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gravitational waves are a radically new way to peer into the darkest depths of the cosmos. pulsars can be used to make direct detections of gravitational waves through precision timing. when a gravitational wave passes between a pulsar and the earth, it stretches and squeezes the intermediate space-time, leading to deviations of the measured pulse arrival times away from model expectations. combining the data from many galactic pulsars can corroborate such a signal, and enhance its detection significance. this technique is known as a pulsar timing array (pta). here i provide an overview of ptas as a precision gravitational-wave detection instrument, then review the types of signal and noise processes that we encounter in typical pulsar data analysis. i take a pragmatic approach, illustrating how searches are performed in real life, and where possible directing the reader to codes or techniques that they can explore for themselves. the goal is to provide theoretical background and practical recipes for data exploration that allow the reader to join in the exciting hunt for very low frequency gravitational waves. | the nanohertz gravitational wave astronomer |
we present 3d full-sphere supernova simulations of non-rotating low-mass (∼9 m⊙) progenitors, covering the entire evolution from core collapse through bounce and shock revival, through shock breakout from the stellar surface, until fallback is completed several days later. we obtain low-energy explosions (∼0.5-1.0 × 1050 erg) of iron-core progenitors at the low-mass end of the core-collapse supernova (lmccsn) domain and compare to a super-agb (sagb) progenitor with an oxygen-neon-magnesium core that collapses and explodes as electron-capture supernova (ecsn). the onset of the explosion in the lmccsn models is modelled self-consistently using the vertex-prometheus code, whereas the ecsn explosion is modelled using parametric neutrino transport in the prometheus-hotb code, choosing different explosion energies in the range of previous self-consistent models. the sagb and lmccsn progenitors that share structural similarities have almost spherical explosions with little metal mixing into the hydrogen envelope. a lmccsn with less second dredge-up results in a highly asymmetric explosion. it shows efficient mixing and dramatic shock deceleration in the extended hydrogen envelope. both properties allow fast nickel plumes to catch up with the shock, leading to extreme shock deformation and aspherical shock breakout. fallback masses of $\mathord {\lesssim }\, 5\, \mathord {\times }\, 10^{-3}$ m⊙ have no significant effects on the neutron star (ns) masses and kicks. the anisotropic fallback carries considerable angular momentum, however, and determines the spin of the newly born ns. the lmccsn model with less second dredge-up results in a hydrodynamic and neutrino-induced ns kick of >40 km s-1 and a ns spin period of ∼30 ms, both not largely different from those of the crab pulsar at birth. | three-dimensional models of core-collapse supernovae from low-mass progenitors with implications for crab |
in this article we study the hydrostatic equilibrium configuration of neutron stars and strange stars, whose fluid pressure is computed from the equations of state p=ωρ5/3 and p=0.28(ρ-4script b), respectively, with ω and script b being constants and ρ the energy density of the fluid. we start by deriving the hydrostatic equilibrium equation for the f(r,t) theory of gravity, with r and t standing for the ricci scalar and trace of the energy-momentum tensor, respectively. such an equation is a generalization of the one obtained from general relativity, and the latter can be retrieved for a certain limit of the theory. for the f(r,t)=r+2λ t functional form, with λ being a constant, we find that some physical properties of the stars, such as pressure, energy density, mass and radius, are affected when λ is changed. we show that for a fixed central star energy density, the mass of neutron and strange stars can increase with λ. concerning the star radius, it increases for neutron stars and it decreases for strange stars with the increment of λ. thus, in f(r,t) theory of gravity we can push the maximum mass above the observational limits. this implies that the equation of state cannot be eliminated if the maximum mass within general relativity lies below the limit given by observed pulsars. | stellar equilibrium configurations of compact stars in f(r,t) theory of gravity |
magnetars are neutron stars in which a strong magnetic field is the main energy source. about two dozens of magnetars, plus several candidates, are currently known in our galaxy and in the magellanic clouds. they appear as highly variable x-ray sources and, in some cases, also as radio and/or optical pulsars. their spin periods (2-12 s) and spin-down rates (∼10-13-10-10 s s-1) indicate external dipole fields of ∼1013-15 g, and there is evidence that even stronger magnetic fields are present inside the star and in non-dipolar magnetospheric components. here we review the observed properties of the persistent emission from magnetars, discuss the main models proposed to explain the origin of their magnetic field and present recent developments in the study of their evolution and connection with other classes of neutron stars. | magnetars: properties, origin and evolution |
the event horizon telescope is a millimeter vlbi array that is taking the first horizon-scale pictures of the black hole in the center of the m87 galaxy and, in the near future, of the one in the center of the milky way. measurements of the shape and size of the shadows cast by the black holes on the surrounding emission can test the cosmic censorship conjecture and the no-hair theorem and may find evidence for classical effects of the quantum structure of black holes. observations of coherent structures in the accretion flows may lead to accurate measurements of the spins of the black holes and of other properties of their spacetimes. for sgr a*, the black hole in the center of the milky way, measurements of the precession of stellar orbits and timing monitoring of orbiting pulsars offer complementary avenues to the gravitational tests with the event horizon telescope. | testing general relativity with the event horizon telescope |
einstein's theory of gravity—the general theory of relativity1—is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field. in contrast to almost all alternative theories of gravity2, the strong equivalence principle of general relativity requires universality of free fall to apply even to bodies with strong self-gravity. direct tests of this principle using solar system bodies3,4 are limited by the weak self-gravity of the bodies, and tests using pulsar-white-dwarf binaries5,6 have been limited by the weak gravitational pull of the milky way. psr j0337+1715 is a hierarchical system of three stars (a stellar triple system) in which a binary consisting of a millisecond radio pulsar and a white dwarf in a 1.6-day orbit is itself in a 327-day orbit with another white dwarf. this system permits a test that compares how the gravitational pull of the outer white dwarf affects the pulsar, which has strong self-gravity, and the inner white dwarf. here we report that the accelerations of the pulsar and its nearby white-dwarf companion differ fractionally by no more than 2.6 × 10-6. for a rough comparison, our limit on the strong-field nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) solar system tests3,4 and a factor of almost a thousand smaller than that obtained from other strong-field tests5,6. | universality of free fall from the orbital motion of a pulsar in a stellar triple system |
we search for a stochastic gravitational wave background (sgwb) generated by a network of cosmic strings using six millisecond pulsars from data release 2 (dr2) of the european pulsar timing array (epta). we perform a bayesian analysis considering two models for the network of cosmic string loops, and compare it to a simple power-law model which is expected from the population of supermassive black hole binaries. our main strong assumption is that the previously reported common red noise process is a sgwb. we find that the one-parameter cosmic string model is slightly favored over a power-law model thanks to its simplicity. if we assume a two-component stochastic signal in the data (supermassive black hole binary population and the signal from cosmic strings), we get a 95% upper limit on the string tension of log10(g μ )<-9.9 (-10.5 ) for the two cosmic string models we consider. in extended two-parameter string models, we were unable to constrain the number of kinks. we test two approximate and fast bayesian data analysis methods against the most rigorous analysis and find consistent results. these two fast and efficient methods are applicable to all sgwbs, independent of their source, and will be crucial for analysis of extended datasets. | practical approaches to analyzing pta data: cosmic strings with six pulsars |
we report the discovery and initial follow-up of a double neutron star (dns) system, psr j1946+2052, with the arecibo l-band feed array pulsar (palfa) survey. psr j1946+2052 is a 17 ms pulsar in a 1.88 hr, eccentric (e = 0.06) orbit with a ≳1.2 m ⊙ companion. we have used the jansky very large array to localize psr j1946+2052 to a precision of 0.″09 using a new phase binning mode. we have searched multiwavelength catalogs for coincident sources but did not find any counterparts. the improved position enabled a measurement of the spin period derivative of the pulsar (\dot{p}=9+/- 2× {10}-19); the small inferred magnetic field strength at the surface (bs= 4 × 109 g) indicates that this pulsar has been recycled. this and the orbital eccentricity lead to the conclusion that psr j1946+2052 is in a dns system. among all known radio pulsars in dns systems, psr j1946+2052 has the shortest orbital period and the shortest estimated merger timescale, 46 myr; at that time it will display the largest spin effects on gravitational-wave waveforms of any such system discovered to date. we have measured the advance of periastron passage for this system, \dot{ω }=25.6+/- 0.3 \deg {yr}}-1, implying a total system mass of only 2.50 ± 0.04 m ⊙, so it is among the lowest-mass dns systems. this total mass measurement combined with the minimum companion mass constrains the pulsar mass to ≲1.3 m ⊙. | palfa discovery of a highly relativistic double neutron star binary |
supermassive black hole binaries (smbhs) are a fascinating byproduct of galaxy mergers in the hierarchical universe. in the last stage of their orbital evolution, gravitational wave radiation drives the binary inspiral and produces the loudest siren awaiting to be detected by gravitational wave observatories. periodically varying emission from active galactic nuclei has been proposed as a powerful approach to probe such systems, although none of the identified candidates are close to their final coalescence such that the observed periods stay constant in time. in this work, we report on the first system with rapid decaying periods revealed by its optical and x-ray light curves, which has decreased from about one year to one month in three years. together with its optical hydrogen line spectroscopy, we propose that the system is an uneven mass-ratio, highly eccentric smbh binary which will merge within three years, as predicted by the trajectory evolution model. if the interpretation is true, coordinated, multi-band electromagnetic campaign should be planned for this first binary smbh merger event observed in human history, together with possible neutrino measurements. gravitational wave memory from this event may also be detectable by pulsar timing array with additional five-to-ten year observation. | tick-tock: the imminent merger of a supermassive black hole binary |
since pulsars were discovered as emitters of bright coherent radio emission more than half a century ago, the cause of the emission has remained a mystery. in this letter we demonstrate that coherent radiation can be directly generated in nonstationary pair plasma discharges which are responsible for filling the pulsar magnetosphere with plasma. by means of large-scale two-dimensional kinetic plasma simulations, we show that if pair creation is nonuniform across magnetic field lines, the screening of electric field by freshly produced pair plasma is accompanied by the emission of waves which are electromagnetic in nature. using localized simulations of the screening process, we identify these waves as superluminal ordinary (o ) modes, which should freely escape from the magnetosphere as the plasma density drops along the wave path. the spectrum of the waves is broadband and the frequency range is comparable to that of observed pulsar radio emission. | origin of pulsar radio emission |
the ligo/virgo detection of the gravitational waves from a binary merger system, gw170817, has put a clean and strong constraint on the tidal deformability of the merging objects. from this constraint, deep insights can be obtained in compact star equation of states, which has been one of the most puzzling problems for nuclear physicists and astrophysicists. employing one of the most widely used quark star eos models, we characterize the star properties by the strange quark mass (ms ), an effective bag constant (beff), the perturbative qcd correction (a4), as well as the gap parameter (δ ) when considering quark pairing, and investigate the dependences of the tidal deformablity on them. we find that the tidal deformability is dominated by beff and insensitive to ms, a4. we discuss the correlation between the tidal deformability and the maximum mass (mtov) of a static quark star, which allows the model possibility to rule out the existence of quark stars with future gravitational wave observations and mass measurements. the current tidal deformability measurement implies mtov≤2.18 m⊙ (2.32 m⊙ when pairing is considered) for quark stars. combining with two-solar-mass pulsar observations, we also make constraints on the poorly known gap parameter δ for color-flavor-locked quark matter. | constraints on interquark interaction parameters with gw170817 in a binary strange star scenario |
the detection of cyclotron resonance scattering features (crsfs) is the only way to directly and reliably measure the magnetic field near the surface of a neutron star (ns). the broad energy coverage and large collection area of insight-hxmt in the hard x-ray band allowed us to detect the crsf with the highest energy known to date, reaching about 146 kev during the 2017 outburst of the first galactic pulsing ultraluminous x-ray source (pulx) swift j0243.6+6124. during this outburst, the crsf was only prominent close to the peak luminosity of ~2 × 1039 erg s-1, the highest to date in any of the galactic pulsars. the crsf is most significant in the spin-phase region corresponding to the main pulse of the pulse profile, and its centroid energy evolves with phase from 120 to 146 kev. we identify this feature as the fundamental crsf because no spectral feature exists at 60-70 kev. this is the first unambiguous detection of an electron crsf from an ulx. we also estimate a surface magnetic field of ~1.6 × 1013 g for swift j0243.6+6124. considering that the dipole magnetic field strengths, inferred from several independent estimates of magnetosphere radius, are at least an order of magnitude lower than our measurement, we argue that the detection of the highest-energy crsf reported here unambiguously proves the presence of multipole field components close to the surface of the neutron star. such a scenario has previously been suggested for several pulsating ulxs, including swift j0243.6+6124, and our result represents the first direct confirmation of this scenario. | insight-hxmt discovery of the highest-energy crsf from the first galactic ultraluminous x-ray pulsar swift j0243.6+6124 |
