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as a fundamental process converting magnetic to plasma energy in high-energy astrophysical plasmas, relativistic magnetic reconnection is a leading explanation for the acceleration of particles to the ultrarelativistic energies that are necessary to power nonthermal emission (especially x-rays and gamma-rays) in pulsar magnetospheres and pulsar wind nebulae, coronae and jets of accreting black holes, and gamma-ray bursts. an important objective of plasma astrophysics is therefore the characterization of nonthermal particle acceleration (ntpa) effected by reconnection. reconnection-powered ntpa has been demonstrated over a wide range of physical conditions using large 2d kinetic simulations. however, its robustness in realistic 3d reconnection—in particular, whether the 3d relativistic drift-kink instability (rdki) disrupts ntpa—has not been systematically investigated, although pioneering 3d simulations have observed ntpa in isolated cases. here, we present the first comprehensive study of ntpa in 3d relativistic reconnection in collisionless electron-positron plasmas, characterizing ntpa as the strength of 3d effects is varied systematically via the length in the third dimension and the strength of the guide magnetic field. we find that, while the rdki prominently perturbs 3d reconnecting current sheets, it does not suppress particle acceleration, even for zero guide field; fully 3d reconnection robustly and efficiently produces nonthermal power-law particle spectra closely resembling those obtained in 2d. this finding provides strong support for reconnection as the key mechanism powering high-energy flares in various astrophysical systems. we also show that strong guide fields significantly inhibit ntpa, slowing reconnection and limiting the energy available for plasma energization, yielding steeper and shorter power-law spectra. | nonthermal particle acceleration in 3d relativistic magnetic reconnection in pair plasma |
in the framework of teleparallel gravity, we derive a charged non-vacuum solution for a physically symmetric tetrad field with two unknown functions of radial coordinate. the field equations result in a closed-form adopting particular metric potentials and a suitable anisotropy function combined with the charge. under these circumstances, it is possible to obtain a set of configurations compatible with observed pulsars. specifically, boundary conditions for the interior spacetime are applied to the exterior reissner-nordström metric to constrain the radial pressure that has to vanish through the boundary. starting from these considerations, we are able to fix the model parameters. the pulsar psr j 1614 -2230 , with estimated mass m =1.97 ±0.04 m⊚, and radius r =9.69 ±0.2 km is used to test numerically the model. the stability is studied, through the causality conditions and adiabatic index, adopting the tolman-oppenheimer-volkov equation. the mass-radius (m, r) relation is derived. furthermore, the compatibility of the model with other observed pulsars is also studied. we reasonably conclude that the model can represent realistic compact objects. | stable and self-consistent compact star models in teleparallel gravity |
observations by hawc and milagro have detected bright and spatially extended tev γ -ray sources surrounding the geminga and monogem pulsars. we argue that these observations, along with a substantial population of other extended tev sources coincident with pulsar wind nebulae, constitute a new morphological class of spatially extended tev halos. we show that hawcs wide field of view unlocks an expansive parameter space of tev halos not observable by atmospheric cherenkov telescopes. under the assumption that geminga and monogem are typical middle-aged pulsars, we show that ten-year hawc observations should eventually observe 37-13+17 middle-aged tev halos that correspond to pulsars whose radio emission is not beamed towards earth. depending on the extrapolation of the tev halo efficiency to young pulsars, hawc could detect more than 100 tev halos from misaligned pulsars. these pulsars have historically been difficult to detect with existing multiwavelength observations. tev halos will constitute a significant fraction of all hawc sources, allowing follow-up observations to efficiently find pulsar wind nebulae and thermal pulsar emission. the observation and subsequent multi-wavelength follow-up of tev halos will have significant implications for our understanding of pulsar beam geometries, the evolution of pulsar wind nebulae, the diffusion of cosmic rays near energetic pulsars, and the contribution of pulsars to the cosmic-ray positron excess. | using hawc to discover invisible pulsars |
the fundamental strong interaction determines the nature of pulsar-like compact stars which are essentially in the form of bulk strong matter. from an observational point of view, it is proposed that bulk strong matter could be composed of strangeons, i.e. quark-clusters with three-light-flavor symmetry of quarks, and therefore pulsar-like compact objects could actually be strangeon stars. the equation of state (eos) of strangeon stars is described in a lennard-jones model for the purpose of constraining the eos by both the tidal deformability λ of gw 170817 and mtov. it is found that the allowed parameter space is quite large as most of the lennard-jones eos models satisfy the tidal deformability constraint by gw 170817. the future gw detections for smaller values of λ and mass measurement for larger values of mtov will help find a better constraint on the strangeon star model. | strangeons constitute bulk strong matter: test using gw 170817 |
observations of the γ-ray emission around star clusters, isolated supernova remnants, and pulsar wind nebulae indicate that the cosmic-ray (cr) diffusion coefficient near acceleration sites can be suppressed by a large factor compared to the galaxy average. we explore the effects of such local suppression of cr diffusion on galaxy evolution using simulations of isolated disk galaxies with regular and high gas fractions. our results show that while cr propagation with constant diffusivity can make gaseous disks more stable by increasing the midplane pressure, large-scale cr pressure gradients cannot prevent local fragmentation when the disk is unstable. in contrast, when cr diffusivity is suppressed in star-forming regions, the accumulation of crs in these regions results in strong local pressure gradients that prevent the formation of massive gaseous clumps. as a result, the distribution of dense gas and star formation changes qualitatively: a globally unstable gaseous disk does not violently fragment into massive star-forming clumps but maintains a regular grand-design spiral structure. this effect regulates star formation and disk structure and is qualitatively different from and complementary to the global role of crs in vertical hydrostatic support of the gaseous disk and in driving galactic winds. | cosmic-ray diffusion suppression in star-forming regions inhibits clump formation in gas-rich galaxies |
we investigate the nonrotating neutron stars in f(t) gravity with f (t ) =t +α t2 , where t is the torsion scalar in the teleparallel formalism of gravity. in particular, we utilize the sly and bsk family of equations of state for perfect fluid to describe the neutron stellar matter and search for the effects of the f(t) modification on the models of neutron stars. for positive α , the modification results in a smaller stellar mass in comparison to general relativity, while the neutron stars will contain larger amount of matter for negative α . moreover, there seems to be an upper limit for the central density of the neutron stars with α >0 , beyond which the effective f(t) fluid would have a steplike phase transition in density and pressure profiles, collapsing the numerical system. we obtain the mass-radius relations of the realistic models of neutron stars and subject them to the joint constraints from the observed massive pulsars psr j0030+0451, psr j0740+6620, and psr j2215+5135, and gravitational wave events gw170817 and gw190814. for the neutron star model in f(t) gravity to be able to accommodate all the mentioned data, the model parameter α needs to be smaller than -4.295 , -6.476 , -4.4 , and -2.12 (in the unit of g2m⊙2/c4 ) for sly, bsk19, bsk20, and bsk21 equations of state, respectively. if one considers the unknown compact object in the event gw190814 not to be a neutron star and hence excludes this dataset, the constraints can be loosened to α <-0.594 , -3.5 , 0.4 and 1.9 (in the unit of g2m⊙2/c4 ), respectively. | realistic neutron star models in f(t) gravity |
this review focuses on applications of the ideas of superfluidity and superconductivity in neutron stars in a broader context, ranging from the microphysics of pairing in nucleonic superfluids to macroscopic manifestations of superfluidity in pulsars. the exposition of the basics of pairing, vorticity and mutual friction can serve as an introduction to the subject. we also review some topics of recent interest, including the various types of pinning of vortices, glitches, and oscillations in neutron stars containing superfluid phases of baryonic matter. | superfluidity and superconductivity in neutron stars |
the merger of a neutron star (ns) binary may result in the formation of a long-lived, or indefinitely stable, millisecond magnetar remnant surrounded by a low-mass ejecta shell. a portion of the magnetar’s prodigious rotational energy is deposited behind the ejecta in a pulsar wind nebula, powering luminous optical/x-ray emission for hours to days following the merger. ions in the pulsar wind may also be accelerated to ultra-high energies, providing a coincident source of high-energy cosmic rays and neutrinos. at early times, the cosmic rays experience strong synchrotron losses; however, after a day or so, pion production through photomeson interaction with thermal photons in the nebula comes to dominate, leading to efficient production of high-energy neutrinos. after roughly a week, the density of background photons decreases sufficiently for cosmic rays to escape the source without secondary production. these competing effects result in a neutrino light curve that peaks on a few day timescale near an energy of ∼1018ev. this signal may be detectable for individual mergers out to ∼10 (100) mpc by current (next generation) neutrino telescopes, providing clear evidence for a long-lived ns remnant, the presence of which may otherwise be challenging to identify from the gravitational waves alone. under the optimistic assumption that a sizable fraction of ns mergers produce long-lived magnetars, the cumulative cosmological neutrino background is estimated to be ∼ {10}-9{--}{10}-8 {gev} {{cm}}-2 {{{s}}}-1 {{sr}}-1 for an ns merger rate of {10}-7 {{mpc}}-3 {{yr}}-1, overlapping with icecube’s current sensitivity and within the reach of next-generation neutrino telescopes. | high-energy neutrinos from millisecond magnetars formed from the merger of binary neutron stars |
current prescriptions for supernova natal kicks in rapid binary population synthesis simulations are based on fits of simple functions to single pulsar velocity data. we explore a new parametrization of natal kicks received by neutron stars in isolated and binary systems developed by mandel & müller, which is based on 1d models and 3d supernova simulations, and accounts for the physical correlations between progenitor properties, remnant mass, and the kick velocity. we constrain two free parameters in this model using very long baseline interferometry velocity measurements of galactic single pulsars. we find that the inferred values of natal kick parameters do not differ significantly between single and binary evolution scenarios. the best-fitting values of these parameters are $v$ns = 520 km s-1 for the scaling prefactor for neutron star kicks, and σns = 0.3 for the fractional stochastic scatter in the kick velocities. | calibration of neutron star natal kick velocities to isolated pulsar observations |
we consider the scenario where dark matter (dm) is represented by an ultralight classical scalar field performing coherent periodic oscillations. we point out that such dm perturbs the dynamics of binary systems either through its gravitational field or via direct coupling to ordinary matter. this perturbation gets resonantly amplified if the frequency of dm oscillations is close to a (half-)integer multiple of the orbital frequency of the system and leads to a secular variation of the orbital period. we suggest using binary pulsars as probes of this scenario and estimate their sensitivity. while the current accuracy of observations is not yet sufficient to probe the purely gravitational effect of dm, it already yields constraints on direct coupling that are competitive with other bounds. the sensitivity will increase with the upcoming radio observatories such as the square kilometer array. | ultralight dark matter resonates with binary pulsars |
by directly inverting several neutron star (ns) observables in the three-dimensional parameter space for the equation of state of super-dense neutron-rich nuclear matter, we show that the lower radius limit for psr j0740+6620 of mass 2.08 ± 0.07 m⊙ from neutron star interior composition explorer (nicer)'s very recent observation sets a much tighter lower boundary than previously known for nuclear symmetry energy in the density range of (1.0 ~ 3.0) times the saturation density ρ0 of nuclear matter. the super-soft symmetry energy leading to the formation of proton polarons in this density region of nss is clearly disfavored by the first radius measurement for the most massive ns observed reliably so far. | impact of nicer's radius measurement of psr j0740+6620 on nuclear symmetry energy at suprasaturation densities |
we investigate interacting quark matter (iqm), including the perturbative qcd correction and color superconductivity, for both up-down quark matter and strange quark matter. we first derive an equation of state (eos) unifying all cases by a simple reparametrization and rescaling, through which we manage to maximally reduce the number of degrees of freedom. we find, in contrast to the conventional eos p =1 /3 (ρ -4 beff) for noninteracting quark matter, that taking the extreme strongly interacting limit on the unified iqm eos gives p =ρ -2 beff , where beff is the effective bag constant. we employ the unified eos to explore the properties of pure interacting quark stars (iqss) composed of iqm. we describe how recent astrophysical observations, such as the pulsar-mass measurements, the nicer analysis, and the binary merger gravitational-wave events gw170817, gw190425, and gw190814, further constrain the parameter space. an upper bound for the maximum allowed mass of iqss is found to be mtov≲3.23 m⊙ . our analysis indicates a new possibility that the currently observed compact stars, including the recently reported gw190814's secondary component (m =2.5 9-0.09+0.08 m⊙ ), can be quark stars composed of interacting quark matter. | unified interacting quark matter and its astrophysical implications |
we investigate the impact of coleman-weinberg inflation on the stochastic gravity wave background spectrum emitted by intermediate-scale cosmic strings. the string network is partially inflated and reenters the horizon at later times after the end of inflation, such that the short string loops are not produced. this leads to a significant modification of the gravity wave spectrum that we explore in detail. we find that coleman-weinberg inflation can help to satisfy the parkes pulsar timing array (ppta) bound for dimensionless string tension values in the range g μ >1.1 ×10-10 . we also identify the modified gravity wave spectra which, in the case of inflation, are compatible with the north american nanohertz observatory for gravitational waves (nanograv) data. we then discuss the formation of monopoles and strings at the same breaking scale and the compatibility of the monopole, astrophysics and cosmic ray observatory (macro) bound with the ppta bound, and also with the nanograv data. finally, an example of a realistic nonsupersymmetric e6 model incorporating successful coleman-weinberg inflation is presented in which monopoles and strings both survive inflation and are present at an observable level. | cosmic strings, inflation, and gravity waves |
the oscillation of neutron n into mirror neutron n', its mass degenerate partner from dark mirror sector, can gradually transform the neutron stars into the mixed stars consisting in part of mirror dark matter. in quark stars n -n' transitions are suppressed. we study the structure of mixed stars and derive the mass-radius scaling relations between the configurations of purely neutron star and maximally mixed star (mms) containing equal amounts of ordinary and mirror components. in particular, we show that the mms masses can be at most mnsmax/√{2 } , where mnsmax is a maximum mass of a pure neutron star allowed by a given equation of state. we evaluate n -n' transition rate in neutron stars, and show that various astrophysical limits on pulsar properties exclude the transition times in a wide range 105year<τε<1015year . for short transition times, τε<105 year, the different mixed stars of the same mass can have different radii, depending on their age, which possibility can be tested by the nicer measurements. we also discuss subtleties related with the possible existence of mixed quark stars, and possible implications for the gravitational waves from the neutron star mergers and associated electromagnetic signals. | neutron-mirror neutron mixing and neutron stars |
