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planck data when combined with ancillary data provide a unique opportunity to separate the diffuse emission components of the inner galaxy. the purpose of the paper is to elucidate the morphology of the various emission components in the strong star-formation region lying inside the solar radius and to clarify the relationship between the various components. the region of the galactic plane covered is l = 300° → 0° → 60° wherestar-formation is highest and the emission is strong enough to make meaningful component separation. the latitude widths in this longitude range lie between 1° and 2°, which correspond to fwhm z-widths of 100-200 pc at a typical distance of 6 kpc. the four emission components studied here are synchrotron, free-free, anomalous microwave emission (ame), and thermal (vibrational) dust emission. these components are identified by constructing spectral energy distributions (seds) at positions along the galactic plane using the wide frequency coverage of planck (28.4-857 ghz) in combination with low-frequency radio data at 0.408-2.3 ghz plus wmap data at 23-94 ghz, along with far-infrared (fir) data from cobe-dirbe and iras. the free-free component is determined from radio recombination line (rrl) data. ame is found to be comparable in brightness to the free-free emission on the galactic plane in the frequency range 20-40 ghz with a width in latitude similar to that of the thermal dust; it comprises 45 ± 1% of the total 28.4 ghz emission in the longitude range l = 300° → 0° → 60°. the free-free component is the narrowest, reflecting the fact that it is produced by current star-formation as traced by the narrow distribution of ob stars. it is the dominant emission on the plane between 60 and 100 ghz. rrls from this ionized gas are used to assess its distance, leading to a free-free z-width of fwhm ≈ 100 pc. the narrow synchrotron component has a low-frequency brightness spectral index βsynch ≈ -2.7 that is similar to the broad synchrotron component indicating that they are both populated by the cosmic ray electrons of the same spectral index. the width of this narrow synchrotron component is significantly larger than that of the other three components, suggesting that it is generated in an assembly of older supernova remnants that have expanded to sizes of order 150 pc in 3 × 105 yr; pulsars of a similar age have a similar spread in latitude. the thermal dust is identified in the seds with average parameters of tdust = 20.4 ± 0.4 k, βfir = 1.94 ± 0.03 (> 353 ghz), and βmm = 1.67 ± 0.02 (< 353 ghz). the latitude distributions of gamma-rays, co, and the emission in high-frequency planck bands have similar widths, showing that they are all indicators of the total gaseous matter on the plane in the inner galaxy. appendices are available in electronic form at http://www.aanda.org | planck intermediate results. xxiii. galactic plane emission components derived from planck with ancillary data |
to have a more comprehensive understanding of the water ecological security status of the yellow river basin, this paper constructs a water ecological security evaluation index system founded on the pressure-state-response (psr) model. the indicators are selected by considering factors such as meteorological conditions, population, economy, water resources, water environment, water ecology, land ecology, ecological service functions, pollution control, and capital investment. then, the "single index quantification-multiple indices syntheses-poly-criteria integration (smi-p) method was used to determine the water ecological security index (wesi) of 62 cities in the yellow river basin, to classify the safety levels, and combined with the spatial autocorrelation analysis to study the regional characteristics. the results prove that: (a) the overall water ecological security of the yellow river basin is relatively poor. half of the 62 cities have reached the second-level warning level, and most of them are concentrated in the upper and middle reaches of the basin. (b) wetland area is a long-term key factor in the construction of water ecological safety, and the greening rate of built-up areas has an increasing impact on water ecological safety. (c) the overall water ecological security index shows a slow upward trend, with the annual average growth rate was 0.59%. (d) the water ecological security of 62 cities in the yellow river basin shows significant spatial autocorrelation. the findings can offer a practical basis for the water ecological management to promote the high-quality development of the yellow river basin. | water ecological security assessment and spatial autocorrelation analysis of prefectural regions involved in the yellow river basin |
superluminous supernovae (slsne) are massive star explosions that are too luminous to be powered by traditional energy sources, such as the radioactive decay of 56ni. instead, they may be powered by a central engine, such as a millisecond pulsar or magnetar, whose relativistic wind inflates a nebula of high-energy particles and radiation behind the expanding supernova ejecta. we present three-dimensional monte carlo radiative transfer calculations of slsne that follow the production of high-energy emission in the nebula and its subsequent thermalization into optical radiation within the surrounding ejecta and, conversely, determine the gamma-ray emission that escapes the ejecta without thermalizing. we identify a novel mechanism by which γγ pair creation in the upstream pulsar wind regulates the mean energy of particles entering the nebula over the first several years after the explosion, rendering our results on this timescale insensitive to the (uncertain) intrinsic wind pair multiplicity. to explain the observed late-time steepening of slsn optical light curves as being the result of gamma-ray leakage, we find that the nebular magnetization must be very low, ${\varepsilon }_{{\rm{b}}}\lesssim {10}^{-6}\mbox{--}{10}^{-4}$ . for higher ${\varepsilon }_{{\rm{b}}}$ , the more efficiently thermalized lower-energy synchrotron emission would overproduce the late-time (≳1 yr) optical radiation, inconsistent with observations. for magnetars to remain as viable contenders for powering slsne, we conclude that either magnetic dissipation in the wind/nebula is extremely efficient or the spin-down luminosity decays significantly faster than the canonical dipole rate ∝t-2 in a way that coincidentally mimics gamma-ray escape. | gamma-ray thermalization and leakage from millisecond magnetar nebulae: toward a self-consistent model for superluminous supernovae |
the galactic center gev excess (gce) has garnered great interest as a possible signal of either dark matter annihilation or some novel astrophysical phenomenon, such as a new population of gamma-ray emitting pulsars. in a companion paper, we showed that in a 10° radius region of interest (roi) surrounding the galactic center, apparent evidence for gce point sources (pss) from non-poissonian template fitting (nptf) is actually an artifact of unmodeled north-south asymmetry of the gce. in this work, we develop a simplified analytic description of how signal mismodeling can drive an apparent preference for a ps population, and demonstrate how the behavior pointed out in the companion paper also appears in simpler simulated datasets that contain no ps signals at all. we explore the generality of this behavior in the real gamma-ray data, and discuss the implications for past and future studies using nptf techniques. while the drop in ps preference once north-south asymmetry is included is not ubiquitous in larger rois, we show that any overly-rigid signal model is expected to yield a spurious ps signal that can appear very convincing: as well as apparent significance comparable to what one would expect from a true ps population, the signal can exhibit stability against a range of variations in the analysis, and a source count function that is very consistent with previous apparent nptf-based detections of a gce ps population. this contrasts with previously studied forms of systematic mismodeling which are unlikely to mimic a ps population in the same way. in the light of this observation, and its explicit realization in the region where the gce is brightest, we argue that a dominantly smooth origin for the gce is not in tension with existing nptf analyses. | the enigmatic galactic center excess: spurious point sources and signal mismodeling |
in this work, large eddy simulations (les) of a gas turbine model combustor (gtmc) are done using a five-step global mechanism that includes separate thermal and non-thermal nox formation parts. to verify the accuracy of the solution, time-averaged profiles of the flow variables and fluctuations are compared to the available experimental and numerical data. the les results show that the vortical structures inside the chamber are highly connected to the temperature field and chemical reactions, and despite having a major role in fast premixing and consequent nox reductions, they contribute to nox generation by forming high temperature spots inclusive of chemical radicals. also, the importance of the baroclinic torque in vorticity creation is demonstrated by comparing the corresponding values to vortex stretching in upstream parts of the chamber. it is shown that the baroclinic torque mostly takes action between high vorticity and high strain regions and can possibly intensify the strong vortices, while the vortex stretching is mostly active near the strong vortices. furthermore, observation of detailed statistics shows that most of the heat release occurs in samples with mixture fractions near the global value, while no generation is highly biased toward the strong vortices and the stoichiometric mixture fraction. to investigate the role of the radicals in more details, a chemical reactor network (crn) is created by clustering the les solution. also, the integration of partially stirred reactors (pasrs) with perfectly stirred reactor (psr) networks is used to improve the accuracy of predicting the reactant jet penetration and ignition radicals. | an investigation on flame structure and nox formation in a gas turbine model combustor using large eddy simulation |
radio pulsars show remarkable clock-like stability, which make them useful astronomy tools in experiments to test equation of state of neutron stars and detecting gravitational waves using pulsar timing techniques. a brief review of relevant astrophysical experiments is provided in this paper highlighting the current state-of-the-art of these experiments. a program to monitor frequently glitching pulsars with indian radio telescopes using high cadence observations is presented, with illustrations of glitches detected in this program, including the largest ever glitch in psr b0531+21. an indian initiative to discover sub-μ hz gravitational waves, called indian pulsar timing array (inpta), is also described briefly, where time-of-arrival uncertainties and post-fit residuals of the order of μ s are already achievable, comparable to other international pulsar timing array experiments. while timing the glitches and their recoveries are likely to provide constraints on the structure of neutron stars, inpta will provide upper limits on sub-μ hz gravitational waves apart from auxiliary pulsar science. future directions for these experiments are outlined. | precision pulsar timing with the ort and the gmrt and its applications in pulsar astrophysics |
we introduce a new method for evaluating the oscillatory integrals which describe natural interference patterns. as an illustrative example of contemporary interest, we consider astrophysical plasma lensing of coherent sources like pulsars and fast radio bursts in radio astronomy. plasma lenses are known to occur near the source, in the interstellar medium, as well as in the solar wind and the earth's ionosphere. such lensing is strongest at long wavelengths, hence it is generally important to go beyond geometric optics and into the full wave optics regime. our computational method is a spinoff of new techniques two of us, and our collaborators, have developed for defining and performing lorentzian path integrals with applications in quantum mechanics, condensed matter physics, and quantum gravity. cauchy's theorem allows one to transform a computationally fragile and expensive, highly oscillatory integral into an exactly equivalent sum of absolutely and rapidly convergent integrals which can be evaluated in polynomial time. we require only that it is possible to analytically continue the lensing phase, expressed in the integrated coordinates, into the complex domain. we give a first-principles derivation of the fresnel-kirchhoff integral, starting from feynman's path integral for a massless particle in a refractive medium. we then demonstrate the effectiveness of our method by computing the detailed diffraction patterns of thom's caustic catastrophes, both in their "normal forms" and within a variety of more realistic, local lens models, for all wavelengths. our numerical method, implemented in a freely downloadable code, provides a fast, accurate tool for modeling interference patterns in radioastronomy and other fields of physics. | oscillatory path integrals for radio astronomy |
we calculate the sensitivity to a circular polarization of an isotropic stochastic gravitational wave background (isgwb) as a function of frequency for ground- and space-based interferometers and observations of the cosmic microwave background. the origin of a circularly polarized isgwb may be due to exotic primordial physics (i.e., parity violation in the early universe) and may be strongly frequency dependent. we present calculations within a coherent framework which clarifies the basic requirements for sensitivity to circular polarization, in distinction from previous work which focused on each of these techniques separately. we find that the addition of an interferometer with the sensitivity of the einstein telescope in the southern hemisphere improves the sensitivity of the ground-based network to circular polarization by about a factor of two. the sensitivity curves presented in this paper make clear that the wide range in frequencies of current and planned observations (10-18 hz ≲f ≲100 hz ) will be critical to determining the physics that underlies any positive detection of circular polarization in the isgwb. we also identify a desert in circular polarization sensitivity for frequencies between 10-15 hz ≲f ≲10-3 hz , given the inability for pulsar timing arrays and indirect-detection methods to distinguish the gravitational wave polarization. | sensitivity to a frequency-dependent circular polarization in an isotropic stochastic gravitational wave background |
with the doors beginning to swing open on the new gravitational wave astronomy, this review provides an up-to-date survey of the most important physical mechanisms that could lead to emission of potentially detectable gravitational radiation from isolated and accreting neutron stars. in particular we discuss the gravitational wave-driven instability and asteroseismology formalism of the f- and r-modes, the different ways that a neutron star could form and sustain a non-axisymmetric quadrupolar "mountain" deformation, the excitation of oscillations during magnetar flares and the possible gravitational wave signature of pulsar glitches. we focus on progress made in the recent years in each topic, make a fresh assessment of the gravitational wave detectability of each mechanism and, finally, highlight key problems and desiderata for future work. | gravitational waves from single neutron stars: an advanced detector era survey |
we explore the burst energy distribution of fast radio bursts (frbs) in the low-twist magnetar model of wadiasingh & timokhin (wt19). motivated by the power-law fluence distributions of frb 121102, we propose an elementary model for the frb luminosity function of individual repeaters with an inversion protocol that directly relates the power-law distribution index of magnetar short burst fluences to that for frbs. the protocol indicates that the frb energy scales virtually linearly with crust/field dislocation amplitude, if magnetar short bursts prevail in the magnetoelastic regime. charge starvation in the magnetosphere during bursts (required in wt19) for individual repeaters implies the predicted burst fluence distribution is narrow, &lsim;3 decades for yielding strains and oscillation frequencies feasible in magnetar crusts. requiring magnetic confinement and charge starvation, we obtain a death line for frbs, which segregates magnetars from the normal pulsar population, suggesting only the former will host recurrent frbs. we convolve the burst energy distribution for individual magnetars to define the distribution of luminosities in evolved magnetar populations. the broken power-law luminosity function's low-energy character depends on the population model, while the high-energy index traces that of individual repeaters. independent of the evolved population, the broken power-law isotropic-equivalent energy/luminosity function peaks at ∼1037-1040 erg with a low-energy cutoff at ∼1037 erg. lastly, we consider the local fluence distribution of frbs and find that it can constrain the subset of frb-producing magnetar progenitors. our model suggests that improvements in sensitivity may reveal a flattening of the global frb fluence distribution and saturation in frb rates. | the fast radio burst luminosity function and death line in the low-twist magnetar model |