we analyze the statistics of pulse arrival times in fast radio burst (frb) 121102 and demonstrate that they are remarkably similar to statistics of magnetar high-energy short bursts. motivated by this correspondence, we propose that repeating frbs are generated during short bursts in the closed field line zone of magnetar magnetospheres via a pulsar-like emission mechanism. crustal slippage events dislocate field line foot points, initiating intense particle acceleration and pair production, giving rise to coherent radio emission similar to that generated near pulsar polar caps. we argue that the energetics of frb 121102 can be readily accounted for if the efficiency of the conversion of poynting flux into coherent radio emission is ∼10-4-10-2 values consistent with empirical efficiencies of radio emission in pulsars and radio-loud magnetars. such a mechanism could operate only in magnetars with preexisting low twist of the magnetosphere, so that the charge density in the closed zone is initially insufficient to screen the electric field provoked by the wiggling of magnetic field lines and is low enough to let ∼1 ghz radio emission escape the magnetosphere, which can explain the absence of frbs from known magnetars. the pair cascades crowd the closed flux tubes with plasma, screening the accelerating electric field, thus limiting the radio pulse duration to ∼1 ms. within the framework of our model, the current data set of the polarization angle variation in frb 121102 suggests a magnetic obliquity α ≲ 40° and viewing angle ζ with respect to the spin axis α < ζ < 180°-α. | repeating fast radio bursts from magnetars with low magnetospheric twist |
recent work has shown that axions can be efficiently produced via non-stationary pair plasma discharges in the polar cap region of pulsars. here, we point out that for axion masses $10^{-9} \, {\rm ev} \lesssim m_a \lesssim 10^{-4} \, \rm ev$, a sizable fraction of the sourced axion population will be gravitationally confined to the neutron star. these axions accumulate over astrophysical timescales, thereby forming a dense `axion cloud' around the star. we argue that the existence of such a cloud, with densities reaching and potentially exceeding $\mathcal{o}(10^{22}) \, {\rm gev \, cm^{-3}}$, is a generic expectation across a wide range of parameter space. for axion masses $m_a \gtrsim 10^{-7} \, \rm ev$, energy is primarily radiated from the axion cloud via resonant axion-photon mixing, generating a number of distinctive signatures that include: a sharp line in the radio spectrum of each pulsar (located at the axion mass, and with a percent-level width), and transient events arising from the reconfiguration of charge densities in the magnetosphere. while a deeper understanding of the systematic uncertainties in these systems is required, our current estimates suggest that existing radio telescopes could improve sensitivity to the axion-photon coupling by more than an order of magnitude. | axion clouds around neutron stars |
we obtain a new static model of the tov equation for an anisotropic fluid distribution by imposing the karmarkar condition. in order to close the system of equations we postulate an interesting form for the g_{rr} gravitational potential, which allows us to solve for g_{tt} metric component via the karmarkar condition. we demonstrate that the new interior solution has well-behaved physical attributes and can be utilized to model relativistic static fluid spheres. by using observational data sets for the radii and masses for compact stars such as 4u 1538-52, lmc x-4, and psr j1614-2230 we show that our solution describes these objects to a very good degree of accuracy. the physical plausibility of the solution depends on a parameter c for a particular star. for 4u 1538-52, lmc x-4, and psr j1614-2230 the solutions are well behaved for 0.1574 ≤ c ≤ 0.46, 0.1235 ≤ c ≤ 0.35 and 0.05 ≤ c ≤ 0.13, respectively. the behavior of the thermodynamical and physical variables of these compact objects leads us to conclude that the parameter c plays an important role in determining the equation of state of the stellar material and observed that smaller values of c lead to stiffer equation of states. | physical viability of fluid spheres satisfying the karmarkar condition |
we present the results of psrπ, a large astrometric project targeting radio pulsars using the very long baseline array (vlba). from our astrometric database of 60 pulsars, we have obtained parallax-based distance measurements for all but 3, with a parallax precision that is typically ∼45 μas and approaches 10 μas in the best cases. our full sample doubles the number of radio pulsars with a reliable (≳5σ) model-independent distance constraint. importantly, many of the newly measured pulsars are well outside the solar neighborhood, and so psrπ brings a near-tenfold increase in the number of pulsars with a reliable model-independent distance at d > 2 kpc. our results show that both widely used galactic electron density distribution models contain significant shortcomings, particularly at high galactic latitudes. when comparing our results to pulsar timing, two of the four millisecond pulsars in our sample exhibit significant discrepancies in their proper motion estimates. with additional vlbi observations that extend our sample and improve the absolute positional accuracy of our reference sources, we will be able to additionally compare pulsar absolute reference positions between vlbi and timing, which will provide a much more sensitive test of the correctness of the solar system ephemerides used for pulsar timing. finally, we use our large sample to estimate the typical accuracy attainable for differential vlba astrometry of pulsars, showing that for sufficiently bright targets observed eight times over 18 months, a parallax uncertainty of 4 μas per arcminute of separation between the pulsar and calibrator can be expected. | microarcsecond vlbi pulsar astrometry with psrπ ii. parallax distances for 57 pulsars |
we study properties of luminous x-ray pulsars using a simplified model of the accretion column. the maximal possible luminosity is calculated as a function of the neutron star (ns) magnetic field and spin period. it is shown that the luminosity can reach values of the order of 1040 erg s-1 for the magnetar-like magnetic field (b ≳ 1014 g) and long spin periods (p ≳ 1.5 s). the relative narrowness of an area of feasible ns parameters which are able to provide higher luminosities leads to the conclusion that l ≃ 1040 erg s-1 is a good estimate for the limiting accretion luminosity of an ns. because this luminosity coincides with the cut-off observed in the high-mass x-ray binaries luminosity function which otherwise does not show any features at lower luminosities, we can conclude that a substantial part of ultraluminous x-ray sources are accreting neutron stars in binary systems. | on the maximum accretion luminosity of magnetized neutron stars: connecting x-ray pulsars and ultraluminous x-ray sources |
the recent discovery of a galactic fast radio burst (frb) occurring simultaneously with an x-ray burst (xrb) from the galactic magnetar sgr j1935+2154 implies that at least some frbs arise from magnetar activities. we propose that frbs are triggered by crust fracturing of magnetars, with the burst event rate depending on the magnetic field strength in the crust. since the crust-fracturing rate is relatively higher in polar regions, frbs are more likely to be triggered near the directions of multipolar magnetic poles. crust fracturing produces alfvén waves, forming a charge-starved region in the magnetosphere and leading to nonstationary pair plasma discharges. an frb is produced by coherent plasma radiation due to nonuniform pair production across magnetic field lines. meanwhile, the frb-associated xrb is produced by the rapid relaxation of the external magnetic field lines. in this picture, the sharp-peak hard x-ray component in association with frb 200428 is from a region between adjacent trapped fireballs, and its spectrum with a high cutoff energy is attributed to resonant compton scattering. the persistent x-ray emission is from a hot spot heated by the magnetospheric activities, and its temperature evolution is dominated by magnetar surface cooling. within this picture, magnetars with stronger fields tend to produce brighter and more frequent repeated bursts. | fast radio bursts and their high-energy counterparts from magnetar magnetospheres |
a large number of extremely low-mass helium white dwarfs (elm wds) have been discovered in recent years. the majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass x-ray binary (lmxb) or a cataclysmic variable (cv) system. here, we investigate the formation of these objects through the lmxb channel with emphasis on the proto-wd evolution in environments with different metallicities. we study for the first time the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-wds and on the cooling properties of the resulting wds. we present state-of-the-art binary stellar evolution models computed with mesa for metallicities of z = 0.02, 0.01, 0.001 and 0.0002, producing wds with masses between 0.16-0.45 m⊙. our results confirm that element diffusion plays a significant role in the evolution of proto-wds that experience hydrogen shell flashes. the occurrence of these flashes produces a clear dichotomy in the cooling timescales of elm wds, which has important consequences e.g. for the age determination of binary millisecond pulsars. in addition, we confirm that the threshold mass at which this dichotomy occurs depends on metallicity. rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated elm proto-wds and therefore plays a key role in determining their surface chemical abundances, i.e. the observed presence of metals in their atmospheres. we predict that these proto-wds have helium-rich envelopes through a significant part of their lifetime. this is of great importance as helium is a crucial ingredient in the driving of the κ-mechanism suggested for the newly observed elm proto-wd pulsators. however, we find that the number of hydrogen shell flashes and, as a result, the hydrogen envelope mass at the beginning of the cooling track, are not influenced significantly by rotational mixing. in addition to being dependent on proto-wd mass and metallicity, the hydrogen envelope mass of the newly formed proto-wds depends on whether or not the donor star experiences a temporary contraction when the h-burning shell crosses the hydrogen discontinuity left behind by the convective envelope. the hydrogen envelope at detachment, although small compared to the total mass of the wd, contains enough angular momentum such that the spin frequency of the resulting wd on the cooling track is well above the orbital frequency. evolutionary model sequences are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/595/a35 | models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing |
pulsar timing array collaborations, such as the north american nanohertz observatory for gravitational waves (nanograv), are seeking to detect nanohertz gravitational waves emitted by supermassive black hole binaries formed in the aftermath of galaxy mergers. we have searched for continuous waves from individual circular supermassive black hole binaries using nanograv's recent 12.5 yr data set. we created new methods to accurately model the uncertainties on pulsar distances in our analysis, and we implemented new techniques to account for a common red-noise process in pulsar timing array data sets while searching for deterministic gravitational wave signals, including continuous waves. as we found no evidence for continuous waves in our data, we placed 95% upper limits on the strain amplitude of continuous waves emitted by these sources. at our most sensitive frequency of 7.65 nhz, we placed a sky-averaged limit of h 0 < (6.82 ± 0.35) × 10-15, and h 0 < (2.66 ± 0.15) × 10-15 in our most sensitive sky location. finally, we placed a multimessenger limit of ${ \mathcal m }\lt (1.41\pm 0.02)\times {10}^{9}\,{m}_{\odot }$ on the chirp mass of the supermassive black hole binary candidate 3c 66b. | the nanograv 12.5 yr data set: bayesian limits on gravitational waves from individual supermassive black hole binaries |
we investigate the propagation of electromagnetic radiation in the magnetic dual chiral density wave (mdcdw) phase of dense quark matter. considering the theory of low-energy fluctuations in this phase, we show how linearly polarized photons reaching the mdcdw medium couple to the fluctuation field to produce two hybridized modes of propagation that we call in analogy with similar phenomenon in condensed matter physics axion polaritons, one of them being gapless and the other gapped. the gapped mode's gap is proportional to the background magnetic field and inversely proportional to the amplitude of the inhomogeneous condensate. the generation of axion polaritons can be traced back to the presence of the chiral anomaly in the low-energy theory of the fluctuations. considering the primakoff effect in the mdcdw medium, we argued that axion polaritons can be generated inside quark stars bombarded by energetic photons coming from gamma-ray bursts and point out that this mechanism could serve to explain the missing pulsar paradox in the galaxy center. | axion-polaritons in the magnetic dual chiral density wave phase of dense qcd |
the existence of a ‘density cusp’—a localized increase in number—of stellar-mass black holes near a supermassive black hole is a fundamental prediction of galactic stellar dynamics. the best place to detect such a cusp is in the galactic centre, where the nearest supermassive black hole, sagittarius a*, resides. as many as 20,000 black holes are predicted to settle into the central parsec of the galaxy as a result of dynamical friction; however, so far no density cusp of black holes has been detected. low-mass x-ray binary systems that contain a stellar-mass black hole are natural tracers of isolated black holes. here we report observations of a dozen quiescent x-ray binaries in a density cusp within one parsec of sagittarius a*. the lower-energy emission spectra that we observed in these binaries is distinct from the higher-energy spectra associated with the population of accreting white dwarfs that dominates the central eight parsecs of the galaxy. the properties of these x-ray binaries, in particular their spatial distribution and luminosity function, suggest the existence of hundreds of binary systems in the central parsec of the galaxy and many more isolated black holes. we cannot rule out a contribution to the observed emission from a population (of up to about one-half the number of x-ray binaries) of rotationally powered, millisecond pulsars. the spatial distribution of the binary systems is a relic of their formation history, either in the stellar disk around sagittarius a* (ref. 7) or through in-fall from globular clusters, and constrains the number density of sources in the modelling of gravitational waves from massive stellar remnants, such as neutron stars and black holes. | a density cusp of quiescent x-ray binaries in the central parsec of the galaxy |
cosmic strings are topological defects which can be formed in grand unified theory scale phase transitions in the early universe. they are also predicted to form in the context of string theory. the main mechanism for a network of nambu-goto cosmic strings to lose energy is through the production of loops and the subsequent emission of gravitational waves, thus offering an experimental signature for the existence of cosmic strings. here we report on the analysis conducted to specifically search for gravitational-wave bursts from cosmic string loops in the data of advanced ligo 2015-2016 observing run (o1). no evidence of such signals was found in the data, and as a result we set upper limits on the cosmic string parameters for three recent loop distribution models. in this paper, we initially derive constraints on the string tension g μ and the intercommutation probability, using not only the burst analysis performed on the o1 data set but also results from the previously published ligo stochastic o1 analysis, pulsar timing arrays, cosmic microwave background and big-bang nucleosynthesis experiments. we show that these data sets are complementary in that they probe gravitational waves produced by cosmic string loops during very different epochs. finally, we show that the data sets exclude large parts of the parameter space of the three loop distribution models we consider. | constraints on cosmic strings using data from the first advanced ligo observing run |
