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gravitational waves with frequencies below 1 nhz are notoriously difficult to detect. with periods exceeding current experimental lifetimes, they induce slow drifts in observables rather than periodic correlations. observables with well-known intrinsic contributions provide a means to probe this regime. in this work, we demonstrate the viability of using observed pulsar timing parameters to discover such ''ultralow'' frequency gravitational waves, presenting two complementary observables for which the systematic shift induced by ultralow-frequency gravitational waves can be extracted. using existing data for these parameters, we search the ultralow frequency regime for continuous-wave signals, finding a sensitivity near the expected prediction from supermassive black hole mergers. we do not see an excess in the data, setting a limit on the strain of $ 7.1 \times 10 ^{ - 14} $ at 1 nhz with a sensitivity dropping approximately quadratically with frequency until 10 phz. our search method opens a new frequency range for gravitational wave detection and has profound implications for astrophysics, cosmology, and particle physics. | using pulsar parameter drifts to detect sub-nanohertz gravitational waves |
the direct detection of gravitational waves provides the opportunity to measure fundamental aspects of gravity which have never been directly probed before, including the polarization of gravitational waves. in the context of searches for continuous waves from known pulsars, we present novel methods to detect signals of any polarization content, measure the modes present and place upper limits on the amplitude of nontensorial components. this will allow us to obtain new model-independent, dynamical constraints on deviations from general relativity. we test this framework on multiple potential sources using simulated data from three advanced-era detectors at design sensitivity. we find that signals of any polarization will become detectable and distinguishable for characteristic strains h ≳3 ×10-27√{1 yr /t }, for an observation time t . we also find that our ability to detect nontensorial components depends only on the power present in those modes, irrespective of the strength of the tensorial strain. | probing dynamical gravity with the polarization of continuous gravitational waves |
recent progress in the determination of both masses and radii of neutron stars has put strong constraints on the equation of state (eos) above the nuclear saturation density. within a confining quark matter model, we propose an anisotropic star consisting of a homogeneous and unpaired charge-neutral 3-flavor interacting quark matter with o (ms4) corrections in the context of einstein-gauss-bonnet gravity theory. this generalized model depends only on three free parameters: the bag constant b, the interaction parameter a and the gauss-bonnet coupling constant α. given the underlying eos, we show the possibility of obtaining the maximal neutron star mass which satisfies the recent observational data for psr j0751+1807. the numerical analysis of mass-radius relations supports the existence of other massive pulsars with a maximum mass consistent and common radii in the range of r ≲ (11 ∼ 14) km [1]. furthermore, we discuss the mass vs central mass density (m -ρc) relation for stability, compactness and binding energy in this gravity theory. our results thus provide circumstantial evidence in favor of super-massive pulsars in egb gravity. | anisotropic quark stars in einstein-gauss-bonnet theory |
ultralight particles (ma∼10-21- 10-22 ev ) with axionlike couplings to other particles can be candidates for fuzzy dark matter (fdm) if the axion decay constant fa∼1017 gev . if a compact star is immersed in such a low-mass axionic potential, it develops a long-range field outside the star. this axionic field is radiated away when the star is in a binary orbit. the orbital period of a compact binary decays mainly due to the gravitational wave radiation, which was confirmed first in the hulse-taylor binary pulsar. the orbital period can also decay by radiation of other light particles like axions and axionlike particles (alps). for axionic radiation to take place, the orbital frequency of the periodic motion of the binary system should be greater than the mass of the scalar particle which can be radiated. this implies that, for most of the observed binaries, particles with mass ma<10-19 ev can be radiated, which includes fdm particles. in this paper, we consider four compact binary systems—psr j 0348 +0432 , psr j 0737 -3039 , psr j 1738 +0333 , and psr b 1913 +16 (hulse-taylor binary)—and show that the observations of the decay in orbital period set a bound on the axion decay constant of fa≲o (1011 gev ) . this implies that fuzzy dark matter cannot couple to gluons. | constraints on ultralight axions from compact binary systems |
an isotropic stochastic background of nanohertz gravitational waves creates excess residual power in pulsar-timing-array datasets, with characteristic interpulsar correlations described by the hellings-downs function. these correlations appear as nondiagonal terms in the noise covariance matrix, which must be inverted to obtain the pulsar-timing-array likelihood. searches for the stochastic background, which require many likelihood evaluations, are therefore quite computationally expensive. we propose a more efficient method: we first compute approximate posteriors by ignoring cross correlations and then reweight them to exact posteriors via importance sampling. we show that this technique results in accurate posteriors and marginal likelihood ratios, because the approximate and exact posteriors are similar, which makes reweighting especially accurate. the bayes ratio between the marginal likelihoods of the exact and approximate models, commonly used as a detection statistic, is also estimated reliably by our method, up to ratios of at least 106 . | accurate characterization of the stochastic gravitational-wave background with pulsar timing arrays by likelihood reweighting |
over the past decade, the discovery of three unique stellar populations and a large number of confirmed pulsars within the globular cluster terzan 5 has raised questions over its classification. using the long-term radio pulsar timing of 36 ms pulsars in the cluster core, we provide new measurements of key physical properties of the system. as terzan 5 is located within the galactic bulge, stellar crowding and reddening make optical and near-infrared observations difficult. pulsar accelerations, however, allow us to study the intrinsic characteristics of the cluster independent of reddening and stellar crowding and probe the mass density profile without needing to quantify the mass-to-light ratio. relating the spin and orbital periods of each pulsar to the acceleration predicted by a king model, we find a core density of {1.58}-0.13+0.13 × 106 {m}⊙pc-3, a core radius of {0.16}-0.01+0.01 pc, a pulsar density profile of n\propto {r}-{3.14-0.53+0.52}, and a total mass of {m}{{t}}({r}\perp < 1.0 pc) ≃ 3.0 × 105 {m}⊙ , assuming a cluster distance of 5.9 kpc. using this information, we argue against terzan 5 being a disrupted dwarf galaxy and discuss the possibility of it being a fragment of the milky way’s proto-bulge. we also discuss whether low-mass pulsars were formed via electron-capture supernovae or exist in a core full of heavy white dwarfs and hard binaries. finally, we provide an upper limit for the mass of a possible black hole at the core of the cluster of {m}bh}≃ 3× {10}4 {m}⊙ . | using long-term millisecond pulsar timing to obtain physical characteristics of the bulge globular cluster terzan 5 |
the aim of this work is to formulate two new solutions by decoupling the field equations via a minimal geometric deformation in the context of self-interacting brans-dicke gravity. we introduce an extra source in the anisotropic fluid distribution to generate new analogs of existing solutions. the radial metric function is transformed to decouple the field equations into two sets such that each array corresponds to one source only. the system corresponding to the original matter distribution is specified by metric functions of well-behaved solutions. on the other hand, the second set is closed by imposing constraints on the additional matter source. for this purpose, we have applied the isotropization condition as well as vanishing complexity condition on the new source. smooth matching of interior and exterior spacetimes at the junction provides values of the unknown constants. interesting physical features of corresponding models are checked by employing the mass and radius of the star psr j1614-2230. it is concluded that both extensions yield viable and stable models for certain values of the decoupling parameter. | isotropization and complexity of decoupled solutions in self-interacting brans-dicke gravity |
we investigate the scalar-tensor gravity of damour and esposito-farèse (def), which predicts nontrivial phenomena in the nonperturbative strong-field regime for neutron stars (nss). instead of solving the modified tolman-oppenheimer-volkoff equations, we construct reduced-order surrogate models, coded in the pyuc(stgrom) package, to predict the relations of a ns radius, mass, and effective scalar coupling to its central density. our models are accurate at ∼1 % level and speed up large-scale calculations by 2 orders of magnitude. as an application, we use pyuc(stgrom) and markov-chain monte carlo techniques to constrain parameters in the def theory, with five well-timed binary pulsars, the binary ns (bns) inspiral gw170817, and a hypothetical bns inspiral in the advanced ligo and next-generation gw detectors. in the future, as more binary pulsars and bns mergers are detected, our surrogate models will be helpful in constraining strong-field gravity with essential speed and accuracy. | reduced-order surrogate models for scalar-tensor gravity in the strong field regime and applications to binary pulsars and gw170817 |
