array (lwa1) obtained during the commissioning period of lwa1 and in early science results. we present detections of periodic emission from 44 previously known pulsars, including 3 millisecond pulsars. the effects of the interstellar medium (ism) on pulsar emission are significantly enhanced at the low frequencies of the lwa1 band (10-88 mhz), making lwa1 a very sensitive instrument for characterizing changes in the dispersion measure (dm) and other effects from the ism. pulsars also often have significant evolution in their pulse profile at low frequency and a break in their spectral index. we report dm measurements for 44 pulsars, mean flux density measurements for 36 pulsars, and multi-frequency component spacing and widths for 15 pulsars with more than one profile component. for 27 pulsars, we report spectral index measurements within our frequency range. we also introduce the lwa1 pulsar data archive, which stores reduced data products from lwa1 pulsar observations. reduced data products for the observations presented here can be found in the archive. reduced data products from future lwa1 pulsar observations will also be made available through the archive. | pulsar observations using the first station of the long wavelength array and the lwa pulsar data archive |
massive black hole binaries (mbhbs) are thought to be the main source of gravitational waves (gws) in the low-frequency domain surveyed by ongoing and forthcoming pulsar timing array campaigns and future space-borne missions, such as elisa. however, many low-redshift mbhbs in realistic astrophysical environments may not reach separations small enough to allow significant gw emission, but rather stall on (sub)pc-scale orbits. this `last-parsec problem' can be eased by the appearance of a third massive black hole (mbh) - the `intruder' - whose action can force, under certain conditions, the inner mbhb on a very eccentric orbit, hence allowing intense gw emission eventually leading to coalescence. a detailed assessment of the process, ultimately driven by the induced kozai-lidov oscillations of the mbhb orbit, requires a general relativistic treatment and the inclusion of external factors, such as the newtonian precession of the intruder orbit in the galactic potential and its hardening by scattering off background stars. in order to tackle this problem, we developed a three-body post-newtonian (pn) code framed in a realistic galactic potential, including both non-dissipative 1pn and 2pn terms, and dissipative terms such as 2.5pn effects, orbital hardening of the outer binary, and the effect of the dynamical friction on the early stages of the intruder dynamics. in this first paper of a series devoted at studying the dynamics of mbh triplets from a cosmological perspective, we describe, test and validate our code. | post-newtonian evolution of massive black hole triplets in galactic nuclei - i. numerical implementation and tests |
we present a new study of the rayleigh-taylor unstable regime of accretion on to rotating magnetized stars in a set of high grid resolution three-dimensional magnetohydrodynamic simulations performed in low-viscosity discs. we find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter ωs = ω⋆/ωk(rm), where ω⋆ is the angular velocity of the star and ωk(rm) is the angular velocity of the keplerian disc at the disc-magnetosphere boundary r = rm. we found that accretion is unstable if ωs ≲ 0.6. accretion through instabilities is present in stars with different magnetospheric sizes. however, only in stars with relatively small magnetospheres, rm/r⋆ ≲ 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. at even smaller values of the fastness parameter, ωs ≲ 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. this transition occurs in stars with even smaller magnetospheres, rm/r⋆ ≲ 4.2. most of our simulations were performed at a small tilt of the dipole magnetosphere, θ = 5°, and a small viscosity parameter α = 0.02. test simulations at higher α values show that many more cases become unstable, and the light curves become even more irregular. test simulations at larger tilts of the dipole θ show that instability is present, however, accretion in two funnel streams dominates if θ ≳ 15°. the results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: classical t tauri stars, accreting millisecond x-ray pulsars, and cataclysmic variables. | boundary between stable and unstable regimes of accretion. ordered and chaotic unstable regimes |