this paper presents the results of a search for generic short-duration gravitational-wave transients in data from the third observing run of advanced ligo and advanced virgo. transients with durations of milliseconds to a few seconds in the 24-4096 hz frequency band are targeted by the search, with no assumptions made regarding the incoming signal direction, polarization, or morphology. gravitational waves from compact binary coalescences that have been identified by other targeted analyses are detected, but no statistically significant evidence for other gravitational wave bursts is found. sensitivities to a variety of signals are presented. these include updated upper limits on the source rate density as a function of the characteristic frequency of the signal, which are roughly an order of magnitude better than previous upper limits. this search is sensitive to sources radiating as little as ∼10-10 m⊙c2 in gravitational waves at ∼70 hz from a distance of 10 kpc, with 50% detection efficiency at a false alarm rate of one per century. the sensitivity of this search to two plausible astrophysical sources is estimated: neutron star f modes, which may be excited by pulsar glitches, as well as selected core-collapse supernova models. | all-sky search for short gravitational-wave bursts in the third advanced ligo and advanced virgo run |
we have detected a bright radio burst from frb 20200120e with the nasa deep space network (dsn) 70 m dish (dss-63) at radio frequencies between 2.2 and 2.3 ghz. this repeating fast radio burst (frb) is reported to be associated with a globular cluster in the m81 galactic system. with high time resolution recording, low scattering, and large intrinsic brightness of the burst, we find a burst duration of ~30 μs, comprised of several narrow components with typical separations of 2-3 μs. the narrowest component has a width of ≲100 ns, which corresponds to a light travel time size as small as 30 m. the peak flux density of the narrowest burst component is 270 jy. we estimate the total spectral luminosity of the narrowest component of the burst to be 4 × 1030 erg s-1 hz-1, which is a factor of ~500 above the luminosities of the so-called "nanoshots" associated with giant pulses from the crab pulsar. this spectral luminosity is also higher than that of the radio bursts detected from the galactic magnetar sgr 1935 + 2154 during its outburst in april 2020, but it falls on the low-end of the currently measured luminosity distribution of extragalatic frbs, further indicating the presence of a continuum of frb luminosities. the temporal separation of the individual components has similarities to the quasiperiodic behavior seen in the microstructure of some pulsars. the known empirical relation between the microstructure quasiperiodicity timescale and the rotation period of pulsars possibly suggests a possible pulsar as the source of this frb, with a rotation period of a few milliseconds. | a bright fast radio burst from frb 20200120e with sub-100 nanosecond structure |
general relativity has passed all solar system experiments and neutron star based tests, such as binary pulsar observations, with flying colors. a more exotic arena for testing general relativity is in systems that contain one or more black holes. black holes are the most compact objects in the universe, providing probes of the strongest-possible gravitational fields. we are motivated to study strong-field gravity since many theories give large deviations from general relativity only at large field strengths, while recovering the weak-field behavior. in this article, we review how one can probe general relativity and various alternative theories of gravity by using electromagnetic waves from a black hole with an accretion disk, and gravitational waves from black hole binaries. we first review model-independent ways of testing gravity with electromagnetic/gravitational waves from a black hole system. we then focus on selected examples of theories that extend general relativity in rather simple ways. some important characteristics of general relativity include (but are not limited to) (i) only tensor gravitational degrees of freedom, (ii) the graviton is massless, (iii) no quadratic or higher curvatures in the action, and (iv) the theory is four-dimensional. altering a characteristic leads to a different extension of general relativity: (i) scalar-tensor theories, (ii) massive gravity theories, (iii) quadratic gravity, and (iv) theories with large extra dimensions. within each theory, we describe black hole solutions, their properties, and current and projected constraints on each theory using black hole based tests of gravity. we close this review by listing some of the open problems in model-independent tests and within each specific theory. | black hole based tests of general relativity |
we present the design, implementation, and performance of the digital pulsar observing system constructed for the canadian hydrogen intensity mapping experiment (chime). using accelerated computing, this system processes independent, digitally steered beams formed by the chime correlator to simultaneously observe up to 10 radio pulsars and transient sources. each of these independent streams is processed by the chime/pulsar back-end system, which can coherently dedisperse, in real time, up to dispersion measure values of 2500 pc cm-3. the tracking beams and real-time analysis system are autonomously controlled by a priority-based algorithm that schedules both known sources and positions of interest for observation with observing cadences as rapid as 1 day. given the distribution of known pulsars and radio-transient sources and the dynamic scheduling, the chime/pulsar system can monitor 400-500 positions once per sidereal day and observe most sources with declinations greater than -20° once every ~4 weeks. we also discuss the extensive science program enabled through the current modes of data acquisition for chime/pulsar that centers on timing and searching experiments. | the chime pulsar project: system overview |
recent measurements of the geminga and b 0656 +14 pulsars by the gamma-ray telescope hawc (along with earlier measurements by milagro) indicate that these objects generate significant fluxes of very high-energy electrons. in this paper, we use the very high-energy gamma-ray intensity and spectrum of these pulsars to calculate and constrain their expected contributions to the local cosmic-ray positron spectrum. among models that are capable of reproducing the observed characteristics of the gamma-ray emission, we find that pulsars invariably produce a flux of high-energy positrons that is similar in spectrum and magnitude to the positron fraction measured by pamela and ams-02. in light of this result, we conclude that it is very likely that pulsars provide the dominant contribution to the long perplexing cosmic-ray positron excess. | hawc observations strongly favor pulsar interpretations of the cosmic-ray positron excess |
we perform global particle-in-cell simulations of pulsar magnetospheres, including pair production, ion extraction from the surface, frame-dragging corrections, and high-energy photon emission and propagation. in the case of oblique rotators, the effects of general relativity increase the fraction of the open field lines that support active pair discharge. we find that the plasma density and particle energy flux in the pulsar wind are highly non-uniform with latitude. a significant fraction of the outgoing particle energy flux is carried by energetic ions, which are extracted from the stellar surface. their energies may extend up to a large fraction of the open field line voltage, making them interesting candidates for ultra-high-energy cosmic rays. we show that pulsar gamma-ray radiation is dominated by synchrotron emission, produced by particles that are energized by relativistic magnetic reconnection close to the y-point and in the equatorial current sheet. in most cases, the calculated light curves contain two strong peaks, which is in general agreement with fermi observations. the radiative efficiency decreases with increasing pulsar inclination and increasing efficiency of pair production in the current sheet, which explains the observed scatter in lγversus \dot{e}. we find that the high-frequency cutoff in the spectra is regulated by the pair-loading of the current sheet. our findings lay the foundation for quantitative interpretation of fermi observations of gamma-ray pulsars. | ab-initio pulsar magnetosphere: particle acceleration in oblique rotators and high-energy emission modeling |
we review the physics of relativistic shocks, which are often invoked as the sources of non-thermal particles in pulsar wind nebulae (pwne), gamma-ray bursts (grbs), and active galactic nuclei (agn) jets, and as possible sources of ultra-high energy cosmic-rays. we focus on particle acceleration and magnetic field generation, and describe the recent progress in the field driven by theory advances and by the rapid development of particle-in-cell (pic) simulations. in weakly magnetized or quasi parallel-shocks (i.e. where the magnetic field is nearly aligned with the flow), particle acceleration is efficient. the accelerated particles stream ahead of the shock, where they generate strong magnetic waves which in turn scatter the particles back and forth across the shock, mediating their acceleration. in contrast, in strongly magnetized quasi-perpendicular shocks, the efficiencies of both particle acceleration and magnetic field generation are suppressed. particle acceleration, when efficient, modifies the turbulence around the shock on a long time scale, and the accelerated particles have a characteristic energy spectral index of s_{γ}∼eq2.2 in the ultra-relativistic limit. we discuss how this novel understanding of particle acceleration and magnetic field generation in relativistic shocks can be applied to high-energy astrophysical phenomena, with an emphasis on pwne and grb afterglows. | relativistic shocks: particle acceleration and magnetization |
we propose a new extra but non-cosmological explanation for fast radio bursts (frbs) based on very young pulsars in supernova remnants. within a few hundred years of a core-collapse supernova, the ejecta is confined within ∼1 pc, providing a high enough column density of free electrons for the observed 375-1600 pc cm-3 of dispersion measure (dm). by extrapolating a crab-like pulsar to its infancy in an environment like that of sn 1987a, we hypothesize such an object could emit supergiant pulses sporadically which would be bright enough to be seen at a few hundred megaparsecs. we hypothesize that such supergiant pulses would preferentially occur early in the pulsar's life when the free electron density is still high, which is why we do not see large numbers of moderate dm frbs (≲300 pc cm-3). in this scenario, faraday rotation at the source gives rotation measures (rms) much larger than the expected cosmological contribution. if the emission were pulsar-like, then the polarization vector could swing over the duration of the burst, which is not expected from non-rotating objects. in this model, the scattering, large dm, and commensurate rm all come from one place which is not the case for the cosmological interpretation. the model also provides testable predictions of the flux distribution and repeat rate of frbs, and could be furthermore verified by spatial coincidence with optical supernovae of the past several decades and cross-correlation with nearby galaxy maps. | non-cosmological frbs from young supernova remnant pulsars |
benefitting from the unequaled precision of the pulsar timing technique, binary pulsars are important testbeds of gravity theories, providing some of the tightest bounds on alternative theories of gravity. one class of well-motivated alternative gravity theories, the scalar-tensor gravity, predict large deviations from general relativity for neutron stars through a nonperturbative phenomenon known as spontaneous scalarization. this effect, which cannot be tested in the solar system, can now be tightly constrained using the latest results from the timing of a set of seven binary pulsars (psrs j0348+0432, j1012+5307, j1738+0333, j1909-3744, j2222-0137, j0737-3039a, and j1913+1102), especially with the updated parameters of psrs j2222-0137, j0737-3039a and j1913+1102. using new timing results, we constrain the neutron star's effective scalar coupling, which describes how strongly neutron stars couple to the scalar field, to a level of $\left\vert {\alpha }_{\mathrm{a}}\right\vert \lesssim 6\times 1{0}^{-3}$ in a bayesian analysis. our analysis is thorough, in the sense that our results apply to all neutron star masses and all reasonable equations of state of dense matters, in the full relevant parameter space. it excludes the possibility of spontaneous scalarization of neutron stars, at least within a class of scalar-tensor gravity theories. | closing a spontaneous-scalarization window with binary pulsars |
we present relativistic analyses of 9257 measurements of times-of-arrival from the first binary pulsar, psr b1913+16, acquired over the last 35 years. the determination of the “keplerian” orbital elements plus two relativistic terms completely characterizes the binary system, aside from an unknown rotation about the line of sight, leading to a determination of the masses of the pulsar and its companion: 1.438 ± 0.001 m ⊙ and 1.390 ± 0.001 m ⊙, respectively. in addition, the complete system characterization allows for the creation of relativistic gravitation test by comparing measured and predicted sizes of various relativistic phenomena. we find that the ratio of the observed orbital period decrease caused by gravitational wave damping (corrected by a kinematic term) to the general relativistic prediction is 0.9983 ± 0.0016, thereby confirms the existence and strength of gravitational radiation as predicted by general relativity. for the first time in this system, we have also successfully measured the two parameters characterizing the shapiro gravitational propagation delay, and found that their values are consistent with general relativistic predictions. for the first time in any system, we have also measured the relativistic shape correction to the elliptical orbit, δθ , although its intrinsic value is obscured by currently unquantified pulsar emission beam aberration. we have also marginally measured the time derivative of the projected semimajor axis, which, when improved in combination with beam aberration modeling from geodetic precession observations, should ultimately constrain the pulsar’s moment of inertia. | relativistic measurements from timing the binary pulsar psr b1913+16 |
neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. in particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. in many respects, its magnetic field determines the appearance of a neutron star. thus, understanding the field properties is important for the interpretation of observational data. complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. in this review, we first briefly describe theoretical models of the formation and evolution of the magnetic field of neutron stars, paying special attention to field decay processes. then, we present important observational results related to the field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, and sources in binary systems. after that, we discuss which observations can shed light on the obscure characteristics of neutron star magnetic fields and their behaviour. we end the review with a subjective list of open problems. | evolution of neutron star magnetic fields |
we have used the central 44 antennas of the new 64-dish meerkat radio telescope array to conduct a deep search for new pulsars in the core of nine globular clusters (gcs). this has led to the discovery of eight new millisecond pulsars in six different clusters. two new binaries, 47 tuc ac and 47 tuc ad, are eclipsing 'spiders', featuring compact orbits (≲0.32 d), very low mass companions, and regular occultations of their pulsed emission. the other three new binary pulsars (ngc 6624g, m62g, and ter 5 an) are in wider (>0.7 d) orbits, with companions that are likely to be white dwarfs or neutron stars. ngc 6624g has a large eccentricity of e ≃ 0.38, which enabled us to detect the rate of advance of periastron. this suggests that the system is massive, with a total mass of mtot = 2.65 ± 0.07 m⊙. likewise, for ter 5 an, with e ≃ 0.0066, we obtain mtot = 2.97 ± 0.52 m⊙. the other three new discoveries (ngc 6522d, ngc 6624h, and ngc 6752f) are faint isolated pulsars. finally, we have used the whole meerkat array and synthesized 288 beams, covering an area of ~2 arcmin in radius around the centre of ngc 6624. this has allowed us to localize many of the pulsars in the cluster, demonstrating the beamforming capabilities of the trapum software backend and paving the way for the upcoming meerkat gc pulsar survey. | eight new millisecond pulsars from the first meerkat globular cluster census |