precise and reliable measurements of the masses and radii of neutron stars with a variety of masses would provide valuable guidance for improving models of the properties of cold matter with densities above the saturation density of nuclear matter. several different approaches for measuring the masses and radii of neutron stars have been tried or proposed, including analyzing the x-ray fluxes and spectra of the emission from neutron stars in quiescent low-mass x-ray binary systems and thermonuclear burst sources; fitting the energy-dependent x-ray waveforms of rotation-powered millisecond pulsars, burst oscillations with millisecond periods, and accretion-powered millisecond pulsars; and modeling the gravitational radiation waveforms of coalescing double neutron star and neutron star - black hole binary systems. we describe the strengths and weaknesses of these approaches, most of which currently have substantial systematic errors, and discuss the prospects for decreasing the systematic errors in each method. | observational constraints on neutron star masses and radii |
we report the detection of 48 millisecond pulsars (msps) out of 75 observed thus far using the low-frequency array (lofar) in the frequency range 110-188 mhz. we have also detected three msps out of nine observed in the frequency range 38-77 mhz. this is the largest sample of msps ever observed at these low frequencies, and half of the detected msps were observed for the first time atfrequencies below 200 mhz. we present the average pulse profiles of the detected msps, their effective pulse widths, and flux densities and compare these with higher observing frequencies. the flux-calibrated, multifrequency lofar pulse profiles are publicly available via the european pulsar network database of pulsar profiles. we also present average values of dispersion measures (dm) and discuss dm and profile variations. about 35% of the msps show strong narrow profiles, another 25% exhibit scattered profiles, and the rest are only weakly detected. a qualitative comparison of the lofar msp profiles with those at higher radio frequencies shows constant separation between profile components. similarly, the profile widths are consistent with those observed at higher frequencies, unless scattering dominates at the lowest frequencies. this is very different from what is observed for normal pulsars and suggests a compact emission region in the msp magnetosphere. the amplitude ratio of the profile components, on the other hand, can dramatically change towards low frequencies, often with the trailing component becoming dominant. as previously demonstrated this can be caused by aberration and retardation. this data set enables high-precision studies of pulse profile evolution with frequency, dispersion, faraday rotation, and scattering in the interstellar medium. characterising and correcting these systematic effects may improve pulsar-timing precision at higher observing frequencies, where pulsar timing array projects aim to directly detect gravitational waves. | a lofar census of millisecond pulsars |
this paper investigates a novel intelligent reflecting surface (irs)-based symbiotic radio (sr) system architecture consisting of a transmitter, an irs, and an information receiver (ir). the primary transmitter communicates with the ir and at the same time assists the irs in forwarding information to the ir. based on the irs's symbol period, we distinguish two scenarios, namely, commensal sr (csr) and parasitic sr (psr), where two different techniques for decoding the irs signals at the ir are employed. we formulate bit error rate (ber) minimization problems for both scenarios by jointly optimizing the active beamformer at the base station and the phase shifts at the irs, subject to a minimum primary rate requirement. specifically, for the csr scenario, a penalty-based algorithm is proposed to obtain a high-quality solution, where semi-closed-form solutions for the active beamformer and the irs phase shifts are derived based on lagrange duality and majorization-minimization methods, respectively. for the psr scenario, we apply a bisection search-based method, successive convex approximation, and difference of convex programming to develop a computationally efficient algorithm, which converges to a locally optimal solution. simulation results demonstrate the effectiveness of the proposed algorithms and show that the proposed sr techniques are able to achieve a lower ber than benchmark schemes. | a novel wireless communication paradigm for intelligent reflecting surface based symbiotic radio systems |
47 tuc x9 is a low-mass x-ray binary (lmxb) in the globular cluster 47 tucanae, and was previously thought to be a cataclysmic variable. however, miller-jones et al. recently identified a radio counterpart to x9 (inferring a radio/x-ray luminosity ratio consistent with black hole lmxbs), and suggested that the donor star might be a white dwarf. we report simultaneous observations of x9 performed by chandra, nustar and australia telescope compact array. we find a clear 28.18 ± 0.02-min periodic modulation in the chandra data, which we identify as the orbital period, confirming this system as an ultracompact x-ray binary. our x-ray spectral fitting provides evidence for photoionized gas having a high oxygen abundance in this system, which indicates a c/o white dwarf donor. we also identify reflection features in the hard x-ray spectrum, making x9 the faintest lmxb to show x-ray reflection. we detect an ∼6.8-d modulation in the x-ray brightness by a factor of 10, in archival chandra, swiftand rosat data. the simultaneous radio/x-ray flux ratio is consistent with either a black hole primary or a neutron star primary, if the neutron star is a transitional millisecond pulsar. considering the measured orbital period (with other evidence of a white dwarf donor), and the lack of transitional millisecond pulsar features in the x-ray light curve, we suggest that this could be the first ultracompact black hole x-ray binary identified in our galaxy. | the ultracompact nature of the black hole candidate x-ray binary 47 tuc x9 |
psr j2032+4127 is a γ-ray and radio-emitting pulsar which has been regarded as a young luminous isolated neutron star. however, its recent spin-down rate has extraordinarily increased by a factor of 2. we present evidence that this is due to its motion as a member of a highly-eccentric binary system with an ∼15-m⊙ be star, mt91 213. timing observations show that, not only are the positions of the two stars coincident within 0.4 arcsec, but timing models of binary motion of the pulsar fit the data much better than a model of a young isolated pulsar. mt91 213, and hence the pulsar, lie in the cyg ob2 stellar association, which is at a distance of only 1.4-1.7 kpc. the pulsar is currently on the near side of, and accelerating towards, the be star, with an orbital period of 20-30 yr. the next periastron is well constrained to occur in early 2018, providing an opportunity to observe enhanced high-energy emission as seen in other be-star binary systems. | the binary nature of psr j2032+4127 |
galactic plane radio surveys play a key role in improving our understanding of a wide range of astrophysical phenomena. performing such a survey using the latest interferometric telescopes produces large data rates necessitating a shift towards fully or quasi-real-time data analysis with data being stored for only the time required to process them. we present here the overview and set-up for the 3000-h max-planck-institut für radioastronomie (mpifr)-meerkat galactic plane survey (mmgps). the survey is unique by operating in a commensal mode, addressing key science objectives of the survey including the discovery of new pulsars and transients and studies of galactic magnetism, the interstellar medium and star formation rates. we explain the strategy coupled with the necessary hardware and software infrastructure needed for data reduction in the imaging, spectral, and time domains. we have so far discovered 78 new pulsars including 17 confirmed binary systems of which two are potential double neutron star systems. we have also developed an imaging pipeline sensitive to the order of a few tens of micro-jansky ($\mu{\rm jy}$) with a spatial resolution of a few arcseconds. further science operations with an in-house built s-band receiver operating between 1.7 and 3.5 ghz are about to commence. early spectral line commissioning observations conducted at s-band, targeting transitions of the key molecular gas tracer ch at 3.3 ghz already illustrate the spectroscopic capabilities of this instrument. these results lay a strong foundation for future surveys with telescopes like the square kilometre array (ska). | the mpifr-meerkat galactic plane survey - i. system set-up and early results |