the origins of matter and radiation in the universe lie in a hot big bang. we present a number of well-motivated cosmologies in which the big bang occurs through a strong first-order phase transition—either at the end of inflation, after a period of kination ("kination-induced big bang"), or after a second period of vacuum domination in the early universe ("supercooled big bang"); we also propose a "dark big bang" where only the dark matter in the universe is created in a first-order phase transition much after inflation. in all of these scenarios, the resulting gravitational radiation can explain the tentative signals reported by the nanograv, parkes, and european pulsar timing array experiments if the reheating temperature of the hot big bang, and correspondingly the energy scale of the false vacuum, falls in the range t*∼ρvac1 /4 =mev - 100 gev . all of the same models at higher reheating temperatures will be of interest to upcoming ground- and space-based interferometer searches for gravitational waves at larger frequency. | have pulsar timing arrays detected the hot big bang: gravitational waves from strong first order phase transitions in the early universe |
the estimation of p- to s-wave velocity ratio (psr) in the subsurface has many applications in gas-bearing reservoir prospecting, lithology discrimination, and anomalous pore-pressure prediction. conventionally, it is estimated with the p- and s-wave velocities/moduli/impedances that are directly obtained from prestack seismic data using the existing reflection coefficient equation (e.g., the aki-richards approximation). however, this indirect inversion method creates cumulative errors for the estimated psr results. to eliminate cumulative errors, we first develop a novel generalized elastic impedance that has more explicit physical meaning compared to conventional elastic impedance. then, we derive a linear p-wave reflection coefficient equation in terms of the psr, p-wave velocity, and density under the assumption of weak contrast. furthermore, a robust amplitude-variation-with-offset inversion method is constructed with the proposed reflection coefficient equation in the bayesian framework. cauchy and gaussian probability distributions are used as the prior probability distribution of the model parameter and likelihood function, respectively, to predict the maximum posterior probability solution for psr, p velocity, and density. synthetic and field examples illustrate that the proposed direct inversion method performs with higher accuracy compared with the indirect method that inverts for the p- and s-wave velocities and also illustrate the feasibility of our method for inverting the three parameters even with strong noise. our inverted results conform to the drilling results, which validate the robustness of the proposed direct inversion method. | amplitude-variation-with-offset inversion using p- to s-wave velocity ratio and p-wave velocity |
in this article we try to present spherically symmetric isotropic strange star model under the framework of f(r,script t) theory of gravity. to this end, we consider that the lagrangian density is a linear function of the ricci scalar r and the trace of the energy momentum tensor script t given as f(r,script t)=r+2χ script t. we also assume that the quark matter distribution is governed by the simplest form of the mit bag model equation of state (eos) as p=1/3(ρ-4b), where b is the bag constant. we have obtained an exact solution of the modified form of the tolman-oppenheimer-volkoff (tov) equation in the framework of f(r,script t) gravity theory and have studied the dependence of different physical properties, viz., the total mass, radius, energy density and pressure for the chosen values of χ. further, to examine physical acceptability of the proposed stellar model, we have conducted different tests in detail, viz., the energy conditions, modified tov equation, mass-radius relation, causality condition etc. we have precisely explained the effects arising due to the coupling of the matter and geometry on the compact stellar system. for a chosen value of the bag constant, we have predicted numerical values of the different physical parameters in tabular form for the different strange star candidates. it is found that as the factor χ decreases the strange star candidates become gradually massive and larger in size with less dense stellar configuration. however, when χ increases the stars shrink gradually and become less massive to turn into a more compact stellar system. hence for χ>0 our proposed model is suitable to explain the ultra-dense compact stars well within the observational limits and for χ<0 case allows to represent the recent massive pulsars and super-chandrasekhar stars. for χ=0 we retrieve as usual the standard results of the general relativity (gr). | strange stars in f(r,script t) gravity |
this review discusses the physics of magnetic reconnection—a process in which the magnetic field topology changes and magnetic energy is converted to kinetic energy—in pair plasmas in the relativistic regime. we focus on recent progress in the field driven by theory advances and the maturity of particle-in-cell codes. this work shows that fragmentation instabilities at the current sheet can play a critical role in setting the reconnection speed and affect the resulting particle acceleration, anisotropy, bulk flows, and radiation. then, we discuss how this novel understanding of relativistic reconnection can be applied to high-energy astrophysical phenomena, with an emphasis on pulsars, pulsar wind nebulae, and active galactic nucleus jets. | relativistic magnetic reconnection in pair plasmas and its astrophysical applications |
we estimate the properties of the double neutron star (dns) population that will be observable by the planned space-based interferometer laser interferometer space antenna (lisa). by following the gravitational radiation-driven evolution of dnss generated from rapid population synthesis of massive binary stars, we estimate that around 35 dnss will accumulate a signal-to-noise ratio above 8 over a 4-yr lisa mission. the observed population mainly comprises galactic dnss (94 per cent), but detections in the lmc (5 per cent) and smc (1 per cent) may also be expected. the median orbital frequency of detected dnss is expected to be 0.8 mhz, and many of them will be eccentric (median eccentricity of 0.11). lisa is expected to localize these dnss to a typical angular resolution of 2°. we expect the best-constrained dnss to have eccentricities known to a few parts in a thousand, chirp masses measured to better than 1 per cent fractional uncertainty, and sky localization at the level of a few arcminutes. the orbital properties will provide insights into dns progenitors and formation channels. the localizations may allow neutron star natal kick magnitudes to be constrained through the galactic distribution of dnss, and make it possible to follow up the sources with radio pulsar searches. lisa is also expected to resolve ∼104 galactic double white dwarfs, many of which may have binary parameters that resemble dnss; we discuss how the combined measurement of binary eccentricity, chirp mass, and sky location may aid the identification of a dns. | detecting double neutron stars with lisa |
gravitational waves at ultra-low frequencies (≲100 nhz) are key to understanding the assembly and evolution of astrophysical black hole binaries with masses ~106-109 m⊙ at low redshifts1-3. these gravitational waves also offer a unique window into a wide variety of cosmological processes4-11. pulsar timing arrays12-14 are beginning to measure15 this stochastic signal at ~1-100 nhz and the combination of data from several arrays16-19 is expected to confirm a detection in the next few years20. the dominant physical processes generating gravitational radiation at nhz frequencies are still uncertain. pulsar timing array observations alone are currently unable21 to distinguish a binary black hole astrophysical foreground22 from a cosmological background due to, say, a first-order phase transition at a temperature ~1-100 mev in a weakly interacting dark sector8-11. this letter explores the extent to which incorporating integrated bounds on the ultra-low-frequency gravitational wave spectrum from any combination of cosmic microwave background23,24, big bang nucleosynethesis25,26 or astrometric27,28 observations can help to break this degeneracy. | ultra-low-frequency gravitational waves from cosmological and astrophysical processes |
it has been proposed that a recent outburst of cosmic-ray electrons could account for the excess of gev-scale gamma rays observed from the region surrounding the galactic center. after studying this possibility in some detail, we identify scenarios in which a series of leptonic cosmic-ray outbursts could plausibly generate the observed excess. the morphology of the emission observed outside of ~1̂-2̂ from the galactic center can be accommodated with two outbursts, one which took place approximately ~106 years ago, and another (injecting only about 10% as much energy as the first) about ~105 years ago. the emission observed from the innermost ~1̂-2̂ requires one or more additional recent outbursts and/or a contribution from a centrally concentrated population of unresolved millisecond pulsars. in order to produce a spectrum that is compatible with the measured excess (whose shape is approximately uniform over the region of the excess), the electrons from the older outburst must be injected with significantly greater average energy than those injected more recently, enabling their spectra to be similar after ~106 years of energy losses. | the galactic center gev excess from a series of leptonic cosmic-ray outbursts |
hundreds of millions of supermassive black hole binaries are expected to contribute to the gravitational-wave signal in the nanohertz frequency band. their signal is often approximated either as an isotropic gaussian stochastic background with a power-law spectrum or as an individual source corresponding to the brightest binary. in reality, the signal is best described as a combination of a stochastic background and a few of the brightest binaries modeled individually. we present a method that uses this approach to efficiently create realistic pulsar timing array data sets using synthetic catalogs of binaries based on the illustris cosmological hydrodynamic simulation. we explore three different properties of such realistic backgrounds that could help distinguish them from those formed in the early universe: (i) their characteristic strain spectrum, (ii) their statistical isotropy, and (iii) the variance of their spatial correlations. we also investigate how the presence of confusion noise from a stochastic background affects detection prospects of individual binaries. we calculate signal-to-noise ratios of the brightest binaries in different realizations for a simulated pulsar timing array based on the nanograv 12.5 yr data set extended to a time span of 15 yr. we find that ~6% of the realizations produce systems with signal-to-noise ratios larger than 5, suggesting that individual systems might soon be detected (the fraction increases to ~41% at 20 yr). these can be taken as a pessimistic prediction for the upcoming nanograv 15 yr data set, since it does not include the effect of potentially improved timing solutions and newly added pulsars. | exploring realistic nanohertz gravitational-wave backgrounds |
the recent hawc observations of a very-high-energy γ-ray halo around geminga and monogem indicate a very slow diffusion of cosmic rays that results in a tiny contribution of positrons from these two pulsars to the local flux. this makes the cosmic positron excess anomaly observed by pamela and ams-02 even more puzzling. however, from the boron-to-carbon ratio data one can infer that the average diffusion coefficient in the galaxy should be much larger. in this work we propose a two-zone diffusion model in which the diffusion is slow only in a small region around the source, outside of which the propagation is as fast as usual. we find that this scenario can naturally explain the positron excess data with parameters even more reasonable than those in the conventional one-zone diffusion model. the reason is that during the lifetime of geminga (∼300 kyr), the electrons/positrons have propagated too far away with a fast diffusion and led to a low local flux. the slow-diffusion region in the two-zone model helps to confine the electrons/positrons for a long time and lead to an enhancement of the local flux. so under the constraint of the hawc observations, pulsars are still the probable origin of the cosmic-ray positron excess. | two-zone diffusion of electrons and positrons from geminga explains the positron anomaly |
the dispersion measure (dm) is one of the key attributes of radio pulsars and fast radio bursts (frbs). there is a mistaken view that the dm is an accurate measure of the column density of electrons between the observer and the source. to start with, the dm, unlike a true column density, is not a lorentz invariant. next, the dm also includes contribution from ions and is sensitive to the temperature of the plasma in the intervening clouds. separately, the primary observable is the dispersion slope, $\mathcal{d}\equiv \delta{(t)}/\delta{(\nu^{-2}})$, where $t(\nu)$ is the arrival time at frequency, $\nu$. a scaling factor composed of physical and astronomical constants is needed to convert $\mathcal{d}$ to dm. in the early days of pulsar astronomy the relevant constants were defined to parts per million (ppm). as a result, a convention arose in which this conversion factor was fixed. over time, several such conventions came about -- recipe for confusion. meanwhile, over the past several years, the si system has been restructured and the parsec is now exactly defined. as a result, the present accuracy of the conversion factor is below a part per billion -- many orders of magnitude better than the best measurement errors of $\mathcal{d}$. we are now in an awkward situation wherein the primary "observable", the dm, has incorrect scaling factor(s). to address these two concerns i propose that astronomers report the primary measurement, $\mathcal{d}$ (with a suggested normalization of $10^{15}\,$hz), and not the dm. interested users can convert $\mathcal{d}$ to dm without the need to know secret handshakes of the pulsar timing communities. | dispersion measure: confusion, constants & clarity |
we present a search for optical bursts from the repeating fast radio burst frb 121102 using simultaneous observations with the high-speed optical camera ultraspec on the 2.4-m thai national telescope and radio observations with the 100-m effelsberg radio telescope. a total of 13 radio bursts were detected, but we found no evidence for corresponding optical bursts in our 70.7-ms frames. the 5σ upper limit to the optical flux density during our observations is 0.33 mjy at 767 nm. this gives an upper limit for the optical burst fluence of 0.046 jy ms, which constrains the broad-band spectral index of the burst emission to α ≤ -0.2. two of the radio pulses are separated by just 34 ms, which may represent an upper limit on a possible underlying periodicity (a rotation period typical of pulsars), or these pulses may have come from a single emission window that is a small fraction of a possible period. | a search for optical bursts from the repeating fast radio burst frb 121102 |
pulsar timing arrays (ptas) are exceptionally sensitive detectors in the frequency band nhz ≲f ≲μ hz . ultralight dark matter (uldm), with mass in the range 10-23 ev ≲mϕ≲10-20 ev , is one class of dm models known to generate signals in this frequency window. while purely gravitational signatures of uldm have been studied previously, in this work we consider two signals in ptas which arise in the presence of direct couplings between uldm and ordinary matter. these couplings induce variations in fundamental constants, i.e., particle masses and couplings. these variations can alter the moment of inertia of pulsars, inducing pulsar spin fluctuations via conservation of angular momentum, or induce apparent timing residuals due to reference clock shifts. by using mock data mimicking current pta datasets, we show that pta experiments outperform torsion balance and atomic clock constraints for uldm coupled to electrons, muons, or gluons. in the case of coupling to quarks or photons, we find that ptas and atomic clocks set similar constraints. additionally, we discuss how future ptas can further improve these constraints, and detail the unique properties of these signals relative to the previously studied effects of uldm on ptas. | constraining fundamental constant variations from ultralight dark matter with pulsar timing arrays |