we show that an ultralight primordial black hole (pbh) dominated phase makes blue-tilted inflationary gravitational waves (bgw) compatible with the recent detection of an nhz stochastic gw background by pulsar-timing arrays (ptas), for high reheating temperatures. this pbh-dominated phase suppresses the bgw spectrum via entropy dilution and generates a new gw spectrum from pbh density fluctuations. this combined spectrum is detectable at ongoing and planned near-future gw detectors and exhibits a unique shape with a low-frequency peak explaining pta data, a mid-range dip, and a sharp peak followed by a third peak at high-frequency. this distinctive shape sets it apart from spectra generated by other matter dominations or exotic physics. therefore, while important for studying gws in the nhz range, the recent pta result also sets the stage for testing and constraining various well-studied mechanisms following a pbh domination, using low-frequency measurements and correlated observations of unique high-frequency gw spectral features. | explaining pta data with inflationary gws in a pbh-dominated universe |
the abundance of primordial black holes changes in the presence of local non-gaussianity. a positive nonlinear parameter fnl increases the abundance while a negative one reduces it. we show that in nonattractor single-field models of inflation which enhance the curvature power spectrum and may give rise to primordial black holes, fnl is always positive, when computed in correspondence of the peak of the curvature power spectrum where the primordial black hole abundance has its maximum. this implies that the interpretation of the recent pulsar timing arrays data from scalar-induced gravitational waves generated at primordial black hole formation may not be supported by invoking non-gaussianity within nonattractor single-field models. | sign of non-gaussianity and the primordial black holes abundance |
with the enhancement mechanism provided by a noncanonical kinetic term with a peak, the amplitude of primordial curvature perturbations can be enhanced by seven orders of magnitude at small scales while keeping to be consistent with observations at large scales. the peak function and inflationary potential are not restricted in this mechanism. we use the higgs model and t-model as examples to show how abundant primordial black hole dark matter with different mass and scalar induced secondary gravitational waves with different peak frequency are generated. we also show that the enhanced power spectrum for the primordial curvature perturbations and the energy density of the scalar induced secondary gravitational waves can have either a sharp peak or a broad peak. the primordial black holes with the mass around 10-14-10-12 m⊙ produced with the enhancement mechanism can make up almost all dark matter, and the scalar induced secondary gravitational waves accompanied with the production of primordial black holes can be tested by the pulsar timing arrays and spaced based gravitational wave observatory. therefore, the mechanism can be tested by primordial black hole dark matter and gravitational wave observations. | primordial black holes and scalar-induced secondary gravitational waves from inflationary models with a noncanonical kinetic term |
in this work we present an exact solution of the einstein-maxwell field equations describing compact charged objects within the framework of classical general relativity. our model is constructed by embedding a four-dimensional spherically symmetric static metric into a five-dimensional flat metric. the source term for the matter field is composed of a perfect fluid distribution with charge. we show that our model obeys all the physical requirements and stability conditions necessary for a realistic stellar model. our theoretical model approximates observations of neutron stars and pulsars to a very good degree of accuracy. | generating physically realizable stellar structures via embedding |
we present broadband x-ray analyses of a sample of bright ultraluminous x-ray sources (ulx) with the goal of investigating the spectral similarity of this population to the known ulx pulsars, m82 x-2, ngc 7793 p13, and ngc 5907 ulx. we perform a phase-resolved analysis of the broadband xmm-newton+nustar data set of ngc 5907 ulx, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both m82 x-2 and ngc 7793 p13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. we then demonstrate that similar “hard” excesses are seen in all ulxs in the broadband sample. finally, for ulxs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in m82 x-2, ngc 7793 p13, and ngc 5907 ulx. based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ulx sample may be dominated by neutron star accretors. compared to the known pulsar ulxs, our spectral fits for the remaining ulxs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. if these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations. | evidence for pulsar-like emission components in the broadband ulx sample |
a graduate-level textbook on the astrophysics of binary star systems and their evolution. physics of binary star evolution is an up-to-date textbook on the astrophysics and evolution of binary star systems. theoretical astrophysicists thomas tauris and edward van den heuvel cover a wide range of phenomena and processes, including mass transfer and ejection, common envelopes, novae and supernovae, x-ray binaries, millisecond radio pulsars, and gravitational wave (gw) sources, and their links to stellar evolution. the authors walk through the observed properties and evolution of different types of binaries, with special emphasis on those containing compact objects (neutron stars, black holes, and white dwarfs). attention is given to the formation mechanisms of gw sources - merging double neutron stars and black holes as well as ultra-compact gw binaries hosting white dwarfs - and to the progenitors of these sources and how they are observed with radio telescopes, x-ray satellites, and gw detectors (ligo, virgo, kagra, einstein telescope, cosmic explorer, and lisa). supported by illustrations, equations, and exercises, physics of binary star evolution combines theory and observations to guide readers through the wonders of a field that will play a central role in modern astrophysics for decades to come. [864 pages, 465 equations, 47 tables, 350+ figures, 80+ student exercises]. | physics of binary star evolution. from stars to x-ray binaries and gravitational wave sources |
we study a specific model of anisotropic strange stars in the modified f(r,t)-type gravity by deriving solutions to the modified einstein field equations representing a spherically symmetric anisotropic stellar object. we take a standard assumption that f(r,t)=r+2χ t, where r is ricci scalar, t is the trace of the energy-momentum tensor of matter, and χ is a coupling constant. to obtain our solution to the modified einstein equations, we successfully apply the `embedding class one' techniques. we also consider the case when the strange quark matter (sqm) distribution is governed by the simplified mit bag model equation of state given by p_r=1/3(ρ -4b), where b is bag constant. we calculate the radius of the strange star candidates by directly solving the modified tov equation with the observed values of the mass and some parametric values of b and χ. the physical acceptability of our solutions is verified by performing several physical tests. interestingly, besides the sqm, another type of matter distribution originates due to the effect of coupling between the matter and curvature terms in the f(r,t) gravity theory. our study shows that with decreasing the value of χ, the stellar systems under investigations become gradually massive and larger in size, turning them into less dense compact objects. it also reveals that for χ < 0 the f(r,t) gravity emerges as a suitable theory for explaining the observed massive stellar objects like massive pulsars, super-chandrasekhar stars, and magnetars, etc., which remain obscure in the standard framework of general relativity. | exploring physical features of anisotropic strange stars beyond standard maximum mass limit in f(r,t) gravity |
an excess of $\gamma$-ray emission from the galactic center (gc) region with respect to predictions based on a variety of interstellar emission models and $\gamma$-ray source catalogs has been found by many groups using data from the {\it fermi} large area telescope (lat). several interpretations of this excess have been invoked. in this paper we search for members of an unresolved population of $\gamma$-ray pulsars located in the inner galaxy that are predicted by the interpretation of the gc excess as being due to a population of such sources. we use cataloged lat sources to derive criteria that efficiently select pulsars with very small contamination from blazars. we search for point sources in the inner $40^\circ\times40^\circ$ region of the galaxy, derive a list of approximately 400 sources, and apply pulsar selection criteria to extract pulsar candidates among our source list. we performed the entire data analysis chain with two different interstellar emission models (iems), and found a total of 135 pulsar candidates, of which 66 were selected with both iems. | characterizing the population of pulsars in the inner galaxy with the fermi large area telescope |
isolated neutron stars that are asymmetric with respect to their spin axis are possible sources of detectable continuous gravitational waves. this paper presents a fully coherent search for such signals from eighteen pulsars in data from ligo and virgo's third observing run (o3). for known pulsars, efficient and sensitive matched-filter searches can be carried out if one assumes the gravitational radiation is phase-locked to the electromagnetic emission. in the search presented here, we relax this assumption and allow both the frequency and the time derivative of the frequency of the gravitational waves to vary in a small range around those inferred from electromagnetic observations. we find no evidence for continuous gravitational waves, and set upper limits on the strain amplitude for each target. these limits are more constraining for seven of the targets than the spin-down limit defined by ascribing all rotational energy loss to gravitational radiation. in an additional search, we look in o3 data for long-duration (hours-months) transient gravitational waves in the aftermath of pulsar glitches for six targets with a total of nine glitches. we report two marginal outliers from this search, but find no clear evidence for such emission either. the resulting duration-dependent strain upper limits do not surpass indirect energy constraints for any of these targets. | narrowband searches for continuous and long-duration transient gravitational waves from known pulsars in the ligo-virgo third observing run |
we have searched for continuous gravitational wave (cgw) signals produced by individually resolvable, circular supermassive black hole binaries (smbhbs) in the latest european pulsar timing array (epta) data set, which consists of ultraprecise timing data on 41-ms pulsars. we develop frequentist and bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. none of the adopted algorithms show evidence for the presence of such a cgw signal, indicating that the data are best described by pulsar and radiometer noise only. depending on the adopted detection algorithm, the 95 per cent upper limit on the sky-averaged strain amplitude lies in the range 6 × 10-15 < a < 1.5 × 10-14 at 5 nhz < f < 7 nhz. this limit varies by a factor of five, depending on the assumed source position and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. the most robust upper limit - obtained via a full bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars - is a ≈ 10-14. these limits, the most stringent to date at f < 10 nhz, exclude the presence of sub-centiparsec binaries with chirp mass m_c>10^9 m_{⊙} out to a distance of about 25 mpc, and with m_c>10^{10} m_{⊙} out to a distance of about 1gpc (z ≈ 0.2). we show that state-of-the-art smbhb population models predict <1 per cent probability of detecting a cgw with the current epta data set, consistent with the reported non-detection. we stress, however, that pta limits on individual cgw have improved by almost an order of magnitude in the last five years. the continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby smbhbs in the coming years. | european pulsar timing array limits on continuous gravitational waves from individual supermassive black hole binaries |
a physically realistic stellar model with a simple expression for the energy density and conformally flat interior is found. the relations between the different conditions are used without graphic proofs. it may represent a real pulsar. | a conformally flat realistic anisotropic model for a compact star |
we perform a direct search for an isotropic stochastic gravitational-wave background (sgwb) produced by cosmic strings in the parkes pulsar timing array (ppta) data release 2 (dr2). we find no evidence for such an sgwb, and therefore place a 95% confidence level upper limit on the cosmic string tension, g μ, as a function of the reconnection probability, p, which can be less than 1 in the string-theory-inspired models or pure yang-mills theory. the upper bound on the cosmic string tension is g μ ≲ 5.1 × 10-10 for p = 1, which is about five orders of magnitude tighter than the bound derived from the null search of individual gravitational-wave bursts from cosmic string cusps in the ppta dr2, and comparable to previous bounds derived from the null search of the sgwb from cosmic strings. | search for the gravitational-wave background from cosmic strings with the parkes pulsar timing array second data release |
motivated by the direct discovery of gravitational waves (gws) from black holes and neutron stars, there is a growing interest in investigating gws from other sources. among them, gws from cosmic strings are particularly fascinating since they naturally appear in a large class of grand unified theories (guts). remarkably, a series of pulsar-timing arrays (ptas) might have already observed gws in the nhz regime, hinting towards forming a cosmic string network in the early universe, which could originate from phase transition associated with the seesaw scale emerging from gut. in this work, we show that if these observations from ptas are confirmed, gws from cosmic strings, when combined with fermion masses, gauge coupling unification, and proton decay constraints, the parameter space of the minimal so(10) gut becomes exceedingly restrictive. the proposed minimal model is highly predictive and will be fully tested in a number of upcoming gravitational wave observatories. | probing minimal grand unification through gravitational waves, proton decay, and fermion masses |
new radio (meerkat and parkes) and x-ray (xmm-newton, swift, chandra, and nustar) observations of psr j1622-4950 indicate that the magnetar, in a quiescent state since at least early 2015, reactivated between 2017 march 19 and april 5. the radio flux density, while variable, is approximately 100× larger than during its dormant state. the x-ray flux one month after reactivation was at least 800× larger than during quiescence, and has been decaying exponentially on a 111 ± 19 day timescale. this high-flux state, together with a radio-derived rotational ephemeris, enabled for the first time the detection of x-ray pulsations for this magnetar. at 5%, the 0.3-6 kev pulsed fraction is comparable to the smallest observed for magnetars. the overall pulsar geometry inferred from polarized radio emission appears to be broadly consistent with that determined 6-8 years earlier. however, rotating vector model fits suggest that we are now seeing radio emission from a different location in the magnetosphere than previously. this indicates a novel way in which radio emission from magnetars can differ from that of ordinary pulsars. the torque on the neutron star is varying rapidly and unsteadily, as is common for magnetars following outburst, having changed by a factor of 7 within six months of reactivation. | revival of the magnetar psr j1622-4950: observations with meerkat, parkes, xmm-newton, swift, chandra, and nustar |