an association of gw190425 and frb 20190425a had been claimed recently. given the $\sim 2.5$ hour delay of the occurrence of frb 20190425a, a uniformly rotating supramassive magnetar remnant is favored. the required maximum gravitational mass of the nonrotating neutron star (ns) is $m_{\rm tov}\approx 2.77m_\odot$, which is strongly in tension with the low $m_{\rm tov}\approx 2.25m_\odot$ obtained in current equation of state (eos) constraints incorporating perturbative quantum chromodynamics (pqcd) information. however, the current mass-radius and mass-tidal deformability measurements of nss alone do not convincingly exclude the high $m_{\rm tov}$ possibility. by performing eos constraints with mock measurements, we find that with a $2\%$ determination for the radius of psr j0740+6620-like ns it is possible to distinguish between the low and high $m_{\rm tov}$ scenarios. we further explore the prospect to resolve the issue of the appropriate density to impose the pqcd constraints with future massive ns observations or determinations of $m_{\rm tov}$ and/or $r_{\rm tov}$. it turns out that measuring the radius of a psr j0740+6620-like ns is insufficient to probe the eoss around 5 nuclear saturation density, where the information from pqcd becomes relevant. the additional precise $m_{\rm tov}$ measurements, anyhow, could help. indeed, supposing the central engine of grb 170817a is a black hole formed via the collapse of a supramassive ns, the resulting $m_{\rm tov}\approx 2.2m_\odot$ considerably softens the eos at the center of the most massive ns, which is in favor of imposing the pqcd constraint at density beyond the one achievable in the nss. | mass and radius of the most massive neutron star: the probe of the equation of state and perturbative qcd |
the fermi large area telescope gamma-ray source 3fgl j2039.6-5618 contains a periodic optical and x-ray source that was predicted to be a 'redback' millisecond pulsar (msp) binary system. however, the conclusive identification required the detection of pulsations from the putative msp. to better constrain the orbital parameters for a directed search for gamma-ray pulsations, we obtained new optical light curves in 2017 and 2018, which revealed long-term variability from the companion star. the resulting orbital parameter constraints were used to perform a targeted gamma-ray pulsation search using the einstein@home-distributed volunteer computing system. this search discovered pulsations with a period of 2.65 ms, confirming the source as a binary msp now known as psr j2039-5617. optical light-curve modelling is complicated, and likely biased, by asymmetric heating on the companion star and long-term variability, but we find an inclination i ≳ 60°, for a low pulsar mass between $1.1\, \mathrm{m}_{\odot } \lt m_{\rm psr} \lt $ 1.6 m⊙, and a companion mass of 0.15- $0.22\, \mathrm{m}_{\odot }$ , confirming the redback classification. timing the gamma-ray pulsations also revealed significant variability in the orbital period, which we find to be consistent with quadrupole moment variations in the companion star, suggestive of convective activity. we also find that the pulsed flux is modulated at the orbital period, potentially due to inverse compton scattering between high-energy leptons in the pulsar wind and the companion star's optical photon field. | einstein@home discovery of the gamma-ray millisecond pulsar psr j2039-5617 confirms its predicted redback nature |
lorentz invariance is one of the fundamental tenets of special relativity, and has been extensively tested with laboratory and astrophysical observations. however, many quantum gravity models and theories beyond the standard model of particle physics predict a violation of lorentz invariance at energies close to the planck scale. this article reviews observational and experimental tests of lorentz invariance violation (liv) with photons, neutrinos and gravitational waves. most astrophysical tests of liv using photons are based on searching for a correlation of the spectral lag data with redshift and energy. these have been primarily carried out using compact objects such as pulsars, active galactic nuclei (agn), and gamma-ray bursts (grb). there have also been some claims for liv from some of these spectral lag observations with grbs, which however are in conflict with the most stringent limits obtained from other liv searches. searches have also been carried out using polarization measurements from grbs and agns. for neutrinos, tests have been made using both astrophysical observations at mev energies (from sn 1987a) as well as in the tev-pev energy range based on icecube observations, atmospheric neutrinos, and long-baseline neutrino oscillation experiments. cosmological tests of liv entail looking for a constancy of the speed of light as a function of redshift using multiple observational probes, as well as looking for birefringence in cosmic microwave background observations. this article will review all of these aforementioned observational tests of liv, including results which are in conflict with each other. | astrophysical and cosmological searches for lorentz invariance violation |
tests of gravity are important to the development of our understanding of gravitation and spacetime. binary pulsars provide a superb playground for testing gravity theories. in this chapter we pedagogically review the basics behind pulsar observations and pulsar timing. we illustrate various recent strong-field tests of the general relativity (gr) from the hulse-taylor pulsar psr b1913+16, the double pulsar psr j0737$-$3039, and the triple pulsar psr j0337+1715. we also overview the inner structure of neutron stars (nss) that may influence some gravity tests, and have used the scalar-tensor gravity and massive gravity theories as examples to demonstrate the usefulness of pulsar timing in constraining specific modified gravity theories. outlooks to new radio telescopes for pulsar timing and synergies with other strong-field gravity tests are also presented. | tests of classical gravity with radio pulsars |
the high-energy diffuse gamma-ray emission and neutrino emission are expected from the galactic plane, generated by hadronuclear interactions between cosmic rays (cr) and interstellar medium (ism). therefore, measurements of these diffuse emissions will provide important clues on the origin and nature of galactic crs. comparing the latest observations of lhaaso and icecube on the diffuse galactic gamma-ray and neutrino emissions respectively, we suggest that the diffuse gamma-ray emission at multi-tev energies contains a considerable contribution of a leptonic component. by modelling the gamma-ray halos powered by middle-aged pulsars in our galaxy with taking into account the magnetic field configuration and the interstellar radiation field in the galaxy, we demonstrate that the collective contribution of pulsar halos can account for the excess in the measured diffuse gamma-ray emission with respect to the predicted flux from cr-ism interactions. the resulting one-dimensional profile along the galactic longitude is also consistent with the observation. | on the origin of galactic diffuse tev-pev emission: insight from lhaaso and icecube |
we study the motion of electrically charged particles, magnetic monopoles, and magnetic dipoles around electrically and magnetically charged stringy black holes. from the analysis of the radius of the innermost stable circular orbit of electrically charged particles, we show that the electric charge q of stringy black holes can mimic well the spin of kerr black holes; the black hole magnetic charge qm can mimic spins up to a*≃0.85 for magnetic dipoles; the magnetic charge parameter g of a magnetic monopole can mimic spins up to a*≃0.8 . this is due to the destructive character of the magnetic field, and such a result excludes the existence of an appreciable black hole magnetic charge in astrophysical black holes from the observation of rapidly rotating objects with dimensionless spin up to a*≃0.99 . we then consider the magnetar sgr (psr) j1745-2900 as a magnetic dipole orbiting the supermassive black hole sagittarius a* (sgr a*). we show that sgr a* may be interpreted as a stringy black hole with magnetic charge qm/m ≤0.4118 . | can the dynamics of test particles around charged stringy black holes mimic the spin of kerr black holes? |
we show that some or all of the inventory of r -process nucleosynthesis can be produced in interactions of primordial black holes (pbhs) with neutron stars (nss) if pbhs with masses 10-14 m⊙<mpbh<10-8 m⊙ make up a few percent or more of dark matter. a pbh captured by a ns sinks to the center of the ns and consumes it from the inside. when this occurs in a rotating millisecond-period ns, the resulting spin-up ejects ∼0.1 m⊙- 0.5 m⊙ of relatively cold neutron-rich material. this ejection process and the accompanying decompression and decay of nuclear matter can produce electromagnetic transients, such as a kilonova-type afterglow and fast radio bursts. these transients are not accompanied by significant gravitational radiation or neutrinos, allowing such events to be differentiated from compact object mergers occurring within the distance sensitivity limits of gravitational-wave observatories. the pbh-ns destruction scenario is consistent with pulsar and ns statistics, the dark-matter content, and spatial distributions in the galaxy and ultrafaint dwarfs, as well as with the r -process content and evolution histories in these sites. ejected matter is heated by beta decay, which leads to emission of positrons in an amount consistent with the observed 511-kev line from the galactic center. | primordial black holes and r -process nucleosynthesis |
we present the results of a bayesian search for gravitational wave (gw) memory in the nanograv 12.5-yr data set. we find no convincing evidence for any gravitational wave memory signals in this data set (bayes factor = 2.8). as such, we go on to place upper limits on the strain amplitude of gw memory events as a function of sky location and event epoch. these upper limits are computed using a signal model that assumes the existence of a common, spatially uncorrelated red noise in addition to a gw memory signal. the median strain upper limit as a function of sky position is approximately $3.3 \times 10^{-14}$. we also find that there are some differences in the upper limits as a function of sky position centered around psr j0613$-$0200. this suggests that this pulsar has some excess noise which can be confounded with gw memory. finally, the upper limits as a function of burst epoch continue to improve at later epochs. this improvement is attributable to the continued growth of the pulsar timing array. | the nanograv 12.5-year data set: search for gravitational wave memory |
the neutron star interior composition explorer observed several rotation-powered millisecond pulsars (msps) to search for or confirm the presence of x-ray pulsations. when broad and sine-like, these pulsations may indicate thermal emission from hot polar caps at the magnetic poles on the neutron star surface. we report confident detections (≥4.7σ after background filtering) of x-ray pulsations for five of the seven pulsars in our target sample: psr j0614-3329, psr j0636+5129, psr j0751+1807, psr j1012+5307, and psr j2241-5236, while psr j1552+5437 and psr j1744-1134 remain undetected. of those, only psr j0751+1807 and psr j1012+5307 had pulsations previously detected at the 1.7σ and almost 3σ confidence levels, respectively, in xmm-newton data. all detected sources exhibit broad sine-like pulses, which are indicative of surface thermal radiation. as such, these msps are promising targets for future x-ray observations aimed at constraining the neutron star mass-radius relation and the dense matter equation of state using detailed pulse profile modeling. furthermore, we find that three of the detected msps exhibit a significant phase offset between their x-ray and radio pulses. | nicer x-ray observations of seven nearby rotation-powered millisecond pulsars |
the hypothesis that pulsar wind nebulae (pwne) can significantly contribute to the excess of the positron (e+) cosmic-ray flux has been consolidated after the observation of a γ -ray emission at tev energies of a few degree size around geminga and monogem pwne, and at gev energies for geminga at a much larger extension. the γ -ray halos around these pwne are interpreted as due to electrons (e-) and e+ accelerated and escaped by their pwne, and inverse compton scattering low-energy photons of the interstellar radiation fields. the extension of these halos suggests that the diffusion around these pwne is suppressed by 2 orders of magnitude with respect to the average in the galaxy. we implement a two-zone diffusion model for the propagation of e+ accelerated by the galactic population of pwne. we consider pulsars from source catalogs and build up simulations of the pwn galactic population. in both scenarios, we find that within a two-zone diffusion model, the total contribution from pwne and secondary e+ is at the level of ams-02 data, for an efficiency of conversion of the pulsar spin-down energy in e± of η ∼0.1 . for the simulated pwne, a 1 σ uncertainty band is determined, which is of at least 1 order of magnitude from 10 gev up to few tev. the hint for a decreasing e+ flux at tev energies is found, even if it is strongly connected to the chosen value of the radius of the low diffusion bubble around each source. | contribution of pulsars to cosmic-ray positrons in light of recent observation of inverse-compton halos |
we consider critically the three most widely favoured pulsar radio emission mechanisms: coherent curvature emission (cce), beam-driven relativistic plasma emission (rpe), and anomalous doppler emission (ade). we assume that the pulsar plasma is 1d, streaming outwards with a bulk lorentz factor γs ≫ <γ> - 1 ≳ 1, where <γ> is the intrinsic spread in the rest frame of the plasma. we argue that the formation of beams in a multicloud model is ineffective in the intrinsically relativistic case for plausible parameters because the overtaking takes too long. we argue that the default choice for the particle distribution in the rest frame is a jüttner distribution and that relativistic streaming should be included by applying a lorentz transformation to the rest-frame distribution, rather than the widely assumed relativistically streaming gaussian distribution. we find that beam-driven wave growth is severely restricted by (a) the wave properties in pulsar plasma, (b) a separation condition between beam and background, and (c) the inhomogeneity of the plasma in the pulsar frame. the growth rate for the kinetic instability is much smaller and the bandwidth of the growing waves is much larger for a jüttner distribution than for a relativistically streaming gaussian distribution. no reactive instability occurs at all for a jüttner distribution. we conclude that none of cce, rpe, and ade is tenable as the generic pulsar radio emission mechanism for 'plausible' assumptions about the pulsar plasma. | pulsar radio emission mechanisms: a critique |
pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. however, it was pointed out by the parkes pulsar timing array (ppta) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. in this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. the former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. we test the methodology on simulated data sets. we find that the reported common process in ppta pulsars is indeed consistent with the spectral feature of a pulsar term. we recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background. | consistency of the parkes pulsar timing array signal with a nanohertz gravitational-wave background |
the damping effect of the free-streaming neutrinos on the second order gravitational waves is investigated in detail. we solve the boltzmann equation and give the anisotropic stress induced by neutrinos to second order. the first order tensor and its coupling with scalar perturbations induced gravitational waves are considered. we give the analytic equations of the damping kernel functions and finally obtain the energy density spectrum. the results show that the free-streaming neutrinos suppress the density spectrum significantly for low frequency gravitational waves and enlarge the logarithmic slope n in the infrared region (k ≪k∗ ) of the spectrum. for the spectrum of k∗∼10-7hz , the damping effect in the range of k <k∗ is significant. the combined effect of the first and second order could reduce the amplitude by 30 % and make n jump from 1.54 to 1.63 at k ∼10-9hz , which may be probed by the pulsar timing arrays (pta) in the future. | impact of the free-streaming neutrinos to the second order induced gravitational waves |