we report on the discovery of a new member of the magnetar class, sgr j1935+2154, and on its timing and spectral properties measured by an extensive observational campaign carried out between 2014 july and 2015 march with chandra and xmm-newton (11 pointings). we discovered the spin period of sgr j1935+2154 through the detection of coherent pulsations at a period of about 3.24 s. the magnetar is slowing down at a rate of dot{p} = 1.43(1)× 10^{-11} s s-1 and with a decreasing trend due to a negative ddot{p} of -3.5(7) × 10-19 s s-2. this implies a surface dipolar magnetic field strength of ∼2.2 × 1014 g, a characteristic age of about 3.6 kyr and a spin-down luminosity lsd ∼1.7 × 1034 erg s-1. the source spectrum is well modelled by a blackbody with temperature of about 500 ev plus a power-law component with photon index of about 2. the source showed a moderate long-term variability, with a flux decay of about 25 per cent during the first four months since its discovery, and a re-brightening of the same amount during the second four months. the x-ray data were also used to study the source environment. in particular, we discovered a diffuse emission extending on spatial scales from about 1 arcsec up to at least 1 arcmin around sgr j1935+2154 both in chandra and xmm-newton data. this component is constant in flux (at least within uncertainties) and its spectrum is well modelled by a power-law spectrum steeper than that of the pulsar. though a scattering halo origin seems to be more probable we cannot exclude that part, or all, of the diffuse emission is due to a pulsar wind nebula. | the discovery, monitoring and environment of sgr j1935+2154 |
the gravitational waves measured at ligo are presumed here to come from merging primordial black holes. we ask how these primordial black holes could arise through inflationary models while not conflicting with current experiments. among the approaches that work, we investigate the opportunity for corroboration through experimental probes of gravitational waves at pulsar timing arrays. we provide examples of theories that are already ruled out, theories that will soon be probed, and theories that will not be tested in the foreseeable future. the models that are most strongly constrained are those with a relatively broad primordial power spectrum. | inflationary theory and pulsar timing investigations of primordial black holes and gravitational waves |
the aim of this paper is to generalize the definition of complexity for the static self-gravitating structure in f (r, t, q) gravitational theory, where r is the ricci scalar, t is the trace part of energy-momentum tensor, and q ≡ rαβt αβ. in this context, we have considered locally anisotropic spherical matter distribution and calculated field equations and conservation laws. after the orthogonal splitting of the riemann curvature tensor, we found the corresponding complexity factor with the help of structure scalars. it is seen that the system may have zero complexity factor if the effects of energy density inhomogeneity and pressure anisotropy cancel the effects of each other. all of our results reduce to general relativity on assuming f (r, t, q) = r condition. | influence of modification of gravity on the complexity factor of static spherical structures |
aims: we report a new microscopic equation of state (eos) of dense symmetric nuclear matter, pure neutron matter, and asymmetric and β-stable nuclear matter at zero temperature using recent realistic two-body and three-body nuclear interactions derived in the framework of chiral perturbation theory (chpt) and including the δ(1232) isobar intermediate state. this eos is provided in tabular form and in parametrized form ready for use in numerical general relativity simulations of binary neutron star merging. here we use our new eos for β-stable nuclear matter to compute various structural properties of non-rotating neutron stars.methods: the eos is derived using the brueckner-bethe-goldstone quantum many-body theory in the brueckner-hartree-fock approximation. neutron star properties are next computed solving numerically the tolman-oppenheimer-volkov structure equations.results: our eos models are able to reproduce the empirical saturation point of symmetric nuclear matter, the symmetry energy esym, and its slope parameter l at the empirical saturation density n0. in addition, our eos models are compatible with experimental data from collisions between heavy nuclei at energies ranging from a few tens of mev up to several hundreds of mev per nucleon. these experiments provide a selective test for constraining the nuclear eos up to 4n0. our eos models are consistent with present measured neutron star masses and particularly with the mass m = 2.01 ± 0.04 m⊙ of the neutron stars in psr j0348+0432. | equation of state of dense nuclear matter and neutron star structure from nuclear chiral interactions |
recent tev observations of nearby pulsars with the hawc telescope have been interpreted as evidence that the diffusion of high-energy electrons and positrons within pulsar wind nebulae is highly inefficient compared to the rest of the interstellar medium. if the diffusion coefficient well outside the nebula is close to the value inferred for the region inside the nebula, high-energy electrons and positrons produced by the two observed pulsars could not contribute significantly to the local measured cosmic-ray flux. the hawc collaboration thus concluded that, under the assumption of isotropic and homogeneous diffusion, the two pulsars are ruled out as sources of the anomalous high-energy positron flux. here, we argue that since the diffusion coefficient is likely not spatially homogeneous, the assumption leading to this conclusion is flawed. we solve the diffusion equation with a radially dependent diffusion coefficient, and show that the pulsars observed by hawc produce potentially perfect matches to the observed high-energy positron fluxes. we also study the implications of inefficient diffusion within pulsar wind nebulae on galactic scales, and show that cosmic rays are likely to have very long residence times in regions of inefficient diffusion. we describe how this prediction can be tested with studies of the diffuse galactic emission. | lessons from hawc pulsar wind nebulae observations: the diffusion constant is not a constant; pulsars remain the likeliest sources of the anomalous positron fraction; cosmic rays are trapped for long periods of time in pockets of inefficient diffusion |
we present a phase-coherent timing solution for psr j1640-4631, a young 206 ms pulsar using x-ray timing observations taken with nustar. over this timing campaign, we have measured the braking index of psr j1640-4631 to be n = 3.15 ± 0.03. using a series of simulations, we argue that this unusually high braking index is not due to timing noise, but is intrinsic to the pulsar's spin-down. we cannot, however, rule out contamination due to an unseen glitch recovery, although the recovery timescale would have to be longer than most yet observed. if this braking index is eventually proven to be stable, it demonstrates that pulsar braking indices greater than three are allowed in nature; hence, other physical mechanisms such as mass or magnetic quadrupoles are important in pulsar spin-down. we also present a 3σ upper limit on the pulsed flux at 1.4 ghz of 0.018 mjy. | a high braking index for a pulsar |
several groups of authors have analyzed fermi lat data in a region around the galactic center finding an unaccounted gamma-ray excess over diffuse backgrounds in the gev energy range. it has been argued that it is difficult or even impossible to explain this diffuse emission by the leading astrophysical candidates-millisecond pulsars (msps). here we provide a new estimate of the contribution to the excess by a population of yet unresolved msp located in the bulge of the milky way. we simulate this population with the galplot package by adopting a parametric approach, with the range of free parameters gauged on the msp characteristics reported by the second pulsar catalogue (2pc). we find that the conclusions strongly depend on the details of the msp luminosity function (in particular, its high luminosity end) and other explicit or tacit assumptions on the msp statistical properties, which we discuss. notably, for the first time we study the importance of the possible secondary emission of the msps in the galactic center, i.e. the emission via inverse compton losses of electrons injected in the interstellar medium. differently from a majority of other authors, we find that within current uncertainties a large if not dominant contribution of msps to the excess cannot be excluded. we also show that the sensitivities of future instruments or possibly already of the latest lat data analysis (pass 8) provide good perspectives to test this scenario by resolving a significant number of msps. | millisecond pulsars and the galactic center gamma-ray excess: the importance of luminosity function and secondary emission |
recently, a fully covariant version of the theory of f (t ) torsion gravity has been introduced by m. kršśák and e. saridakis [classical quantum gravity 33, 115009 (2016)]. in covariant f (t ) gravity, the schwarzschild solution is not a vacuum solution for f (t )≠t , and therefore determining the spherically symmetric vacuum is an important open problem. within the covariant framework, we perturbatively solve the spherically symmetric vacuum gravitational equations around the schwarzschild solution for the scenario with f (t )=t +(α /2 )t2 , representing the dominant terms in theories governed by lagrangians analytic in the torsion scalar. from this, we compute the perihelion shift correction to solar system planetary orbits as well as perturbative gravitational effects near neutron stars. this allows us to set an upper bound on the magnitude of the coupling constant, α , which governs deviations from general relativity. we find the bound on this nonlinear torsion coupling constant by specifically considering the uncertainty in the perihelion shift of mercury. we also analyze a bound from a similar comparison with the periastron orbit of the binary pulsar psr j0045-7319 as an independent check for consistency. setting bounds on the dominant nonlinear coupling is important in determining if other effects in the solar system or greater universe could be attributable to nonlinear torsion. | spherically symmetric vacuum in covariant f (t )=t +α/2 t2+o (tγ) gravity theory |
magnetized neutron stars power at least some ultraluminous x-ray sources. the accretion flow in these cases is interrupted at the magnetospheric radius and then reaches the surface of a neutron star following magnetic field lines. accreting matter moving along magnetic field lines forms the accretion envelope around the central object. we show that in case of high-mass accretion rates ≳ 1019 g s-1 the envelope becomes closed and optically thick, which influences the dynamics of the accretion flow and the observational manifestation of the neutron star hidden behind the envelope. particularly, the optically thick accretion envelope results in a multi-colour blackbody spectrum originating from the magnetospheric surface. the spectrum and photon energy flux vary with the viewing angle, which gives rise to pulsations characterized by high pulsed fraction and typically smooth pulse profiles. the reprocessing of radiation due to interaction with the envelope leads to the disappearance of cyclotron scattering features from the spectrum. we speculate that the super-orbital variability of ultraluminous x-ray sources powered by accreting neutron stars can be attributed to precession of the neutron star due to interaction of magnetic dipole with the accretion disc. | optically thick envelopes around ulxs powered by accreating neutron stars |
context. the recently claimed discovery of a massive (mbh = 68-13+11 m⊙) black hole in the galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution.aims: a crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the b-type star ls v+22 25. because stars of different mass can exhibit b-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass.methods: to this end, we study the spectral energy distribution of ls v+22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for he, c, n, o, ne, mg, al, si, s, ar, and fe.results: our analysis clearly shows that ls v+22 25 is not an ordinary main sequence b-type star. the derived abundance pattern exhibits heavy imprints of the cno bi-cycle of hydrogen burning, that is, he and n are strongly enriched at the expense of c and o. moreover, the elements mg, al, si, s, ar, and fe are systematically underabundant when compared to normal main-sequence b-type stars. we suggest that ls v+22 25 is a stripped helium star and discuss two possible formation scenarios. combining our photometric and spectroscopic results with the gaia parallax, we infer a stellar mass of 1.1 ± 0.5 m⊙. based on the binary system's mass function, this yields a minimum mass of 2-3 m⊙ for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star.conclusions: the star ls v+22 25 has become famous for possibly having a very massive black hole companion. however, a closer look reveals that the star itself is a very intriguing object. further investigations are necessary for complete characterization of this object. | a stripped helium star in the potential black hole binary lb-1 |
we present the first nonlinear lattice simulation of an axion field coupled to a u(1) gauge field during inflation. we use it to fully characterize the statistics of the primordial curvature perturbation ζ . we find high-order statistics to be essential in describing non-gaussianity of ζ in the linear regime of the theory. on the contrary, non-gaussianity is suppressed when the dynamics become nonlinear. this relaxes the bounds from overproduction of primordial black holes, allowing for an observable gravitational waves signal at pulsar timing array and interferometer scales. our work establishes lattice simulations as a crucial tool to study the inflationary epoch and its predictions. | lattice simulations of axion-u(1) inflation |
context. repeating fast radio bursts (frbs) present excellent opportunities to identify frb progenitors and host environments as well as to decipher the underlying emission mechanism. detailed studies of repeating frbs might also hold clues as to the origin of frbs as a population.aims: we aim to detect bursts from the first two repeating frbs, frb 121102 (r1) and frb 180814.j0422+73 (r2), and to characterise their repeat statistics. we also want to significantly improve the sky localisation of r2 and identify its host galaxy.methods: we used the westerbork synthesis radio telescope to conduct extensive follow-up of these two repeating frbs. the new phased-array feed system, apertif, allows one to cover the entire sky position uncertainty of r2 with fine spatial resolution in a single pointing. the data were searched for bursts around the known dispersion measures of the two sources. we characterise the energy distribution and the clustering of detected r1 bursts.results: we detected 30 bursts from r1. the non-poissonian nature is clearly evident from the burst arrival times, which is consistent with earlier claims. our measurements indicate a dispersion measure (dm) of 563.5(2) pc cm-3, suggesting a significant increase in dm over the past few years. assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. we did not detect any bursts from r2.conclusions: a single power-law might not fit the r1 burst energy distribution across the full energy range or widely separated detections. our observations provide improved constraints on the clustering of r1 bursts. our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium at 1400 mhz, which is not observed at higher frequencies. the non-detection of any bursts from r2, despite nearly 300 h of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of the two assuming that the source is still active. another possibility is that r2 has turned off completely, either permanently or for an extended period of time. | repeating fast radio bursts with wsrt/apertif |
gravitational waves in the sensitivity band of ground-based detectors can be emitted by a number of astrophysical sources, including not only binary coalescences, but also individual spinning neutron stars. the most promising signals from such sources, although not yet detected, are long-lasting, quasi-monochromatic continuous waves (cws). the pyfstat package provides tools to perform a range of cw data-analysis tasks. it revolves around the f-statistic, a matched-filter detection statistic for cw signals that has been one of the standard methods for ligo-virgo cw searches for two decades. pyfstat is built on top of established routines in lalsuite but through its more modern python interface it enables a flexible approach to designing new search strategies. hence, it serves a dual function of (i) making lalsuite cw functionality more easily accessible through a python interface, thus facilitating the new user experience and, for developers, the exploratory implementation of novel methods; and (ii) providing a set of production-ready search classes for use cases not yet covered by lalsuite itself, most notably for mcmc-based followup of promising candidates from wide-parameter-space searches. | pyfstat: a python package for continuous gravitational-wave data analysis |