we examine the spectra of 23 fast radio bursts (frbs) detected in a fly’s-eye survey with the australian ska pathfinder, including those of three bursts not previously reported. the mean spectral index of α =-{1.5}-0.3+0.2 (fν∝ να ) is close to that of the galactic pulsar population. the sample is dominated by bursts exhibiting a large degree of spectral modulation: 17 exhibit fine-scale spectral modulation with an rms exceeding 50% of the mean, with decorrelation bandwidths (half-maximum) ranging from ≈1 to 49 mhz. most decorrelation bandwidths are an order of magnitude lower than the ≳30 mhz expected of galactic interstellar scintillation at the galactic latitude of the survey, | b| =50^\circ +/- 5^\circ . however, these bandwidths are consistent with the ∼ν 4 scaling expected of diffractive scintillation when compared against the spectral structure observed in bright utmost frbs detected at 843 mhz. a test of the amplitude distribution of the spectral fluctuations reveals only 12 bursts consistent at better than a 5% confidence level with the prediction of 100%-modulated diffractive scintillation. five of six frbs with a signal-to-noise ratio exceeding 20 are only consistent with this prediction at less than 1% confidence. nonetheless, there is weak evidence (92%-94% confidence) of an anti-correlation between the amplitude of the spectral modulation and dispersion measure (dm), which suggests that it originates as a propagation effect. this effect is corroborated by the smoothness of the higher-dm parkes frbs, and could arise due to quenching of diffractive scintillation (e.g., in the interstellar medium of the host galaxy) by angular broadening in the intergalactic medium. | the spectral properties of the bright fast radio burst population |
we revisit the predictions for the merger rate of massive black hole binaries detectable by the laser interferometer space antenna (lisa) and their background signal for pulsar-timing arrays. we focus on the effect of the delays between the merger of galaxies and the final coalescence of black hole binaries, and on supernova feedback on the black hole growth. by utilizing a semianalytic galaxy formation model, not only do we account for the driving of the evolution of binaries at separations ≲1 pc (gas-driven migration, stellar hardening, and triple/quadruple massive black hole systems), but we also improve on previous studies by accounting for the time spent by black hole pairs from kiloparsec down to parsec separation. we also include the effect of supernova feedback, which may eject gas from the nuclear region of low-mass galaxies, thus hampering the growth of black holes via accretion and suppressing their orbital migration in circumbinary disks. despite including these novel physical effects, we predict that the lisa detection rate should still be $\gtrsim 2{{\rm{yr}}}^{-1}$ , irrespective of the model for the black hole seeds at high redshifts. scenarios where black holes form from $\sim 100{m}_{\odot }$ seeds are more significantly impacted by supernova feedback. we also find that for detectable events, the merging black holes typically have mass ratios between ∼0.1 and 1. predictions for the stochastic background in the band of pulsar-timing array experiments are instead rather robust and show only a mild dependence on the model. | massive black hole merger rates: the effect of kiloparsec separation wandering and supernova feedback |
we report constraints on the sources of ultrahigh-energy cosmic rays (uhecrs) above 1 09 gev , based on an analysis of seven years of icecube data. this analysis efficiently selects very high- energy neutrino-induced events which have deposited energies from 5 ×1 05 gev to above 1 011 gev . two neutrino-induced events with an estimated deposited energy of (2.6 ±0.3 )×1 06 gev , the highest neutrino energy observed so far, and (7.7 ±2.0 )×1 05 gev were detected. the atmospheric background-only hypothesis of detecting these events is rejected at 3.6 σ . the hypothesis that the observed events are of cosmogenic origin is also rejected at >99 % cl because of the limited deposited energy and the nonobservation of events at higher energy, while their observation is consistent with an astrophysical origin. our limits on cosmogenic neutrino fluxes disfavor the uhecr sources having a cosmological evolution stronger than the star formation rate, e.g., active galactic nuclei and γ -ray bursts, assuming proton-dominated uhecrs. constraints on uhecr sources including mixed and heavy uhecr compositions are obtained for models of neutrino production within uhecr sources. our limit disfavors a significant part of parameter space for active galactic nuclei and new-born pulsar models. these limits on the ultrahigh-energy neutrino flux models are the most stringent to date. | constraints on ultrahigh-energy cosmic-ray sources from a search for neutrinos above 10 pev with icecube |
primordial black holes (pbhs) can constitute the predominant fraction of dark matter (dm) if pbhs reside in the currently unconstrained "sublunar" mass range. pbhs originating from scalar perturbations generated during inflation can naturally appear with a broad spectrum in a class of models. the resulting stochastic gravitational wave (gw) background generated from such pbh production can account for the recently reported north american nanohertz observatory for gravitational waves (nanograv) pulsar timing array data signal, and will be testable in future gw observations by interferometer-type experiments such as laser interferometer space antenna (lisa). we show that the broad mass function of such pbh dm is already being probed by subaru hyper suprime-cam (hsc) microlensing data and is consistent with a detected candidate event. upcoming observations of hsc will be able to provide an independent definitive test of the stochastic gw signals originating from such pbh dm production scenarios. | testing stochastic gravitational wave signals from primordial black holes with optical telescopes |
quantum fluctuations of the gravitational field in the early universe, amplified by inflation, produce a primordial gravitational-wave background across a broad frequency band. we derive constraints on the spectrum of this gravitational radiation, and hence on theories of the early universe, by combining experiments that cover 29 orders of magnitude in frequency. these include planck observations of cosmic microwave background temperature and polarization power spectra and lensing, together with baryon acoustic oscillations and big bang nucleosynthesis measurements, as well as new pulsar timing array and ground-based interferometer limits. while individual experiments constrain the gravitational-wave energy density in specific frequency bands, the combination of experiments allows us to constrain cosmological parameters, including the inflationary spectral index nt and the tensor-to-scalar ratio r . results from individual experiments include the most stringent nanohertz limit of the primordial background to date from the parkes pulsar timing array, ωgw(f )<2.3 ×10-10 . observations of the cosmic microwave background alone limit the gravitational-wave spectral index at 95% confidence to nt≲5 for a tensor-to-scalar ratio of r =0.11 . however, the combination of all the above experiments limits nt<0.36 . future advanced ligo observations are expected to further constrain nt<0.34 by 2020. when cosmic microwave background experiments detect a nonzero r , our results will imply even more stringent constraints on nt and, hence, theories of the early universe. | gravitational-wave cosmology across 29 decades in frequency |
we present a search for gravitational waves from 222 pulsars with rotation frequencies ≳10 hz. we use advanced ligo data from its first and second observing runs spanning 2015-2017, which provides the highest-sensitivity gravitational-wave data so far obtained. in this search we target emission from both the l = m = 2 mass quadrupole mode, with a frequency at twice that of the pulsar’s rotation, and the l = 2, m = 1 mode, with a frequency at the pulsar rotation frequency. the search finds no evidence for gravitational-wave emission from any pulsar at either frequency. for the l = m = 2 mode search, we provide updated upper limits on the gravitational-wave amplitude, mass quadrupole moment, and fiducial ellipticity for 167 pulsars, and the first such limits for a further 55. for 20 young pulsars these results give limits that are below those inferred from the pulsars’ spin-down. for the crab and vela pulsars our results constrain gravitational-wave emission to account for less than 0.017% and 0.18% of the spin-down luminosity, respectively. for the recycled millisecond pulsar j0711-6830 our limits are only a factor of 1.3 above the spin-down limit, assuming the canonical value of 1038 kg m2 for the star’s moment of inertia, and imply a gravitational-wave-derived upper limit on the star’s ellipticity of 1.2 × 10-8. we also place new limits on the emission amplitude at the rotation frequency of the pulsars. | searches for gravitational waves from known pulsars at two harmonics in 2015-2017 ligo data |
the black hole merging rates inferred after the gravitational-wave detections by advanced ligo/virgo and the relatively high mass of the progenitors are consistent with models of dark matter made of massive primordial black holes (pbh). pbh binaries emit gravitational waves in a broad range of frequencies that will be probed by future space interferometers (lisa) and pulsar timing arrays (pta). the amplitude of the stochastic gravitational-wave background expected for pbh dark matter is calculated taking into account various effects such as initial eccentricity of binaries, pbh velocities, mass distribution and clustering. it allows a detection by the lisa space interferometer, and possibly by the pta of the ska radio-telescope. interestingly, one can distinguish this background from the one of non-primordial massive binaries through a specific frequency dependence, resulting from the maximal impact parameter of binaries formed by pbh capture, depending on the pbh velocity distribution and their clustering properties. moreover, we find that the gravitational wave spectrum is boosted by the width of pbh mass distribution, compared with that of the monochromatic spectrum. the current pta constraints already rule out broad-mass pbh models covering more than six decades of masses, but evading the microlensing and cmb constraints because black holes appear spatially distributed in clusters. | detecting the gravitational wave background from primordial black hole dark matter |
we investigate a mechanism of primordial black hole (pbh) formation that avoids any dependence on specific inflationary features or exotic physics. in this scenario, the required large curvature fluctuations leading to pbh formation are generated after inflation by the quantum fluctuations of a light stochastic spectator field during inflation, when this field transiently dominates the energy density. we calculate the dynamics of such a spectator field during and after inflation, the distribution of induced curvature perturbations and their non-gaussian tails leading to the copious production of pbhs. for a plateau-like potential, this scenario produces an extended pbh mass distribution with a peak at the solar-mass scale when one takes into account the effects of the thermal history. what is remarkable in this scenario is the absence of parameter fine-tuning. instead, it invokes an anthropic selection over all the realizations of pbh abundances predicted by the field stochasticity. this scenario offers a novel perspective for the formation of pbhs with minimal ingredients and without the need of fine-tuning. it is amenable to observational tests, notably with the gravitational-wave observations of black hole mergers and of a background at nanohertz frequency, as recently observed by pulsar timing arrays. | primordial black holes without fine-tuning from a light stochastic spectator field |
recently we found compelling evidence for a gravitational wave background with hellings and downs (hd) correlations in our 15-year data set. these correlations describe gravitational waves as predicted by general relativity, which has two transverse polarization modes. however, more general metric theories of gravity can have additional polarization modes which produce different interpulsar correlations. in this work we search the nanograv 15-year data set for evidence of a gravitational wave background with quadrupolar hellings and downs (hd) and scalar transverse (st) correlations. we find that hd correlations are the best fit to the data, and no significant evidence in favor of st correlations. while bayes factors show strong evidence for a correlated signal, the data does not strongly prefer either correlation signature, with bayes factors $\sim 2$ when comparing hd to st correlations, and $\sim 1$ for hd plus st correlations to hd correlations alone. however, when modeled alongside hd correlations, the amplitude and spectral index posteriors for st correlations are uninformative, with the hd process accounting for the vast majority of the total signal. using the optimal statistic, a frequentist technique that focuses on the pulsar-pair cross-correlations, we find median signal-to-noise-ratios of 5.0 for hd and 4.6 for st correlations when fit for separately, and median signal-to-noise-ratios of 3.5 for hd and 3.0 for st correlations when fit for simultaneously. while the signal-to-noise-ratios for each of the correlations are comparable, the estimated amplitude and spectral index for hd are a significantly better fit to the total signal, in agreement with our bayesian analysis. | the nanograv 15-year data set: search for transverse polarization modes in the gravitational-wave background |
the recent nicer measurement of the radius of the neutron star psr j0740+6620, and the inferred small variation of radii from 1.4 to 2.1 m ⊙, reveal key features of the equation of state of neutron star matter. the pressure rises rapidly in the regime of baryon density n ~ 2-4 times nuclear saturation density, n 0-the region where we expect hadronic matter to be undergoing transformation into quark matter-and the pressure in the nuclear regime is greater than predicted by microscopic many-body variational calculations of nuclear matter. to incorporate these insights into the microscopic physics from the nuclear to the quark matter regimes, we construct an equation of state, qhc21, within the framework of quark-hadron crossover. we include nuclear matter results primarily based on the state-of-the-art chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces. we employ explicit nuclear degrees of freedom only up to n ~ 1.5 n 0, in order to explore the possibility of further physical degrees of freedom than nucleonic here. the resulting qhc21, which has a peak in sound velocity in ~2-4 n 0, is stiffer than the earlier qhc19 below 2 n 0, predicting larger radii in substantial agreement with the nicer data. | implications of nicer for neutron star matter: the qhc21 equation of state |
we present a monte carlo-based population synthesis study of fast radio burst (frb) dispersion and scattering focusing on the first catalog of sources detected with the canadian hydrogen intensity mapping experiment fast radio burst (chime/frb) project. we simulate intrinsic properties and propagation effects for a variety of frb population models and compare the simulated distributions of dispersion measures and scattering timescales with the corresponding distributions from the chime/frb catalog. our simulations confirm the results of previous population studies, which suggested that the interstellar medium of the host galaxy alone (simulated based on the ne2001 model) cannot explain the observed scattering timescales of frbs. we therefore consider additional sources of scattering, namely, the circumgalactic medium (cgm) of intervening galaxies and the circumburst medium whose properties are modeled based on typical galactic plane environments. we find that a population of frbs with scattering contributed by these media is marginally consistent with the chime/frb catalog. in this scenario, our simulations favor a population of frbs offset from their galaxy centers over a population that is distributed along the spiral arms. however, if the models proposing the cgm as a source of intense scattering are incorrect, then we conclude that frbs may inhabit environments with more extreme properties than those inferred for pulsars in the milky way. | modeling fast radio burst dispersion and scattering properties in the first chime/frb catalog |