strong evidence of the existence of the stochastic gravitational wave background (sgwb) has been reported recently by the north american nanohertz observatory for gravitational waves, the parkes pulsar timing array, the european pulsar timing array, and the chinese pulsar timing array collaborations. the bayesian posteriors of the sgwb amplitude and spectrum are compatible with astrophysical predictions for the sgwb from a population of supermassive black hole binaries (smbhbs). we discuss the modifications arising from the extra scalar or vector radiation to the characteristic dimensionless strain of gravitational waves from smbhbs and explore the possibility of detecting charges carried by supermassive black holes in modified gravity. the bayesian posteriors on the tensor amplitude and the spectral exponent of sgwb are log10at=-14.6 4-0.27+0.19 and α =-0.3 5-0.25+0.23, and a rough upper limit log10av ,s≤-15.2 is obtained for the sgwb scalar or vector amplitude. the bayesian factor between the models with and without charge is only 0.6, so the current pta data do not favor charged smbhbs over neutral smbhbs. | detecting new fundamental fields with pulsar timing arrays |
after nanograv, the ipta collaboration also reports a strong evidence of a stochastic gravitation wave background. this hint has very important implications for fundamental physics. with the recent ipta data release two, we attempt to search signals of light new physics. and give new constraints on the audible axion, domain walls and cosmic strings models. we find that the best fit point corresponding to a decay constant $f\approx5\times10^{17}$ gev and an axion mass $m_a\approx2\times10^{-13}$ ev from nanograv data is ruled out by ipta at beyond $2\sigma$ confidence level. fixing the coupling strength $\lambda=1$, we obtain a $2\sigma$ lower bound on the breaking scale of $z_2$ symmetry $\eta>135$ tev. interestingly, we give a very strong restriction on the cosmic-string tension $\mathrm{log}_{10}\,g\mu=-8.93_{-0.06}^{+0.12}$ at $1\sigma$ confidence level. employing the rule of bayes factor, we find that ipta data has a moderate, strong and inconclusive preference of an uncorrelated common power-law (cpl) model over audible axion, domain walls and cosmic strings, respectively. this means that it is hard to distinguish cpl from cosmic strings with current observations and more pulsar timing data with high precision are required to give new clues of underlying physics. | novel physics with international pulsar timing array: axionlike particles, domain walls and cosmic strings |
we study the early universe evolution of axion-like particle (alp) domain walls taking into account the effect of friction from particles in the surrounding plasma, including the case of particles in thermal equilibrium and frozen out species. we characterize the friction force from interactions within the alp effective theory, providing new results for the fermion contribution as well as identifying simple conditions for friction to be relevant during the domain wall life time. when friction dominates, the domain wall network departs from the standard scaling regime and the corresponding gravitational wave emission is affected. as a relevant example, we show how this can be the case for alp domain walls emitting at the typical frequencies of pulsar timing array experiments, when the alp couples to the sm leptons. we then move to a general exploration of the gravitational wave prospects in the alp parameter space. we finally illustrate how the gravitational wave signal from alp domain walls is correlated with the quality of the underlying u(1) symmetry. | friction on alp domain walls and gravitational waves |
it was recently found that, in certain flavors of scalar-gauss-bonnet gravity, linearly stable bald black holes can coexist with stable scalarized solutions. the transition between both can be ignited by a large nonlinear perturbation, thus the process was dubbed nonlinear scalarization, and it happens with a jump that leads to interesting astrophysical implications. generalizing these results to the case of nonzero scalar field potential is important because a massive self-interacting scalar field can have interesting theoretical and observational consequences, e.g., reconcile scalar-gauss-bonnet gravity with binary pulsar observation, stabilize black hole solutions, etc. that is why, in the present paper, we address this open problem. we pay special attention to the influence of a scalar field mass and self-interaction on the existence of scalarized phases and the presence of a jump between stable bald and hairy black holes. our results show that both the addition of a mass and positive self-interaction of the scalar field result in suppression or quenching of the overall scalarization phenomena. a negative scalar field self-interaction results in an increase of the scalarization. the presence and the size of the jump, though, are not so sensitive to the scalar field potential. | effects of mass and self-interaction on nonlinear scalarization of scalar-gauss-bonnet black holes |
in the frame of gauss–bonnet gravity and in the limit of d→4, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory, we derive a new interior spherically symmetric solution assuming specific forms of the metric potentials that have two constants. using the junction condition we determine these two constants. by using the data of the star exo 1785-248, whose mass is m=1.3±0.2m⊙ and radius l=8.849±0.4 km, we calculate the numerical values of these constants, in terms of the dimensionful coupling parameter of the gauss–bonnet term, and eventually, we get real values for these constants. in this regard, we show that the components of the energy–momentum tensor have a finite value at the center of the star as well as a smaller value to the surface of the star. moreover, we show that the equations of the state behave in a non-linear way due to the impact of the gauss–bonnet term. using the tolman–oppenheimer–volkoff equation, the adiabatic index, and stability in the static state we show that the model under consideration is always stable. finally, the solution of this study is matched with observational data of other pulsars showing satisfactory results. | anisotropic compact stars in d → 4 limit of gauss–bonnet gravity |
following the idea that a stable sexaquark state with quark content (u u d d s s ) would have gone unnoticed by experiments so far and that such a particle would be a good dark matter candidate, we investigate the possible role of a stable sexaquark in the physics of compact stars given the stringent constraints on the equation of state that stem from observations of high mass pulsars and gw170817 bounds on the compactness of intermediate mass stars. we find that there is a "sexaquark dilemma" (analogous to the hyperon dilemma) for which the dissociation of the sexaquark in quark matter is a viable solution fulfilling all present constraints from multimessenger astronomy. the parameters needed to model the hybrid star, including sexaquarks, are in line with parameters of preexisting quark- and hadronic-matter models. we find that current constraints—tidal deformability in accordance with gw170817 and maximum mass above the lower limit from psr j 0740 +6620 —can be satisfied two ways: with early quark deconfinement such that neither sexaquarks nor hyperons are present in any neutron star interiors, or with later deconfinement such that a neutron-sexaquark shell surrounds the inner quark matter core. | sexaquark dilemma in neutron stars and its solution by quark deconfinement |
<p id="par1">millisecond pulsars in tight binaries have recently challenged our understanding of physical processes governing the evolution of binaries and the interaction between astrophysical plasma and electromagnetic fields. transitional systems that showed changes from rotation-powered to accretion-powered states and vice versa have bridged the populations of radio and accreting millisecond pulsars, eventually demonstrating the tight evolutionary link envisaged by the recycling scenario. a decade of discoveries and theoretical efforts have just grasped the complex phenomenology of transitional millisecond pulsars from the radio to the gamma-ray bands. this chapter summarizes the main properties of the three transitional millisecond pulsars discovered so far, as well as of candidates and related systems, discussing the various models proposed to cope with their multifaceted behaviour. | transitional millisecond pulsars |
context. the psr j2222−0137 binary system has a set of features that make it a unique laboratory for tests of gravity theories.aims: to fully exploit the system's potential for these tests, we aim to improve the measurements of its physical parameters, spin and orbital orientation, and post-keplerian parameters, which quantify the observed relativistic effects.methods: we describe an improved analysis of archival very long baseline interferometry (vlbi) data, which uses a coordinate convention in full agreement with that used in timing. we have also obtained much improved polarimetry of the pulsar with the five hundred meter aperture spherical telescope (fast). we provide an improved analysis of significantly extended timing datasets taken with the effelsberg, nançay, and lovell radio telescopes; this also includes previous timing data from the green bank telescope.results: from the vlbi analysis, we have obtained a new estimate of the position angle of the ascending node, ω = 189−18+19 deg (all uncertainties are 68% confidence limits), and a new reference position for the pulsar with an improved and more conservative uncertainty estimate. the fast polarimetric results, and in particular the detection of an interpulse, yield much improved estimates for the spin geometry of the pulsar, in particular an inclination of the spin axis of the pulsar of ∼84 deg. from the timing, we obtain a new ∼1% test of general relativity (gr) from the agreement of the shapiro delay parameters and the rate of advance of periastron. assuming gr in a self-consistent analysis of all effects, we obtain much improved masses: 1.831(10) m⊙ for the pulsar and 1.319(4) m⊙ for the white dwarf companion; the total mass, 3.150(14) m⊙, confirms this as the most massive double degenerate binary known in the galaxy. this analysis also yields the orbital orientation; in particular, the orbital inclination is 85.27(4) deg - indicating a close alignment between the spin of the pulsar and the orbital angular momentum - and ω = 187.7(5.7) deg, which matches our new vlbi estimate. finally, the timing also yields a precise measurement of the variation in the orbital period, ṗb = 0.251(8) × 10−12 ss−1; this is consistent with the expected variation in the doppler factor plus the orbital decay caused by the emission of gravitational waves predicted by gr. this agreement introduces stringent constraints on the emission of dipolar gravitational waves. | psr j2222−0137. i. improved physical parameters for the system |
we demonstrate that next-generation cosmic microwave background spectral distortion and pulsar timing array experiments can either detect signatures of primordial black holes in the mass range o (0.1 - 1012) m⊙ , which are sourced from primordial overdensities within the standard thermal history of the universe, or constrain their existence to a negligible abundance, ruling them out as super-massive black hole seeds. our conclusions are robust to changes in (i) the statistical properties of primordial density fluctuations, (ii) primordial black hole merger and accretion history and (iii) clustering statistics. | multimessenger probes of inflationary fluctuations and primordial black holes |
as the era of gravitational-wave astronomy has well and truly begun, gravitational radiation from rotating neutron stars remains elusive. rapidly spinning neutron stars are the main targets for continuous-wave searches since, according to general relativity, provided they are asymmetrically deformed, they will emit gravitational waves. it is believed that detecting such radiation will unlock the answer to why no pulsars have been observed to spin close to the break-up frequency. we review existing studies on the maximum mountain that a neutron star crust can support, critique the key assumptions and identify issues relating to boundary conditions that need to be resolved. in light of this discussion, we present a new scheme for modelling neutron star mountains. the crucial ingredient for this scheme is a description of the fiducial force which takes the star away from sphericity. we consider three examples: a source potential which is a solution to laplace's equation, another solution which does not act in the core of the star and a thermal pressure perturbation. for all the cases, we find that the largest quadrupoles are between a factor of a few to two orders of magnitude below previous estimates of the maximum-mountain size. | modelling neutron star mountains |
the radio galaxy 3c 66b has been hypothesized to host a supermassive black hole binary (smbhb) at its center based on electromagnetic observations. its apparent 1.05-year period and low redshift ($\sim0.02$) make it an interesting testbed to search for low-frequency gravitational waves (gws) using pulsar timing array (pta) experiments. this source has been subjected to multiple searches for continuous gws from a circular smbhb, resulting in progressively more stringent constraints on its gw amplitude and chirp mass. in this paper, we develop a pipeline for performing bayesian targeted searches for eccentric smbhbs in pta data sets, and test its efficacy by applying it on simulated data sets with varying injected signal strengths. we also search for a realistic eccentric smbhb source in 3c 66b using the nanograv 12.5-year data set employing pta signal models containing earth term-only as well as earth+pulsar term contributions using this pipeline. due to limitations in our pta signal model, we get meaningful results only when the initial eccentricity $e_0<0.5$ and the symmetric mass ratio $\eta>0.1$. we find no evidence for an eccentric smbhb signal in our data, and therefore place 95% upper limits on the pta signal amplitude of $88.1\pm3.7$ ns for the earth term-only and $81.74\pm0.86$ ns for the earth+pulsar term searches for $e_0<0.5$ and $\eta>0.1$. similar 95% upper limits on the chirp mass are $(1.98 \pm 0.05) \times 10^9\,m_{\odot}$ and $(1.81 \pm 0.01) \times 10^9\,m_{\odot}$. these upper limits, while less stringent than those calculated from a circular binary search in the nanograv 12.5-year data set, are consistent with the smbhb model of 3c 66b developed from electromagnetic observations. | the nanograv 12.5-year data set: a computationally efficient eccentric binary search pipeline and constraints on an eccentric supermassive binary candidate in 3c 66b |
the recent data from nanograv provide strong evidence of the existence of the \acp{sgwb}. we investigate \acp{sigw} from finslerian inflation as a potential source of stochastic gravitational wave background. small-scale ($\lesssim$1 mpc) statistically anisotropic primordial scalar perturbations can be generated in finslerian inflation. the second order \acp{sigw} from finslerian inflation are also anisotropic on small scales. after spatially averaging the small-scale anisotropic \acp{sigw}, we obtain the large-scale isotropic \acp{sgwb}. we find that the parameters of small-scale anisotropic primordial power spectrum generated by finslerian inflation affect the \acp{pta} observations of large-scale isotropic gravitational wave background. | scalar induced gravitational waves from finslerian inflation and pulsar timing arrays observations |
white dwarf stars are the most common stellar fossils. when in binaries, they make up the dominant form of compact object binary within the galaxy and can offer insight into different aspects of binary formation and evolution. one of the most remarkable white dwarf binary systems identified to date is ar scorpii (ar sco). ar sco is composed of an m dwarf star and a rapidly spinning white dwarf in a 3.56 h orbit. it shows pulsed emission with a period of 1.97 min over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. both the pulse mechanism and the evolutionary origin of ar sco provide challenges to theoretical models. here we report the discovery of a sibling of ar sco, j191213.72-441045.1, which harbours a white dwarf in a 4.03 h orbit with an m dwarf and exhibits pulsed emission with a period of 5.30 min. this discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars. | a 5.3-min-period pulsing white dwarf in a binary detected from radio to x-rays |