we provide a systematic study of hybrid neutron star equations of state (eos) consisting of a relativistic density functional for the hadronic phase and a covariant nonlocal nambu--jona-lasinio (nlnjl) model to describe the color superconducting quark matter phase. changing the values of the two free parameters, the dimensionless vector and diquark coupling strengths $\eta_v$ and $\eta_d$ results in a set of eos with varying stiffness and deconfinement onset. the favorable parameters are obtained from a systematic bayesian analysis for which the multi-messenger constraint on the neutron star radius at $1.4~$m$_\odot$ and the combined mass-radius constraint for psr j0740+6620 from nicer experiment are used as the constraints. additionally, the transition from hadronic matter to deconfined quark matter is constrained to occur above nuclear saturation density. hybrid stars modeled with these favorable parameters are compatible with the nicer results for the radius of the highest known mass neutron star, psr j0740+6620. three new observations interesting for neutron star phenomenology are reported: 1) we show that the constant sound speed (css) eos provides an excellent fit to that of the nlnjl model which implies the squared speed of sound at high densities to be about $0.5$ for the optimized parameters; 2) we give a simple functional form for the mapping between the parameter spaces of these two models valid for the whole range of relevant chemical potentials and 3) we observe that the special point property of hybrid eos based on css quark matter generalizes to a set of lines consisting of special points when two eos parameters are varied instead of one. a lower limit for the maximum mass of hybrid stars as a function of the vector coupling strength is obtained. | constraining free parameters of a color superconducting non-local nambu-jona-lasinio model using bayesian analysis of neutron stars mass and radius measurements |
we study the prospects for the detection of continuous gravitational signals from normal galactic neutron stars, i.e., nonrecycled stars. we use a synthetic population generated by evolving stellar remnants in time, according to several models. we consider the most recent constraints set by all-sky searches for continuous gravitational waves and use them for our detectability criteria. we discuss the detection prospects for the current and the next generation of gravitational-wave detectors. we find that neutron stars whose ellipticity is solely caused by magnetic deformations cannot produce any detectable signal, not even by third-generation detectors. the currently detectable sources all have b ≲ 1012 g and deformations that are not solely due to the magnetic field. for these, we find in fact that the larger the magnetic field, the higher the ellipticity required for the signal to be detectable, and this ellipticity is well above the value induced by the magnetic field. third-generation detectors such as the einstein telescope and cosmic explorer will be able to detect up to ≈250 more sources than current detectors. we briefly treat the case of recycled neutron stars with a simplified model. we find that continuous gravitational waves from these objects will likely remain elusive to detection by current detectors, but should be detectable with the next generation of detectors. | continuous gravitational waves from galactic neutron stars: demography, detectability, and prospects |
the nature of the fermi γ -ray galactic center excess (gce) has remained a persistent mystery for over a decade. although the excess is broadly compatible with emission expected due to dark matter annihilation, an explanation in terms of a population of unresolved astrophysical point sources, e.g., millisecond pulsars, remains viable. the effort to uncover the origin of the gce is hampered in particular by an incomplete understanding of diffuse emission of galactic origin. this can lead to spurious features that make it difficult to robustly differentiate smooth emission, as expected for a dark matter origin, from more "clumpy" emission expected from a population of relatively bright, unresolved point sources. we use recent advancements in the field of simulation-based inference, in particular density estimation techniques using normalizing flows, in order to characterize the contribution of modeled components, including unresolved point source populations, to the gce. compared to traditional techniques based on the statistical distribution of photon counts, our machine-learning-based method is able to utilize more of the information contained in a given model of the galactic center emission and in particular can perform posterior parameter estimation while accounting for pixel-to-pixel spatial correlations in the γ -ray map. this makes the method demonstrably more resilient to certain forms of model misspecification. on application to fermi data, the method generically attributes a smaller fraction of the gce flux to unresolved point sources when compared to traditional approaches. we nevertheless infer such a contribution to make up a non-negligible fraction of the gce across all analysis variations considered, with at least 38-19+9% of the excess attributed to unresolved point sources in our baseline analysis. | neural simulation-based inference approach for characterizing the galactic center γ -ray excess |
the low-mass companions of evaporating binary pulsars (black widows and similar) are strongly heated on the side facing the pulsar. however, in high-quality photometric and spectroscopic data, the heating pattern does not match that expected for direct pulsar illumination. here we explore a model where the pulsar power is intercepted by an intra-binary shock (ibs) before heating the low-mass companion. we develop a simple analytic model and implement it in the popular “icarus” light curve code. the model is parameterized by the wind momentum ratio β and the companion wind speed {f}v{v}{{orb}}, and assumes that the reprocessed pulsar wind emits prompt particles or radiation to heat the companion surface. we illustrate an interesting range of light curve asymmetries controlled by these parameters. the code also computes the ibs synchrotron emission pattern, and thus can model black widow x-ray light curves. as a test, we apply the results to the high-quality asymmetric optical light curves of psr j2215+5135; the resulting fit gives a substantial improvement upon direct heating models and produces an x-ray light curve consistent with that seen. the ibs model parameters imply that at the present loss rate, the companion evaporation has a characteristic timescale of {τ }{{evap}}≈ 150 myr. still, the model is not fully satisfactory, indicating that there are additional unmodeled physical effects. | intra-binary shock heating of black widow companions |
the hawc collaboration has reported the detection of an extended γ -ray emission around the geminga and monogem pulsars of a few degree extension. very recently, the lhaaso collaboration released also the data for an extended γ -ray emission around the pulsar psr j0622+3749. this flux can be explained with electrons and positrons injected from these sources and their inverse compton scattering on the interstellar radiation fields. so far the size of such γ -ray halos has been interpreted as the result of the diffusion coefficient around the sources being about 2 orders of magnitude smaller than the average in the galaxy. however, this conclusion is driven by the assumption that particles propagate diffusively right away after the injection without taking into account the ballistic propagation. the propagation of cosmic-ray leptons in the proximity of the geminga, monogem and psr j0622+3749 pulsars is examined here considering the transition from the quasiballistic, valid for the most recently injected particles, to the diffusive transport regime. for typical interstellar values of the diffusion coefficient, the quasiballistic regime dominates the lepton distribution up to distances of a few tens of parsec from the pulsar for particle energies above ∼10 tev . in this regime the resulting γ -ray source tends to be rather compact, despite particles traveling a long distance. indeed, for larger values of the diffusion coefficient, particles propagate ballistically up to larger distances with the result of a more pointlike γ -ray source. when such transition is taken into account, a good fit to the hawc and lhaaso γ -ray data around geminga, monogem and psr j0622+3749 is obtained without the need to invoke a strong suppression of the diffusion coefficient. | do the geminga, monogem and psr j0622+3749 γ -ray halos imply slow diffusion around pulsars? |
fast radio bursts (frbs) are millisecond bursts of radio radiation whose progenitors, so far, remain mysterious. nevertheless, the timescales and energetics of these events have lead to many theories associating frbs with young neutron stars (nss). motivated by this, i explore the interaction of frbs with young supernova remnants (snrs), and i discuss the potential observational consequences and constraints of such a scenario. as the supernova (sn) ejecta plows into the interstellar medium (ism), a reverse shock is generated that passes back through the material and ionizes it. this leads to a dispersion measure (dm) associated with the snr as well as a time derivative for dm. the times when dm is high are generally overshadowed by free-free absorption, which, depending on the mass of the ejecta and the density of the ism, may be probed at frequencies of 400 {{mhz}}-1.4 {{ghz}} on timescales of ∼100-500 years after the sn. magnetic fields generated at the reverse shock may be high enough to explain faraday rotation that has been measured for one frb. if frbs are powered by the spin energy of a young ns (rather than by magnetic energy), the ns must have a magnetic field ≲ {10}11{--}{10}12 {{g}} to ensure that it does not spin down too quickly while the snr is still optically thick at radio frequencies. in the future, once there are distance measurements to frbs and their energetics are better understood, the spin of the ns can also be constrained. | the impact of a supernova remnant on fast radio bursts |
the interaction of a rotating star’s magnetic field with a surrounding plasma disk lies at the heart of many questions posed by neutron stars in x-ray binaries. we consider the opening of stellar magnetic flux due to differential rotation along field lines coupling the star and disk, using a simple model for the disk-opened flux, the torques exerted on the star by the magnetosphere, and the power extracted by the electromagnetic wind. we examine the conditions under which the system enters an equilibrium spin state, in which the accretion torque is instantaneously balanced by the pulsar wind torque alone. for magnetic moments, spin frequencies, and accretion rates relevant to accreting millisecond pulsars, the spin-down torque from this enhanced pulsar wind can be substantially larger than that predicted by existing models of the disk-magnetosphere interaction, and is in principle capable of maintaining spin equilibrium at frequencies less than 1 khz. we speculate that this mechanism may account for the non-detection of frequency increases during outbursts of sax j1808.4-3658 and xte j1814-338, and may be generally responsible for preventing spin-up to sub-millisecond periods. if the pulsar wind is collimated by the surrounding environment, the resulting jet can satisfy the power requirements of the highly relativistic outflows from cir x-1 and sco x-1. in this framework, the jet power scales relatively weakly with accretion rate, {l}{{j}}\propto {\dot{m}}4/7, and would be suppressed at high accretion rates only if the stellar magnetic moment is sufficiently low. | torque enhancement, spin equilibrium, and jet power from disk-induced opening of pulsar magnetic fields |
programmable arrays of hundreds of rydberg atoms have recently enabled the exploration of remarkable phenomena in many-body quantum physics. in addition, the development of high-fidelity quantum gates are making them promising architectures for the implementation of quantum circuits.we present herepulser, an open-source python library for programming neutral-atom devices at the pulse level. the low-level nature of pulser makes it a versatile framework for quantum control both in the digital and analog settings. the library also contains simulation routines for studying and exploring the outcome of pulse sequences for small systems. | pulser: an open-source package for the design of pulse sequences in programmable neutral-atom arrays |
gravitational waves encode invaluable information about the nature of the relatively unexplored extreme gravity regime, where the gravitational interaction is strong, nonlinear and highly dynamical. recent gravitational wave observations by advanced ligo have provided the first glimpses into this regime, allowing for the extraction of new inferences on different aspects of theoretical physics. for example, these detections provide constraints on the mass of the graviton, lorentz violation in the gravitational sector, the existence of large extra dimensions, the temporal variability of newton's gravitational constant, and modified dispersion relations of gravitational waves. many of these constraints, however, are not yet competitive with constraints obtained, for example, through solar system observations or binary pulsar observations. in this paper, we study the degree to which theoretical physics inferences drawn from gravitational wave observations will strengthen with detections from future detectors. we consider future ground-based detectors, such as the ligo-class expansions a + , voyager, cosmic explorer and the einstein telescope, as well as space-based detectors, such as various configurations of elisa and the recently proposed lisa mission. we find that space-based detectors will place constraints on general relativity up to 12 orders of magnitude more stringently than current aligo bounds, but these space-based constraints are comparable to those obtained with the ground-based cosmic explorer or the einstein telescope (a + and voyager only lead to modest improvements in constraints). we also generically find that improvements in the instrument sensitivity band at low frequencies lead to large improvements in certain classes of constraints, while sensitivity improvements at high frequencies lead to more modest gains. these results strengthen the case for the development of future detectors, while providing additional information that could be useful in future design decisions. | theoretical physics implications of gravitational wave observation with future detectors |
assuming that the common-spectrum process in the nanograv 12.5-year dataset has an origin of scalar-induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the bayesian parameter inference for the first time. we obtain lower limits on the spectral amplitude, i.e., a≳10‑2 at 95% confidence level, when assuming the power spectrum of primordial curvature perturbations to follow a log-normal distribution function with width σ. in the case of σ→0, we find that the primordial black holes with 2×10‑4‑10‑2 solar mass are allowed to compose at least a fraction 10‑6 of dark matter. such a mass range is shifted to more massive regimes for larger values of σ, e.g., to a regime of 4×10‑3‑0.2 solar mass in the case of σ=1. we expect the planned gravitational-wave experiments to have their best sensitivity to a in the range of 10‑4 to 10‑7, depending on the experimental setups. with this level of sensitivity, we can search for primordial black holes throughout the entire parameter space, especially in the mass range of 10‑16 to 10‑11 solar masses, where they could account for all dark matter. in addition, the importance of multi-band detector networks is emphasized to accomplish our theoretical expectation. | bayesian implications for the primordial black holes from nanograv's pulsar-timing data using the scalar-induced gravitational waves |
roughly every 2-10 min, a pair of stellar-mass black holes merge somewhere in the universe. a small fraction of these mergers are detected as individually resolvable gravitational-wave events by advanced detectors such as ligo and virgo. the rest contribute to a stochastic background. we derive the statistically optimal search strategy (producing minimum credible intervals) for a background of unresolved binaries. our method applies bayesian parameter estimation to all available data. using monte carlo simulations, we demonstrate that the search is both "safe" and effective: it is not fooled by instrumental artifacts such as glitches and it recovers simulated stochastic signals without bias. given realistic assumptions, we estimate that the search can detect the binary black hole background with about 1 day of design sensitivity data versus ≈40 months using the traditional cross-correlation search. this framework independently constrains the merger rate and black hole mass distribution, breaking a degeneracy present in the cross-correlation approach. the search provides a unified framework for population studies of compact binaries, which is cast in terms of hyperparameter estimation. we discuss a number of extensions and generalizations, including application to other sources (such as binary neutron stars and continuous-wave sources), simultaneous estimation of a continuous gaussian background, and applications to pulsar timing. | optimal search for an astrophysical gravitational-wave background |
using as references the posterior probability distribution functions of the equation of state (eos) parameters inferred from the radii of canonical neutron stars (nss) reported by the ligo/virgo and nicer collaborations based on their observations of gw170817 and psr j0030+0451, we investigate how future radius measurements of more massive nss will improve our current knowledge about the eos of superdense neutron-rich nuclear matter, especially its symmetry energy term. within the bayesian statistical approach using an explicitly isospin-dependent parametric eos for the core of nss, we infer the eos parameters of superdense neutron-rich nuclear matter from three sets of imagined mass-radius correlation data representing typical predictions by various nuclear many-body theories, that is, the radius stays the same, decreases, or increases with increasing ns mass within ±15% between 1.4 and 2.0 m⊙. the corresponding ns average density increases quickly or slowly or slightly decreases as the ns mass increases from 1.4 to 2.0 m⊙. while the eoss of symmetric nuclear matter (snm) inferred from the three data sets are approximately the same, the corresponding symmetry energies above about twice the saturation density of nuclear matter are very different, indicating that the radii of massive nss carry important information about the high-density behavior of nuclear symmetry energy with little influence from the remaining uncertainties of the snm eos at suprasaturation densities. | bayesian inference of the symmetry energy of superdense neutron-rich matter from future radius measurements of massive neutron stars |
magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. with large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e. with magnetization σ ≫ 1) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cut-off of the spectrum of accelerated particles. we find that the high-energy spectral cut-off does not saturate at γcut ∼ 4σ, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. at late times, the cut-off scales approximately as γ _cut∝ √{t}, regardless of the flow magnetization and initial temperature. we show that the particles dominating the high-energy spectral cut-off reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core. the growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. we also find that the power-law slope of the spectrum (p = -d log n/d log γ) evolves with time. for σ ≳ 10, the spectrum is hard at early times (p ≲ 2), but it tends to asymptote to p ∼ 2; the steepening of the power-law slope allows the spectral cut-off to extend to higher and higher energies, without violating the fixed energy budget of the system. our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources. | the steady growth of the high-energy spectral cut-off in relativistic magnetic reconnection |
we synthesize the known information about fast radio bursts (frbs) and radio magnetars, and describe an allowed origin near nuclei of external, but non-cosmological, galaxies. this places them at z\ll 1, within a few hundred megaparsecs. in this scenario, the high dispersion measure (dm) is dominated by the environment of the frb, modeled on the known properties of the milky way center, whose innermost 100 pc provides 1000 pc cm-3. a radio loud magnetar is known to exist in our galactic center, within ∼2 arcsec of sgr a*. based on the polarization, dm, and scattering properties of this known magnetar, we extrapolate its properties to those of crab-like giant pulses and sgr flares and point out their consistency with observed frbs. we conclude that galactic center magnetars could be the source of frbs. this scenario is readily testable with very long baseline interferometry measurements as well as with flux count statistics from large surveys such as chime or utmost. | local circumnuclear magnetar solution to extragalactic fast radio bursts |
supermassive black hole binaries, cosmic strings, relic gravitational waves from inflation, and first-order phase transitions in the early universe are expected to contribute to a stochastic background of gravitational waves in the 1 0-9- 1 0-7 h z frequency band. pulsar timing arrays (ptas) exploit the high-precision timing of radio pulsars to detect signals at such frequencies. here we present a time-domain implementation of the optimal cross-correlation statistic for stochastic background searches in pta data. due to the irregular sampling typical of pta data as well as the use of a timing model to predict the times of arrival of radio pulses, time-domain methods are better-suited for gravitational-wave data analysis of such data. we present a derivation of the optimal cross-correlation statistic starting from the likelihood function, a method to produce simulated stochastic background signals, and a rigorous derivation of the scaling laws for the signal-to-noise ratio of the cross-correlation statistic in the two relevant pta regimes: the weak-signal limit where instrumental noise dominates over the gravitational-wave signal at all frequencies, and a second regime where the gravitational-wave signal dominates at the lowest frequencies. | time-domain implementation of the optimal cross-correlation statistic for stochastic gravitational-wave background searches in pulsar timing data |
the direct detection of gravitational waves with the next-generation detectors, like advanced ligo, provides the opportunity to measure deviations from the predictions of general relativity. one such departure would be the existence of alternative polarizations. to measure these, we study a single detector measurement of a continuous gravitational wave from a triaxial pulsar source. we develop methods to detect signals of any polarization content and distinguish between them in a model-independent way. we present ligo science run 5 sensitivity estimates for 115 pulsars. | detecting beyond-einstein polarizations of continuous gravitational waves |
we present results from particle-in-cell simulations of driven turbulence in magnetized, collisionless, and relativistic pair plasmas. we find that the fluctuations are consistent with the classical k⊥-5 /3 magnetic energy spectrum at fluid scales and a steeper k⊥-4 spectrum at sub-larmor scales, where k⊥ is the wave vector perpendicular to the mean field. we demonstrate the development of a nonthermal, power-law particle energy distribution f (e )∼e-α, with an index α that decreases with increasing magnetization and increases with an increasing system size (relative to the characteristic larmor radius). our simulations indicate that turbulence can be a viable source of energetic particles in high-energy astrophysical systems, such as pulsar wind nebulae, if scalings asymptotically become insensitive to the system size. | kinetic turbulence in relativistic plasma: from thermal bath to nonthermal continuum |
precise characterization of detector time resolution is of crucial importance for next-generation cryogenic-bolometer experiments searching for neutrinoless double-beta decay, such as cupid, in order to reject background due to pileup of two-neutrino double-beta decay events. in this paper, we describe a technique developed to study the pileup rejection capability of cryogenic bolometers. our approach, which consists of producing controlled pileup events with a programmable wave-form generator, has the benefit that we can reliably and reproducibly control the time separation and relative energy of the individual components of the generated pileup events. the resulting data allow us to optimize and benchmark analysis strategies to discriminate between individual and pileup pulses. we describe a test of this technique performed with a small array of detectors at the laboratori nazionali del gran sasso, in italy; we obtain a 90 % rejection efficiency against pulser-generated pileup events with rise time of ∼15 ms down to time separation between the individual events of about 2 ms . | novel technique for the study of pileup events in cryogenic bolometers |
the chime/frb collaboration reports the detection of a radio burst from the direction of the known galactic magnetar sgr 1935+2154, which is currently in an active state (gcns #32675, #32698, #32706, #32708, #32737, and atels #15667, #15672, #15674). | chime/frb detection of a bright radio burst from sgr 1935+2154 |
the packed ultra-wideband mapping array (puma) is a proposed low-resolution transit interferometric radio telescope operating over the frequency range 200 - 1100mhz. its rich science portfolio will include measuring structure in the universe from redshift z = 0.3 to 6 using 21cm intensity mapping, detecting one million fast radio bursts, and monitoring thousands of pulsars. it will allow puma to advance science in three different areas of physics (the physics of dark energy, the physics of cosmic inflation and time-domain astrophysics). this document is a response to a request for information (rfi) by the panel on radio, millimeter, and submillimeter observations from the ground (rms) of the decadal survey on astronomy and astrophysics 2020. we present the science case of puma, the development path and major risks to the project. | packed ultra-wideband mapping array (puma): astro2020 rfi response |
breakthrough listen is the most comprehensive and sensitive search for extraterrestrial intelligence (seti) to date, employing a collection of international observational facilities including both radio and optical telescopes. during the first three years of the listen program, thousands of targets have been observed with the green bank telescope (gbt), parkes telescope and automated planet finder. at gbt and parkes, observations have been performed ranging from 700 mhz to 26 ghz, with raw data volumes averaging over 1 pb day-1. a pseudo-real time software spectroscopy suite is used to produce multi-resolution spectrograms amounting to approximately 400 gb h-1 ghz-1 beam-1. for certain targets, raw baseband voltage data is also preserved. observations with the automated planet finder produce both two-dimensional and one-dimensional high-resolution (r ∼ 105) echelle spectral data. although the primary purpose of listen data acquisition is for seti, a range of secondary science has also been performed with these data, including studies of fast radio bursts. other current and potential research topics include spectral line studies, searches for certain kinds of dark matter, probes of interstellar scattering, pulsar searches, radio transient searches and investigations of stellar activity. listen data are also being used in the development of algorithms, including machine-learning approaches to modulation scheme classification and outlier detection, that have wide applicability not just for astronomical research but for a broad range of science and engineering. in this paper, we describe the hardware and software pipeline used for collection, reduction, archival, and public dissemination of listen data. we describe the data formats and tools, and present breakthrough listen data release 1.0 (bldr 1.0), a defined set of publicly available raw and reduced data totaling 1 pb. | the breakthrough listen search for intelligent life: public data, formats, reduction, and archiving |
in agreement with the constantly increasing gravitational wave events, new aspects of the internal structure of compact stars can be considered. a scenario in which a first-order transition takes place inside these stars is of particular interest, as it can lead, under certain conditions, to a third gravitationally stable branch (besides white dwarfs and neutron stars), the twin stars. the new branch yields stars with the same mass as normal compact stars but quite different radii. we focus on hybrid stars undergoing a hadron-to-quark phase transition near their core and how this new stable configuration arises. emphasis is given on the aspects of the phase transition and its parametrization in two different ways—namely, with the maxwell and gibbs constructions. we systematically study the gravitational mass, the radius, and the tidal deformability, and we compare them with the predictions of the recent observation by the ligo/virgo collaboration, the gw170817 event, and the mass and radius limits, suggesting possible robust constraints. moreover, we extend the study to include rotation effects on the twin star configurations. the recent discovery of the fast rotating supermassive pulsar psr j0952-0607 triggered the efforts to constrain the equation of state and, moreover, to examine possible predictions related to the phase transition in dense nuclear matter. we pay special attention to relate the psr j0952-0607 pulsar properties to the twin star predictions, and mainly to explore the possibility that the existence of such a massive object would rule out the existence of twin stars. finally, we discuss the constraints on the radius and mass of the recently observed compact object within the supernova remnant hess j1731-347. the estimations imply that this object is either the lightest known neutron star or a star with a more exotic equation of state. | twin stars as probes of the nuclear equation of state: effects of rotation through the psr j0952-0607 pulsar and constraints via the tidal deformability from the gw170817 event |
the gravitational wave gw170817 is associated with the inspiral phase of a binary neutron star coalescence event. the ligo-virgo detectors' sensitivity at high frequencies was not sufficient to detect the signal corresponding to the merger and postmerger phases. hence, the question whether the merger outcome was a prompt black-hole formation or not must be answered using either the premerger gravitational-wave signal or electromagnetic counterparts. in this work we present two methods to infer the probability of prompt black-hole formation, using the analysis of the inspiral gravitational-wave signal. both methods combine the posterior distribution from the gravitational-wave data analysis with numerical-relativity results. one method relies on the use of phenomenological models for the equation of state and on the estimate of the collapse threshold mass. the other is based on the estimate of the tidal polarizability parameter λ ∼ that is correlated in an equation-of-state agnostic way with the prompt black-hole formation. we analyze gw170817 data and find that the two methods consistently predict a probability of ∼50 %- 70 % for prompt black-hole formation, which however may significantly decrease below 10% if the maximum mass constraint from psr j0348+0432 or psr j0740+6620 is imposed. | inferring prompt black-hole formation in neutron star mergers from gravitational-wave data |
the first x-ray pulsar, cen x-3, was discovered 50 yr ago. radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with propagation of photons in the presence of the strong magnetic field. here we present results of the observations of cen x-3 performed with the imaging x-ray polarimetry explorer. the source exhibited significant flux variability and was observed in two states different by a factor of ~20 in flux. in the low-luminosity state, no significant polarization was found in either pulse phase-averaged (with a 3σ upper limit of 12%) or phase-resolved (the 3σ upper limits are 20%-30%) data. in the bright state, the polarization degree of 5.8% ± 0.3% and polarization angle of 49.°6 ± 1.°5 with a significance of about 20σ were measured from the spectropolarimetric analysis of the phase-averaged data. the phase-resolved analysis showed a significant anticorrelation between the flux and the polarization degree, as well as strong variations of the polarization angle. the fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49° and a magnetic obliquity of 17°. the detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. a fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain. | the x-ray polarimetry view of the accreting pulsar cen x-3 |
we present the measurements of faraday rotation for 477 pulsars observed by the parkes 64 m radio telescope and the green bank 100 m radio telescope. using these results, along with previous measurements for pulsars and extragalactic sources, we analyze the structure of the large-scale magnetic field in the galactic disk. comparisons of rotation measures of pulsars in the disk at different distances, as well as with rotation measures of background radio sources beyond the disk, reveal large-scale reversals of the field directions between the spiral arms and interarm regions. we develop a model for the disk magnetic field, which can reproduce not only these reversals but also the distribution of the observed rotation measures of background sources. | pulsar rotation measures and large-scale magnetic field reversals in the galactic disk |
the x-ray afterglow plateau emission observed in many gamma-ray bursts (grbs) has been interpreted as being fueled either by fallback onto a newly formed black hole or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. if the latter model is assumed, grb x-ray afterglow light curves can be reproduced analytically. we fit a sample of grb x-ray plateaus, interestingly yielding a distribution in the diagram of magnetic field versus spin period (b-p) consistent with b ∝ p 7/6, which is consistent with grb expectations of the well-established physics of the spin-up line for accreting galactic x-ray pulsars. the normalization of the relation that we obtain perfectly matches spin-up line predictions for typical neutron star masses (∼1 m ⊙) and radii (∼10 km), and for mass accretion rates typically expected in grbs, {10}-4 {m}⊙{{{s}}}-1< \dot{m}< {10}-1 {m}⊙{{{s}}}-1. short grbs with extended emission (sees) appear toward the long-period end of the distribution, and long grbs (lgrbs) toward the short-period end. this result is consistent with expectations from the spin-up limit, where the total accreted mass determines the position of the neutron star in the b-p diagram. the b-p distributions for lgrbs and sees are statistically different, further supporting the idea that the fundamental plane relation—a tri-dimensional correlation between the x-ray luminosity at the end of the plateau, the end time of the plateau, and the 1 s peak luminosity in the prompt emission—is a powerful discriminant among those populations. our conclusions are robust against suppositions regarding the collimation angle of the grb and the magnetar braking index, which shift the resulting properties of the magnetar parallel to the spin-up line, and strongly support a magnetar origin for grbs presenting x-ray plateaus. | on the magnetar origin of the grbs presenting x-ray afterglow plateaus |