the galactic center excess (gce) remains one of the most intriguing discoveries from the fermi large area telescope (lat) observations. we revisit the characteristics of the gce by first producing a new set of high-resolution galactic diffuse gamma-ray emission templates. this diffuse emission, which accounts for the bulk of the observed gamma rays, is ultimately due to cosmic-ray interactions with the interstellar medium. using recent high-precision cosmic-ray observations, in addition to the continuing fermi-lat observations and observations from lower energy photons, we constrain the properties of the galactic diffuse emission. we describe a large set of diffuse gamma-ray emission templates which account for a very wide range of initial assumptions on the physical conditions in the inner galaxy. the broad properties of the gce that we find in this work are qualitatively unchanged despite the introduction of this new set of templates, though its quantitative features appear mildly different than those obtained in previous analyses. in particular, we find a high-energy tail at higher significance than previously reported. this tail is very prominent in the northern hemisphere, and less so in the southern hemisphere. this strongly affects one prominent interpretation of the excess: known millisecond pulsars are incapable of producing this high-energy emission, even in the relatively softer southern hemisphere, and are therefore disfavored as the sole explanation of the gce. the annihilation of dark matter particles of mass 40-7+10 gev (95% cl) to b quarks with a cross-section of ⟨σav ⟩=1. 4-0.3+0.6×10-26 cm3 s−1 provides a good fit to the excess especially in the relatively cleaner southern sky. dark matter of the same mass range annihilating to b quarks or heavier dark matter particles annihilating to heavier standard model bosons can combine with millisecond pulsars to provide a good fit to the southern hemisphere emission as well, as can a broken power-law spectrum which would be related to recent cosmic-ray burst activity. as part of this paper, we make publicly available all of our templates and the data covariance matrix we have generated to account for systematic uncertainties. | return of the templates: revisiting the galactic center excess with multimessenger observations |
we study graviton-photon conversion in magnetosphere of a pulsar and explore the possibility of detecting high frequency gravitational waves with pulsar observations. it is shown that conversion of one polarization mode of photons can be enhanced significantly due to strong magnetic fields around a pulsar. we also constrain stochastic gravitational waves in frequency range of $10^{8}-10^{9}\,$hz and $10^{13}-10^{27}\,$hz by using data of observations of the crab pulsar and the geminga pulsar. our method widely fills the gap among existing high frequency gravitational wave experiments and boosts the frequency frontier in gravitational wave observations. | probing high frequency gravitational waves with pulsars |
we study possible effects of a dark matter (dm) core on the maximum mass of a neutron star (ns), on the mass-radius relation and on the ns tidal deformability parameter λ . we show that all these quantities would in general be reduced in the presence of a dm core. in particular, our calculations indicate that the presence of a dm core with a mass fraction ∼5 % could affect significantly the interpretation of these ns data as constraints on the nuclear equation of state (eos), potentially excluding some eos models on the basis of the measured mass of psr j 0348 +0432 , while allowing other eos models to become consistent with the ligo/virgo upper limit on λ . specific scenarios for generating such dm cores are explored in an appendix. | dark matter effects on neutron star properties |
binaries comprised of a neutron star (ns) and a black hole (bh) have so far eluded observations as pulsars and with gravitational waves (gws). we model the formation and evolution of these ns+bh binaries - including pulsar evolution - using the binary population synthesis code compas. we predict the presence of a total of 50-2000 binaries containing a pulsar and a bh (psr+bhs) in the galactic field. we find the population observable by the next generation of radio telescopes, represented by the ska and meerkat, current (ligo/virgo) and future (lisa) gw detectors. we conclude that the ska will observe 1-80 psr+bhs, with 0-4 binaries containing millisecond pulsars. meerkat is expected to observe 0-40 psr+bh systems. future radio detections of ns+bhs will constrain uncertain binary evolution processes such as bh natal kicks. we show that systems in which the ns formed first (nsbh) can be distinguished from those where the bh formed first (bhns) by their pulsar and binary properties. we find 40 per cent of the ligo/virgo observed ns+bhs from a milky way like field population will have a chirp mass ≥3.0 m⊙. we estimate the spin distributions of ns+bhs with two models for the spins of bhs. the remnants of bhns mergers will have a spin of ~0.4, whilst nsbh merger remnants can have a spin of ~0.6 or ~0.9 depending on the model for bh spins. we estimate that approximately 25-1400 psr+bhs will be radio alive whilst emitting gws in the lisa frequency band, raising the possibility of joint observation by the ska and lisa. | modelling neutron star-black hole binaries: future pulsar surveys and gravitational wave detectors |
pulsars are known for their superb timing precision, although glitches can interrupt the regular timing behavior when the stars are young. these glitches are thought to be caused by interactions between normal and superfluid matter in the crust of the star. however, glitching pulsars such as vela have been shown to require a superfluid reservoir that greatly exceeds that available in the crust. we examine a model in which glitches tap the superfluid in the core. we test a variety of theoretical superfluid models against the most recent glitch data and find that only one model can successfully explain up to 45 years of observational data. we develop a new technique for combining radio and x-ray data to measure pulsar masses, thereby demonstrating how current and future telescopes can probe fundamental physics such as superfluidity near nuclear saturation. | pinning down the superfluid and measuring masses using pulsar glitches |
natal kicks are a matter of debate and they significantly affect the merger rate density of compact objects. here, we present a new simple formalism for natal kicks of neutron stars (nss) and black holes (bhs). we describe the magnitude of the kick as {v}kick∝ {f}h05 mej mrem-1, where fh05 is a normalization factor, drawn from a maxwellian distribution with one-dimensional rms velocity σ = 265 km s-1, mej is the mass of the supernova (sn) ejecta, and mrem is the mass of the compact object. this formalism matches the proper motions of young galactic pulsars and can naturally account for the differences between core-collapse sne of single stars, electron-capture sne and ultra-stripped sne occurring in interacting binaries. finally, we use our new kick formalism to estimate the local merger rate density of binary nss (rbns), bh-ns binaries (rbhns), and binary bhs (rbbh), based on the cosmic star formation rate density and metallicity evolution. in our fiducial model, we find rbns ∼ 600 gpc-3 yr-1, rbhns ∼ 10 gpc-3 yr-1, and rbbh ∼ 50 gpc-3 yr-1, fairly consistent with the numbers inferred from the ligo-virgo collaboration. | revising natal kick prescriptions in population synthesis simulations |
screening mechanisms are essential features of dark energy models mediating a fifth force on large scales. we study the regime of strong scalar field nonlinearities, known as vainshtein screening, in the most general scalar-tensor theories propagating a single scalar degree of freedom. we first develop an effective approach to parametrize cosmological perturbations beyond linear order for these theories. in the quasistatic limit, the fully nonlinear effective lagrangian contains six independent terms, one of which starts at cubic order in perturbations. we compute the two gravitational potentials around a spherical body. outside and near the body, screening reproduces standard gravity, with a modified gravitational coupling. inside the body, the two potentials are different and depend on the density profile, signalling the breaking of the vainshtein screening. we provide the most general expressions for these modifications, revising and extending previous results. we apply our findings to show that the combination of the gw170817 event, the hulse-taylor pulsar and stellar structure physics, constrain the parameters of these general theories at the level of 10-1, and of gleyzes-langlois-piazza-vernizzi theories at the level of 10-2. | vainshtein screening in scalar-tensor theories before and after gw170817: constraints on theories beyond horndeski |
we explore the implications of the recent radius determination of psr j 0740 +6620 by the nicer experiment, combined with the neutron skin measurement by the prex-ii experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. we combine a covariant density-functional nucleonic equation of state (eos) with a constant-speed-of-sound eos for quark matter. we show that the radius and tidal deformability ranges obtained from gw170817 can be reconciled with the implication of the prex-ii experiment if there is a phase transition to quark matter in the low-mass compact star. in the high-mass segment, the eos needs to be stiff to comply with the large-radius inference for psr j 0740 +6620 and j 0030 +0451 with masses m ≃2 m⊙ and m ≃1.4 m⊙. we show that twin stars are not excluded, but the mass and radius ranges (with m ≥m⊙) are restricted to narrow domains δ mtwin≲0.05 m⊙ and δ rtwin∼1.0 km . we also show that the existence of twin configurations is compatible with the light companion in the gw190814 event being a hybrid star in the case of values of the sound-speed square s =0.6 and s =1 /3 . | relativistic hybrid stars in light of the nicer psr j 0740 +6620 radius measurement |
we present the result of searches for gravitational waves from 200 pulsars using data from the first observing run of the advanced ligo detectors. we find no significant evidence for a gravitational-wave signal from any of these pulsars, but we are able to set the most constraining upper limits yet on their gravitational-wave amplitudes and ellipticities. for eight of these pulsars, our upper limits give bounds that are improvements over the indirect spin-down limit values. for another 32, we are within a factor of 10 of the spin-down limit, and it is likely that some of these will be reachable in future runs of the advanced detector. taken as a whole, these new results improve on previous limits by more than a factor of two. | first search for gravitational waves from known pulsars with advanced ligo |
if the black holes detected by ligo/virgo are primordial black holes (pbhs) sourcing from a large primordial curvature perturbation on small scales, the corresponding induced gravitational waves (gws) would peak at nanohertz that is detectable by the current and future observations of pulsar timing array (pta). in this paper we show that with the mass function estimated from the merger rate of ligo o1 and o2 events, the induced gws from such a curvature perturbation with a gaussian narrow peak at some small scale would be in a seemingly mild tension with current constraints from pta. however, if the curvature perturbation is of local-type non-gaussianity with a non-linear parameter fnlgtrsimscript o(10), the tension could be relieved. nevertheless, such an induced gws must be detectable by the square kilometer array in a decade or less. | pulsar timing array constraints on the induced gravitational waves |
pulsars act as accurate clocks, sensitive to gravitational redshift and acceleration induced by transiting clumps of matter. we study the sensitivity of pulsar timing arrays (ptas) to single transiting compact objects, focusing on primordial black holes and compact subhalos in the mass range from 10-12 m⊙ to well above 100 m⊙. we find that the square kilometer array can constrain such objects to be a subdominant component of the dark matter over this entire mass range, with sensitivity to a dark matter subcomponent reaching the subpercent level over significant parts of this range. we also find that ptas offer an opportunity to probe substantially less dense objects than lensing because of the large effective radius over which such objects can be observed, and we quantify the subhalo concentration parameters which can be constrained. | pulsar timing probes of primordial black holes and subhalos |
cosmic rays (protons and other atomic nuclei) are believed to gain energies of petaelectronvolts (pev) and beyond at astrophysical particle accelerators called `pevatrons' inside our galaxy. although a characteristic feature of a pevatron is expected to be a hard gamma-ray energy spectrum that extends beyond 100 teraelectronvolts (tev) without a cut-off, none of the currently known sources exhibit such a spectrum owing to the low maximum energy of accelerated cosmic rays or owing to insufficient detector sensitivity around 100 tev. here, we report the observation of gamma-ray emission from the supernova remnant g106.3+2.7 (refs. 1,2) above 10 tev. this work provides flux data points up to and above 100 tev and indicates that the very-high-energy gamma-ray emission above 10 tev is well correlated with a molecular cloud3 rather than with the pulsar psr j2229+6114 (refs. 4-8). regarding the gamma-ray emission mechanism of g106.3+2.7, this morphological feature appears to favour a hadronic origin via the π0 decay caused by accelerated relativistic protons9 over a leptonic origin via the inverse compton scattering by relativistic electrons10,11. furthermore, we point out that an x-ray flux upper limit on the synchrotron spectrum would provide important information to firmly establish the hadronic scenario as the mechanism of particle acceleration at the source. | potential pevatron supernova remnant g106.3+2.7 seen in the highest-energy gamma rays |
the strong magnetic field of neutron stars is intimately coupled to the observed temperature and spectral properties, as well as to the observed timing properties (distribution of spin periods and period derivatives). thus, a proper theoretical and numerical study of the magnetic field evolution equations, supplemented with detailed calculations of microphysical properties (heat and electrical conductivity, neutrino emission rates) is crucial to understand how the strength and topology of the magnetic field vary as a function of age, which in turn is the key to decipher the physical processes behind the varied neutron star phenomenology. in this review, we go through the basic theory describing the magneto-thermal evolution models of neutron stars, focusing on numerical techniques, and providing a battery of benchmark tests to be used as a reference for present and future code developments. we summarize well-known results from axisymmetric cases, give a new look at the latest 3d advances, and present an overview of the expectations for the field in the coming years. | magnetic, thermal and rotational evolution of isolated neutron stars |
the aligo detection of the black-hole binary gw150914 opens a new era for probing extreme gravity. many gravity theories predict the emission of dipole gravitational radiation by binaries. this is excluded to high accuracy in binary pulsars, but entire classes of theories predict this effect predominantly (or only) in binaries involving black holes. joint observations of gw150914-like systems by aligo and elisa will improve bounds on dipole emission from black-hole binaries by 6 orders of magnitude relative to current constraints, provided that elisa is not dramatically descoped. | theory-agnostic constraints on black-hole dipole radiation with multiband gravitational-wave astrophysics |