many powerful and variable gamma-ray sources, including pulsar wind nebulae, active galactic nuclei and gamma-ray bursts, seem capable of accelerating particles to gamma-ray emitting energies efficiently over very short timescales. these are likely due to the rapid dissipation of electromagnetic energy in a highly magnetized, relativistic plasma. in order to understand the generic features of such processes, we have investigated simple models based on the relaxation of unstable force-free magnetostatic equilibria. in this work, we make the connection between the corresponding plasma dynamics and the expected radiation signal, using 2d particle-in-cell simulations that self-consistently include synchrotron radiation reactions. we focus on the lowest order unstable force-free equilibrium in a 2d periodic box. we find that rapid variability, with modest apparent radiation efficiency as perceived by a fixed observer, can be produced during the evolution of the instability. the “flares” are accompanied by an increased polarization degree in the high energy band, with rapid variation in the polarization angle. furthermore, the separation between the acceleration sites and the synchrotron radiation sites for the highest energy particles facilitates acceleration beyond the synchrotron radiation reaction limit. we also discuss the dynamical consequences of the radiation reaction, and some astrophysical applications of this model. our current simulations with numerically tractable parameters are not yet able to reproduce the most dramatic gamma-ray flares, e.g., from the crab nebula. higher magnetization studies are promising and will be carried out in the future. | kinetic study of radiation-reaction-limited particle acceleration during the relaxation of unstable force-free equilibria |
psr j0537-6910, also known as the big glitcher, is the most prolific glitching pulsar known, and its spin-induced pulsations are only detectable in x-ray. we present results from analysis of 2.7 yr of nicer timing observations, from 2017 august to 2020 april. we obtain a rotation phase-connected timing model for the entire time span, which overlaps with the third observing run of ligo/virgo, thus enabling the most sensitive gravitational wave searches of this potentially strong gravitational wave-emitting pulsar. we find that the short-term braking index between glitches decreases towards a value of 7 or lower at longer times since the preceding glitch. by combining nicer and rxte data, we measure a long-term braking index n = -1.25 ± 0.01. our analysis reveals eight new glitches, the first detected since 2011, near the end of rxte, with a total nicer and rxte glitch activity of $8.88\times 10^{-7}\, \mathrm{yr^{-1}}$. the new glitches follow the seemingly unique time-to-next-glitch-glitch-size correlation established previously using rxte data, with a slope of $5\, \rm {d} \, \mu \mathrm{hz}^{-1}$. for one glitch around which nicer observes 2 d on either side, we search for but do not see clear evidence of spectral nor pulse profile changes that may be associated with the glitch. | return of the big glitcher: nicer timing and glitches of psr j0537-6910 |
the dark matter particle explorer (dampe) experiment has recently announced the first results for the measurement of total electron plus positron fluxes between 25 gev and 4.6 tev. a spectral break at about 0.9 tev and a tentative peak excess around 1.4 tev have been found. however, it is very difficult to reproduce both the peak signal and the smooth background including spectral break simultaneously. we point out that the numbers of events in the two energy ranges (bins) close to the 1.4 tev excess have 1σ deficits. with the basic physics principles such as simplicity and naturalness, we consider the -2σ , +2σ , and -1σ deviations due to statistical fluctuations for the 1229.3 gev bin, 1411.4 gev bin, and 1620.5 gev bin. interestingly, we show that all the dampe data can be explained consistently via both the continuous distributed pulsar and dark matter interpretations, which have χ 2 ∼eq 17.2 and χ 2 ∼eq 13.9 ( for all the 38 points in dampe electron/positron spectrum with 3 of them revised), respectively. these results are different from the previous analysis by neglecting the 1.4 tev excess. at the same time, we do a similar global fitting on the newly released calet lepton data, which could also be interpreted by such configurations. moreover, we present a u(1)_d dark matter model with the breit-wigner mechanism, which can provide the proper dark matter annihilation cross section and escape the cmb constraint. furthermore, we suggest a few ways to test our proposal. | a simple and natural interpretations of the dampe cosmic-ray electron/positron spectrum within two sigma deviations |