context. we present a detailed view of the pulsar wind nebula (pwn) hess j1825-137. we aim to constrain the mechanisms dominating the particle transport within the nebula, accounting for its anomalously large size and spectral characteristics.aims: the nebula was studied using a deep exposure from over 12 years of h.e.s.s. i operation, together with data from h.e.s.s. ii that improve the low-energy sensitivity. enhanced energy-dependent morphological and spatially resolved spectral analyses probe the very high energy (vhe, e > 0.1 tev) γ-ray properties of the nebula.methods: the nebula emission is revealed to extend out to 1.5° from the pulsar, 1.5 times farther than previously seen, making hess j1825-137, with an intrinsic diameter of 100 pc, potentially the largest γ-ray pwn currently known. characterising the strongly energy-dependent morphology of the nebula enables us to constrain the particle transport mechanisms. a dependence of the nebula extent with energy of r ∝ eα with α = -0.29 ± 0.04stat ± 0.05sys disfavours a pure diffusion scenario for particle transport within the nebula. the total γ-ray flux of the nebula above 1 tev is found to be (1.12 ± 0.03stat ± 0.25sys) × 10-11 cm-2 s-1, corresponding to 64% of the flux of the crab nebula.results: hess j1825-137 is a pwn with clearly energy-dependent morphology at vhe γ-ray energies. this source is used as a laboratory to investigate particle transport within intermediate-age pwne. based on deep observations of this highly spatially extended pwn, we produce a spectral map of the region that provides insights into the spectral variation within the nebula. sky maps as fits files and spectra are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/621/a116 | particle transport within the pulsar wind nebula hess j1825-137 |
modeling of the nicer x-ray waveform of the pulsar psr j0030+0451, aimed at constraining the neutron star mass and radius, has inferred surface hot spots (the magnetic polar caps) that imply significantly nondipolar magnetic fields. to this end, we investigate magnetic field configurations that comprise offset dipole-plus-quadrupole components using a static vacuum field and force-free global magnetosphere models. taking into account the compactness and observer angle values provided by miller et al. and riley et al., we compute geodesics from the observer plane to the polar caps to compute the resulting x-ray light curve. we explore, through markov chain monte carlo techniques, the detailed magnetic field configurations that can reproduce the observed x-ray light curve and have discovered degeneracies, i.e., diverse field configurations, which can provide sufficient descriptions of the nicer x-ray waveforms. having obtained the force-free field structures, we then compute the corresponding synchronous γ-ray light curves following kalapotharakos et al.; these we compare to those obtained by fermi-lat, to provide models consistent with both the x-ray and the γ-ray data, thereby restricting further the multipole field parameters. an essential aspect of this approach is the proper computation of the relative phase between the synchronous x- and γ-ray light curves. we conclude with a discussion of the broader implications of our study. | the multipolar magnetic field of the millisecond pulsar psr j0030+0451 |
the recent discovery of a new population of ultrahigh-energy gamma-ray sources with spectra extending beyond 100 tev revealed the presence of galactic pevatrons-cosmic-ray factories accelerating particles to pev energies. these sources, except for the one associated with the crab nebula, are not yet identified. with an extension of 1° or more, most of them contain several potential counterparts, including supernova remnants, young stellar clusters, and pulsar wind nebulae (pwne), which can perform as pevatrons and thus power the surrounding diffuse ultrahigh-energy gamma-ray structures. in the case of pwne, gamma-rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse compton scattering of 2.7 k cosmic microwave background (cmb)radiation. the high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of ${l}_{\gamma }\sim 0.1\dot{e}$ . the pulsar spin-down luminosity, $\dot{e}$ , also determines the absolute maximum energy of individual photons: ${e}_{\gamma ,max}\approx 0.9{\dot{e}}_{36}^{0.65}\,\,{\rm{pev}}$ . this fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young pwne with typical magnetic field of ≈100 μg with $\dot{e}\lesssim {10}^{37}\ \mathrm{erg}\,{{\rm{s}}}^{-1}$ . we discuss the implications of eγ ,max by comparing it with the highest-energy photons reported by lhaaso from a dozen of ultrahigh-energy sources. whenever a pwn origin of the emission is possible, we use the lhaaso measurements to set upper limits on the nebular magnetic field. | on the potential of bright, young pulsars to power ultrahigh gamma-ray sources |
binary neutron stars have been observed as millisecond pulsars, gravitational-wave sources, and as the progenitors of short gamma-ray bursts and kilonovae. massive stellar binaries that evolve into merging double neutron stars are believed to experience a common-envelope episode. during this episode, the envelope of a giant star engulfs the whole binary. the energy transferred from the orbit to the envelope by drag forces or from other energy sources can eject the envelope from the binary system, leading to a stripped short-period binary. in this paper, we use one-dimensional single stellar evolution to explore the final stages of the common-envelope phase in progenitors of neutron star binaries. we consider an instantaneously stripped donor star as a proxy for the common-envelope phase and study the star's subsequent radial evolution. we determine a range of stripping boundaries that allow the star to avoid significant rapid re-expansion and that thus represent plausible boundaries for the termination of the common-envelope episode. we find that these boundaries lie above the maximum compression point, a commonly used location of the core/envelope boundary. we conclude that stars may retain fractions of a solar mass of hydrogen-rich material even after the common-envelope episode. if we consider orbital energy as the only energy source available, all of our models would overfill their roche lobe after ejecting the envelope, whose binding energy includes gravitational, thermal, radiation, and recombination energy terms. | stellar response after stripping as a model for common-envelope outcomes |
ligo and virgo have initiated the era of gravitational-wave (gw) astronomy; but in order to fully explore gw frequency spectrum, we must turn our attention to innovative techniques for gw detection. one such approach is to use binary systems as dynamical gw detectors by studying the subtle perturbations to their orbits caused by impinging gws. we present a powerful new formalism for calculating the orbital evolution of a generic binary coupled to a stochastic background of gws, deriving from first principles a secularly-averaged fokker-planck equation which fully characterizes the statistical evolution of all six of the binary's orbital elements. we also develop practical tools for numerically integrating this equation, and derive the necessary statistical formalism to search for gws in observational data from binary pulsars and laser-ranging experiments. | detecting stochastic gravitational waves with binary resonance |
the crab pulsar is a typical example of a young, rapidly spinning, strongly magnetized neutron star that generates broadband electromagnetic radiation by accelerating charged particles to near light speeds in its magnetosphere1. details of this emission process so far remain poorly understood. measurement of polarization in x-rays, particularly as a function of pulse phase, is thought to be a key element necessary to unravel the mystery of pulsar radiation2-4. such measurements are extremely difficult, however: to date, crab is the only pulsar to have been detected in polarized x-rays5-8 and the measurements have not been sensitive enough to adequately reveal the variation of polarization characteristics across the pulse7. here, we present the most sensitive measurement to date of polarized hard x-ray emission from the crab pulsar and nebula in the 100-380 kev band, using the cadmium-zinc-telluride imager9 instrument on-board the indian astronomy satellite astrosat10. we confirm with high significance the earlier indication6,7 of a strongly polarized off-pulse emission. however, we also find a variation in polarization properties within the off-pulse region. in addition, our data hint at a swing of the polarization angle across the pulse peaks. this behaviour cannot be fully explained by the existing theoretical models of high-energy emission from pulsars. | phase-resolved x-ray polarimetry of the crab pulsar with the astrosat czt imager |
while a growing body of research indicates that relativistic magnetic reconnection is a prodigious source of particle acceleration in high-energy astrophysical systems, the dominant acceleration mechanism remains controversial. using a combination of fully kinetic simulations and theoretical analysis, we demonstrate that fermi-type acceleration within the large-scale motional electric fields dominates over direct acceleration from non-ideal electric fields within small-scale diffusion regions. this result has profound implications for modeling particle acceleration in large-scale astrophysical problems, as it opens up the possibility of modeling the energetic spectra without resolving microscopic diffusion regions. | determining the dominant acceleration mechanism during relativistic magnetic reconnection in large-scale systems |