nanograv and other pulsar timing arrays (ptas) have discovered a common-spectrum process in the nhz range that may be due to gravitational waves (gws): if so, they are likely to have been generated by black hole (bh) binaries with total masses > 109 m⊙. using the extended press-schechter formalism to model the galactic halo mass function and a simple relation between the halo and bh masses suggests that these binaries have redshifts z = 𝒪(1) and mass ratios ≳10, and that the gw signal at frequencies above 𝒪(10) nhz may be dominated by relatively few binaries that could be distinguished experimentally and would yield observable circular polarization. extrapolating the model to higher frequencies indicates that future gw detectors such as lisa and aedge could extend the pta observations to lower bh masses ≳103 m⊙. | prospects for future binary black hole gravitational wave studies in light of pta measurements |
dark matter (dm) from the galactic halo can accumulate in neutron stars and transmute them into sub-2.5 m⊙ black holes if the dark matter particles are heavy, stable, and have interactions with nucleons. we show that nondetection of gravitational waves from mergers of such low-mass black holes can constrain the interactions of nonannihilating dark matter particles with nucleons. we find benchmark constraints with ligo o3 data, viz., σχ n≥o (10-47) cm2 for bosonic dm with mχ∼pev (or mχ∼gev , if they can bose-condense) and ≥o (10-46) cm2 for fermionic dm with mχ∼103 pev . these bounds depend on the priors on dm parameters and on the currently uncertain binary neutron star merger rate density. however, with increased exposure by the end of this decade, ligo will probe cross sections that are many orders of magnitude below the neutrino floor and completely test the dark matter solution to missing pulsars in the galactic center, demonstrating a windfall science case for gravitational wave detectors as probes of particle dark matter. | can ligo detect nonannihilating dark matter? |
very recently the nicer collaboration published the first-ever accurate measurement of mass and radius together for psr j0030+0451, a nearby isolated quickly rotating neutron star (ns). in this work we set the joint constraints on the equation of state (eos) and some bulk properties of nss with the data of psr j0030+0451, gw170817, and some nuclear experiments. the piecewise polytropic expansion method and the spectral decomposition method have been adopted to parameterize the eos. the resulting constraints are consistent with each other. assuming the maximal gravitational mass of nonrotating ns mtov lies between 2.04m⊙ and 2.4m⊙, with the piecewise method the pressure at twice nuclear saturation density is measured to be 3.19-1.35+2.63 × 1034 dyn cm-2 at the 90% level. for an ns with canonical mass of 1.4m⊙, we have the moment of inertia i1.4= 1.43-0.13+0.30× 1038 kg \cdot m2, tidal deformability λ1.4 = 370-130+360, radius r1.4 = 12.1-0.8+1.2km, and binding energy be1.4= 0.16-0.02+0.01 m⊙ at the 90% level, which are improved in comparison to the constraints with the sole data of gw170817. these conclusions are drawn for the mass/radius measurements of psr j0030+0451 by riley et al. for the measurements of miller et al., the results are rather similar. | psr j0030+0451, gw170817, and the nuclear data: joint constraints on equation of state and bulk properties of neutron stars |
results are presented of searches for continuous gravitational waves from 20 accreting millisecond x-ray pulsars with accurately measured spin frequencies and orbital parameters, using data from the third observing run of the advanced ligo and advanced virgo detectors. the search algorithm uses a hidden markov model, where the transition probabilities allow the frequency to wander according to an unbiased random walk, while the j -statistic maximum-likelihood matched filter tracks the binary orbital phase. three narrow subbands are searched for each target, centered on harmonics of the measured spin frequency. the search yields 16 candidates, consistent with a false alarm probability of 30% per subband and target searched. these candidates, along with one candidate from an additional target-of-opportunity search done for sax j 1808.4 −3658 , which was in outburst during one month of the observing run, cannot be confidently associated with a known noise source. additional follow-up does not provide convincing evidence that any are a true astrophysical signal. when all candidates are assumed nonastrophysical, upper limits are set on the maximum wave strain detectable at 95% confidence, h095 %. the strictest constraint is h095 %=4.7 ×10-26 from igr j 17062 −6143 . constraints on the detectable wave strain from each target lead to constraints on neutron star ellipticity and r -mode amplitude, the strictest of which are ε95 %=3.1 ×10-7 and α95 %=1.8 ×10-5 respectively. this analysis is the most comprehensive and sensitive search of continuous gravitational waves from accreting millisecond x-ray pulsars to date. | search for continuous gravitational waves from 20 accreting millisecond x-ray pulsars in o3 ligo data |
the main objective of the paper is to provide a new family of solutions of embedding class one describing the interior of a spherically symmetric anisotropic stellar configuration. for n =6 ,8 ,10 and 12, all the physical parameters are well-behaved within the stellar interior and our model satisfies all the required conditions to be physically viable. due to the well-behaved nature of the solution of the above n values, we develop the model of psr j1614-2230 (nature 467, 1081 (2010)) and discuss the behavior of the class of solutions extensively. by analyzing the adiabatic index (γ ) we observe that for lower values of n, i.e., n =6 , the star is soft and as we increase the value of n the star becomes stiff. | stellar modelling of psr j1614-2230 using the karmarkar condition |
we study exact models for anisotropic gravitating stars with conformal symmetry. the gravitational potentials are related explicitly by the conformal vector. we use this relationship between the metric potentials to find new classes of exact solutions to the field equations. we identify a particular model to study the physical features and demonstrate that the model is well behaved. in particular the criteria for stability are satisfied. we regain masses, radii and surface redshifts for the compact objects psr j1614-2230 and sax j1808.4-3658. | relativistic stars with conformal symmetry |
blobs, or quasi-spherical emission regions containing relativistic particles and magnetic fields, are often assumed ad hoc in emission models of relativistic astrophysical jets, yet their physical origin is still not well understood. here, we employ a suite of large-scale 2d particle-in-cell simulations in electron-positron plasmas to demonstrate that relativistic magnetic reconnection can naturally account for the formation of quasi-spherical plasmoids filled with high-energy particles and magnetic fields. our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic physics independently of the initial setup. we characterize the properties of the plasmoids, continuously generated as a self-consistent by-product of the reconnection process: they are in rough energy equipartition between particles and magnetic fields; the upper energy cutoff of the plasmoid particle spectrum is proportional to the plasmoid width w, corresponding to a larmor radius ∼0.2 w; the plasmoids grow in size at ∼0.1 of the speed of light, with most of the growth happening while they are still non-relativistic (`first they grow'); their growth is suppressed once they get accelerated to relativistic speeds by the field line tension, up to the alfvén speed (`then they go'). the largest plasmoids reach a width wmax ∼ 0.2 l independently of the system length l, they have nearly isotropic particle distributions and contain the highest energy particles, whose larmor radius is ∼0.03 l. the latter can be regarded as the hillas criterion for relativistic reconnection. we briefly discuss the implications of our results for the high-energy emission from relativistic jets and pulsar winds. | plasmoids in relativistic reconnection, from birth to adulthood: first they grow, then they go |
motivated by the unknown nature of the 2.50 -2.67 m⊙ compact object in the binary merger event gw190814, we study the maximum neutron star mass based on constraints from low-energy nuclear physics, neutron star tidal deformabilities from gw170817, and simultaneous mass-radius measurements of psr j0030+045 from nicer. our prior distribution is based on a combination of nuclear modeling valid in the vicinity of normal nuclear densities together with the assumption of a maximally stiff equation of state at high densities, a choice that enables us to probe the connection between observed heavy neutron stars and the transition density at which conventional nuclear physics models must break down. we demonstrate that a modification of the highly uncertain suprasaturation density equation of state beyond 2.64 times normal nuclear density is required in order for chiral effective field theory models to be consistent with current neutron star observations and the existence of 2.6 m⊙ neutron stars. we also show that the existence of very massive neutron stars strongly impacts the radii of ≈2.0 m⊙ neutron stars (but not necessarily the radii of 1.4 m⊙ neutron stars), which further motivates future nicer radius measurements of psr j1614 −2230 and psr j0740+6620. | radius and equation of state constraints from massive neutron stars and gw190814 |
recent observations of fast radio bursts (frbs) indicate a perplexing, inconsistent picture. we propose a unified scenario to interpret diverse frbs observed. a regular pulsar, otherwise unnoticeable at a cosmological distance, may produce a bright frb if its magnetosphere is suddenly “combed” by a nearby, strong plasma stream toward the anti-stream direction. if the earth is to the night side of the stream, the combed magnetic sheath would sweep across the direction of earth and make a detectable frb. the stream could be an agn flare, a grb or supernova blastwave, a tidal disruption event, or even a stellar flare. since it is the energy flux received by the pulsar rather than the luminosity of the stream origin that defines the properties of the frb, this model predicts a variety of counterparts of frbs, including a possible connection between frb 150418 and an agn flare, a possible connection between frb 131104 and a weak grb, a steady radio nebula associated with the repeating frb 121102, and probably no bright counterparts for some frbs. | a “cosmic comb” model of fast radio bursts |
the nature of dark matter remains obscure in spite of decades of experimental efforts. the mass of dark matter candidates can span a wide range, and its coupling with the standard model sector remains uncertain. all these unknowns make the detection of dark matter extremely challenging. ultralight dark matter, with m ∼10−22 ev, is proposed to reconcile the disagreements between observations and predictions from simulations of small-scale structures in the cold dark matter paradigm while remaining consistent with other observations. because of its large de broglie wavelength and large local occupation number within galaxies, ultralight dark matter behaves like a coherently oscillating background field with an oscillating frequency dependent on its mass. if the dark matter particle is a spin-1 dark photon, such as the u (1) b or u (1) b −l gauge boson, it can induce an external oscillating force and lead to displacements of test masses. such an effect would be observable in the form of periodic variations in the arrival times of radio pulses from highly stable millisecond pulsars. in this study, we search for evidence of ultralight dark photon dark matter (dpdm) using 14-year high-precision observations of 26 pulsars collected with the parkes pulsar timing array. while no statistically significant signal is found, we place constraints on coupling constants for the u (1) b and u (1) b −l dpdm. compared with other experiments, the limits on the dimensionless coupling constant ε achieved in our study are improved by up to two orders of magnitude when the dark photon mass is smaller than 3 ×10−22 ev (10−22 ev) for the u (1) b (u (1) b −l ) scenario. | high-precision search for dark photon dark matter with the parkes pulsar timing array |
some models (such as the skyrme model, a low-energy effective field theory for quantum chromodynamics) suggest that the high-density matter prevailing in neutron star (ns) interiors may be significantly anisotropic. anisotropy is known to affect the bulk properties of nonrotating nss in general relativity (gr). in this paper we study the effects of anisotropy on slowly rotating stars in gr. we also consider one of the most popular extensions of einstein's theory, namely scalar-tensor theories allowing for spontaneous scalarization (a phase transition similar to spontaneous magnetization in ferromagnetic materials). anisotropy affects the moment of inertia of nss (a quantity that could potentially be measured in binary pulsar systems) in both theories. we find that the effects of scalarization increase (decrease) when the tangential pressure is bigger (smaller) than the radial pressure, and we present a simple criterion to determine the onset of scalarization by linearizing the scalar-field equation. our calculations suggest that binary pulsar observations may constrain the degree of anisotropy or even, more optimistically, provide evidence for anisotropy in ns cores. | slowly rotating anisotropic neutron stars in general relativity and scalar-tensor theory |
we present a new class of solutions to the einstein field equations for an anisotropic matter distribution in which the interior space-time obeys the karmarkar condition. the necessary and sufficient condition required for a spherically symmetric space-time to be of class one reduces the gravitational behavior of the model to a single metric function. by assuming a physically viable form for the grr metric potential we obtain an exact solution of the einstein field equations which is free from any singularities and satisfies all the physical criteria. we use this solution to predict the masses and radii of well-known compact objects such as cen x-3, psr j0348+0432, psr b0943+10 and xte j1739-285. | anisotropic compact stars in karmarkar spacetime |
first discovered in 2007, fast radio bursts (frbs) are highly luminous (10-1-102 jy), millisecond-scale, highly dispersive single radio pulses whose record high brightness temperatures suggest a nonthermal emission mechanism. as of march 2018, a total of 32 frbs have been recorded. there is also one repeating source, from which hundreds of bursts have already been detected. the rate of events is estimated to be several thousand per day per sky (disregarding bursts from the repeater), and their isotropic distribution in the sky suggests a likely cosmological origin. while numerous hypotheses have been proposed for frbs since their discovery, the origin of these transients is not yet known. the most promising models either relate them to burst-type radiation from magnetars (neutron stars powered by the dissipation of their magnetic energy) or consider them analogous to giant pulses from some radio pulsars (strongly magnetized rotating neutron stars). the increasing statistics on the observed bursts and improvements in characterizing the frb population will allow frbs to become another tool for probing the intergalactic medium, estimating the cosmological parameters, and testing fundamental physical theories. | fast radio bursts |
qed cascades are complex avalanche processes of hard photon emission and electron-positron pair creation driven by ultrastrong electromagnetic fields. they play a fundamental role in astrophysical environments such as a pulsars' magnetosphere, rendering an earth-based implementation with intense lasers attractive. in the literature, qed cascades were also predicted to limit the attainable intensity in a set-up of colliding laser beams in a tenuous gas such as the residual gas of a vacuum chamber, therefore severely hindering experiments at extreme field intensities. here, we demonstrate that the onset of qed cascades may be either prevented even at intensities around 1026 w/cm2 with tightly focused laser pulses and low-z gases, or facilitated at intensities below 1024 w/cm2 with enlarged laser focal areas or high-z gases. these findings pave the way for the control of novel experiments such as the generation of pure electron-positron-photon plasmas from laser energy, and for probing qed in the extreme-intensity regime where the quantum vacuum becomes unstable. | laser-pulse-shape control of seeded qed cascades |