we report on a search for high-energy counterparts to fast radio bursts (frbs) with the fermi gamma-ray burst monitor, fermi large area telescope, and the neil gehrels swift observatory burst alert telescope. we find no significant associations for any of the 23 frbs in our sample, but report upper limits to the high-energy fluence for each on timescales of 0.1, 1, 10, and 100 s. we report lower limits on the ratio of the radio to high-energy fluence, fr /fγ , for timescales of 0.1 and 100 s. we discuss the implications of our non-detections on various proposed progenitor models for frbs, including analogs of giant pulses from the crab pulsar and hyperflares from magnetars. this work demonstrates the utility of analyses of high-energy data for frbs in tracking down the nature of these elusive sources. | a search for high-energy counterparts to fast radio bursts |
pulsars can act as an excellent probe of the milky way magnetic field. the average strength of the galactic magnetic field component parallel to the line of sight can be estimated as $\langle b_\parallel \rangle = 1.232 \, \text{rm}/\text{dm}$ , where rm and dm are the rotation and dispersion measure of the pulsar. however, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. using numerical simulations and observations, we test the validity of this assumption. based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing mach number of the turbulence. we find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and trans-sonic flows, but for supersonic turbulence, the field strength can be severely overestimated by using $1.232 \, \text{rm}/\text{dm}$ . we then correlate existing pulsar observations from the australia telescope national facility with regions of enhanced thermal electron density and magnetic fields probed by 12co data of molecular clouds, magnetic fields from the zeeman splitting of the 21 cm line, neutral hydrogen column density, and h α observations. using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. thus, we conclude that the relation $\langle b_\parallel \rangle = 1.232 \, \text{rm}/\text{dm}$ provides a good estimate of the magnetic field on galactic scales, but might break down on sub-kpc scales. | magnetic fields in the milky way from pulsar observations: effect of the correlation between thermal electrons and magnetic fields |
magnetic field evolution in neutron-star crusts is driven by the hall effect and ohmic dissipation, for as long as the crust is sufficiently strong to absorb maxwell stresses exerted by the field and thus makes the momentum equation redundant. for the strongest neutron-star fields, however, stresses build to the point of crustal failure, at which point the standard evolution equations are no longer valid. here, we study the evolution of the magnetic field of the crust up to and beyond crustal failure, whence the crust begins to flow plastically. we perform global axisymmetric evolutions, exploring different types of failures affecting a limited region of the crust. we find that a plastic flow does not simply suppress the hall effect even in the regime of a low plastic viscosity, but it rather leads to non-trivial evolution - in some cases even overreacting and enhancing the impact of the hall effect. its impact is more pronounced in the toroidal field, with the differences on the poloidal field being less substantial. we argue that both the nature of magnetar bursts and their spin-down evolution will be affected by plastic flow, so that observations of these phenomena may help us to constrain the way the crust fails. | axisymmetric magneto-plastic evolution of neutron-star crusts |
context. interstellar scintillation (iss) of pulsar emission can be used both as a probe of the ionized interstellar medium (iism) and cause corruptions in pulsar timing experiments. of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision.aims: the primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale iism studies and pulsar-timing related propagation studies with the low-frequency array (lofar) high band antennae (hba).methods: we use observations from five international lofar stations and the lofar core in the netherlands. we analyze the 2d auto-covariance function of the dynamic spectra of these observations to determine the characteristic bandwidth and timescale of the iss toward the pulsars in our sample and investigate the 2d power spectra of the dynamic spectra to determine the presence of scintillation arcs.results: in this initial set of 31 sources, 15 allow for the full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120-180 mhz. eight of the observed sources show unresolved scintillation; and the final eight do not display diffractive scintillation. some correlation between scintillation detectability and pulsar brightness and a dispersion measure is apparent, although no clear cut-off values can be determined. our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations.conclusions: our results indicate the powerful advantage and great potential of iss studies at low frequencies and the complex dependence of scintillation detectability on parameters such as pulsar brightness and interstellar dispersion. this work provides the first installment of a larger-scale census and longer-term monitoring of iss effects at low frequencies. | pulsar scintillation studies with lofar. i. the census |