the accretion-induced collapse (aic) scenario was proposed 40 years ago as an evolutionary end state of oxygen-neon white dwarfs (one wds), linking them to the formation of neutron star (ns) systems. however, there has been no direct detection of any aic event so far, even though there exists a lot of indirect observational evidence. meanwhile, the evolutionary pathways resulting in ns formation through aic are still not thoroughly investigated. in this article, we review recent studies on the two classic progenitor models of aic events, i.e., the single-degenerate model (including the one wd+ms/rg/he star channels and the co wd+he star channel) and the double-degenerate model (including the double co wd channel, the double one wd channel and the one wd+co wd channel). recent progress on these progenitor models is reviewed, including the evolutionary scenarios leading to aic events, the initial parameter space for producing aic events and the related objects (e.g., the pre-aic systems and the post-aic systems). for the single-degenerate model, the pre-aic systems (i.e., the progenitor systems of aic events) could potentially be identified as supersoft x-ray sources, symbiotics and cataclysmic variables (such as classical novae, recurrent novae, ne novae and he novae) in the observations, whereas the post-aic systems (i.e., ns systems) could potentially be identified as low-/intermediate-mass x-ray binaries, and the resulting low-/intermediate-mass binary pulsars, most notably millisecond pulsars. for the double-degenerate model, the pre-aic systems are close double wds with short orbital periods, whereas the post-aic systems are single isolated nss that may correspond to a specific kind of ns with peculiar properties. we also review the predicted rates of aic events, the mass distribution of nss produced via aic and the gravitational wave (gw) signals from double wds that are potential gw sources in the galaxy in the context of future space-based gw detectors, such as lisa, tianqin, taiji, etc. recent theoretical and observational constraints on the detection of aic events are summarized. in order to confirm the existence of the aic process, and resolve this long-term issue presented by current stellar evolution theories, more numerical simulations and observational identifications are required. | the formation of neutron star systems through accretion-induced collapse in white-dwarf binaries |
the light curves of some luminous supernovae are suspected to be powered by the spindown energy of a rapidly rotating magnetar. here we describe a possible signature of the central engine: a burst of shock breakout emission occurring several days after the supernova explosion. the energy input from the magnetar inflates a high-pressure bubble that drives a shock through the pre-exploded supernova ejecta. if the magnetar is powerful enough, that shock will near the ejecta surface and become radiative. at the time of shock breakout, the ejecta will have expanded to a large radius (∼ {10}14 cm) so that the radiation released is at optical/ultraviolet wavelengths ({t}{{eff}} ≈ 20,000 k) and lasts for several days. the luminosity and timescale of this magnetar-driven shock breakout are similar to the first peak observed recently in the double-peaked light curve of sn-lsq14bdq. however, for a large region of model parameter space, the breakout emission is predicted to be dimmer than the diffusive luminosity from direct magnetar heating. a distinct double-peaked light curve may therefore only be conspicuous if thermal heating from the magnetar is suppressed at early times. we describe how such a delay in heating may naturally result from inefficient dissipation and thermalization of the pulsar wind magnetic energy. without such suppression, the breakout may only be noticeable as a small bump or kink in the early luminosity or color evolution, or as a small but abrupt rise in the photospheric velocity. a similar breakout signature may accompany other central engines in supernovae, such as a black hole accreting fallback material. | magnetar-driven shock breakout and double-peaked supernova light curves |
ultraluminous x-ray sources (ulxs) are a population of extragalactic objects whose luminosity exceeds the eddington limit for a 10 m ⊙ black hole (bh). their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass bhs. however at least three neutron stars (nss) have been recently identified in ulxs through the discovery of periodic pulsations. motivated by these findings we studied the spectral properties of a sample of bright ulxs using a simple continuum model which was extensively used to fit the x-ray spectra of accreting magnetic nss in the galaxy. we found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits most of the ulx spectra analyzed here very well, at a level comparable to that of models involving an accreting bh. on these grounds alone we suggest that other non-pulsating ulxs may host nss. we also found that above 2 kev the spectrum of known pulsating ulxs is harder than that of the majority of the other ulxs of the sample, with only ic 342 x-1 and ho ix x-1 displaying spectra of comparable hardness. we thus suggest that these two ulxs may host an accreting ns and encourage searches for periodic pulsations in the flux. | pulsator-like spectra from ultraluminous x-ray sources and the search for more ultraluminous pulsars |
we extend the quark mean-field (qmf) model for nuclear matter and study the possible presence of quark matter inside the cores of neutron stars. a sharp first-order hadron-quark phase transition is implemented combining the qmf for the hadronic phase with "constant-speed-of-sound" parameterization for the high-density quark phase. the interplay of the nuclear symmetry energy slope parameter, l, and the dimensionless phase transition parameters (the transition density ntrans/n0, the transition strength δɛ/ɛtrans, and the sound speed squared in quark matter ${c}_{\mathrm{qm}}^{2}$ ) are then systematically explored for the hybrid star properties, especially the maximum mass mmax and the radius and the tidal deformability of a typical 1.4 m⊙ star. we show the strong correlation between the symmetry energy slope l and the typical stellar radius r1.4, similar to that previously found for neutron stars without a phase transition. with the inclusion of phase transition, we obtain robust limits on the maximum mass (mmax < 3.6 m⊙) and the radius of 1.4 m⊙ stars (r1.4 ≳ 9.6 km), and we find that a phase transition that is too weak (δɛ/ɛtrans ≲ 0.2) taking place at low densities ≲1.3-1.5 n0 is strongly disfavored. we also demonstrate that future measurements of the radius and tidal deformability of ∼1.4 m⊙ stars, as well as the mass measurement of very massive pulsars, can help reveal the presence and amount of quark matter in compact objects. | constraining hadron-quark phase transition parameters within the quark-mean-field model using multimessenger observations of neutron stars |
we have compiled a significantly updated and comprehensive census of massive stars in the nearby cygnus ob2 association by gathering and homogenizing data from across the literature. the census contains 169 primary ob stars, including 52 o-type stars and 3 wolf-rayet stars. spectral types and photometry are used to place the stars in a hertzsprung-russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. the star formation history and mass function of the association are assessed, and both are found to be heavily influenced by the evolution of the most massive stars to their end states. we find that the mass function of the most massive stars is consistent with a `universal' power-law slope of γ = 1.3. the age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7 myr ago, in agreement with studies of low- and intermediate-mass stars in the association. we identify a nearby young pulsar and runaway o-type star that may have originated in cyg ob2 and suggest that the association has already seen its first supernova. finally we use the census and mass function to calculate the total mass of the association of 16 500^{+3800}_{-2800} m⊙, at the low end, but consistent with, previous estimates of the total mass of cyg ob2. despite this cyg ob2 is still one of the most massive groups of young stars known in our galaxy making it a prime target for studies of star formation on the largest scales. | the massive star population of cygnus ob2 |
aims: in the present work we search for a new model of compact star within embedding class one spacetime i.e., four dimensional spacetime embedded in five dimensional pseudo euclidean space. methods: in particular we propose a new mass function to obtain an exact analytic solutions of the einstein field equations. for this specific mass function, obtained solutions are well-behaved at the centre of the star, satisfy all energy conditions and the mass-radius relation fall within the limit proposed by buchdahl (1959). results: the static equilibrium condition has been maintained under different forces. we have discussed the solutions in detail and compare with a set of astrophysical objects like 4u1608-52, psr j1903+327, psr j1614-2230 and x-ray pulsar vela x-1 is also explored. | compact stars with specific mass function |
neutron stars spin down over time due to a number of energy-loss processes. we provide tantalizing population-based evidence that millisecond pulsars (msps) have a minimum ellipticity of ɛ ≈ 10-9 around their spin axis and that, consequently, some spin down mostly through gravitational-wave emission. we discuss the implications of such a minimum ellipticity in terms of the internal magnetic field strengths and nuclear matter composition of neutron stars and show it would result in the advanced ligo and virgo gravitational-wave detectors, or their upgrades, detecting gravitational waves from some known msps in the near future. | evidence for a minimum ellipticity in millisecond pulsars |
the large magellanic cloud, a satellite galaxy of the milky way, has been observed with the high energy stereoscopic system (h.e.s.s.) above an energy of 100 billion electron volts for a deep exposure of 210 hours. three sources of different types were detected: the pulsar wind nebula of the most energetic pulsar known, n 157b; the radio-loud supernova remnant n 132d; and the largest nonthermal x-ray shell, the superbubble 30 dor c. the unique object sn 1987a is, unexpectedly, not detected, which constrains the theoretical framework of particle acceleration in very young supernova remnants. these detections reveal the most energetic tip of a γ-ray source population in an external galaxy and provide via 30 dor c the unambiguous detection of γ-ray emission from a superbubble. | the exceptionally powerful tev γ-ray emitters in the large magellanic cloud |
the realm of strong classical gravity and perhaps even quantum gravity are waiting to be explored. in this letter we consider the recently detected triple system composed of two stars and a non-accreting black hole. using published observations of this system we conduct the most sensitive test to date for whether the black hole is actually a wormhole by looking for orbital perturbations due to an object on the other side of the wormhole. the mass limit obtained on the perturber is $\sim4$ orders of magnitude better than for observations of s2 orbiting the supermassive black hole at sgr a*. we also consider how observations of a pulsar could test for whether the black hole in a pulsar-black hole binary is a wormhole. a pulsar in a similar orbit to s2 would be $\sim10$ orders of magnitude more sensitive than observations of s2. for a nominal pulsar-black hole binary of stellar masses, with orbital size similar to that of the hulse-taylor binary pulsar, one year of observations could set a mass limit on a perturber that is $\sim6$ orders of magnitude better than observations of a pulsar around sgr~a*. a range of limits between the pulsar-sgr~a* and hulse-taylor cases could be obtained for a possible population of pulsar-black hole binaries that may exist near the galactic center. | a sensitive search for wormholes |
corrections to general relativity that introduce long-ranged scalar fields which are nonminimally coupled to curvature typically predict that neutron stars possess a nontrivial scalar field profile anchored to the star. an observer far from a star is most sensitive to the spherically symmetric piece of this profile that decays linearly with the inverse of the distance to the source, the so-called scalar monopole charge, which is related to the emission of dipolar radiation from compact binary systems. the presence of dipolar radiation has the potential to rule out or very strongly constrain extended theories of gravity. these facts may lead people to believe that gravitational theories that introduce long-ranged scalar fields have already been constrained strongly from binary pulsar observations. here we challenge this "lore" by investigating the decoupling limit of gauss-bonnet gravity as an example, in which the scalar field couples linearly to the gauss-bonnet density in the action. we prove a theorem that neutron stars in this theory cannot possess a scalar charge, due to the topological nature of the gauss-bonnet density. thus gauss-bonnet gravity evades the strong binary pulsar constraints on dipole radiation. we discuss the astrophysical systems which will yield the best constraints on gauss-bonnet gravity and related quadratic gravity theories. to achieve this we compute the scalar charge in quadratic gravity theories by performing explicit analytic and numerical matching calculations for slowly rotating neutron stars. in generic quadratic gravity theories, either neutron star-binary or neutron star-black hole systems can be used to constrain the theory, but because of the vanishing charge, gauss-bonnet gravity evades the neutron star-binary constraints. however, in contrast to neutron stars, black holes in gauss-bonnet gravity do anchor scalar charge, because of the difference in topology. the best constraints on gauss-bonnet gravity will thus come from accurate black hole observations, for example through gravitational waves from inspiraling binaries or the timing of pulsar-black hole binaries with radio telescopes. we estimate these constraints to be a factor of 10 better than the current estimated bound, and also include estimated constraints on generic quadratic gravity theories from pulsar timing. | challenging the presence of scalar charge and dipolar radiation in binary pulsars |
we analyze families of hybrid equations of state of cold qcd matter, which combine input from gauge-gravity duality and from various ab initio methods for nuclear matter at low density, and predict that all neutron stars are fully hadronic without quark matter cores. we focus on constraints from recent measurements by the nicer telescope on the radius and mass of the millisecond pulsar psr j 0740 +6620 . these results are found to be consistent with our approach: they set only mild constraints on the hybrid equations of state and favor the most natural models which are relatively stiff at low density. adding an upper bound on the maximal mass of neutron stars, as suggested by the analysis of the gw170817 neutron star merger event, tightens the constraints considerably. we discuss updated predictions on observables such as the transition density and latent heat of the nuclear to quark matter transition as well as the masses, radii, and tidal deformabilities of neutron stars. | holographic qcd in the nicer era |
the measurement of the macroscopic properties of a neutron star, whether in binary or in an isolated system, provides us with a key opportunity to place a stringent constraint on its equation of state. in this paper, we perform bayesian model selection on a wide variety of neutron star equations of state using multi-messenger observations. in particular, (i) we use the mass and tidal deformability measurements from two binary neutron star merger events, gw170817 and gw190425; and (ii) we use the simultaneous mass-radius measurements of psr j0030+0451 and psr j0740+6620 by the nicer collaboration, while the latter has been analyzed by the joint nicer/radio/xmm-newton collaboration. among the 31 equations of state considered in this analysis, we are able to rule out different variants of the ms1 family, ski5, h4, and wff1 decisively, which are either extremely stiff or soft equations of state. the most preferred equation of state model turns out to be ap3 (or mpa1), which predicts the radius and dimensionless tidal deformability of a 1.4 m ⊙ neutron star to be 12.10 (12.50) km and 393 (513), respectively. | bayesian model selection of neutron star equations of state using multi-messenger observations |
the coherent oscillation of ultralight dark matter in the mass regime around 10-23 ev induces changes in gravitational potential with the frequency in the nanohertz range. this effect is known to produce a monochromatic signal in the pulsar timing residuals. here we discuss a multifield scenario that produces a wide spectrum of frequencies, such that the ultralight particle oscillation can mimic the pulsar timing signal of stochastic common spectrum process. we discuss how ultralight dark matter with various spins produces such a wide band spectrum on pulsar timing residuals and perform the bayesian analysis to constrain the parameters. it turns out that the stochastic background detected by nanograv can be associated with a wideband ultralight dark matter. | pulsar timing residual induced by wideband ultralight dark matter with spin 0,1,2 |