coupling of axions or axion-like particles (alps) with photons may lead to photons escaping optically opaque regions by oscillating into alps. this phenomenon may be viewed as the light shining through wall (lsw) scenario. while this lsw technique has been used previously in controlled laboratory settings to constrain the alp-photon coupling ($g_{a\gamma}$), we show that this can also be applied in astrophysical environments. we find that obscured magnetars in particular are excellent candidates for this purpose. a fraction of photons emitted by the magnetar may convert to alps in the magnetar neighborhood, cross the large absorption column densities, and convert back into photons due to the interstellar magnetic field. comparing the observed flux with the estimated intrinsic flux from the magnetar, we can constrain the contribution of this process, and hence constrain $g_{a\gamma}$. the effects of resonant conversion near the magnetar as well as alp-photon oscillations in the interstellar medium are carefully considered. taking a suitable magnetar candidate psr j1622-4950, we find that the alp-photon coupling can be constrained at $g_{a\gamma} \lesssim (10^{-10} - 10^{-11})$ gev$^{-1}$ for low mass axions ($m_a \lesssim 10^{-12}$ ev). our study reveals the previously unrealized potential for employing the lsw technique for obscured magnetars for probing and constraining alp-photon couplings. | light shining through wall bounds on axions from obscured magnetars |
significant evidence for a stochastic gravitational-wave background has recently been reported by several pulsar timing array observations. these studies have shown that, in addition to astrophysical explanations based on supermassive black hole binaries (smbhbs), cosmological origins are considered equally important sources for these signals. to further explore these cosmological sources, in this study, we discuss the anisotropies in the cosmological gravitational wave background (cgwb) in a model-independent way. taking the north american nanohertz observatory for gravitational waves (nanograv) 15-year dataset as a benchmark, we estimate the angular power spectra of the cgwb and their cross-correlations with cosmic microwave background (cmb) fluctuations and weak gravitational lensing. we find that the nanograv 15-year data implies suppressed sachs-wolf (sw) effects in the cgbw spectrum, leading to a marginally negative cross-correlation with the cmb at large scales. this procedure is applicable to signals introduced by different early universe processes and is potentially useful for identifying unique features about anisotropies of cgwb from future space-based interferometers and astrometric measurements. | on the anisotropies of the cosmological gravitational-wave background from pulsar timing array observations |
we report the properties of more than 800 bursts detected from the repeating fast radio burst (frb) source frb 20201124a with the five-hundred-meter aperture spherical radio telescope during an extremely active episode on utc 2021 september 25th-28th in a series of four papers. in this fourth paper of the series, we present a systematic search of the spin period and linear acceleration of the source object from both 996 individual pulse peaks and the dedispersed time series. no credible spin period was found from this data set. we rule out the presence of significant periodicity in the range between 1 ms and 100 s with a pulse duty cycle <0.49 ± 0.08 (when the profile is defined by a von-mises function, not a boxcar function) and linear acceleration up to 300 m s-2 in each of the four one-hour observing sessions, and up to 0.6 m s-2 in all 4 days. these searches contest theoretical scenarios involving a 1 ms-100 s isolated magnetar/pulsar with surface magnetic field <1015 g and a small duty cycle (such as in a polar-cap emission mode) or a pulsar with a companion star or black hole up to 100 m ⊙ and pb> 10 hr. we also perform a periodicity search of the fine structures and identify 53 unrelated millisecond-timescale "periods" in multi-components with the highest significance of 3.9σ. the "periods" recovered from the fine structures are neither consistent nor harmonically related. thus they are not likely to come from a spin period. we caution against claiming spin periodicity with significance below ~4σ with multi-components from one-off frbs. we discuss the implications of our results and the possible connections between frb multi-components and pulsar microstructures. | fast observations of an extremely active episode of frb 20201124a. iv. spin period search |
the apparent n-independence observed in the evolution of massive black hole binaries (mbhbs) in recent simulation of merging stellar bulges suggests a simple interpretation beyond complex time-dependent relaxation processes. we conjecture that the mbhb hardening rate is roughly equivalent to that of a binary immersed in a field of unbound stars with density ρ and typical velocity dispersion σ, provided that ρ and σ are the stellar density and the velocity dispersion at the influence radius of the mbhb. by comparing direct n-body simulations to a hybrid model based on three-body scattering experiments, we verify that the hardening rates matches reasonably well (within 30 per cent) in the two cases. this result is particularly practical because it provides an approximate estimate the lifetime of mbhbs forming in dry mergers based solely on the stellar density profile of the host galaxy. we briefly discuss some implications of our finding for the gravitational wave signal observable by pulsar timing arrays and for the expected population of mbhbs lurking in massive ellipticals. | scattering experiments meet n-body - i. a practical recipe for the evolution of massive black hole binaries in stellar environments |
the location of radio pulsars in the period-period derivative (p-\dot{p}) plane has been a key diagnostic tool since the early days of pulsar astronomy. of particular importance is how pulsars evolve through the p-\dot{p} diagram with time. here we show that the decay of the inclination angle (\dot{α }) between the magnetic and rotation axes plays a critical role. in particular, \dot{α } strongly impacts on the braking torque, an effect that has been largely ignored in previous work. we carry out simulations that include a negative \dot{α } term, and show that it is possible to reproduce the observational p-\dot{p} diagram without the need for either pulsars with long birth periods or magnetic field decay. our best model indicates a birth rate of one radio pulsar per century and a total galactic population of ∼20 000 pulsars beaming towards earth. | pulsar braking and the p-dot{p} diagram |
we present results from the first directed search for nontensorial gravitational waves. while general relativity allows for tensorial (plus and cross) modes only, a generic metric theory may, in principle, predict waves with up to six different polarizations. this analysis is sensitive to continuous signals of scalar, vector, or tensor polarizations, and does not rely on any specific theory of gravity. after searching data from the first observation run of the advanced ligo detectors for signals at twice the rotational frequency of 200 known pulsars, we find no evidence of gravitational waves of any polarization. we report the first upper limits for scalar and vector strains, finding values comparable in magnitude to previously published limits for tensor strain. our results may be translated into constraints on specific alternative theories of gravity. | first search for nontensorial gravitational waves from known pulsars |
based on phase-resolved broadband spectroscopy using xmm-newton and nustar, we report on a potential cyclotron resonant scattering feature (crsf) at e ∼ 13 kev in the pulsed spectrum of the recently discovered ultraluminous x-ray source (ulx) pulsar ngc 300 ulx1. if this interpretation is correct, the implied magnetic field of the central neutron star is b ∼ 1012 g (assuming scattering by electrons), similar to that estimated from the observed spin-up of the star, and also similar to known galactic x-ray pulsars. we discuss the implications of this result for the connection between ngc 300 ulx1 and the other known ulx pulsars, particularly in light of the recent discovery of a likely proton cyclotron line in another ulx, m51 ulx-8. | a potential cyclotron resonant scattering feature in the ultraluminous x-ray source pulsar ngc 300 ulx1 seen by nustar and xmm-newton |
decade-long timing observations of arrays of millisecond pulsars have placed highly constraining upper limits on the amplitude of the nanohertz gravitational-wave stochastic signal from the mergers of supermassive black hole binaries (∼10-15 strain at f = 1 yr-1). these limits suggest that binary merger rates have been overestimated, or that environmental influences from nuclear gas or stars accelerate orbital decay, reducing the gravitational-wave signal at the lowest, most sensitive frequencies. this prompts the question whether nanohertz gravitational waves (gws) are likely to be detected in the near future. in this letter, we answer this question quantitatively using simple statistical estimates, deriving the range of true signal amplitudes that are compatible with current upper limits, and computing expected detection probabilities as a function of observation time. we conclude that small arrays consisting of the pulsars with the least timing noise, which yield the tightest upper limits, have discouraging prospects of making a detection in the next two decades. by contrast, we find large arrays are crucial to detection because the quadrupolar spatial correlations induced by gws can be well sampled by many pulsar pairs. indeed, timing programs that monitor a large and expanding set of pulsars have an ∼80% probability of detecting gws within the next 10 years, under assumptions on merger rates and environmental influences ranging from optimistic to conservative. even in the extreme case where 90% of binaries stall before merger and environmental coupling effects diminish low-frequency gravitational-wave power, detection is delayed by at most a few years. | are we there yet? time to detection of nanohertz gravitational waves based on pulsar-timing array limits |
we study the efficiency of pair production in polar caps of young pulsars under a variety of conditions to estimate the maximum possible multiplicity of pair plasma in pulsar magnetospheres. we develop a semi-analytic model for calculation of cascade multiplicity which allows efficient exploration of the parameter space and corroborate it with direct numerical simulations. pair creation processes are considered separately from particle acceleration in order to assess different factors affecting cascade efficiency, with acceleration of primary particles described by recent self-consistent non-stationary model of pair cascades. we argue that the most efficient cascades operate in the curvature radiation/synchrotron regime, the maximum multiplicity of pair plasma in pulsar magnetospheres is ∼few × 105. the multiplicity of pair plasma in magnetospheres of young energetic pulsars weakly depends on the strength of the magnetic field and the radius of curvature of magnetic field lines and has a stronger dependence on pulsar inclination angle. this result questions assumptions about very high pair plasma multiplicity in theories of pulsar wind nebulae. | on the polar cap cascade pair multiplicity of young pulsars |
we perform a systematic assessment of models for the equation of state (eos) of dense matter in the context of recent neutron star mass and radius measurements to obtain a broad picture of the structure of neutron stars. we demonstrate that currently available neutron star mass and radius measurements provide strong constraints on moments of inertia, tidal deformabilities, and crust thicknesses. a measurement of the moment of inertia of psr j0737-3039a with a 10% error, without any other information from observations, will constrain the eos over a range of densities to within 50%-60%. we find tidal deformabilities between 0.6 and 6 ×1036g cm2s2 (to 95% confidence) for m =1.4 m⊙ , and any measurement which constrains this range will provide an important constraint on dense matter. the crustal fraction of the moment of inertia can be as large as 10% for m =1.4 m⊙ permitting crusts to have a large enough moment of inertia reservoir to explain glitches in the vela pulsar even with a large amount of superfluid entrainment. finally, due to the uncertainty in the equation of state, there is at least a 40% variation in the thickness of the crust for a fixed mass and radius, which implies that future simulations of the cooling of a neutron star crust which has been heated by accretion will need to take this variation into account. | using neutron star observations to determine crust thicknesses, moments of inertia, and tidal deformabilities |
the psr j1023+0038 binary system hosts a neutron star and a low-mass, main-sequence-like star. it switches on year timescales between states as an eclipsing radio millisecond pulsar and a low-mass x-ray binary (lmxb). we present a multi-wavelength observational campaign of psr j1023+0038 in its most recent lmxb state. two long xmm-newton observations reveal that the system spends ∼70% of the time in a ≈3 × 1033 erg s-1 x-ray luminosity mode, which, as shown in archibald et al., exhibits coherent x-ray pulsations. this emission is interspersed with frequent lower flux mode intervals with ≈ 5× {10}32 erg s-1 and sporadic flares reaching up to ≈1034 erg s-1, with neither mode showing significant x-ray pulsations. the switches between the three flux modes occur on timescales of order 10 s. in the uv and optical, we observe occasional intense flares coincident with those observed in x-rays. our radio timing observations reveal no pulsations at the pulsar period during any of the three x-ray modes, presumably due to complete quenching of the radio emission mechanism by the accretion flow. radio imaging detects highly variable, flat-spectrum continuum radiation from psr j1023+0038, consistent with an origin in a weak jet-like outflow. our concurrent x-ray and radio continuum data sets do not exhibit any correlated behavior. the observational evidence we present bears qualitative resemblance to the behavior predicted by some existing “propeller” and “trapped” disk accretion models although none can account for key aspects of the rich phenomenology of this system. | coordinated x-ray, ultraviolet, optical, and radio observations of the psr j1023+0038 system in a low-mass x-ray binary state |
we present results from a circular polarization survey for radio stars in the rapid askap continuum survey (racs). racs is a survey of the entire sky south of δ = +41○ being conducted with the australian square kilometre array pathfinder telescope (askap) over a 288 mhz wide band centred on 887.5 mhz. the data we analyse include stokes i and v polarization products to an rms sensitivity of 250 μjy psf-1. we searched racs for sources with fractional circular polarization above 6 per cent, and after excluding imaging artefacts, polarization leakage, and known pulsars we identified radio emission coincident with 33 known stars. these range from m-dwarfs through to magnetic, chemically peculiar a- and b-type stars. some of these are well-known radio stars such as yz cmi and cu vir, but 23 have no previous radio detections. we report the flux density and derived brightness temperature of these detections and discuss the nature of the radio emission. we also discuss the implications of our results for the population statistics of radio stars in the context of future askap and square kilometre array surveys. | a circular polarization survey for radio stars with the australian ska pathfinder |
the interaction between refraction from a distribution of inhomogeneous plasma and gravitational lensing introduces novel effects to the paths of light rays passing by a massive object. the plasma contributes additional terms to the equations of motion, and the resulting ray trajectories are frequency-dependent. lensing phenomena and circular orbits are investigated for plasma density distributions n ∝ 1/rh with h ≥ 0 in the schwarzschild space-time. for rays passing by the mass near the plasma frequency refractive effects can dominate, effectively turning the gravitational lens into a mirror. we obtain the turning points, circular orbit radii and angular momentum for general h. previous results have shown that light rays behave like massive particles with an effective mass given by the plasma frequency for a constant density h = 0. we study the behaviour for general h and show that when h = 2 the plasma term acts like an additional contribution to the angular momentum of the passing ray. when h = 3 the potential and radii of circular orbits are analogous to those found in studies of massless scalar fields on the schwarzschild background. as a physically motivated example we study the pulse profiles of a compact object with antipodal hotspots sheathed in a dense plasma, which shows dramatic frequency-dependent shifts from the behaviour in vacuum. finally, we consider the potential observability and applications of such frequency-dependent plasma effects in general relativity for several types of neutron star. | frequency-dependent effects of gravitational lensing within plasma |