we report the results of an observational campaign using the effelsberg 100-m telescope of the pulsars j1746-2849, j1746-2850, j1746-2856, and j1745-2912 located in the central molecular zone (cmz) close to the galactic centre in order to study rotation measure (rm) variations. we report for the first time the rm value of psr j1746-2850 to be -12234 ± 181 rad m-2. this pulsar shows significant variations of rm of 300-400 rad m-2 over the course of months to years that suggest a strongly magnetized environment. the structure function analysis of the rm of psr j1746-2850 revealed a steep power-law index of $1.87_{-0.3}^{+0.4}$ comparable to the value expected for isotropic turbulence. this pulsar also showed large dispersion measure (dm) variation of ~50 pc cm-3 in an event lasting a few months where the rm increased by ~200 rad m-2. the large difference in rm between psr j1746-2849 and psr j1746-2850 despite the small angular separation reveals the presence of a magnetic field of at least 70 μg in the cmz and can explain the lack of polarization in the radio images of the region. these results contribute to our understanding of the magnetic field in the cmz and show similarities between the rm behaviours of these pulsars and some fast radio bursts. | rotation measure variations in galactic centre pulsars |
as one of the major components of the interstellar medium, the ionized gas in our milky way, especially the low-density diffuse component, has not been extensively observed in the radio band. the galactic plane pulsar snapshot (gpps) survey covers the sky area within the galactic latitude of ±10° around the galactic plane visible by the five-hundred-meter aperture spherical radio telescope (fast), and the spectral line data are simultaneously recorded during the pulsar survey observations. with an integration time of 5 min for each beam, the gpps survey project provides the most sensitive piggyback spectra for tens of radio recombination lines (rrls) in the band of 1000-1500 mhz for hnα, henα, cnα, as well as hnβ and hnγ. we processed the spectral data of rrls, and obtained a sensitive averaged hnα rrl map of a sky area of 88 square degrees in the inner galaxy of 33° ≼ l ≼ 55° and ∣b∣ ≼ 2°0. the final spectral data of the hnα rrls have a spatial resolution of ∼3', a spectral resolution of 2.2 km s−1, and a typical spectral rms noise of 0.25 mjy beam−1 or 6.3 mk in main-beam brightness temperature. the fast gpps hnα rrl observations are sensitive down to an emission measure of about 200 cm−6 pc if a 3σ detection limit is required. the new hnα rrl map shows complex structural features dominated by a number of hii regions and large extended diffuse ionized gas regions. we detect about 94% of the known hii regions and confirm 43 wise hii regions in the observed sky area. several large hii regions or star-forming complexes in the distant outer galaxy are resolved in the map of hnα rrls. extended rrl features of the diffuse ionized gas are detected. in addition, the gpps piggyback spectral-line data also provide sensitive detection for other kinds of rrls, such as the henα, cnα, hnβ, and hnγ rrls. the rrl data products of the gpps survey will be published and updated at http://zmtt.bao.ac.cn/milkywayfast/. | peering into the milky way by fast: ii. ionized gas in the inner galactic disk revealed by the piggyback line observations of the fast gpps survey |
gravitational waves are ripples in the fabric of spacetime that are caused by events such as the merging of black holes. in principle, many types of events occur that could create gravitational waves with frequencies ranging from as high as a few kilohertz to as low as a few nanohertz. sources of gravitational waves in the nanohertz frequency range include cosmic strings, quantum fluctuations from the early universe, and, notably, supermassive black hole binaries (smbhbs). some gravitational wave sources are so numerous that they are all expected to contribute to a gravitational wave background (gwb). this gwb has been the target of pulsar timing arrays (ptas) for decades. | seeing the gravitational wave universe |