psr j0540-6919 is the second-most energetic radio pulsar known and resides in the large magellanic cloud. like the crab pulsar, it is observed to emit giant radio pulses (gps). we used the newly commissioned ptuse instrument on the meerkat radio telescope to search for gps across three observations. in a total integration time of 5.7 h, we detected 865 pulses above our 7σ threshold. with full polarization information for a subset of the data, we estimated the faraday rotation measure, $\rm {rm}=-245.8 \pm 1.0$ rad m-2 towards the pulsar. the brightest of these pulses is ~60 per cent linearly polarized but the pulse-to-pulse variability in the polarization fraction is significant. we find that the cumulative gp flux distribution follows a power-law distribution with index -2.75 ± 0.02. although the detected gps make up only ~10 per cent of the mean flux, their average pulse shape is indistinguishable from the integrated pulse profile, and we postulate that, unlike in the crab pulsar, there are no additional regular emission components. the pulses are scattered at l-band frequencies with the brightest pulse exhibiting a scattering time-scale of τ = 0.92 ± 0.02 ms at 1.2 ghz. we find several of the giants display very narrow-band flux knots similar to those seen in many fast radio bursts, which we assert cannot be due to scintillation or plasma lensing. the gp time-of-arrival distribution is found to be poissonian on all but the shortest time-scales where we find four gps in six rotations, which if gps are statistically independent is expected to occur in only 1 of 7000 observations equivalent to our data. | the thousand-pulsar-array programme on meerkat - iii. giant pulse characteristics of psr j0540-6919 |
magnetic fields permeate the diffuse interstellar medium (ism) of the milky way, and are essential to explain the dynamical evolution and current shape of the galaxy. magnetic fields reveal themselves via their influence on the surrounding matter, and as such are notoriously hard to measure independently of other tracers. in this work, we attempt to disentangle an all sky map of the line-of-sight parallel component of the galactic magnetic field from the faraday effect, utilizing several tracers of the galactic thermal electron density. additionally, we aim to produce a galactic electron dispersion measure map and quantify several tracers of the structure of the ionized medium of the milky way. we rely on compiled catalogs of extragalactic faraday rotation measures and galactic pulsar dispersion measures, a well as data on bremsstrahlung and the hydrogen $\alpha$ spectral line to trace the ionized medium of the milky way. we present the first full sky map of the line-of-sight averaged galactic magnetic field. within this map, we find los parallel and los-averaged magnetic field strengths of up to 4 $\mu$g, with an all-sky root-mean-square of 1.1 $\mu$g, which is consistent with previous local measurements and global magnetic field models. additionally, we produce a detailed electron dispersion measure map, which agrees with already existing parametric models at high latitudes, but suffers from systematic effects in the disk. further analysis of our results with regard to the 3d structure of $n_{th}$ reveals that it follows a kolmogorov-type turbulence for most of the sky. from the reconstructed dispersion measure and emission measure maps we construct several tracers of variability of $n_{th}$ along the los. | disentangling the faraday rotation sky |
we investigate the joint effect of cosmological phase transitions, thermal light dark matter, and lepton asymmetry on big bang nucleosynthesis and the cosmic microwave background. we find that all of them can modify the predictions of the effective number of neutrino species and primordial nucleosynthesis. in turn, we observe that (i) cosmological observations can exclude slow and strong phase transitions with strength even smaller than o (10-3- 10-2) ; (ii) a much larger portion of the dark matter mass region is excluded when the phase transition temperature is closer to 1 mev; and (iii) the magnitude of the nonvanishing neutrino lepton asymmetry is limited to be around o (10-2- 10-1) depending on the phase transition strength. these phase transitions can produce stochastic gravitational wave background to be probed by pulsar timing array experiments. | constraints on new physics around the mev scale with cosmological observations |
the recent discovery of a radio-emitting neutron star with an ultralong spin period of 76 s, psr j0901-4046, raises a fundamental question on how such a slowly rotating magnetized object can be active in the radio band. a canonical magnetic field of 1.3 ×1014 g estimated from the pulsar period and its time derivative is wholly insufficient for psr j0901-4046 to operate. consideration of a magnetic inclination angle of 10° estimated from the pulse width gives a higher magnetic field of 1.5 ×1015 g , which is still an order of magnitude lower than the necessary minimum of 2.5 ×1016 g following from the death line for radio pulsars with magnetic fields exceeding the critical value 4.4 ×1013 g . we show that if the observed microstructure of single pulses reflects relativistic beaming, the inferred surface magnetic field appears to be 3.2 ×1016 g , and without this assumption it is no less than 2.7 ×1016 g , which explains the existence of radio emission from psr j0901-4046. this estimation makes psr j0901-4046 a radio pulsar with the strongest magnetic field known and is a sign that psr j0901-4046 slows down not by magnetic-dipole radiation, but rather by an electric current of 56 ma, when rotational energy is expended in accelerating charged particles over the polar cap. | ultraslow psr j0901-4046 with an ultrahigh magnetic field of 3.2 ×1016 g |
the sensitivity and field of view of the canadian hydrogen intensity mapping experiment (chime) has enabled its fast radio burst (frb) backend to detect thousands of frbs. however, the low angular resolution of chime prevents it from localizing most frbs to their host galaxies. very long baseline interferometry (vlbi) can readily provide the subarcsecond resolution needed to localize many frbs to their hosts. thus we developed tone: an interferometric array of eight $6~\mathrm{m}$ dishes to serve as a pathfinder for the chime/frb outriggers project, which will use wide field of view cylinders to determine the sky positions for a large sample of frbs, revealing their positions within their host galaxies to subarcsecond precision. in the meantime, tone's $\sim3333~\mathrm{km}$ baseline with chime proves to be an excellent testbed for the development and characterization of single-pulse vlbi techniques at the time of discovery. this work describes the tone instrument, its sensitivity, and its astrometric precision in single-pulse vlbi. we believe that our astrometric errors are dominated by uncertainties in the clock measurements which build up between successive crab pulsar calibrations which happen every $\approx 24~\mathrm{h}$; the wider fields of view and higher sensitivity of the outriggers will provide opportunities for higher-cadence calibration. at present, chime-tone localizations of the crab pulsar yield systematic localization errors of ${0.1}-{0.2}~\mathrm{arcsec}$ - comparable to the resolution afforded by state-of-the-art optical instruments ($\sim 0.05 ~\mathrm{arcsec}$). | tone: a chime/frb outrigger pathfinder for localizations of fast radio bursts using very long baseline interferometry |
the coherent oscillation of axionic fields naturally drives copious production of dark photon particles in the early universe, due to resonance and tachyonic enhancement. during the process, energy is abruptly transferred from the former to the latter, sourcing gravitational-wave generation. the resulting gravitational waves are eventually observed today as stochastic background. we report analytical results of this production and connect them to the recent pulsar-timing results from the nanograv collaboration. we show an available parameter space for our mechanism to account for the signal around the mass mϕ∼10-13 ev and the decay constant fϕ∼1016 gev , with a dimensionless coupling of o (1 ). a mechanism to keep the axion from dominating the universe is a necessary ingredient of this model, and we discuss a possibility to recover a symmetry and render the axion massless after the production. we also comment on potential implications of the required effective number of relativistic species for the determination of the present hubble constant. | implications of gravitational-wave production from dark photon resonance to pulsar-timing observations and effective number of relativistic species |
we report on analysis of observations of the bright transient x-ray pulsar (xrp) swift j0243.6+6124 obtained during its 2017-2018 giant outburst with insight-hxmt, nustar, and swift observatories. we focus on the discovery of a sharp state transition of the timing and spectral properties of the source at super-eddington accretion rates, which we associate with the transition of the accretion disc to a radiation pressure dominated state, the first ever directly observed for magnetized neutron star. this transition occurs at slightly higher luminosity compared to already reported transition of the source from sub- to supercritical accretion regime associate with onset of an accretion column. we argue that this scenario can only be realized for comparatively weakly magnetized neutron star, not dissimilar to other ultra-luminous x-ray pulsars, which accrete at similar rates. further evidence for this conclusion is provided by the non-detection of the transition to the propeller state in quiescence which strongly implies compact magnetosphere and thus rules out magnetar-like fields. | hot disc of the swift j0243.6+6124 revealed by insight-hxmt |