the fast folding algorithm (ffa) is a phase-coherent search technique for periodic signals. it has rarely been used in radio pulsar searches, having been historically supplanted by the less computationally expensive fast fourier transform (fft) with incoherent harmonic summing (ihs). here, we derive from first principles that an ffa search closely approaches the theoretical optimum sensitivity to all periodic signals; it is analytically shown to be significantly more sensitive than the standard fft+ihs method, regardless of pulse period and duty cycle. a portion of the pulsar phase space has thus been systematically underexplored for decades; pulsar surveys aiming to fully sample the pulsar population should include an ffa search as part of their data analysis. we have developed an ffa software package, riptide, fast enough to process radio observations on a large scale; riptide has already discovered sources undetectable using existing fft+ihs implementations. our sensitivity comparison between search techniques also shows that a more realistic radiometer equation is needed, which includes an additional term: the search efficiency. we derive the theoretical efficiencies of both the ffa and the fft+ihs methods and discuss how excluding this term has consequences for pulsar population synthesis studies. | optimal periodicity searching: revisiting the fast folding algorithm for large-scale pulsar surveys |
the recent discovery of pulsating ultraluminous x-ray sources (ulxs) shows that the apparent luminosity of accreting neutron stars can exceed the eddington luminosity by a factor of 100s. the relation between the actual and apparent luminosity is a key ingredient in theoretical models of ulxs, but it is still under debate. a typical feature of the discovered pulsating ulxs is a large pulsed fraction (pf). using monte carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high pf. we argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ulx pulsars can show pf above 10 per cent. discrepancy between this conclusion and current observations indicates that pulsating ulxs are not strongly beamed and their apparent luminosity is close to the actual one. | pulsating ulxs: large pulsed fraction excludes strong beaming |
nonthermal relativistic plasmas are ubiquitous in astrophysical systems like pulsar wind nebulae and active galactic nuclei, as inferred from their emission spectra. the underlying nonthermal particle acceleration (ntpa) processes have traditionally been modeled with a fokker-planck (fp) diffusion-advection equation in momentum space. in this letter, we directly test the fp framework in ab initio kinetic simulations of driven magnetized turbulence in relativistic pair plasma. by statistically analyzing the motion of tracked particles, we demonstrate the diffusive nature of ntpa and measure the fp energy diffusion (d) and advection (a) coefficients as functions of particle energy $\gamma {m}_{e}{c}^{2}$ . we find that $d(\gamma )$ scales as ${\gamma }^{2}$ in the high-energy nonthermal tail, in line with second-order fermi acceleration theory, but has a much weaker scaling at lower energies. we also find that a is not negligible and reduces ntpa by tending to pull particles toward the peak of the particle energy distribution. this study provides strong support for the fp picture of turbulent ntpa, thereby enhancing our understanding of space and astrophysical plasmas. | first-principles demonstration of diffusive-advective particle acceleration in kinetic simulations of relativistic plasma turbulence |
we investigate the hadron-quark phase transition in cold neutron stars in light of (i) the observed limits on the maximum-mass of heavy pulsars, (ii) constraints on the tidal properties inferred from the gravitational waves emitted in binary neutron-star mergers, and (iii) mass and radius constraints derived from the observation of hot spots on neutron star observed with the neutron star interior composition explorer instrument. special attention is directed to the possible presence of δ (1232 ) baryons in neutron star matter. our results indicate that this particle could make up a large fraction of the baryons in neutron stars and thus have a significant effect on the properties of such objects, particularly on their radii. this is partially caused by the low density appearance of δ s for a wide range of theoretically defensible sets of meson-hyperon, su(3) esc08 model, and meson-δ coupling constants. the transition of hadronic matter to quark matter, treated in the 2 sc +s condensation phase, is found to occur only in neutron stars very close to the mass peak. nevertheless, quark matter may still constitute an appreciable fraction of the stars' total matter if the phase transition is treated as maxwell-like (sharp), in which case the neutron stars located beyond the gravitational mass peak would remain stable against gravitational collapse. in this case, the instability against gravitational collapse is shifted to a new (terminal) mass different from the maximum-mass of the stellar sequence, giving rise to stable compact objects with the same gravitational masses as those of the neutron stars on the traditional branch, but whose radii are smaller by up to 1 km. all models for the equation of state of our study fall comfortably within the bound established very recently by annala et al. [nat. phys., , 10.1038/s41567-020-0914-9 (2020)]. | delta baryons and diquark formation in the cores of neutron stars |
we analyze dispersion measure (dm) variations of 37 millisecond pulsars in the nine-year north american nanohertz observatory for gravitational waves (nanograv) data release and constrain the sources of these variations. dm variations can result from a changing distance between earth and the pulsar, inhomogeneities in the interstellar medium, and solar effects. variations are significant for nearly all pulsars, with characteristic timescales comparable to or even shorter than the average spacing between observations. five pulsars have periodic annual variations, 14 pulsars have monotonically increasing or decreasing trends, and 14 pulsars show both effects. of the four pulsars with linear trends that have line-of-sight velocity measurements, three are consistent with a changing distance and require an overdensity of free electrons local to the pulsar. several pulsars show correlations between dm excesses and lines of sight that pass close to the sun. mapping of the dm variations as a function of the pulsar trajectory can identify localized interstellar medium features and, in one case, an upper limit to the size of the dispersing region of 4 au. four pulsars show roughly kolmogorov structure functions (sfs), and another four show sfs less steep than kolmogorov. one pulsar has too large an uncertainty to allow comparisons. we discuss explanations for apparent departures from a kolmogorov-like spectrum, and we show that the presence of other trends and localized features or gradients in the interstellar medium is the most likely cause. | the nanograv nine-year data set: measurement and analysis of variations in dispersion measures |
we use compiled high-precision pulsar timing measurements to directly measure the galactic acceleration of binary pulsars relative to the solar system barycenter. given the vertical accelerations, we use the poisson equation to derive the oort limit, i.e., the total volume mass density in the galactic mid-plane. our best-fitting model gives an oort limit of ${0.08}_{-0.02}^{0.05}{m}_{\odot }\,{\mathrm{pc}}^{-3}$ , which is close to estimates from recent jeans analyses. given the accounting of the baryon budget from mckee et al., we obtain a local dark matter density of $-{0.004}_{-0.02}^{0.05}\,{m}_{\odot }\,{\mathrm{pc}}^{-3}$ , which is slightly below other modern estimates but consistent within the current uncertainties of our method. the error bars are currently about five times larger than kinematical estimates, but should improve in the future for this novel dynamical method. we also constrain the oblateness of the potential, finding it consistent with that expected from the disk and inconsistent with a potential dominated by a spherical halo, as is appropriate for our sample that is within a ∼kpc of the sun. we find that current measurements of binary pulsar accelerations lead to large uncertainties in the slope of the rotation curve. we give a fitting function for the vertical acceleration az: az = - α1z; ${\mathrm{log}}_{10}({\alpha }_{1}/{\mathrm{gyr}}^{-2})={3.69}_{-0.12}^{0.19}$ . by analyzing interacting simulations of the milky way, we find that large asymmetric variations in daz/dz as a function of vertical height may be a signature of sub-structure. we end by discussing the power of combining constraints from pulsar timing and high-precision radial velocity measurements toward lines-of-sight near pulsars, to test theories of gravity and constrain dark matter sub-structure. | a measurement of the galactic plane mass density from binary pulsar accelerations |
aims: we investigate the extension of the very high-energy spectral tail of the crab pulsar at energies above 400 gev.methods: we analyzed ~320 h of good-quality crab data obtained with the magic telescope from february 2007 to april 2014.results: we report the most energetic pulsed emission ever detected from the crab pulsar reaching up to 1.5 tev. the pulse profile shows two narrow peaks synchronized with those measured in the gev energy range. the spectra of the two peaks follow two different power-law functions from 70 gev up to 1.5 tev and connect smoothly with the spectra measured above 10 gev by the large area telescope (lat) on board the fermi satellite. when making a joint fit of the lat and magic data above 10 gev the photon indices of the spectra differ by 0.5 ± 0.1.conclusions: using data from the magic telescopes we measured the most energetic pulsed photons from a pulsar to date. such tev pulsed photons require a parent population of electrons with a lorentz factor of at least 5 × 106. these results strongly suggest ic scattering off low-energy photons as the emission mechanism and a gamma-ray production region in the vicinity of the light cylinder. | teraelectronvolt pulsed emission from the crab pulsar detected by magic |
we describe the north american nanohertz observatory for gravitational waves (nanograv) and its efforts to directly detect and study gravitational waves and other synergistic physics and astrophysics using radio timing observations of millisecond pulsars. | the nanograv program for gravitational waves and fundamental physics |
we study the implications of the very recently reported mass m={2.17}-0.10+0.11 m ⊙ of psr j0740+6620 on the equation of state (eos) of super-dense neutron-rich nuclear matter with respect to existing constraints on the eos based on the mass m = 2.01 ± 0.04 m ⊙ of psr j0348+0432, the maximum tidal deformability of gw170817, and earlier results of various terrestrial nuclear laboratory experiments. the lower limit of the skewness j 0 measuring the stiffness of super-dense isospin-symmetric nuclear matter is raised from about -220 mev to -150 mev, significantly reducing its current uncertainty range. the lower bound of the high-density symmetry energy also increases appreciably leading to a rise of the minimum proton fraction in neutron stars at β-equilibrium from about 0% to 5% around three times the saturation density of nuclear matter. the difficulties for some of the most widely used and previously well tested model eoss to simultaneously predict both a maximum mass higher than 2.17 m ⊙ and a pressure consistent with that extracted from gw170817 present some interesting new challenges for nuclear theories. | implications of the mass m={2.17}_{-0.10}^{+0.11} m ⊙ of psr j0740+6620 on the equation of state of super-dense neutron-rich nuclear matter |
the square kilometre array (ska) will make ground breaking discoveries in pulsar science. in this chapter we outline the ska surveys for new pulsars, as well as how we will perform the necessary follow-up timing observations. the ska's wide field-of-view, high sensitivity, multi-beaming and sub-arraying capabilities, coupled with advanced pulsar search backends, will result in the discovery of a large population of pulsars. these will enable the ska's pulsar science goals (tests of general relativity with pulsar binary systems, investigating black hole theorems with pulsar-black hole binaries, and direct detection of gravitational waves in a pulsar timing array). using ska1-mid and ska1-low we will survey the milky way to unprecedented depth, increasing the number of known pulsars by more than an order of magnitude. ska2 will potentially find all the galactic radio-emitting pulsars in the ska sky which are beamed in our direction. this will give a clear picture of the birth properties of pulsars and of the gravitational potential, magnetic field structure and interstellar matter content of the galaxy. targeted searches will enable detection of exotic systems, such as the ~1000 pulsars we infer to be closely orbiting sgr a*, the supermassive black hole in the galactic centre. in addition, the ska's sensitivity will be sufficient to detect pulsars in local group galaxies. to derive the spin characteristics of the discoveries we will perform live searches, and use sub-arraying and dynamic scheduling to time pulsars as soon as they are discovered, while simultaneously continuing survey observations. the large projected number of discoveries suggests that we will uncover currently unknown rare systems that can be exploited to push the boundaries of our understanding of astrophysics and provide tools for testing physics, as has been done by the pulsar community in the past. | a cosmic census of radio pulsars with the ska |
the dispersion measure (dm), the column density of free electrons to a pulsar, is shown to be frequency dependent because of multipath scattering from small-scale electron-density fluctuations. dms vary between propagation paths whose transverse extent varies strongly with frequency, yielding arrival times that deviate from the high-frequency scaling expected for a cold, uniform, unmagnetized plasma (1/frequency2). scaling laws for thin phase screens are verified with simulations; extended media are also analyzed. the rms dm difference across an octave band near 1.5 ghz is ∼ 4 × 10-5 pc cm-3 for pulsars at ∼1 kpc distance. the corresponding arrival-time variations are a few to hundreds of nanoseconds for dm ≲ 30 pc cm-3 but increase rapidly to microseconds or more for larger dms and wider frequency ranges. chromatic dms introduce correlated noise into timing residuals with a power spectrum of “low pass” form. the correlation time is roughly the geometric mean of the refraction times for the highest and lowest radio frequencies used, ranging from days to years, depending on the pulsar. we discuss implications for methodologies that use large frequency separations or wide bandwidth receivers for timing measurements. chromatic dms are partially mitigable by including an additional chromatic term in arrival time models. without mitigation, an additional term in the noise model for pulsar timing is implied. in combination with measurement errors from radiometer noise, an arbitrarily large increase in total frequency range (or bandwidth) will yield diminishing benefits and may be detrimental to overall timing precision. | frequency-dependent dispersion measures and implications for pulsar timing |
the repeating fast radio burst (frb) localized to a globular cluster (gc) in m81 challenges our understanding of frb models. in this letter, we explore dynamical formation scenarios for objects in old gcs that may plausibly power frbs. using n-body simulations, we demonstrate that young neutron stars (nss) may form in gcs at a rate of up to ~50 gpc-3 yr-1 through a combination of binary white dwarf (wd) mergers, wd-ns mergers, binary ns mergers, and accretion-induced collapse of massive wds in binary systems. we consider two frb emission mechanisms: first, we show that a magnetically powered source (e.g., a magnetar with field strength ≳1014 g) is viable for radio emission efficiencies ≳10-4. this would require magnetic activity lifetimes longer than the associated spin-down timescales and longer than empirically constrained lifetimes of galactic magnetars. alternatively, if these dynamical formation channels produce young rotation-powered nss with spin periods of ~10 ms and magnetic fields of ~1011 g (corresponding to spin-down lifetimes of ≳105 yr), the inferred event rate and energetics can be reasonably reproduced for order unity duty cycles. additionally, we show that recycled millisecond pulsars or low-mass x-ray binaries similar to those well-observed in galactic gcs may also be plausible channels, but only if their duty cycle for producing bursts similar to the m81 frb is small. | dynamical formation channels for fast radio bursts in globular clusters |