we present the largest single survey to date of average profiles of radio pulsars, observed and processed using the same telescope and data reduction software. specifically, we present measurements for 1170 pulsars, observed by the thousand pulsar array programme at the 64-dish sarao meerkat radio telescope, in a frequency band from 856 to 1712 mhz. we provide rotation measures (rm), dispersion measures, flux densities, and polarization properties. the catalogue includes 254 new rms that substantially increase the total number of known pulsar rms. our integration times typically span over 1000 individual rotations per source. we show that the radio (pseudo-) luminosity has a strong, shallow dependence on the spin-down energy, proportional to $\dot{e}^{0.15\pm 0.04}$, that contradicts some previous proposals of population synthesis studies. in addition, we find a significant correlation between the steepness of the observed flux density spectra and $\dot{e}$, and correlations of the fractional linear polarization with $\dot{e}$, the spectral index, and the pulse width, which we discuss in the context of what is known about pulsar radio emission and how pulsars evolve with time. on the whole, we do not see significant correlations with the estimated surface magnetic field strength, and the correlations with $\dot{e}$ are much stronger than those with the characteristic age. this finding lends support to the suggestion that magnetic dipole braking may not be the dominant factor for the evolution of pulsar rotation over the lifetimes of pulsars. a public data release of the high-fidelity time-averaged pulse profiles in full polarization accompanies our catalogue. | the thousand pulsar array program on meerkat - ix. the time-averaged properties of the observed pulsar population |
we investigate pre-merger coherent radio emission from neutron star mergers arising due to the magnetospheric interaction between compact objects. we consider two plausible radiation mechanisms, and show that if one neutron star has a surface magnetic field bs ≥ 1012g, coherent millisecond radio bursts with characteristic temporal morphology and inclination angle dependence are observable to gpc distances with next-generation radio facilities. we explore multi-messenger and multi-wavelength methods of identification of a neutron star merger origin of radio bursts, such as in fast radio burst surveys, triggered observations of gamma-ray bursts and gravitational wave events, and optical/radio follow-up of fast radio bursts in search of kilonova and radio afterglow emission. we present our findings for current and future observing facilities, and make recommendations for verifying or constraining the model. | pulsar revival in neutron star mergers: multimessenger prospects for the discovery of pre-merger coherent radio emission |
psr j1012+5307, a millisecond pulsar in orbit with a helium white dwarf (wd), has been timed with high precision for about 25 yr. one of the main objectives of this long-term timing is to use the large asymmetry in gravitational binding energy between the neutron star and the wd to test gravitational theories. such tests, however, will be eventually limited by the accuracy of the distance to the pulsar. here, we present very long baseline interferometry (vlbi) astrometry results spanning approximately 2.5 yr for psr j1012+5307, obtained with the very long baseline array as part of the $\mathrm{mspsr}\pi $ project. these provide the first proper motion and absolute position for psr j1012+5307 measured in a quasi-inertial reference frame. from the vlbi results, we measure a distance of ${0.83}_{-0.02}^{+0.06}$ kpc (all the estimates presented in the abstract are at 68% confidence) for psr j1012+5307, which is the most precise obtained to date. using the new distance, we improve the uncertainty of measurements of the unmodeled contributions to orbital period decay, which, combined with three other pulsars, places new constraints on the coupling constant for dipole gravitational radiation ${\kappa }_{d}=(-1.7\pm 1.7)\times {10}^{-4}$ and the fractional time derivative of newton's gravitational constant $\dot{g}/g=-{1.8}_{-4.7}^{+5.6}\times {10}^{-13}\,{\mathrm{yr}}^{-1}$ in the local universe. as the uncertainties of the observed decays of orbital period for the four leading pulsar-wd systems become negligible in ≈10 yr, the uncertainties for $\dot{g}/g$ and κd will be improved to ≤1.5 × 10-13 yr-1 and ≤1.0 × 10-4, respectively, predominantly limited by the distance uncertainties. | very long baseline astrometry of psr j1012+5307 and its implications on alternative theories of gravity |
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