psr b1820-30a is located in the globular cluster ngc 6624 and is the closest known pulsar to the centre of any globular cluster. we present more than 25 yr of high-precision timing observations of this millisecond pulsar and obtain four rotational frequency time derivative measurements. modelling these higher order derivatives as being due to orbital motion, we find solutions that indicate the pulsar is in either a low-eccentricity (0.33 ≲ e ≲ 0.4) smaller orbit with a low-mass companion (such as a main-sequence star, white dwarf, neutron star or stellar mass black hole) or a high-eccentricity (e ≳ 0.9) larger orbit with a massive companion. the cluster mass properties and the observed properties of 4u 1820-30 and the other pulsars in the cluster argue against the low-eccentricity possibility. the high-eccentricity solution reveals that the pulsar is most likely orbiting around an intermediate-mass black hole (imbh) of mass >7500 m⊙ located at the cluster centre. a gravitational model for the globular cluster, which includes such a central bh, predicts an acceleration that is commensurate with that measured for the pulsar. it further predicts that the model-dependent minimum mass of the imbh is ∼60 000 m⊙. accounting for the associated contribution to the observed period derivative indicates that the γ-ray efficiency of the pulsar should be between 0.08 and 0.2. our results suggest that other globular clusters may also contain central bhs and they may be revealed by the study of new pulsars found sufficiently close to their centres. | evidence for an intermediate-mass black hole in the globular cluster ngc 6624 |
the frequency-dependent periodic active window of the fast radio burst frb 180916.j0158+65 (frb 180916b) was observed recently. in this letter, we propose that a be/x-ray binary (bexrb) system, which is composed of a neutron star (ns) and a be star with a circumstellar disk, might be the source of a repeating frb with periodic activities, and we apply this model to explain the activity window of frb 180916b. the interaction between the ns magnetosphere and the accreted material results in evolution of the spin period and the centrifugal force of the ns, leading to the change of the stress in the ns crust. when the stress of the crust reaches the critical value, a starquake occurs and further produces frbs. the interval between starquakes is estimated to be a few days, which is smaller than the active window of frb 180916b. when the ns moves out of the disk of the be star, the interval between starquakes becomes much longer than the orbital period, which corresponds to the nonactive phase. in this model, due to the absorption of the disk of the be star, a frequency-dependent active window would appear for the frbs, which is consistent with the observed properties of frb 180916b. and the contribution of the dispersion measure from the disk of the be star is small. in addition, the location of frb 180916b in the host galaxy is consistent with a bexrb system. | periodic activities of repeating fast radio bursts from be/x-ray binary systems |
the hawc collaboration has observed gamma rays at energies above 56 tev from a collection of nine sources. it has been suggested that this emission could be hadronic in nature, requiring that these systems accelerate cosmic-ray protons or nuclei up to pev-scale energies. in this paper, we instead show that the spectra of these objects favor a leptonic (inverse compton) origin for their emission. more specifically, the gamma-ray emission from these objects can be straightforwardly accommodated within a model in which $\sim \mathcal{o}(10\%)$ of the host pulsar's spindown power is transferred into the acceleration of electrons and positrons with a power-law spectrum that extends to several hundred tev or higher. the spectral break that is observed among these sources is naturally explained within the context of this simple model, and occurs at the energy where the timescale for energy losses matches the age of the pulsar. in contrast, this spectral feature cannot be straightforwardly accommodated in hadronic scenarios. furthermore, hadronic models predict that these sources should produce more emission at gev-scale energies than is observed. in light of these considerations, we conclude that hawc's highest energy sources should be interpreted as tev halos or pulsar wind nebulae, which produce their emission through inverse compton scattering, and are powered by the rotational kinetic energy of their host pulsar. | the highest energy hawc sources are likely leptonic and powered by pulsars |
we develop a new nonparametric method to reconstruct the equation of state (eos) of a neutron star with multimessenger data. as a universal function approximator, the feed-forward neural network (ffnn) with one hidden layer and a sigmoidal activation function can approximately fit any continuous function. thus, we are able to implement the nonparametric ffnn representation of the eoss. this new representation is validated by its capability of fitting the theoretical eoss and recovering the injected parameters. then, we adopt this nonparametric method to analyze the real data, including the mass-tidal deformability measurement from the binary neutron star merger gravitational-wave event gw170817 and mass-radius measurement of psr j0030+0451 by nicer. we take the publicly available samples to construct the likelihood and use the nested sampling to obtain the posteriors of the parameters of the ffnn according to the bayesian theorem, which in turn can be translated to the posteriors of the eos parameters. combining all of these data for a canonical 1.4 m⊙ neutron star, we get a radius ${r}_{1.4}={11.83}_{-1.08}^{+1.25}$ km and tidal deformability ${{\rm{\lambda }}}_{1.4}={323}_{-165}^{+334}$ (90% confidence interval). furthermore, we find that in the high-density region (≥3ρsat), the 90% lower limits of ${c}_{{\rm{s}}}^{2}/{c}^{2}$ (where cs is the sound speed and c is the velocity of light in vacuum) are above 1/3, which means that the so-called conformal limit (i.e., ${c}_{{\rm{s}}}^{2}/{c}^{2}\lt 1/3$ ) is not always valid in the neutron stars. | bayesian nonparametric inference of the neutron star equation of state via a neural network |
in this work we investigate neutron stars (ns) in f(script r,t) gravity for the case r+2λscript t, script r is the ricci scalar and script t the trace of the energy-momentum tensor. the hydrostatic equilibrium equations are solved considering realistic equations of state (esos). the ns masses and radii obtained are subject to a joint constrain from massive pulsars and the event gw170817. the parameter λ needs to be negative as in previous ns studies, however we found a minimum value for it. the value should be |λ|lesssim0.02 and the reason for so small value in comparison with previous ones obtained with simpler esos is due to the existence of the ns crust. the pressure in theory of gravity depends on the inverse of the sound velocity vs. since, vs is low in the crust, |λ| need to be very small. we found that the increment in the star mass is less than 1%, much smaller than previous ones obtained not considering the realistic stellar structure, and the star radius cannot become larger, its changes compared to gr is less than 3.6% in all cases. the finding that using several relativistic and non-relativistic models the variation on the ns mass and radius are almost the same for all the esos, manifests that our results are insensitive to the high density part of the esos. it confirms that stellar mass and radii changes depend only on crust, where the eos is essentially the same for all the models. the ns crust effect implying very small values of |λ| does not depend on the theory's function chosen, since for any other one the hydrostatic equilibrium equation would always have the dependence 1/vs. finally, we highlight that our results indicate that conclusions obtained from ns studies done in modified theories of gravity without using realistic esos that describe correctly the ns interior can be unreliable. | neutron stars in f(script r,t) gravity using realistic equations of state in the light of massive pulsars and gw170817 |
pulsar timing array collaborations have recently reported evidence for a noise process with a common spectrum among the millisecond pulsars in the arrays. the spectral properties of this common-noise process are consistent with expectations for an isotropic gravitational-wave background (gwb) from inspiralling supermassive black hole binaries. however, recent simulation analyses based on parkes pulsar timing array data indicate that such a detection may arise spuriously. in this paper, we use simulated pulsar timing array data sets to further test the robustness of the inference methods for spectral and spatial correlations from a gwb. expanding on our previous results, we find strong support (bayes factors exceeding 105) for the presence of a common-spectrum noise process in data sets where no common process is present, under a wide range of timing noise prescriptions per pulsar. we show that these results are highly sensitive to the choice of bayesian priors on timing noise parameters, with priors that more closely match the injected distributions of timing noise parameters resulting in diminished support for a common-spectrum noise process. these results emphasize shortcomings in current methods for inferring the presence of a common-spectrum process, and imply that the detection of a common process is not a reliable precursor to detection of the gwb. future searches for the nanohertz gwb should remain focused on detecting spatial correlations, and make use of more tailored specifications for a common-spectrum noise process. | evaluating the prevalence of spurious correlations in pulsar timing array data sets |