we compare the results of a large grid of n-body simulations with the surface brightness and velocity dispersion profiles of the globular clusters ω cen and ngc 6624. our models include clusters with varying stellar-mass black hole retention fractions and varying masses of a central intermediate-mass black hole (imbh). we find that an ∼ 45 000 m⊙ imbh, whose presence has been suggested based on the measured velocity dispersion profile of ω cen, predicts the existence of about 20 fast-moving, m > 0.5 m⊙, main-sequence stars with a (1d) velocity v > 60 km s-1 in the central 20 arcsec of ω cen. however, no such star is present in the hst/acs proper motion catalogue of bellini et al. (2017), strongly ruling out the presence of a massive imbh in the core of ω cen. instead, we find that all available data can be fitted by a model that contains 4.6 per cent of the mass of ω cen in a centrally concentrated cluster of stellar-mass black holes. we show that this mass fraction in stellar-mass bhs is compatible with the predictions of stellar evolution models of massive stars. we also compare our grid of n-body simulations with ngc 6624, a cluster recently claimed to harbour a 20 000 m⊙ black hole based on timing observations of millisecond pulsars. however, we find that models with mimbh > 1000 m⊙ imbhs are incompatible with the observed velocity dispersion and surface brightness profile of ngc 6624, ruling out the presence of a massive imbh in this cluster. models without an imbh provide again an excellent fit to ngc 6624. | no evidence for intermediate-mass black holes in the globular clusters ω cen and ngc 6624 |
the main aim of this study is to explore the existence and salient features of spherically symmetric relativistic quark stars in the background of massive brans-dicke gravity. the exact solutions to the modified einstein field equations are derived for specific forms of coupling and scalar field functions using the equation of state relating to the strange quark matter that stimulates the phenomenological mit-bag model as a free fermi gas of quarks. we use a well-behaved function along with the karmarkar condition for class-one embedding as well as junction conditions to determine the unknown metric tensors. the radii of strange compact stars viz., psr j1416-2230, psr j1903+327, 4u 1820-30, cenx-3, and exo1785-248, are predicted via their observed mass for different values of the massive brans-dicke parameters. we explore the influences of the mass of scalar field $m_{\phi}$ , coupling parameter $\omega_{\rm bd}$ , and bag constant ${\cal{b}}$ on state determinants and perform several tests on the viability and stability of the constructed stellar model. conclusively, we find that our stellar system is physically viable and stable as it satisfies all the energy conditions and necessary stability criteria under the influence of a gravitational scalar field. *supported by the postdoctoral fellowship at zhejiang normal university (zc304022919) | exploring physical features of anisotropic quark stars in brans-dicke theory with a massive scalar field via embedding approach |
in this work some families of relativistic anisotropic charged fluid spheres have been obtained by solving the einstein-maxwell field equations with a preferred form of one of the metric potentials, and suitable forms of electric charge distribution and pressure anisotropy functions. the resulting equation of state (eos) of the matter distribution has been obtained. physical analysis shows that the relativistic stellar structure for the matter distribution considered in this work may reasonably model an electrically charged compact star whose energy density associated with the electric fields is on the same order of magnitude as the energy density of fluid matter itself (e.g., electrically charged bare strange stars). furthermore these models permit a simple method of systematically fixing bounds on the maximum possible mass of cold compact electrically charged self-bound stars. it has been demonstrated, numerically, that the maximum compactness and mass increase in the presence of an electric field and anisotropic pressures. based on the analytic models developed in this present work, the values of some relevant physical quantities have been calculated by assuming the estimated masses and radii of some well-known potential strange star candidates like psr j1614-2230, psr j1903+327, vela x-1, and 4u 1820-30. | some new wyman-leibovitz-adler type static relativistic charged anisotropic fluid spheres compatible to self-bound stellar modeling |
in this work, we investigate anisotropic effects on the equations of state (eos) used to describe neutron and quark stars in the framework of rastall-rainbow gravity. all our calculations are computed using two different eos to describe the matter contained within the star: the mit bag model for quark stars and the iu-fsu parameterisation for the standard hadronic matter. from the values of masses and radii obtained, we can conclude that anisotropic pressure has significant consequences on the structure of compact objects. specifically, when anisotropy is considered within general relativity, it significantly modifies the maximum stellar mass. on the other hand, when rastall-rainbow gravity and anisotropy are simultaneously considered, they provide the best results for the masses and radii of some important astrophysical objects such as the low-mass x-ray binary (lmxb) ngc 6397 and the extremely massive millisecond pulsating source of radio (psr) j0740 + 6620, for instance. radii results inferred from the lead radius experiment (prex-2) and the compact object in the mass-gap of gw190814 event can also be described for certain values of the rastall-rainbow and anisotropy parameters. | anisotropic compact stars in rastall-rainbow gravity |
nonequilibrium conditions imposed by neutrino cooling through the liquid-solid transition lead to disorder in the solid crust of neutron stars. disorder reduces the superfluid fraction, $\rho_s/\rho$, at densities above that of neutron drip, $\rho_d \approx 4\times 10^{11}\,g/cm^3$. for an amorphous solid crust the suppression of $\rho_s$ is small, except in the highest density regions of the crust. in contrast to the strong reduction in neutron conduction predicted for coherent bragg scattering in a crystalline crust, the disordered solid crust supports sufficient neutron superfluid density to account for pulsar glitches. | superfluidity in disordered neutron stars crusts |
we explore the possible mass radius relation of compact stars for the equation of states with a first order phase transition. the low density matter is described by a nuclear matter equation of state resulting from fits to nuclear properties. a constant speed of sound parametrization is used to describe the high density matter phase with the speed of sound cs2 = 1. a classification scheme of four distinct categories including twin star solutions, i.e. solutions with the same mass but differing radii, is found which is compatible with the m ≥ 2m_{⊙} pulsar mass constraint. we show the dependence of the mass and radius differences on the transition parameters and delineate that higher twin star masses are more likely to be accompanied by large radius differences. these massive twin stars are generated by high values of the discontinuity in the energy density and the lowest possible values of the transition pressure that still result in masses of m ≥ 2m_{⊙} at the maximum of the hadronic branch. | classifications of twin star solutions for a constant speed of sound parameterized equation of state |
there are two outstanding issues regarding the neutron-star merger event gw170817: the nature of the compact remnant and the interstellar shock. the mass of the remnant of gw170817, ∼2.7 {m}⊙ , implies that the remnant could be either a massive rotating neutron star, or a black hole. we report chandra director’s discretionary time observations made in 2017 december and 2018 january, and we reanalyze earlier observations from 2017 august and 2017 september, in order to address these unresolved issues. we estimate the x-ray flux from a neutron star remnant and compare that to the measured x-ray flux. if we assume that the spin-down luminosity of any putative neutron star is converted to pulsar wind nebula x-ray emission in the 0.5-8 kev band with an efficiency of 10-3, for a dipole magnetic field with 3 × 1011 g < b < 1014 g, a rising x-ray signal would result and would be brighter than that observed by day 107; we therefore conclude that the remnant of gw170817 is most likely a black hole. independent of any assumptions of x-ray efficiency, however, if the remnant is a rapidly rotating magnetized neutron star, the total energy in the external shock should rise by a factor ∼102 (to ∼1052 erg) after a few years; therefore, chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant. the same observations can distinguish between two different models for the relativistic outflow, either an angular or radially varying structure. | gw170817 most likely made a black hole |
we have constructed a new time-scale, tt(ipta16), based on observations of radio pulsars presented in the first data release from the international pulsar timing array (ipta). we used two analysis techniques with independent estimates of the noise models for the pulsar observations and different algorithms for obtaining the pulsar time-scale. the two analyses agree within the estimated uncertainties and both agree with tt(bipm17), a post-corrected time-scale produced by the bureau international des poids et mesures (bipm). we show that both methods could detect significant errors in tt(bipm17) if they were present. we estimate the stability of the atomic clocks from which tt(bipm17) is derived using observations of four rubidium fountain clocks at the us naval observatory. comparing the power spectrum of tt(ipta16) with that of these fountain clocks suggests that pulsar-based time-scales are unlikely to contribute to the stability of the best time-scales over the next decade, but they will remain a valuable independent check on atomic time-scales. we also find that the stability of the pulsar-based time-scale is likely to be limited by our knowledge of solar-system dynamics, and that errors in tt(bipm17) will not be a limiting factor for the primary goal of the ipta, which is to search for the signatures of nano-hertz gravitational waves. | a pulsar-based time-scale from the international pulsar timing array |
the detection of gravitational waves from neutron star merger events has opened up a new field of multimessenger astronomy linking gravitational-wave events to short gamma-ray bursts and kilonova afterglows. a further - yet to be discovered - electromagnetic counterpart is a precursor emission produced by the nontrivial interaction of the magnetospheres of the two neutron stars prior to merger. by performing special-relativistic force-free simulations of orbiting neutron stars we discuss the effect of different magnetic field orientations and show how the emission can be significantly enhanced by differential motion present in the binary, either due to stellar spins or misaligned stellar magnetospheres. we find that the buildup of twist in the magnetic flux tube connecting the two stars can lead to the repeated emission of powerful flares for a variety of orbital configurations. we also discuss potential coherent radio emission mechanisms in the flaring process. | electromagnetic precursors to gravitational-wave events: numerical simulations of flaring in pre-merger binary neutron star magnetospheres |
strong magnetic fields, synchrotron emission, and compton scattering are omnipresent in compact celestial x-ray sources. emissions in the x-ray energy band are consequently expected to be linearly polarized. x-ray polarimetry provides a unique diagnostic to study the location and fundamental mechanisms behind emission processes. the polarization of emissions from a bright celestial x-ray source, the crab, is reported here for the first time in the hard x-ray band ( 20-160 kev). the crab is a complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebula including a jet and torus. measurements are made by a purpose-built and calibrated polarimeter, pogo+. the polarization vector is found to be aligned with the spin axis of the pulsar for a polarization fraction, pf = (20.9 ± 5.0)%. this is higher than that of the optical diffuse nebula, implying a more compact emission site, though not as compact as, e.g., the synchrotron knot. contrary to measurements at higher energies, no significant temporal evolution of phase-integrated polarisation parameters is observed. the polarization parameters for the pulsar itself are measured for the first time in the x-ray energy band and are consistent with observations at optical wavelengths. | shedding new light on the crab with polarized x-rays |
relativistic jets are observed throughout the universe and strongly affect their surrounding environments on a range of physical scales, from galactic binary systems1 to galaxies and clusters of galaxies2. all types of accreting black hole and neutron star have been observed to launch jets3, with the exception of neutron stars with strong magnetic fields4,5 (higher than 1012 gauss), leading to the conclusion that their magnetic field strength inhibits jet formation6. however, radio emission recently detected from two such objects could have a jet origin, among other possible explanations7,8, indicating that this long-standing idea might need to be reconsidered. but definitive observational evidence of such jets is still lacking. here we report observations of an evolving jet launched by a strongly magnetized neutron star accreting above the theoretical maximum rate given by the eddington limit. the radio luminosity of the jet is two orders of magnitude fainter than those seen in other neutron stars with similar x-ray luminosities9, implying an important role for the properties of the neutron star in regulating jet power. our result also shows that the strong magnetic fields of ultra-luminous x-ray pulsars do not prevent such sources from launching jets. | an evolving jet from a strongly magnetized accreting x-ray pulsar |
gravitational waves (gws) create correlations in the arrival times of pulses from different pulsars. the expected correlation μ (γ ) as a function of the angle γ between the directions to two pulsars was calculated by hellings and downs for an isotropic and unpolarized gw background, and several pulsar timing array (pta) collaborations are working to observe these. we ask: given a set of noise-free observations, are they consistent with that expectation? to answer this, we calculate the expected variance σ2(γ ) in the correlation for a single gw point source, as pulsar pairs with fixed separation angle γ are swept around the sky. we then use this to derive simple analytic expressions for the variance produced by a set of discrete point sources uniformly scattered in space for two cases of interest: (1) point sources radiating gws at the same frequency, generating confusion noise, and (2) point sources radiating gws at distinct nonoverlapping frequencies. by averaging over all pulsar sky positions at fixed separation angle γ , we show how this variance may be cleanly split into cosmic variance and pulsar variance, also demonstrating that measurements of the variance can provide information about the nature of gw sources. in a series of technical appendices, we calculate the mean and variance of the hellings-downs correlation for an arbitrary (polarized) point source, quantify the impact of neglecting pulsar terms, and calculate the pulsar and cosmic variance for a gaussian ensemble. the mean and variance of the gaussian ensemble may be obtained from the previous discrete-source confusion-noise model in the limit of a high density of weak sources. | variance of the hellings-downs correlation |
pulsar wind nebulae are highly intriguing astrophysical objects in many respects. they are the brightest and closest class of relativistic sources, and hence the ultimate laboratory for the physics of relativistic plasmas: several processes observed (or inferred to occur) in other classes of relativistic sources can here be studied with unique detail, like the acceleration and collimation of relativistic outflows, or the acceleration of particles at relativistic shocks. here i review the current status of our theoretical understanding of pulsar wind nebulae in light of the most recent 2d and 3d mhd modelling of these sources. i will discuss how these studies are taking us to the point when we can reliably use multi-wavelength observations of these nebulae as a diagnostics of the hidden physics of the pulsar wind and of the mechanism(s) through which particles are accelerated at the highly relativistic shock that terminates the wind. finally i will briefly discuss recent progress in the modelling of evolved pulsar wind nebulae and of the escape of particles from these systems. this effort is instrumental to credibly assess the role of pulsar winds as sources of cosmic ray leptons, and has recently been recognised to have important implications also on cosmic ray transport in the galaxy. | the theory of pulsar wind nebulae: recent progress |