most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (smbhb), and hence periodic signatures due to orbital motion. we report unique periodic radio flux density variations in the blazar pks 2131-021, which strongly suggest an smbhb with an orbital separation of ~0.001-0.01 pc. our 45.1 yr radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within ≲2% and ~10%, respectively, straddling a 20 yr period when this variation was absent. our simulated light curves accurately reproduce the "red noise" of this object, and lomb-scargle, weighted wavelet z-transform and least-squares sine-wave analyses demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. the observed period translates to 2.082 ± 0.003 yr in the rest frame at the z = 1.285 redshift of pks 2131-021. the periodic variation in pks 2131-021 is remarkably sinusoidal. we present a model in which orbital motion, combined with the strong doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density that easily fits the observations. given the rapidly developing field of gravitational-wave experiments with pulsar timing arrays, closer counterparts to pks 2131-021 and searches using the techniques we have developed are strongly motivated. these results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field. | the unanticipated phenomenology of the blazar pks 2131-021: a unique supermassive black hole binary candidate |
we report the detection of steady radio emission from the known x-ray source x9 in the globular cluster 47 tuc. with a double-peaked c iv emission line in its ultraviolet spectrum providing a clear signature of accretion, this source had been previously classified as a cataclysmic variable. in deep atca (australia telescope compact array) imaging from 2010 and 2013, we identified a steady radio source at both 5.5 and 9.0 ghz, with a radio spectral index (defined as sν ∝ να) of α = -0.4 ± 0.4. our measured flux density of 42 ± 4 μjy beam-1 at 5.5 ghz implies a radio luminosity (νlν) of 5.8 × 1027 erg s-1, significantly higher than any previous radio detection of an accreting white dwarf. transitional millisecond pulsars, which have the highest radio-to-x-ray flux ratios among accreting neutron stars (still a factor of a few below accreting black holes at the same lx), show distinctly different patterns of x-ray and radio variability than x9. when combined with archival x-ray measurements, our radio detection places 47 tuc x9 very close to the radio/x-ray correlation for accreting black holes, and we explore the possibility that this source is instead a quiescent stellar-mass black hole x-ray binary. the nature of the donor star is uncertain; although the luminosity of the optical counterpart is consistent with a low-mass main-sequence donor star, the mass transfer rate required to produce the high quiescent x-ray luminosity of 1033 erg s-1 suggests the system may instead be ultracompact, with an orbital period of order 25 min. this is the fourth quiescent black hole candidate discovered to date in a galactic globular cluster, and the only one with a confirmed accretion signature from its optical/ultraviolet spectrum. | deep radio imaging of 47 tuc identifies the peculiar x-ray source x9 as a new black hole candidate |
numerical and observational evidence suggests that massive white dwarfs dominate the innermost regions of core-collapsed globular clusters by both number and total mass. using ngc 6397 as a test case, we constrain the features of white dwarf populations in core-collapsed clusters, both at present day and throughout their lifetimes. the dynamics of these white dwarf subsystems have a number of astrophysical implications. we demonstrate that the collapse of globular cluster cores is ultimately halted by the dynamical burning of white dwarf binaries. we predict that core-collapsed clusters in the local universe yield a white dwarf merger rate of ${ \mathcal o }(10)\,{{\rm{gpc}}}^{-3}\,{{\rm{yr}}}^{-1}$ , roughly 0.1%-1% of the observed type ia supernova rate. we show that prior to merger, inspiraling white dwarf binaries will be observable as gravitational-wave sources at millihertz and decihertz frequencies. over 90% of these mergers have a total mass greater than the chandrasekhar limit. we argue that the merger/collision remnants, if not destroyed completely in an explosive transient, may be observed in core-collapsed clusters either as young neutron stars/pulsars/magnetars (in the event of accretion-induced collapse) or as young massive white dwarfs offset from the standard white dwarf cooling sequence. finally, we show that collisions between white dwarfs and main-sequence stars, which may be detectable as bright transients, occur at a rate of ${ \mathcal o }(100)\,{{\rm{gpc}}}^{-3}\,{{\rm{yr}}}^{-1}$ in the local universe. we find that these collisions lead to depletion of blue straggler stars and main-sequence star binaries in the centers of core-collapsed clusters. | white dwarf subsystems in core-collapsed globular clusters |
radio pulsars scintillate because their emission travels through the ionized interstellar medium along multiple paths, which interfere with each other. it has long been realized that, independent of their nature, the regions responsible for the scintillation could be used as `interstellar lenses' to localize pulsar emission regions1,2. most such lenses, however, resolve emission components only marginally, limiting results to statistical inferences and detections of small positional shifts3-5. as lenses situated close to their source offer better resolution, it should be easier to resolve emission regions of pulsars located in high-density environments such as supernova remnants6 or binaries in which the pulsar's companion has an ionized outflow. here we report observations of extreme plasma lensing in the `black widow' pulsar, b1957+20, near the phase in its 9.2-hour orbit at which its emission is eclipsed by its companion's outflow7-9. during the lensing events, the observed radio flux is enhanced by factors of up to 70-80 at specific frequencies. the strongest events clearly resolve the emission regions: they affect the narrow main pulse and parts of the wider interpulse differently. we show that the events arise naturally from density fluctuations in the outer regions of the outflow, and we infer a resolution of our lenses that is comparable to the pulsar's radius, about 10 kilometres. furthermore, the distinct frequency structures imparted by the lensing are reminiscent of what is observed for the repeating fast radio burst frb 121102, providing observational support for the idea that this source is observed through, and thus at times strongly magnified by, plasma lenses10. | pulsar emission amplified and resolved by plasma lensing in an eclipsing binary |
alternative theories of gravity not only modify the polarization contents of the gravitational wave, but also affect the motions of the stars and the energy radiated away via the gravitational radiation. these aspects leave imprints in the observational data, which enables the test of general relativity and its alternatives. in this work, the nordtvedt effect and the shapiro time delay are calculated in order to constrain horndeski theory using the observations of lunar laser ranging experiments and cassini time-delay data. the effective stress-energy tensor is also obtained using the method of isaacson. gravitational wave radiation of a binary system is calculated, and the change of the period of a binary system is deduced for the elliptical orbit. these results can be used to set constraints on horndeski theory with the observations of binary systems, such as psr j1738 + 0333. constraints have been obtained for some subclasses of horndeski theory, in particular, those satisfying the gravitational wave speed limits from gw170817 and grb 170817a. | constraints on horndeski theory using the observations of nordtvedt effect, shapiro time delay and binary pulsars |
intensity scintillations of radio pulsars are known to originate from interference between waves scattered by the electron density irregularities of interstellar plasma, often leading to parabolic arcs in the two-dimensional power spectrum of the recorded dynamic spectrum. the degree of arc curvature depends on the distance to the scattering plasma and its transverse velocity with respect to the line of sight. we report the observation of annual and orbital variations in the curvature of scintillation arcs over a period of 16 yr for the bright millisecond pulsar, psr j0437-4715. these variations are the signature of the relative transverse motions of earth, the pulsar, and the scattering medium, which we model to obtain precise measurements of parameters of the pulsar's binary orbit and the scattering medium itself. we observe two clear scintillation arcs in most of our >5000 observations, and we show that they originate from scattering by thin screens located at distances d1 = 89.8 ± 0.4 pc and d2 = 124 ± 3 pc from earth. the best-fit scattering model we derive for the brightest arc yields the pulsar's orbital inclination angle, i = 137°1 ± 0°3, and longitude of ascending node, ω = 206°3 ± 0°4. using scintillation arcs for precise astrometry and orbital dynamics can be superior to modeling variations in the diffractive scintillation timescale, because the arc curvature is independent of variations in the level of turbulence of interstellar plasma. this technique can be used in combination with pulsar timing to determine the full three-dimensional orbital geometries of binary pulsars and provides parameters essential for testing theories of gravity and constraining neutron star masses. | precision orbital dynamics from interstellar scintillation arcs for psr j0437-4715 |
we study strange stars in the framework of f (r ,t ) theory of gravity. to provide exact solutions of the field equations it is considered that the gravitational lagrangian can be expressed as the linear function of the ricci scalar r and the trace of the stress-energy tensor t , i.e. f (r ,t )=r +2 χ t , where χ is a constant. we also consider that the strange quark matter (sqm) distribution inside the stellar system is governed by the phenomenological mit bag model equation of state (eos), given as pr=1/3 (ρ -4 b ) , where b is the bag constant. further, for a specific value of b and observed values of mass of the strange star candidates we obtain the exact solution of the modified tolman-oppenheimer-volkoff (tov) equation in the framework of f (r ,t ) gravity and have studied in detail the dependence of the different physical parameters, like the metric potentials, energy density, radial and tangential pressures and anisotropy etc., due to the chosen different values of χ . likewise in gr, as have been shown in our previous work [deb et al., ann. phys. (amsterdam) 387, 239 (2017), 10.1016/j.aop.2017.10.010] in the present work also we find maximum anisotropy at the surface which seems an inherent property of the strange stars in modified f (r ,t ) theory of gravity. to check the physical acceptability and stability of the stellar system based on the obtained solutions we have performed different physical tests, viz., the energy conditions, herrera cracking concept, adiabatic index etc. in this work, we also have explained the effects, those are arising due to the interaction between the matter and the curvature terms in f (r ,t ) gravity, on the anisotropic compact stellar system. it is interesting to note that as the values of χ increase the strange stars become more massive and their radius increase gradually so that eventually they gradually turn into less dense compact objects. the present study reveals that the modified f (r ,t ) gravity is a suitable theory to explain massive stellar systems like recent magnetars, massive pulsars and super-chandrasekhar stars, which cannot be explained in the framework of gr. however, for χ =0 the standard results of einsteinian gravity are retrieved. | anisotropic strange stars under simplest minimal matter-geometry coupling in the f (r ,t ) gravity |
the psrix backend is the primary pulsar timing instrument of the effelsberg 100 m radio telescope since early 2011. this new roach-based system enables bandwidths up to 500 mhz to be recorded, significantly more than what was possible with its predecessor, the effelsberg-berkeley pulsar processor (ebpp). we review the first four years of psrix timing data for 33 pulsars collected as part of the monthly european pulsar timing array (epta) observations. we describe the automated data analysis pipeline, coastguard, that we developed to reduce these observations. we also introduce toaster, the epta timing data base, used to store timing results, processing information and observation metadata. using these new tools, we measure the phase-averaged flux densities at 1.4 ghz of all 33 pulsars. for seven of these pulsars, our flux density measurements are the first values ever reported. for the other 26 pulsars, we compare our flux density measurements with previously published values. by comparing psrix data with ebpp data, we find an improvement of ∼2-5 times in signal-to-noise ratio, which translates to an increase of ∼2-5 times in pulse time-of-arrival (toa) precision. we show that such an improvement in toa precision will improve the sensitivity to the stochastic gravitational wave background. finally, we showcase the flexibility of the new psrix backend by observing several millisecond-period pulsars (msps) at 5 and 9 ghz. motivated by our detections, we discuss the potential for complementing existing pulsar timing array data sets with msp monitoring campaigns at these higher frequencies. | prospects for high-precision pulsar timing with the new effelsberg psrix backend |
the detection of gravitational waves (gws) from a binary neutron star (bns) has opened a new window on gravitational wave astronomy. with current sensitivities, detectable signals coming from compact objects like neutron stars turn out to be a crucial ingredient for probing their structure, composition, and evolution. moreover, astronomical observations on pulsars and their mass-radius relations place important constraints on the dense matter equation of state. in this paper, we consider a homogeneous and unpaired charge-neutral three-flavor interacting quark matter with ${ \mathcal o }({m}_{s}^{4})$ corrections that account for the moderately heavy strange quark instead of the naive mit bag model. we perform a detailed analysis of strange quark stars in the context of the recently proposed 4d einstein-gauss-bonnet (egb) theory of gravity. however, this theory does not have standard 4d equations. thus, we show that the equivalence of the actions in the regularized 4d egb theory and in the original one is satisfied for a spherically symmetric spacetime. we pay particular attention to the possible existence of neutron stars of mass compatible with $m\sim 2{m}_{\odot }$ . our findings suggest that the fourth-order correction parameter (a4) of the quantum chromodynamic perturbation and coupling constant α of the gb term play an important role in the mass-radius relation as well as the stability of the quark star. finally, we compare the results with the well-measured limits of pulsars and their mass and radius extracted from the spectra of several x-ray compact sources. | quark stars in 4d einstein-gauss-bonnet gravity with an interacting quark equation of state |
we apply a number of statistical and machine learning techniques to classify and rank gamma-ray sources from the third fermi large area telescope source catalog (3fgl), according to their likelihood of falling into the two major classes of gamma-ray emitters: pulsars (psr) or active galactic nuclei (agns). using 1904 3fgl sources that have been identified/associated with agns (1738) and psr (166), we train (using 70% of our sample) and test (using 30%) our algorithms and find that the best overall accuracy (>96%) is obtained with the random forest (rf) technique, while using a logistic regression (lr) algorithm results in only marginally lower accuracy. we apply the same techniques on a subsample of 142 known gamma-ray pulsars to classify them into two major subcategories: young (yng) and millisecond pulsars (msp). once more, the rf algorithm has the best overall accuracy (∼90%), while a boosted lr analysis comes a close second. we apply our two best models (rf and lr) to the entire 3fgl catalog, providing predictions on the likely nature of unassociated sources, including the likely type of pulsar (yng or msp). we also use our predictions to shed light on the possible nature of some gamma-ray sources with known associations (e.g., binaries, supernova remnants/pulsar wind nebulae). finally, we provide a list of plausible x-ray counterparts for some pulsar candidates, obtained using swift, chandra, and xmm. the results of our study will be of interest both for in-depth follow-up searches (e.g., pulsar) at various wavelengths and for broader population studies. | classification and ranking of fermi lat gamma-ray sources from the 3fgl catalog using machine learning techniques |
we show that ultraluminous x-ray sources (ulxs) with coherent x-ray pulsing (pulxs) probably have neutron-star spin axes significantly misaligned from their central accretion discs. scattering in the funnels collimating their emission and producing their apparent super-eddington luminosities is the most likely origin of the observed correlation between pulse fraction and x-ray photon energy. pulsing is suppressed in systems with the neutron-star spin closely aligned to the inner disc, explaining why some ulxs show cyclotron features indicating strong magnetic fields, but do not pulse. we suggest that alignment (or conceivably, field suppression through accretion) generally occurs within a fairly short fraction of the ulx lifetime, so that most neutron-star ulxs become unpulsed. as a result we further suggest that almost all ulxs actually have neutron-star accretors, rather than black holes or white dwarfs, reflecting their progenitor high-mass x-ray binary and supersoft x-ray source populations. | pulsing and non-pulsing ulxs: the iceberg emerges |