supernova explosion and the associated neutron star (ns) natal kicks are important events on a pathway of a binary to become a gravitational wave source, an x-ray binary, or a millisecond radio pulsar. weak natal kicks often lead to binary survival, while strong kicks frequently disrupt the binary. in this article, we aim to further constrain ns natal kicks in binaries. we explore binary population synthesis models by varying prescription for natal kick, remnant mass, and mass accretion efficiency. we introduce a robust statistical technique to analyse combined observations of different nature. using this technique, we further test different models using parallax and proper motion measurements for young isolated radio pulsars and similar measurements for galactic be x-ray binaries (bexs). our best model for natal kicks is consistent with both measurements and contains a fraction of w = 0.2 ± 0.1 weak natal kicks with $\sigma _1 = 45^{+25}_{-15}$ km s-1, the remaining natal kicks are drawn from the high-velocity component, same as in previous works: σ2 = 336 km s-1. we found that currently used models for natal kicks of nss produced by electron capture supernova (ecsn; combination of maxwellian σ = 265 km s-1 and σ = 30 km s-1 for electron capture) are inconsistent or marginally consistent with parallaxes and proper motions measured for isolated radio pulsars. we suggest a new model for natal kicks of ecsn, which satisfy both observations of isolated radio pulsars and bexs. | combined analysis of neutron star natal kicks using proper motions and parallax measurements for radio pulsars and be x-ray binaries |
in this paper we study the dynamics of neutral, electrically charged and magnetized particles around deformed electrically and magnetically charged reissner-nordström black holes. for the neutral test particles motion, it is shown that the radius of the innermost stable circular orbits (iscos) decrease with the increase in both the black hole charge and positive spacetime deformation. it reaches up to 3.85 m at the value of deformation parameter ɛ =20.45 in schwarzschild spacetime case (q =0 ). in the extreme charged reissner-nordstrom black hole case, the isco decreases up to 2.26 m at the value of deformation parameter ɛ =6.17 . moreover, the negative deformation results in an increase in the isco radius. comparing effects of positive deformation and the reissner-nordström black hole charge with spin of rotating kerr black holes, it is shown that the extremely charged reissner-nordström black hole can mimic rotating kerr black hole up to the spin parameter a /m =0.48 , while for the positive deformation with ɛ =6.17 the mimic value increases up to a /m =0.88 implying that the supermassive black hole m87 cannot be considered as reissner-nordström black hole. using this comparison, we estimate charge of the supermassive black hole sagittarius a* as q /m ≃0.8287 without deformation which can mimic the spin of the black hole. when the deformation ɛ =1 , the mimic charge increases up to q /m ≃0.8926 . our study of the energy extraction from the accretion disk shows that the maximum energy efficiency increases up to 20.02%, which is almost the same for extreme kerr black hole case (20.6%). we have also considered the behaviour of isco of electrically charged particles showing that the attractive (repulsive) electrostatic interactions cause rapid increase (slightly decrease) of the isco radius. finally, we have explored the dynamics of magnetized particles around deformed magnetically charged reissner-nordström black hole. by treating the magnetar psr j1745-2900, orbiting the supermassive black hole sagittarius a* as a magnetized particle, showing that the magnetic charge of pure reissner-nordström black hole can mimic spin of a rotating kerr black hole up to a /m ≃0.82 for the value to be in the range qm/m ∈(0 ,0.692 ). moreover, we find that the positive values of the deformation parameter lead to shift the isco for the magnetized particles toward the central object. | test particles dynamics around deformed reissner-nordström black hole |
gravitational waves (gws) can offer a novel window into the structure and dynamics of neutron stars. here we present the first search for long-duration quasimonochromatic gw transients triggered by pulsar glitches. we focus on two glitches observed in radio timing of the vela pulsar (psr j0835-4510) on 12 december 2016 and the crab pulsar (psr j 0534 +2200 ) on 27 march 2017, during the advanced ligo second observing run (o2). we assume the gw frequency lies within a narrow band around twice the spin frequency as known from radio observations. using the fully-coherent transient-enabled f -statistic method, we search for transients of up to four months in length. we find no credible gw candidates for either target, and through simulated signal injections we set 90% upper limits on (constant) gw strain as a function of transient duration. for the larger vela glitch, we come close to beating an indirect upper limit for when the total energy liberated in the glitch would be emitted as gws, thus demonstrating that similar postglitch searches at improved detector sensitivity can soon yield physical constraints on glitch models. | first search for long-duration transient gravitational waves after glitches in the vela and crab pulsars |
pulsars with high spin-down power produce relativistic winds radiating a non-negligible fraction of this power over the whole electromagnetic range from radio to gamma-rays in the pulsar wind nebulae (pwne). the rest of the power is dissipated in the interactions of the pwne with the ambient interstellar medium (ism). some of the pwne are moving relative to the ambient ism with supersonic speeds producing bow shocks. in this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may undergo reacceleration in the converging flow system formed by the plasma outflowing from the wind termination shock and the plasma inflowing from the bow shock. the presence of magnetic perturbations in the flow, produced by instabilities induced by the accelerated particles themselves, is essential for the process to work. a generic outcome of this type of reacceleration is the creation of particle distributions with very hard spectra, such as are indeed required to explain the observed spectra of synchrotron radiation with photon indices γ≲ 1.5. the presence of this hard spectral component is specific to pwne with bow shocks (bspwne). the accelerated particles, mainly electrons and positrons, may end up containing a substantial fraction of the shock ram pressure. in addition, for typical ism and pulsar parameters, the e+ released by these systems in the galaxy are numerous enough to contribute a substantial fraction of the positrons detected as cosmic ray (cr) particles above few tens of gev and up to several hundred gev. the escape of ultrarelativistic particles from a bspwn—and hence, its appearance in the far-uv and x-ray bands—is determined by the relative directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. in this respect we review the observed appearance and multiwavelength spectra of three different types of bspwne: psr j0437-4715, the guitar and lighthouse nebulae, and vela-like objects. we argue that high resolution imaging of such objects provides unique information both on pulsar winds and on the ism. we discuss the interpretation of imaging observations in the context of the model outlined above and estimate the bspwn contribution to the positron flux observed at the earth. | pulsar wind nebulae with bow shocks: non-thermal radiation and cosmic ray leptons |
the nanohertz stochastic gravitational wave background (sgwb) is an excellent early universe laboratory for testing the fundamental properties of gravity. in this letter, we elucidate on the full potential of pulsar timing array (pta) by utilizing cosmic variance-limited, or rather experimental noise-free, correlation measurements to understand the sgwb and by extension gravity. we show that measurements of the angular power spectrum play a pivotal role in the pta precision era for scientific inferencing. in particular, we illustrate that cosmic variance-limited measurements of the first few power spectrum multipoles enable us to clearly set apart general relativity from alternative theories of gravity. this ultimately conveys that ptas can be most ambitious for testing gravity in the nanohertz gw regime by zeroing in on the power spectrum. | testing gravity with cosmic variance-limited pulsar timing array correlations |
we investigate the use of harmonic analysis techniques to perform measurements of the angular power spectrum on mock pulsar timing data for an isotropic stochastic gravitational-wave background (sgwb) with a dimensionless strain amplitude $a_{\text{gw}}=2 \times 10^{-15}$ and spectral index $\gamma_{\text{gw}}=13/3$. we examine the sensitivity of our harmonic analysis to the number of pulsars (50, 100, and 150) and length of pulsar observation time (10, 20, and 30 years) for an isotropic distribution of pulsars. we account for intrinsic pulsar red noise and use an average value of white noise of ~100 ns. we are able to detect the quadrupole for all our mock harmonic analyses, and for the analysis with 150 pulsars observed for 30 years, we are able to detect up to the $\ell = 5$ multipole. we provide scaling laws for the sgwb amplitude, the quadrupole, and $\ell = 3$ as a function of pulsar observation time and as a function of number of pulsars. we estimate the sensitivity of our harmonic approach to deviations of general relativity that produce subluminal gravitational wave propagation speeds. | harmonic analysis for pulsar timing arrays |