the crab nebula is so far the only celestial object with a statistically significant detection in soft x-ray polarimetry1-4, a window that has not been explored in astronomy since the 1970s. however, soft x-ray polarimetry is expected to be a sensitive probe of magnetic fields in high-energy astrophysical objects, including rotation-powered pulsars5-7 and pulsar wind nebulae8. here we report the re-detection of soft x-ray polarization after 40 years from the crab nebula and pulsar with polarlight9, a miniature polarimeter utilizing a novel technique10,11 onboard a cubesat. the polarization fraction of the crab in the on-pulse phases was observed to decrease after a glitch of the crab pulsar on 23 july 2019, while that of the pure nebular emission remained constant within uncertainty. the phenomenon may have lasted about 100 days. if the association between the glitch and polarization change can be confirmed with future observations, it will place strong constraints on the physical mechanism of the high-energy emission12-14 and glitch15-17 of pulsars. | re-detection and a possible time variation of soft x-ray polarization from the crab |
ultra-light primordial black holes with masses m bh < 109 g evaporate before big-bang nucleosynthesis producing all matter fields, including dark matter, in particular super-heavy dark matter: m dm ≳ 1010 gev. if the dark matter gets its mass via u(1) symmetry-breaking, the phase transition that gives a mass to the dark matter also produces cosmic strings which radiate gravitational waves. because the symmetry-breaking scale λcs is of the same order as m dm, the gravitational waves radiated by the cosmic strings have a large enough amplitude to be detectable across all frequencies accessible with current and planned experimental facilities. moreover, an epoch of early primordial black hole domination introduces a unique spectral break in the gravitational wave spectrum whose frequency is related to the super-heavy dark matter mass. hence, the features of a stochastic background of primordial gravitational waves could indicate that super-heavy dark matter originated from primordial black holes. in this perspective, the recent finding of a stochastic common-spectrum process across many pulsars by two nano-frequency pulsar timing arrays would fix the dark matter mass to be 3 × 1013 gev ≲ m dm ≲ 1014 gev. the (non-)detection of a spectral break at 0.2 hz ≲ f * ≲ 0.4 hz would (exclude) substantiate this interpretation of the signal. | testing super heavy dark matter from primordial black holes with gravitational waves |
pulsar timing arrays (ptas) consisting of widely distributed and well-timed millisecond pulsars can serve as a galactic interferometer to measure gravitational waves. with the same data acquired for ptas, we propose to develop pulsar polarization arrays (ppas), to explore astrophysics and fundamental physics. as in the case of ptas, ppas are best suited to reveal temporal and spatial correlations at large scales that are hard to mimic by local noise. to demonstrate the physical potential of ppas, we consider detection of ultralight axionlike dark matter (aldm), through cosmic birefringence induced by its chern-simons coupling. because of its tiny mass, the ultralight aldm can be generated as a bose-einstein condensate, characterized by a strong wave nature. incorporating both temporal and spatial correlations of the signal, we show that ppas have a potential to probe the chern-simons coupling up to ∼10-14-10-17 gev-1 , with a mass range ∼10-27- 10-21 ev . | pulsar polarization arrays |
the discovery of a radioactively powered kilonova associated with the binary neutron-star merger gw170817 remains the only confirmed electromagnetic counterpart to a gravitational-wave event1,2. observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. although the large measured ejecta mass3,4 could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio q of 0.7 to 0.8)5, the known galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass (q > 0.9) binaries6. the pulsar psr j1913+1102 is a double system in a five-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii7, and the two neutron stars are predicted to coalesce in 470-11+12? million years owing to gravitational-wave emission. here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. with a measured mass ratio of q = 0.78 ± 0.03, this is the most asymmetric merging system reported so far. on the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confidence) of the total population of merging binaries. the coalescence of a member of this population offers a possible explanation for the anomalous properties of gw170817, including the observed kilonova emission from that event. | asymmetric mass ratios for bright double neutron-star mergers |
the first fast radio burst (frb) to be precisely localized was associated with a luminous persistent radio source (prs). recently, a second frb/prs association was discovered for another repeating source of frbs. however, it is not clear what makes frbs or prs or how they are related. we compile frb and prs properties to consider the population of frb/prs sources. we suggest a practical definition for prs as frb associations with luminosity greater than 1029 erg s-1 hz-1 that are not attributed to star formation activity in the host galaxy. we model the probability distribution of the fraction of frbs with prs for repeaters and nonrepeaters, showing there is not yet evidence for repeaters to be preferentially associated with prs. we discuss how frb/prs sources may be distinguished by the combination of active repetition and an excess dispersion measure local to the frb environment. we use chime/frb event statistics to bound the mean per-source repetition rate of frbs to be between 25 and 440 yr-1. we use this to provide a bound on the density of frb-emitting sources in the local universe of between 2.2 × 102 and 5.2 × 104 gpc-3 assuming a pulsar-like beamwidth for frb emission. this density implies that prs may comprise as much as 1% of compact, luminous radio sources detected in the local universe. the cosmic density and phenomenology of prs are similar to that of the newly discovered, off-nuclear "wandering" active galactic nuclei (agn). we argue that it is likely that some prs have already been detected and misidentified as agn. | on the fast radio burst and persistent radio source populations |
we review electrodynamics of rotating magnetized neutron stars, from the early vacuum model to recent numerical experiments with plasma-filled magnetospheres. significant progress became possible due to the development of global particle-in-cell simulations which capture particle acceleration, emission of high-energy photons, and electron-positron pair creation. the numerical experiments show from first principles how and where electric gaps form, and promise to explain the observed pulsar activity from radio waves to gamma-rays. | electrodynamics of pulsar magnetospheres |
i present an empirical study of the properties of fast radio bursts (frbs): gigahertz-frequency, dispersed pulses of extragalactic origin. i focus my investigation on a sample of 17 frbs detected at the parkes radio telescope with largely self-consistent instrumentation. of this sample, six are temporally unresolved, eight exhibit evidence for scattering in inhomogeneous plasma, and five display potentially intrinsic temporal structure. the characteristic scattering time-scales at a frequency of 1 ghz range between 0.005 and 32 ms; moderate evidence exists for a relation between frb scattering time-scales and dispersion measures. additionally, i present constraints on the fluences of parkes frbs, accounting for their uncertain sky positions, and use the multiple-beam detection of frb 010724 (the lorimer burst) to measure its fluence to be 800 ± 400 jy ms. frbs, including the repeating frb 121102, appear to manifest with a plethora of characteristics, and it is uncertain at present whether they share a common class of progenitor object, or arise from a selection of independent progenitors. | the observed properties of fast radio bursts |
we consider ultraluminous x-ray systems (ulxs) where the accretor is a neutron star rather than a black hole. we show that the recently discovered example (m82 x-2) fits naturally into the simple picture of ulxs as beamed x-ray sources fed at super-eddington rates, provided that its magnetic field is weaker (≃1011g) than a new-born x-ray pulsar, as expected if there has been mass gain. continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ulx containing a black hole. accordingly we suggest that a significant fraction of all ulxs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their x-ray binary progenitors. we emphasize that neutron-star ulxs are likely to have higher apparent luminosities than black hole ulxs for a given mass transfer rate, as their tighter beaming outweighs their lower eddington luminosities. this further increases the likely proportion of neutron-star accretors among all ulxs. cygnus x-2 is probably a typical descendant of neutron-star ulxs, which may therefore ultimately end as millisecond pulsar binaries with massive white dwarf companions. | ulxs: neutron stars versus black holes |
pulsar timing arrays (ptas) detect gravitational waves (gws) via the correlations they induce in the arrival times of pulses from different pulsars. we assume that the gws are described by a gaussian ensemble, which models the confusion noise produced by expected pta sources. the mean correlation h2μu(γ ) as a function of the angle γ between the directions to two pulsars was predicted by hellings and downs in 1983. the variance σtot2(γ ) in this correlation was recently calculated [b. allen, variance of the hellings-downs correlation, phys. rev. d 107, 043018 (2023), 10.1103/physrevd.107.043018] for a single noise-free pulsar pair at angle γ , which shows that after averaging over many pairs, the variance reduces to an intrinsic cosmic variance σcos2(γ ). here, we extend this to an arbitrary set of pulsars at specific sky locations, with pulsar pairs binned by γ . we derive the linear combination of pulsar-pair correlations which is the optimal estimator of the hellings and downs correlation for each bin, illustrating our methods with plots of the expected range of variation away from the hellings and downs curve, for the sets of pulsars monitored by three active pta collaborations. we compute the variance of and the covariance between these binned estimates, and show that these reduce to the cosmic variance and covariance s (γ ,γ') respectively, in the many-pulsar limit. the likely fluctuations away from the hellings and downs curve μu(γ ) are strongly correlated/anticorrelated in the three angular regions where μu(γ ) is successively positive, negative, and positive. we also construct the optimal estimator of the squared strain h2 from pulsar-pair correlation data. remarkably, when there are very many pulsar pairs, this determines h2 with arbitrary precision because (in contrast to ligo-like gw detectors) ptas probe an infinite set of gw modes. to assess if observed deviations away from the hellings and downs curve are consistent with predictions, we propose and characterize several χ2 goodness-of-fit statistics. while our main focus is ideal noise-free data, we also show how pulsar noise and measurement noise can be included. our methods can also be applied to future ptas, where the improved telescopes will provide larger pulsar populations and higher-precision timing. | hellings and downs correlation of an arbitrary set of pulsars |
context. by providing information about the location of scattering material along the line of sight (los) to pulsars, scintillation arcs are a powerful tool for exploring the distribution of ionized material in the interstellar medium (ism). here, we present observations that probe the ionized ism on scales of ~0.001-30 au. aims. we have surveyed pulsars for scintillation arcs in a relatively unbiased sample with dm < 100 pc cm-3. we present multifrequency observations of 22 low to moderate dm pulsars. many of the 54 observations were also observed at another frequency within a few days. methods. for all observations, we present dynamic spectra, autocorrelation functions, and secondary spectra. we analyze these data products to obtain scintillation bandwidths, pulse broadening times, and arc curvatures. results. we detect definite or probable scintillation arcs in 19 of the 22 pulsars and 34 of the 54 observations, showing that scintillation arcs are a prevalent phenomenon. the arcs are better defined in low dm pulsars. we show that well-defined arcs do not directly imply anisotropy of scattering. only the presence of reverse arclets and a deep valley along the delay axis, which occurs in about 20% of the pulsars in the sample, indicates substantial anisotropy of scattering. conclusions. the survey demonstrates substantial patchiness of the ionized ism on both astronomical-unit-size scales transverse to the los and on ~100 pc scales along it. we see little evidence for distributed scattering along most lines of sight in the survey. | a scintillation arc survey of 22 pulsars with low to moderate dispersion measures |
in the framework of the thomas-fermi approximation, we systematically study the eoss and microscopic structures of neutron star matter in a vast density range with n b ≈ 10-10-2 fm-3, where various covariant density functionals are adopted, i.e., those with nonlinear self couplings (nl3, pk1, tm1, gm1, mtvtc) and density-dependent couplings (dd-lz1, ddme-x, pkdd, dd-me2, dd2, tw99). it is found that the eoss generally coincide with each other at n b ≲ 10-4 fm-3 and 0.1 fm-3 ≲ n b ≲ 0.3 fm-3, while in other density regions they are sensitive to the effective interactions between nucleons. by adopting functionals with a larger slope of symmetry energy l, the curvature parameter k sym and neutron drip density generally increases, while the droplet size, proton number of nucleus, core-crust transition density, and onset density of non-spherical nuclei, decrease. all functionals predict neutron stars with maximum masses exceeding the two-solar-mass limit, while those of dd2, dd-lz1, dd-me2, and ddme-x predict optimum neutron star radii according to the observational constraints. nevertheless, the corresponding skewness coefficients j are much larger than expected, while only the functionals mtvtc and tw99 meet the start-of-art constraints on j. more accurate measurements on the radius of psr j0740 + 6620 and the maximum mass of neutron stars are thus essential to identify the functional that satisfies all constraints from nuclear physics and astrophysical observations. approximate linear correlations between neutron stars' radii at m = 1.4m ⊙ and 2m ⊙, the slope l and curvature parameter k sym of symmetry energy are observed as well, which are mainly attributed to the curvature-slope correlations in the functionals adopted here. the results presented here are applicable for investigations of the structures and evolutions of compact stars in a unified manner. | unified neutron star eoss and neutron star structures in rmf models |
the main focus of this paper is to explore the possibility of providing a new family of exact solutions for suitable anisotropic spherically symmetric systems in the realm of general relativity involving the embedding spherically symmetric static metric into the five-dimensional pseudo-euclidean space. in this regard, we ansatz a new metric potential λ (r ) , and we obtained the other metric potential ν (r ) by mains of embedding class one approach. the unknown constants are determined by the matching of interior space-time with the schwarzschild exterior space-time. the physical acceptability of the generating celestial model for anisotropic compact stars is approved via acting several physical tests of the main salient features viz., energy density, radial and tangential pressures, anisotropy effect, dynamical equilibrium, energy conditions, and dynamical stability, which are well-compared with experimental statistics of four different compact stars: psr j1416-2230, psr j1903+327, 4u 1820-30 and cen x-3. conclusively, all the compact stars under observations are realistic, stable, and are free from any physical or geometrical singularities. we find that the embedding class one solution for anisotropic compact stars is viable and stable, plus, it provides circumstantial evidence in favor of super-massive pulsars. | anisotropic compact stars via embedding approach in general relativity: new physical insights of stellar configurations |
gravity theories beyond general relativity typically predict dipolar gravitational emission by compact-star binaries. this emission is sourced by "sensitivity" parameters depending on the stellar compactness. we introduce a general formalism to calculate these parameters, and show that in shift-symmetric horndeski theories stellar sensitivities and dipolar radiation vanish, provided that the binary's dynamics is perturbative (i.e., the post-newtonian formalism is applicable) and cosmological-expansion effects can be neglected. this allows one to reproduce the binary-pulsar-observed orbital decay. | gravitation-wave emission in shift-symmetric horndeski theories |