during the final stages of a compact object merger, if at least one of the binary components is a magnetized neutron star (ns), then its orbital motion substantially expands the ns's open magnetic flux - and hence increases its wind luminosity - relative to that of an isolated pulsar. as the binary orbit shrinks due to gravitational radiation, the power and speed of this binary-induced inspiral wind may (depending on pair loading) secularly increase, leading to self-interaction and internal shocks in the outflow beyond the binary orbit. the magnetized forward shock can generate coherent radio emission via the synchrotron maser process, resulting in an observable radio precursor to binary ns merger. we perform 1d relativistic hydrodynamical simulations of shock interaction in the accelerating binary ns wind, assuming that the inspiral wind efficiently converts its poynting flux into bulk kinetic energy prior to the shock radius. this is combined with the shock maser spectrum from particle-in-cell simulations, to generate synthetic radio light curves. the precursor burst with a fluence of ~1 jy·ms at ~ghz frequencies lasts ~1-500 ms following the merger for a source at ~3 gpc (bd/1012 g)8/9, where bd is the dipole field strength of the more strongly magnetized star. given an outflow geometry concentrated along the binary equatorial plane, the signal may be preferentially observable for high-inclination systems, that is, those least likely to produce a detectable gamma-ray burst. | shock-powered radio precursors of neutron star mergers from accelerating relativistic binary winds |
the fermi large area telescope (lat) has opened the way for comparative studies of cosmic rays (crs) and high-energy objects in the milky way (mw) and in other, external, star-forming galaxies. using 2 yr of observations with the fermi lat, local group galaxy m31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy m33. we revisited the gamma-ray emission in the direction of m31 and m33 using more than 7 yr of lat pass 8 data in the energy range 0.1{--}100 {gev}, presenting detailed morphological and spectral analyses. m33 remains undetected, and we computed an upper limit of 2.0× {10}-12 {erg} {{cm}}-2 {{{s}}}-1 on the 0.1{--}100 {gev} energy flux (95% confidence level). this revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average cr density in m33 that is at most half of that of the mw. m31 is detected with a significance of nearly 10σ . its spectrum is consistent with a power law with photon index {{γ }}=2.4+/- {0.1}{stat+{syst}} and a 0.1{--}100 {gev} energy flux of (5.6+/- {0.6}{stat+{syst}})× {10}-12 {erg} {{cm}}-2 {{{s}}}-1. m31 is detected to be extended with a 4σ significance. the spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0.°4 and no offset from the center of the galaxy, but nonuniform intensity distributions cannot be excluded. the flux from m31 appears confined to the inner regions of the galaxy and does not fill the disk of the galaxy or extend far from it. the gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of m31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called galactic center excess found in fermi-lat observations of the mw. | observations of m31 and m33 with the fermi large area telescope: a galactic center excess in andromeda? |
radio pulsars are believed to have their emission powered by the loss of rotational kinetic energy. by contrast, magnetars show intense x-ray and γ-ray radiation whose luminosity greatly exceeds that due to spin down and magnetar luminosity is believed to be powered by intense internal magnetic fields. a basic prediction of this picture is that radio pulsars of high magnetic field should show magnetar-like emission. here we report on a magnetar-like x-ray outburst from the radio pulsar psr j1119-6127, heralded by two short bright x-ray bursts on 2016 july 27 and 28. using target of opportunity data from the swift x-ray telescope and nustar, we show that this pulsar’s flux has brightened by a factor of \gt 160 in the 0.5-10 kev band, and that its previously soft x-ray spectrum has undergone a strong hardening with strong pulsations appearing for the first time above 2.5 kev, with phase-averaged emission detectable up to 25 kev. by comparing swift-xrt and nustar timing data with a pre-outburst ephemeris derived from fermi large area telescope data, we find that the source has contemporaneously undergone a large spin-up glitch of amplitude {{δ }}ν /ν =5.74(8)× {10}-6. the collection of phenomena observed thus far in this outburst strongly mirrors those in most magnetar outbursts and provides an unambiguous connection between the radio pulsar and magnetar populations. | a magnetar-like outburst from a high-b radio pulsar |
within the next several years, pulsar-timing array programs will likely usher in the next era of gravitational-wave astronomy through the detection of a stochastic background of nanohertz-frequency gravitational waves, originating from a cosmological population of inspiraling supermassive binary black holes. while the source positions will likely be isotropic to a good approximation, the gravitational-wave angular power distribution will be anisotropic, with the most massive and/or nearby binaries producing signals that may resound above the background. we study such a realistic angular power distribution, developing fast and accurate sky-mapping strategies to localize pixels and extended regions of excess power while simultaneously modeling the background signal from the less massive and more distant ensemble. we find that power anisotropy will be challenging to discriminate from isotropy for realistic gravitational-wave skies, requiring snr >10 in order to favor anisotropy with 10 :1 posterior odds in our case study. amongst our techniques, modeling the population signal with multiple point sources in addition to an isotropic background provides the most physically motivated and easily interpreted maps, while spherical-harmonic modeling of the square-root power distribution, p (ω ^)1/2, performs best in discriminating from overall isotropy. our techniques are modular and easily incorporated into existing pulsar-timing array analysis pipelines. | from bright binaries to bumpy backgrounds: mapping realistic gravitational wave skies with pulsar-timing arrays |
the strength of neutron star crust is crucial for modelling magnetar flares, pulsar glitches, and gravitational wave emission. we aim to shed some light on this problem by analysing uniaxial stretch deformation (elongation and contraction) of perfect body-centered cubic coulomb crystals, paying special attention to the inherent anisotropy of this process. our analysis is based on the semi-analytical approach of baiko & kozhberov, which, for any uniform deformation, allows one to calculate, in fully non-linear regime, critical deformation parameters beyond which the lattice loses its dynamic stability. we determine critical strain, pressure anisotropy, and deformation energy for any stretch direction with respect to the crystallographic axes. these quantities are shown to be strongly anisotropic: they vary by a factor of almost 10 depending on the orientation of the deformation axis. for polycrystalline crust, we argue that the maximum strain for the stretch deformation sustainable elastically is 0.04. it is lower than the breaking strain of 0.1 obtained in molecular dynamic simulations of a shear deformation by horowitz & kadau. the maximum pressure anisotropy of polycrystalline matter is estimated to be in the range from 0.005 to 0.014 nz2e2/a, where n is the ion number density, ze is the ion charge, and a is the ion-sphere radius. we discuss possible mechanisms of plastic motion and formation of large crystallites in neutron star crust as well as analyse energy release associated with breaking of such crystallites in the context of magnetic field evolution and magnetar flaring activity. | breaking properties of neutron star crust |
aims: the magnetic field of accreting neutron stars determines their overall behavior including the maximum possible luminosity. some models require an above-average magnetic field strength (≳1013 g) in order to explain super-eddington mass accretion rate in the recently discovered class of pulsating ultraluminous x-ray sources (ulx). the peak luminosity of smc x-3 during its major outburst in 2016-2017 reached 2.5 × 1039 erg s-1 comparable to that in ulxs thus making this source the nearest ulx-pulsar. determination of the magnetic field of smc x-3 is the main goal of this paper.methods: smc x-3 belongs to the class of transient x-ray pulsars with be optical companions, and exhibited a giant outburst in july 2016-march 2017. the source has been observed over the entire outburst with the swift/xrt and fermi/gbm telescopes, as well as the nustar observatory. collected data allowed us to estimate the magnetic field strength of the neutron star in smc x-3 using several independent methods.results: spin evolution of the source during and between the outbursts, and the luminosity of the transition to the so-called propeller regime in the range of (0.3-7) × 1035 erg s-1 imply a relatively weak dipole field of (1-5) × 1012 g. on the other hand, there is also evidence for a much stronger field in the immediate vicinity of the neutron star surface. in particular, transition from super- to sub-critical accretion regime associated with the cease of the accretion column and very high peak luminosity favor a field that is an order of magnitude stronger. this discrepancy makes smc x-3 a good candidate for possessing significant non-dipolar components of the field, and an intermediate source between classical x-ray pulsars and accreting magnetars which may constitute an appreciable fraction of ulx population. | smc x-3: the closest ultraluminous x-ray source powered by a neutron star with non-dipole magnetic field |
we study the current sample of rapidly rotating neutron stars in both accreting and non-accreting binaries in order to determine whether the spin distribution of accreting neutron stars in low-mass x-ray binaries (lmxbs) can be reconciled with current accretion torque models. we perform a statistical analysis of the spin distributions and show that there is evidence for two subpopulations among lmxbs, one at a relatively low spin frequency, with an average of ≈ 300 {hz} and a broad spread, and a peaked population at higher frequency with an average spin frequency of ≈ 575 {hz}. we show that the two subpopulations are separated by a cut-point at a frequency of ≈ 540 {hz}. we also show that the spin frequency of radio millisecond pulsars (rmsps) does not follow a log-normal distribution and shows no evidence for the existence of distinct subpopulations. we discuss the uncertainties of different accretion models and speculate that either the accreting neutron star cut-point marks the onset of gravitational waves as an efficient mechanism to remove angular momentum or some of the neutron stars in the fast subpopulation do not evolve into rmsps. | the spin distribution of fast-spinning neutron stars in low-mass x-ray binaries: evidence for two subpopulations |
we calculate the properties, occurrence rates and detection prospects of individually resolvable `single sources' in the low-frequency gravitational wave (gw) spectrum. our simulations use the population of galaxies and massive black hole binaries from the illustris cosmological hydrodynamic simulations, coupled to comprehensive semi-analytic models of the binary merger process. using mock pulsar timing arrays (pta) with, for the first time, varying red-noise models, we calculate plausible detection prospects for gw single sources and the stochastic gw background (gwb). contrary to previous results, we find that single sources are at least as detectable as the gw background. using mock pta, we find that these `foreground' sources (also `deterministic'/`continuous') are likely to be detected with ∼20 yr total observing baselines. detection prospects, and indeed the overall properties of single sources, are only moderately sensitive to binary evolution parameters - namely eccentricity and environmental coupling, which can lead to differences of ∼5 yr in times to detection. red noise has a stronger effect, roughly doubling the time to detection of the foreground between a white-noise only model (∼10-15 yr) and severe red noise (∼20-30 yr). the effect of red noise on the gwb is even stronger, suggesting that single source detections may be more robust. we find that typical signal-to-noise ratios for the foreground peak near f = 0.1 yr-1, and are much less sensitive to the continued addition of new pulsars to pta. | single sources in the low-frequency gravitational wave sky: properties and time to detection by pulsar timing arrays |
a head-on collision between two dust ion acoustic solitary waves (diasws) travelling in the opposite direction in a weakly relativistic plasma composed of four distinct particle populations, namely, weakly relativistic ion fluid, superthermal electrons as well as positrons, and immobile dust, is investigated. by employing extended poincaré-lighthill-kuo method, two korteweg-de vries (kdv) equations are derived. the analytical phase shift after a head-on collision of two dust ion acoustic (dia) solitary waves is also obtained. the combined effects of relativistic factor (β), electron to positron temperature ratio (α), ion to electron temperature ratio (σ), positron to electron density ratio (p), dust density ratio (d), and superthermality of electrons as well as positrons (via κ) on the phase shifts are numerically studied. all these physical parameters have also changed the potential amplitude and the width of colliding solitary waves. it is found that the presence of superthermal electrons as well as positrons and dust grains has emphatic influence on the phase shifts and potential pulse profiles of compressive dia solitons. our results are general and may be helpful in understanding a head-on collision between two diasws in astrophysical and laboratory plasmas, especially the interaction of pulsar relativistic winds with supernova ejecta that produces the superthermal particles and relativistic ions. | head-on collision of two dust ion acoustic solitary waves in a weakly relativistic multicomponent superthermal plasma |
cooling simulations of neutron stars and their comparison with the data from thermally emitting x-ray sources put constraints on the properties of axions, and by extension, of any light pseudoscalar dark matter particles, whose existence has been postulated to solve the strong-c p problem of qcd. we incorporate the axion emission by pair-breaking and formation processes by s - and p -wave nucleonic condensates in a benchmark code for cooling simulations, as well as provide fit formulas for the rates of these processes. axion cooling of neutron stars has been simulated for 24 models covering the mass range 1 to 1.8 solar masses, featuring nonaccreted iron and accreted light-element envelopes, and a range of nucleon-axion couplings. the models are based on an equation state predicting conservative physics of superdense nuclear matter that does not allow for the onset of fast cooling processes induced by phase transitions to non-nucleonic forms of matter or high proton concentration. the cooling tracks in the temperature vs age plane were confronted with the (time-averaged) measured surface temperature of the central compact object in the cas a supernova remnant as well as surface temperatures of three nearby middle-aged thermally emitting pulsars. we find that the axion coupling is limited to fa/107 gev ≥(5 - 10 ) , which translates into an upper bound on axion mass ma≤(0.06 - 0.12 ) ev for peccei-quinn charges of the neutron |cn|∼0.04 and proton |cp|∼0.4 characteristic for hadronic models of axions. | axion cooling of neutron stars |
the double pulsar (psr j0737-3039) is the only neutron star-neutron star (ns-ns) binary in which both nss have been detectable as radio pulsars. the double pulsar has been assumed to dominate the galactic ns-ns binary merger rate r_g among all known systems, solely based on the properties of the first-born, recycled pulsar (psr j0737-3039a, or a) with an assumption for the beaming correction factor of 6. in this work, we carefully correct observational biases for the second-born, non-recycled pulsar (psr j0737-0737b, or b) and estimate the contribution from the double pulsar on r_g using constraints available from both a and b. observational constraints from the b pulsar favour a small beaming correction factor for a (∼2), which is consistent with a bipolar model. considering known ns-ns binaries with the best observational constraints, including both a and b, we obtain r_g=21_{-14}^{+28} myr-1 at 95 per cent confidence from our reference model. we expect the detection rate of gravitational waves from ns-ns inspirals for the advanced ground-based gravitational-wave detectors is to be 8^{+10}_{-5} yr-1 at 95 per cent confidence. within several years, gravitational-wave detections relevant to ns-ns inspirals will provide us useful information to improve pulsar population models. | implications of psr j0737-3039b for the galactic ns-ns binary merger rate |
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