we present an analysis of a densely repeating sample of bursts from the first repeating fast radio burst, frb 121102. we reanalyzed the data used by gourdji et al. and detected 93 additional bursts using our single-pulse search pipeline. in total, we detected 133 bursts in three hours of data at a center frequency of 1.4 ghz using the arecibo telescope, and develop robust modeling strategies to constrain the spectro-temporal properties of all of the bursts in the sample. most of the burst profiles show a scattering tail, and burst spectra are well modeled by a gaussian with a median width of 230 mhz. we find a lack of emission below 1300 mhz, consistent with previous studies of frb 121102. we also find that the peak of the log-normal distribution of wait times decreases from 207 to 75 s using our larger sample of bursts, as compared to that of gourdji et al. our observations do not favor either poissonian or weibull distributions for the burst rate distribution. we searched for periodicity in the bursts using multiple techniques, but did not detect any significant period. the cumulative burst energy distribution exhibits a broken power-law shape, with the lower- and higher-energy slopes of -0.4 ± 0.1 and -1.8 ± 0.2, with the break at (2.3 ± 0.2) × 1037 erg. we provide our burst fitting routines as a python package burstfit 4 4 https://github.com/thepetabyteproject/burstfit that can be used to model the spectrogram of any complex fast radio burst or pulsar pulse using robust fitting techniques. all of the other analysis scripts and results are publicly available. 5 5 https://github.com/thepetabyteproject/frb121102 | comprehensive analysis of a dense sample of frb 121102 bursts |
the transitional millisecond pulsar (msp) binary system psr j1023+0038 re-entered an accreting state in 2013 june in which it bears many similarities to low-mass x-ray binaries (lmxbs) in quiescence or near-quiescence. at a distance of just 1.37 kpc, psr j1023+0038 offers an unsurpassed ability to study low-level accretion onto a highly magnetized compact object. we have monitored psr j1023+0038 intensively using radio imaging with the karl g. jansky very large array, the european vlbi network and the low frequency array, seeing rapidly variable, flat spectrum emission that persists over a period of six months. the flat spectrum and variability are indicative of synchrotron emission originating in an outflow from the system, most likely in the form of a compact, partially self-absorbed jet, as is seen in lmxbs at higher accretion rates. the radio brightness, however, greatly exceeds extrapolations made from observations of more vigorously accreting neutron star lmxb systems. we postulate that psr j1023+0038 is undergoing radiatively inefficient “propeller-mode” accretion, with the jet carrying away a dominant fraction of the liberated accretion luminosity. we confirm that the enhanced γ-ray emission seen in psr j1023+0038 since it re-entered an accreting state has been maintained; the increased γ-ray emission in this state can also potentially be associated with propeller-mode accretion. similar accretion modes can be invoked to explain the radio and x-ray properties of the other two known transitional msp systems xss j12270-4859 and psr j1824-2452i (m28i), suggesting that radiatively inefficient accretion may be a ubiquitous phenomenon among (at least one class of) neutron star binaries at low accretion rates. | radio imaging observations of psr j1023+0038 in an lmxb state |
we analyze the spectrum of gravitational waves generated by the induced spectrum of tensor fluctuation during warm natural inflation. in our previous work it has been demonstrated that an epoch of warm natural inflation can lead to cosmologically relevant dark matter production in the form of primordial black holes. here we show that models which solve the dark-matter production also produce a contribution to the cosmic gravitational wave background that satisfies current constraints from pulsar timing and big bang nucleosynthesis. more importantly, this gravitational wave background may be observable in the next generation of space-based and ground-based gravitational wave interferometers. | induced gravitational waves via warm natural inflation |
explosive astrophysical transients at cosmological distances can be used to place precision tests of the basic assumptions of relativity theory, such as lorentz invariance, the photon zero-mass hypothesis, and the weak equivalence principle (wep). signatures of lorentz invariance violations (liv) include vacuum dispersion and vacuum birefringence. sensitive searches for liv using astrophysical sources such as gamma-ray bursts, active galactic nuclei, and pulsars are discussed. the most direct consequence of a nonzero photon rest mass is a frequency dependence in the velocity of light propagating in vacuum. a detailed representation of how to obtain a combined severe limit on the photon mass using fast radio bursts at different redshifts through the dispersion method is presented. the accuracy of the wep has been well tested based on the shapiro time delay of astrophysical messengers traveling through a gravitational field. some caveats of shapiro delay tests are discussed. in this article, we review and update the status of astrophysical tests of fundamental physics. | testing fundamental physics with astrophysical transients |
transient gravitational waves (aka gravitational wave bursts) within the nanohertz frequency band could be generated by a variety of astrophysical phenomena such as the encounter of supermassive black holes, the kinks or cusps in cosmic strings, or other as-yet-unknown physical processes. radio pulses emitted from millisecond pulsars could be perturbed by passing gravitational waves; hence, the correlation of the perturbations in a pulsar timing array can be used to detect and characterize burst signals with a duration of o (1 -10 ) years . we propose a fully bayesian framework for the analysis of the pulsar-timing-array data, where the burst waveform is generically modeled by piecewise straight lines, and the waveform parameters in the likelihood can be integrated out analytically. as a result, with merely three parameters (in addition to those describing the pulsars' intrinsic and background noise), one is able to efficiently search for the existence and the sky location of a burst signal. if a signal is present, the posterior of the waveform can be found without further bayesian inference. we demonstrate this model by analyzing simulated datasets containing a stochastic gravitational wave background and a burst signal generated by the parabolic encounter of two supermassive black holes. | searching for gravitational wave burst in pulsar-timing-array data with piecewise linear functions |
recently, a few-degrees extended γ -ray halo in the direction of the geminga pulsar has been detected by hawc, milagro and fermi-lat. these observations can be interpreted with positrons (e+) and electrons (e-) accelerated by the geminga pulsar wind nebula (pwn), released in a galactic environment with a low diffusion coefficient (d0), and inverse compton scattering (ics) with the interstellar radiation fields. we inspect here how the morphology of the ics γ -ray flux depends on the energy, the pulsar age and distance, and the strength and extension of the low-diffusion bubble. in particular we show that γ -ray experiments with a peak of sensitivity at tev energies are the most promising ones to detect ics halos. we perform a study of the sensitivity of hawc, hess and the future cta experiment finding that, with efficiencies of the order of a few %, the first two experiments should have already detected a few tens of ics halos while the latter will increase the number of detections by a factor of 4. we then consider a sample of sources associated to pwne and detected in the hess galactic plane survey and in the second hawc catalog. we use the information available in these catalogs for the γ -ray spatial morphology and flux of these sources to inspect the value of d0 around them and the e± injection spectrum. all sources are detected as extended with a γ -ray emission extended about 15-80 pc. assuming that most of the e± accelerated by these sources have been released in the interstellar medium, the diffusion coefficient is 2 - 30 ×1026 cm2 /s at 1 tev, i.e., 2 orders of magnitude smaller than the value considered to be the average in the galaxy. these observations imply that galactic pwne have low-diffusion bubbles with a size of at least 80 pc. | evidences of low-diffusion bubbles around galactic pulsars |
in this work we provide a framework for modelling compact stars in which the interior matter distribution obeys a generalised chaplygin equation of state. the interior geometry of the stellar object is described by a spherically symmetric line element which is simultaneously co-moving and isotropic with the exterior space-time being vacuum. we are able to integrate the einstein field equations and present closed form solutions which adequately describe compact strange star candidates such as 4u 1538-52, psr j1614-2230, vela x-1 and cen x-3 (gangopadhyay et al, mon. not. r. astron. soc. 431, 3216 (2013)). | anisotropic stars obeying chaplygin equation of state |
with the continuous upgrade of detectors, greater numbers of gravitational wave (gw) events have been captured by the ligo scientific collaboration and virgo collaboration (lvc), which offer a new avenue to test general relativity and explore the nature of gravity. although various model-independent tests have been performed by lvc in previous works, it is still interesting to ask what constraints can be placed on specific models by current gw observations. in this work, we focus on three models of scalar-tensor theories, the brans-dicke theory (bd), the theory with scalarization phenomena proposed by damour and esposito-farèse (def), and screened modified gravity (smg). of the four possible neutron star-black hole events that have occurred so far, we use two of them to place constraints. the other two are excluded in this work because of possible unphysical deviations. we consider the inspiral range with the cutoff frequency at the innermost stable circular orbit and add a modification of dipole radiation into the waveform template. the scalar charges of neutron stars in the dipole term are derived by solving the tolman-oppenheimer-volkoff equations for different equations of state. the constraints are obtained by performing the full bayesian inference with the help of the open source software bilby. the results show that the constraints given by gws are comparable to those given by pulsar timing experiments for def theory, but are not competitive with the current solar system constraints for bd and smg theories. | constraining scalar-tensor theories using neutron star-black hole gravitational wave events |