in present article we extend one of our earlier works bhar et al. (astrophys. space sci. 359: 13, 2015) by considering quadratic equation of state for the matter distribution. the solution has its distinct feature as the eos chosen is quadratic and is presenting for the first time in tolman vii background. the solution is well behaved in nature in all respects and satisfies energy conditions. the solution is also free from central singularities and satisfies buchdahl condition. using this solution, we optimized the masses and radii of few well-known compact stars namely her x-1, rx j1856.5-3754, psr b0943 + 10, psr b1913 + 16 and cyg x-2 with their experimentally observed values. | compact star modeling with quadratic equation of state in tolman vii space-time |
lorentz symmetry violations can be described by an effective field theory framework that contains both general relativity and the standard model of particle physics called the standard model extension (sme). recently, postfit analysis of gravity probe b and binary pulsars led to an upper limit at the 10-4 level on the time-time coefficient s¯t t of the pure-gravity sector of the minimal sme. in this work, we derive the observable of very long baseline interferometry (vlbi) in sme and then implement it into a real data analysis code of geodetic vlbi observations. analyzing all available observations recorded since 1979, we compare estimates of s¯t t and errors obtained with various analysis schemes, including global estimations over several time spans, and with various sun elongation cutoff angles, and by analysis of radio source coordinate time series. we obtain a constraint on s¯ t t=(-5 ±8 )×10-5 , directly fitted to the observations and improving by a factor of 5 previous postfit analysis estimates. | lorentz symmetry and very long baseline interferometry |
continued observation of psr j0737-3039, the double pulsar, is expected to yield a precise determination of its primary component’s moment of inertia in the next few years. since the moment of inertia depends sensitively on the neutron star’s internal structure, such a measurement will constrain the equation of state of ultra-dense matter, which is believed to be universal. independent equation-of-state constraints have already been established by the gravitational-wave measurement of neutron star tidal deformability in gw170817. here, using well-known universal relations among neutron star observables, we translate the reported 90% credible bounds on tidal deformability into a direct constraint, {i}\star ={1.15}-0.24+0.38× {10}45 {{g}} {cm}}2, on the moment of inertia of psr j0737-3039a. should a future astrophysical measurement of i ⋆ disagree with this prediction, it could indicate a breakdown in the universality of the neutron star equation of state. | constraints on the moment of inertia of psr j0737-3039a from gw170817 |
while binary pulsar systems are fantastic laboratories for a wide array of astrophysics, they are particularly difficult to detect. the orbital motion of the pulsar changes its apparent spin frequency over the course of an observation, essentially “smearing” the response of the time series in the fourier domain. we review the fourier domain acceleration search (fdas), which uses a matched filtering algorithm to correct for this smearing by assuming constant acceleration for a small enough portion of the orbit. we discuss the theory and implementation of a fourier domain “jerk” search, developed as part of the presto software package, which extends the fdas to account for a linearly changing acceleration, or constant orbital jerk, of the pulsar. we test the performance of our algorithm on archival green bank telescope observations of the globular cluster terzan 5, and show that while the jerk search has a significantly longer runtime, it improves search sensitivity to binaries when the observation duration is 5%-15% of the orbital period. finally, we present the jerk-search-enabled detection of ter5am (psr j1748-2446am), a new highly accelerated pulsar in a compact, eccentric, and relativistic orbit, with a likely pulsar mass of {1.649}-0.11+0.037 m ⊙. | a fourier domain “jerk” search for binary pulsars |
we construct realistic sensitivity curves for pulsar timing array searches for gravitational waves, incorporating both red and white noise contributions to individual pulsar noise spectra, and the effect of fitting to a pulsar timing model. we demonstrate the method on both simulated pulsars and a realistic array consisting of a subset of nanograv pulsars used in recent analyses. a comparison between the results presented here and measured upper limit curves from actual analyses shows agreement to tens of percent. the resulting sensitivity curves can be used to assess the detectability of predicted gravitational-wave signals in the nanohertz frequency band in a coherent, flexible, and computationally efficient manner. | realistic sensitivity curves for pulsar timing arrays |
we study the formation and evolution of topological defects in an aligned axion model with multiple peccei-quinn scalars, where the qcd axion is realized by a certain combination of the axions with decay constants much smaller than the conventional peccei-quinn breaking scale. when the underlying u(1) symmetries are spontaneously broken, the aligned structure in the axion field space exhibits itself as a complicated string-wall network in the real space. we find that the string-wall network likely survives until the qcd phase transition if the number of the peccei-quinn scalars is greater than two. the string-wall system collapses during the qcd phase transition, producing a significant amount of gravitational waves in the nano-hz range at present. the typical decay constant is constrained to be below o(100) tev by the pulsar timing observations, and the constraint will be improved by a factor of 2 in the future ska observations. | topological defects and nano-hz gravitational waves in aligned axion models |
the canadian hydrogen observatory and radio-transient detector (chord) is a next-generation radio telescope, proposed for construction over the next 3-5 years, which will leverage canadian technology developments to yield breakthrough measurements of the cosmos. chord is a pan-canadian project, designed to work with and build on the success of the canadian hydrogen intensity mapping experiment (chime). it is an ultra-wideband, "large-n, small-d" telescope, combining a large number of small-diameter dishes for extreme sensitivity over a large field-of-view. chord consists of a central array of 512x6-m dishes, supported by a pair of distant outrigger stations, each equipped with chime-like cylinders and a 64-dish array. with breakthrough sensitivity, bandwidth, and localization capabilities, chord will measure the distribution of matter over a huge swath of the universe, detect and localize tens of thousands of fast radio bursts (frbs), and undertake cutting-edge measurements of fundamental physics. global leadership in radio astronomy: canada has emerged as a world leader in radio astronomy, thanks in large part to substantial investments made over the last decade. our national team has built and deployed the world's leading instruments to address some of the most pressing questions in astrophysics, including the nature of fast radio bursts, using radio pulsars to study fundamental physics, mapping large scale structure with intensity mapping, and measuring the cosmic microwave background radiation from the early universe. the cfi-funded canadian hydrogen intensity mapping experiment (chime) in particular has been a game-changer that has elevated the work of canadian astrophysics and has led to intense international interest. although chime only came online in summer 2018, our early results have been so spectacular that they were featured on the cover of nature in february 2019. chord will offer observational capabilities unprecedented in radio astronomy, including higher wideband mapping speed than any other instrument in the world. this world-leading instrument will allow our team to address three of the most exciting areas of astrophysics: - elucidating the nature of fast radio bursts and their precise location within galactic hosts; - mapping the distribution of matter on cosmic scales to reveal the detailed evolution of structure in the universe; and - measuring fundamental physics parameters, such as probing neutrino properties and testing general relativity. we envision chord as a flagship of canadian science built firmly on a foundation of canadian innovation. enhance research capacity while forging productive partnerships: chord will be a truly national effort that deepens the relationships that have developed in canada's radio astronomy community through previous cfi-funded collaborations. the core institutions include uoft, mcgill, u calgary, the perimeter institute, and nrc herzberg. internationally, the team will continue their successful partnerships with world-leading groups at mit, nrao, west virginia, and yale and forge new ones with the deep synoptic array, zwicky transient facility and nobel-winning ligo. moreover, the scientific and technical achievements delivered through chord will strengthen canada's position in other international efforts such as the square kilometre array. generate benefits: chord will elevate the profile and impact of the entire canadian astrophysics community and our high-tech industry. aside from reinforcing the synergies between the academic partners, this collaboration involves several canadian private sector companies active in engineering, wireless communications, digital signal processing and high-performance computing. as a digital telescope, chord will generate and process enormous volumes of data, greater than the total traffic on the canadian internet. chord's technological demands pose rich challenges for our industrial partners, including amd, ibm, intel and coolit. previous collaborations have already led to several innovations that have been adopted into our partner's products. moreover, the end-to-end process of designing, building, and commissioning a cutting-edge scientific instrument to make precision measurements provides an ideal training ground for highly qualified personnel. these trainees will gain skills in advanced hardware design, algorithm design, and data management, and will be highly sought after by academia and canadian high-technology companies. | the canadian hydrogen observatory and radio-transient detector (chord) |
magnetic braking (mb) likely plays a vital role in the evolution of low-mass x-ray binaries (lmxbs). however, there is still uncertainty around the physics of mb, and there are various proposed scenarios for mb in the literature. to examine and discriminate the efficiency of mb, we investigate the lmxb evolution with five proposed mb laws. combining detailed binary evolution calculation with binary population synthesis, we obtain the expected properties of lmxbs and their descendant binary millisecond pulsars. we then discuss the strength and weakness of each mb law by comparing the calculated results with observations. we conclude that the τ-boosted mb law seems to best match the observational characteristics. | evolution of lmxbs under different magnetic braking prescriptions |
∼6% of all known pulsars have been observed to exhibit sudden spin-up events, known as glitches. for more than fifty years, these phenomena have played an important role in helping to understand pulsar (astro)physics. based on the review of pulsar glitches search method, the progress made in observations in recent years is summarized, including the achievements obtained by chinese telescopes. glitching pulsars demonstrate great diversity of behaviours, which can be broadly classified into four categories: normal glitches, slow glitches, glitches with delayed spin-ups, and anti-glitches. the main models of glitches that have been proposed are reviewed and their implications for neutron star structure are critically examined regarding our current understanding. furthermore, the correlations between glitches and emission changes, which suggest that magnetospheric state-change is linked to the pulsar-intrinsic processes, are also described and discussed in some detail. | pulsar glitches: a review |
we describe the ongoing relativistic binary programme (relbin), a part of the meertime large survey project with the meerkat radio telescope. relbin is primarily focused on observations of relativistic effects in binary pulsars to enable measurements of neutron star masses and tests of theories of gravity. we selected 25 pulsars as an initial high priority list of targets based on their characteristics and observational history with other telescopes. in this paper, we provide an outline of the programme, and present polarization calibrated pulse profiles for all selected pulsars as a reference catalogue along with updated dispersion measures. we report faraday rotation measures for 24 pulsars, twelve of which have been measured for the first time. more than a third of our selected pulsars show a flat position angle swing confirming earlier observations. we demonstrate the ability of the rotating vector model, fitted here to seven binary pulsars, including the double pulsar (psr j0737-3039a), to obtain information about the orbital inclination angle. we present a high time resolution light curve of the eclipse of psr j0737-3039a by the companion's magnetosphere, a high-phase-resolution position angle swing for psr j1141-6545, an improved detection of the shapiro delay of psr j1811-2405, and pulse scattering measurements for psrs j1227-6208, j1757-1854, and j1811-1736. finally, we demonstrate that timing observations with meerkat improve on existing data sets by a factor of, typically, 2-3, sometimes by an order of magnitude. | the relativistic binary programme on meerkat: science objectives and first results |
gravitational-wave (gw) astronomy, together with precise pulsar timing and long baseline interferometry, is changing our ability to perform tests of fundamental physics with astrophysical observations. some of these tests are based on electromagnetic (em) probes or electrically charged bodies, and assume an empty universe. however, the cosmos is filled with plasma, a dilute medium which prevents the propagation of low-frequency, small-amplitude em waves. we show that the plasma hinders our ability to perform some strong-field gravity tests, in particular: (i) nonlinear plasma effects dramatically quench plasma-driven super-radiant instabilities; (ii) the contribution of em emission to the inspiral of charged black-hole binaries is strongly suppressed; (iii) em-driven secondary modes, although present in the spectrum of charged black holes, are excited to negligible amplitude in the gw ringdown signal. the last two effects are relevant also in the case of massive fields that propagate in vacuum and can jeopardize tests of modified theories of gravity containing massive degrees of freedom. | the tune of the universe: the role of plasma in tests of strong-field gravity |
we consider the current observed ensemble of pulsing ultraluminous x-ray sources (pulxs). we show that all of their observed properties (luminosity, spin period, and spin-up rate) are consistent with emission from magnetic neutron stars with fields in the usual range 10^{11}-10^{13} g, which is collimated (`beamed') by the outflow from an accretion disc supplied with mass at a super-eddington rate, but ejecting the excess, in the way familiar for other (non-pulsing) ulxs. the observed properties are inconsistent with magnetar-strength fields in all cases. we point out that all proposed pictures of magnetar formation suggest that they are unlikely to be members of binary systems, in agreement with the observation that all confirmed magnetars are single. the presence of magnetars in ulxs is therefore improbable, in line with our conclusions above. | no magnetars in ulxs |
the timing of millisecond pulsars has long been used as an exquisitely precise tool for testing the building blocks of general relativity, including the strong equivalence principle and lorentz symmetry. observations of binary systems involving at least 1 ms pulsar have been used to place bounds on the parameters of einstein-æther theory, a gravitational theory that violates lorentz symmetry at low energies via a preferred and dynamical time threading of the spacetime manifold. however, these studies did not cover the region of parameter space that is still viable after the recent bounds on the speed of gravitational waves from gw170817/grb170817a. the restricted coverage was due to limitations in the methods used to compute the pulsar 'sensitivities', which parameterize violations of the strong-equivalence principle in these systems. we extend here the calculation of pulsar sensitivities to the parameter space of einstein-æther theory that remains viable after gw170817/grb170817a. we show that observations of the damping of the period of quasi-circular binary pulsars and of the triple system psr j0337+1715 further constrain the viable parameter space by about an order of magnitude over previous constraints. | new binary pulsar constraints on einstein-æther theory after gw170817 |
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