pulsar timing arrays seek and study gravitational waves (gws) through the angular two-point correlation function of timing residuals they induce in pulsars. the two-point correlation function induced by the standard transverse-traceless gws is the famous hellings-downs curve, a function only of the angle between the two pulsars. additional polarization modes (vector/scalar) that may arise in alternative-gravity theories have different angular correlation functions. furthermore, anisotropy, linear, or circular polarization in the stochastic gw background gives rise to additional structure in the two-point correlation function that cannot be written simply in terms of the angular separation of the two pulsars. in this paper, we provide a simple formula for the most general two-point correlation function -- or overlap reduction function (orf) -- for a gravitational-wave background with an arbitrary polarization state, possibly containing anisotropies in its intensity and polarization (linear/circular). we provide specific expressions for the orfs sourced by the general-relativistic transverse-traceless gw modes as well as vector (or spin-1) modes that may arise in alternative-gravity theories. | all the pretty overlap reduction functions |
non-relativistic axions can be efficiently produced in in the polar caps of pulsars, resulting in the formation of a dense cloud of gravitationally bound axions. here, we investigate the interplay between such an axion cloud and the electrodynamics in the pulsar magnetosphere, focusing specifically on the dynamics in the polar caps, where the impact of the axion cloud is expected to be most pronounced. for sufficiently light axions $m_a \lesssim 10^{-7}$ ev, we show that the axion cloud can occasionally screen the local electric field responsible for particle acceleration and pair production, inducing a periodic nulling of the pulsar's intrinsic radio emission. at larger axion masses, the small-scale fluctuations in the axion field tend to suppress the back-reaction of the axion on the electrodynamics; however, we point out that the incoherent oscillations of the axion in short-lived regions of vacuum near the neutron star surface can produce a narrow radio line, which provides a complementary source of radio emission to the plasma-resonant emission processes identified in previous work. while this work focuses on the leading order correction to pair production in the magnetosphere, we speculate that there can exist dramatic deviations in the electrodynamics of these systems when the axion back-reaction becomes non-linear. | pulsar nulling and vacuum radio emission from axion clouds |
the chinese pulsar timing array (cpta) collaboration has recently reported the observational evidence of a stochastic gravitational wave background. in light of the latest cpta observation, we aim at exploring the ability of cpta in probing new physics. specifically, we constrain the first-order cosmological phase transitions with cpta data, and find that the constraining result is slightly tighter than that of nanograv's 12.5-yr data but weaker than nanograv's 15-yr data. considering the possible complexity of gravitational wave sources, we give the constraint on a mixed scenario of cosmological phase transitions and astrophysical supermassive binary black holes. our analysis suggests that cpta has a great potential to probe fundamental physics in the near future. | constraining cosmological phase transitions with chinese pulsar timing array data release 1 |
puma is a proposed ultra-wideband low-resolution interferometric transit radio telescope operating at 200-1100 mhz composed of thousands of 6m dishes. it is optimized for intensity mapping of the redshifted 21cm line over z=0.3-6 to study dark energy and inflation. the same design also allows an unprecedented study of fast radio bursts and pulsars. | packed ultra-wideband mapping array (puma): a radio telescope for cosmology and transients |
the angular correlation of pulsar residuals observed by nanograv and other pulsar timing array (pta) collaborations show evidence in support of the hellings-downs correlation expected from stochastic gravitational waves (sgw). in this paper, we offer a non-gravitational wave explanation of the observed pulsar timing correlations as caused by an ultra-light $l_{\mu} - l_{\tau}$ gauge boson dark matter (uldm). uldm can affect the pulsar correlations in two ways. the gravitational potential of vector uldm gives rise to a shapiro time-delay of the pulsar signals and a non-trivial angular correlation (as compared to the scalar uldm case). in addition, if the pulsars have a non-zero charge of the dark matter gauge group then the electric field of the local dark matter causes an oscillation of the pulsar and a corresponding doppler shift of the pulsar signal. we point out that pulsars carry a significant charge of muons and thus the $l_{\mu} - l_{\tau}$ vector dark matter contributes to both the doppler oscillations and the time-delay of the pulsar signals. our analysis shows that the nanograv data has a better fit to the $l_{\mu} - l_{\tau}$ uldm scenario compared to the sgw or the sgw with shapiro time-delay hypotheses. | ultralight $(l_\\mu-l_\\tau)$ vector dark matter interpretation of nanograv observations |
we report a revised analysis for the radius, mass, and hot surface regions of the massive millisecond pulsar psr j0740+6620, studied previously with joint fits to nicer and xmm-newton data by riley et al. (2021) and miller et al. (2021). we perform a similar bayesian estimation for the pulse-profile model parameters, except that instead of fitting simultaneously the xmm-newton data, we use the best available nicer background estimates to constrain the number of photons detected from the source. this approach eliminates any potential issues in the cross-calibration between these two instruments, providing thus an independent check of the robustness of the analysis. the obtained neutron star parameter constraints are compatible with the already published results, with a slight dependence on how conservative the imposed background limits are. a tighter lower limit causes the inferred radius to increase, and a tighter upper limit causes it to decrease. we also extend the study of the inferred emission geometry to examine the degree of deviation from antipodality of the hot regions. we show that there is a significant offset to an antipodal spot configuration, mainly due to the non-half-cycle azimuthal separation of the two emitting spots. the offset angle from the antipode is inferred to be above 25° with 84% probability. this seems to exclude a centered-dipolar magnetic field in psr j0740+6620. | the radius of psr j0740+6620 from nicer with nicer background estimates |
using twenty long-term 3d core-collapse supernova simulations, we find that lower compactness progenitors that explode quasi-spherically due to the short delay to explosion experience smaller neutron star recoil kicks in the $\sim$100$-$200 km s$^{-1}$ range, while higher compactness progenitors that explode later and more aspherically leave neutron stars with kicks in the $\sim$300$-$1000 km s$^{-1}$ range. in addition, we find that these two classes are correlated with the gravitational mass of the neutron star. this correlation suggests that the survival of binary neutron star systems may in part be due to their lower kick speeds. we also find a correlation of the kick with both the mass dipole of the ejecta and the explosion energy. furthermore, one channel of black hole birth leaves masses of $\sim$10 $m_{\odot}$, is not accompanied by a neutrino-driven explosion, and experiences small kicks. a second is through a vigorous explosion that leaves behind a black hole with a mass of $\sim$3.0 $m_{\odot}$ kicked to high speeds. we find that the induced spins of nascent neutron stars range from seconds to $\sim$10 milliseconds, {but do not yet see a significant spin/kick correlation for pulsars.} we suggest that if an initial spin biases the explosion direction, a spin/kick correlation {would be} a common byproduct of the neutrino mechanism of core-collapse supernovae. finally, the induced spin in explosive black hole formation is likely large and in the collapsar range. this new 3d model suite provides a greatly expanded perspective and appears to explain some observed pulsar properties by default. | a theory for neutron star and black hole kicks and induced spins |
intermediate-mass black holes should help us to understand the evolutionary connection between stellar-mass and super-massive black holes. however, the existence of intermediate-mass black holes is still uncertain, and their formation process is therefore unknown. it has long been suspected that black holes with masses 100 to 10,000 times that of the sun should form and reside in dense stellar systems. therefore, dedicated observational campaigns have targeted globular clusters for many decades, searching for signatures of these elusive objects. all candidate signatures appear radio-dim and do not have the x-ray to radio flux ratios required for accreting black holes. based on the lack of an electromagnetic counterpart, upper limits of 2,060 and 470 solar masses have been placed on the mass of a putative black hole in 47 tucanae (ngc 104) from radio and x-ray observations, respectively. here we show there is evidence for a central black hole in 47 tucanae with a mass of 2,300-850+1,500 solar masses when the dynamical state of the globular cluster is probed with pulsars. the existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved. this intermediate-mass black hole might be a member of an electromagnetically invisible population of black holes that grow into supermassive black holes in galaxies. | an intermediate-mass black hole in the centre of the globular cluster 47 tucanae |
swift j0243.6+6124 is a newly discovered galactic be/x-ray binary, revealed in late 2017 september in a giant outburst with a peak luminosity of 2 × 1039(d/7 kpc)2 erg s-1 (0.1-10 kev), with no formerly reported activity. at this luminosity, swift j0243.6+6124 is the first known galactic ultraluminous x-ray pulsar. we describe neutron star interior composition explorer (nicer) and fermi gamma-ray burst monitor (gbm) timing and spectral analyses for this source. a new orbital ephemeris is obtained for the binary system using spin frequencies measured with gbm and 15-50 kev fluxes measured with the neil gehrels swift observatory burst alert telescope to model the system’s intrinsic spin-up. power spectra measured with nicer show considerable evolution with luminosity, including a quasi-periodic oscillation near 50 mhz that is omnipresent at low luminosity and has an evolving central frequency. pulse profiles measured over the combined 0.2-100 kev range show complex evolution that is both luminosity and energy dependent. near the critical luminosity of l ∼ 1038 erg s-1, the pulse profiles transition from single peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both nicer and gbm show a turnover to softening as the intensity increases. this behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. these two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. this is the highest observed (to date) value of the critical luminosity, suggesting a magnetic field of b ∼ 1013 g. | nicer and fermi gbm observations of the first galactic ultraluminous x-ray pulsar swift j0243.6+6124 |
we analyze plasma dispersion and scattering of fast radio bursts (frbs) to identify the dominant locations of free electrons along their lines of sight and thus constrain the distances of the burst sources themselves. we establish the average $\tau$-dm relation for galactic pulsars and use it as a benchmark for discussing frb scattering. though scattering times $\tau$ for frbs are large in the majority of the 17 events we analyze, they are systematically smaller than those of galactic pulsars that have similar dispersion measures (dms). the lack of any correlation between $\tau$ and dm for frbs suggests that the intergalactic medium (igm) cannot account for both $\tau$ and dm. we therefore consider mixed models involving the igm and host galaxies. if the igm contributes significantly to dm while host galaxies dominate $\tau$, the scattering deficit with respect to the mean galactic trend can be explained with a $\tau$-dm relation in the host that matches that for the milky way. however, it is possible that hosts dominate both $\tau$ and dm, in which case the observed scattering deficits require free electrons in the host to be less turbulent than in the galaxy, such as if they are in hot rather than warm ionized regions. our results imply that distances or redshifts of frb sources can be significantly overestimated if they are based on the assumption that the extragalactic portion of dm is dominated by the igm. | radio wave propagation and the provenance of fast radio bursts |
the discovery of extended tev emission around the geminga and psr b0656+14 pulsars, with properties consistent with free particle propagation in the interstellar medium (ism), has led to the suggestion of "tev halos" as a separate source class, which is distinct from pulsar wind nebulae. this has sparked considerable discussion on the possible presence of such halos in other systems. in defining halos as regions where the pulsar no longer dominates the dynamics of the interstellar medium, yet where an over-density of relativistic electrons is present, we make an assessment of the current tev source population associated with energetic pulsars in terms of size and estimated energy density. based on two alternative estimators, we conclude that a large majority of the known tev sources have emission originating in the zone that is energetically and dynamically dominated by the pulsar (i.e. the pulsar wind nebula), rather than from a surrounding halo of escaped particles diffusing into the ism. furthermore, whilst the number of established halos will surely increase in the future since there is a known large population of older, less energetic pulsars, we find that it is unlikely that such halos contribute significantly to the total tev γ-ray luminosity from electrons accelerated in pulsar wind nebulae due to their lower intrinsic surface brightness. | halo fraction in tev-bright pulsar wind nebulae |
pulsar timing array projects measure the pulse arrival times of millisecond pulsars for the primary purpose of detecting nanohertz-frequency gravitational waves. the measurements include contributions from a number of astrophysical and instrumental processes, which can either be deterministic or stochastic. it is necessary to develop robust statistical and physical models for these noise processes because incorrect models diminish sensitivity and may cause a spurious gravitational wave detection. here we characterize noise processes for the 26 pulsars in the second data release of the parkes pulsar timing array using bayesian inference. in addition to well-studied noise sources found previously in pulsar timing array data sets such as achromatic timing noise and dispersion measure variations, we identify new noise sources including time-correlated chromatic noise that we attribute to variations in pulse scattering. we also identify 'exponential dip' events in four pulsars, which we attribute to magnetospheric effects as evidenced by pulse profile shape changes observed for three of the pulsars. this includes an event in psr j1713+0747, which had previously been attributed to interstellar propagation. we present noise models to be used in searches for gravitational waves. we outline a robust methodology to evaluate the performance of noise models and identify unknown signals in the data. the detection of variations in pulse profiles highlights the need to develop efficient profile domain timing methods. | identifying and mitigating noise sources in precision pulsar timing data sets |
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