we investigate evolutionary pathways leading to neutron star (ns) formation through the collapse of oxygen-neon white dwarf (one wd) stars in interacting binaries. we consider (1) non-dynamical mass transfer where an one wd approaches the chandrasekhar mass leading to accretion-induced collapse (aic) and (2) dynamical time-scale merger-induced collapse (mic) between an one wd and another wd. we present rates, delay times, and progenitor properties for two different treatments of common envelope evolution. we show that aic nss are formed via many different channels and the most dominant channel depends on the adopted common envelope physics. most aic and mic nss are born shortly after star formation, though some have delay times >10 gyr. the shortest delay time (25-50 myr) aic nss have stripped-envelope, compact, helium-burning star donors, though many prompt aic nss form via wind accretion from an asymptotic giant branch star. the longest delay time aic nss, which may be observed as young millisecond pulsars among globular clusters, have a red giant or main-sequence donor at the time of ns formation and will eventually evolve into ns + helium wd binaries. we discuss aic and mic binaries as potential gravitational wave sources for laser interferometer space antenna (lisa). nss created via aic undergo a low-mass x-ray binary phase, offering an electromagnetic counterpart for those shortest orbital period sources that lisa could identify. the formation of nss from interacting wds in binaries is likely to be a key mechanism for the production of ligo/virgo gravitational wave sources (ns-ns and bh-ns mergers) in globular clusters. | on the formation of neutron stars via accretion-induced collapse in binaries |
we present results from the first all-sky radio survey in circular polarization. the survey uses the murchison widefield array (mwa) to cover 30 900 sq deg, over declinations south of +30° and north of -86° centred at 200 mhz (over a 169-231 mhz band). we achieve a spatial resolution of ∼3 arcmin and a typical sensitivity of 3.0 mjy psf-1 over most of the survey region. we demonstrate a new leakage mitigation technique that reduces the leakage from total intensity into circular polarization by an order of magnitude. in a blind survey of the imaged region, we detect 14 pulsars in circular polarization above a 6σ threshold. we also detect six transient sources associated with artificial satellites. a targeted survey of 2376 pulsars within the surveyed region yielded 33 detections above 4σ. looking specifically at pulsars previously detected at 200 mhz in total intensity, this represents a 35 per cent detection rate. we also conducted a targeted survey of 2400 known flare stars, this resulted in two tentative detections above 4σ. a similar targeted search for 1506 known exoplanets in the field yielded no detections above 4σ. the success of the survey suggests that similar surveys at longer wavelength bands and of deeper fields are warranted. | an all-sky survey of circular polarization at 200 mhz |
we perform ab initio particle-in-cell (pic) simulations of a pulsar magnetosphere with electron-positron plasma produced only in the regions close to the neutron star surface. we study how the magnetosphere transitions from the vacuum to a nearly force-free configuration. we compare the resulting force-free-like configuration with those obtained in a pic simulation where particles are injected everywhere as well as with macroscopic force-free simulations. we find that, although both pic solutions have similar structure of electromagnetic fields and current density distributions, they have different particle density distributions. in fact, in the injection from the surface solution, electrons and positrons counterstream only along parts of the return current regions and most of the particles leave the magnetosphere without returning to the star. we also find that pair production in the outer magnetosphere is not critical for filling the whole magnetosphere with plasma. we study how the current density distribution supporting the global electromagnetic configuration is formed by analyzing particle trajectories. we find that electrons precipitate to the return current layer inside the light cylinder and positrons precipitate to the current sheet outside the light cylinder by crossing magnetic field lines, contributing to the charge density distribution required by the global electrodynamics. moreover, there is a population of electrons trapped in the region close to the y-point. on the other hand, the most energetic positrons are accelerated close to the y-point. these processes can have observational signatures that, with further modeling effort, would help to distinguish this particular magnetosphere configuration from others. | electron-positron pair flow and current composition in the pulsar magnetosphere |
recently, a 16-day periodicity in a fast radio burst was reported. we propose that this 16-day periodicity may be due to forced precession of the neutron star by a fallback disk. when the rotation axis is misaligned with respect to the normal direction of the disk plane, the neutron star will precess. the eccentricity of the neutron star may be due to rotation or strong magnetic field, or similar reasons. we found that the 16-day period may be understood using typical masses of the fallback disk. polarization observations and information about the neutron star rotation period may help to discriminate different models. the possible precession observations in pulsars, magnetars and fast radio bursts may be understood together considering forced precession by a fallback disk. | periodicity in fast radio bursts due to forced precession by a fallback disk |
the possibility that in the mass range around $10^{-12}\ m_\odot$ most of dark matter constitutes of primordial black holes (pbhs) is a very interesting topic. to produce pbhs with this mass, the primordial scalar power spectrum needs to be enhanced to the order of 0.01 at the scale $k\sim 10^{12}\ \text{mpc}^{-1}$. the enhanced power spectrum also produces large secondary gravitational waves at the mhz band. a phenomenological delta function power spectrum is usually used to discuss the production of pbhs and secondary gravitational waves. based on g and k inflations, we propose a new mechanism to enhance the power spectrum at small scales by introducing a non-canonical kinetic term $[1-2g(\phi)]x$ with the function $g(\phi)$ having a peak. away from the peak, $g(\phi)$ is negligible and we recover the usual slow-roll inflation which is constrained by the cosmic microwave background anisotrpy observations. around the peak, the slow-roll inflation transiently turns to ultra slow-roll inflation. the enhancement of the power spectrum can be obtained with generic potentials, and there is no need to fine tune the parameters in $g(\phi)$. the energy spectrum $\omega_{gw}(f)$ of secondary gravitational waves have the characteristic power law behaviour $\omega_{gw}(f)\sim f^{n}$ and is testable by pulsar timing array and space based gravitational wave detectors. | primordial black holes and secondary gravitational waves from k/g inflation |
radio pulsars in short-period eccentric binary orbits can be used to study both gravitational dynamics and binary evolution. the binary system containing psr j1141-6545 includes a massive white dwarf (wd) companion that formed before the gravitationally bound young radio pulsar. we observed a temporal evolution of the orbital inclination of this pulsar that we infer is caused by a combination of a newtonian quadrupole moment and lense-thirring (lt) precession of the orbit resulting from rapid rotation of the wd. lt precession, an effect of relativistic frame dragging, is a prediction of general relativity. this detection is consistent with an evolutionary scenario in which the wd accreted matter from the pulsar progenitor, spinning up the wd to a period of <200 seconds. | lense-thirring frame dragging induced by a fast-rotating white dwarf in a binary pulsar system |
nearby pulsars have been suggested as sources of ∼tev e+/e- cosmic ray (cr) excess on the earth. the high-altitude water cherenkov observatory (hawc) detected extended tev emission regions in the direction of two nearby middle-aged pulsars, geminga and psr b0656+14. by modelling the tev emission as inverse compton emission from electron-positron pairs diffusing in the interstellar medium (ism), the hawc collaboration derives a diffusion coefficient much smaller than the standard value in the vicinity of the two pulsars, which make them unlikely the origin of the positron excess. we propose that the observed γ-ray emission originate from the relic pulsar wind nebula. a two zone diffusion model with a slow diffusion in the nebula and a fast diffusion in the ism can explain the hawc surface-brightness profile and the positron excess simultaneously. inefficient diffusion in the γ-ray emission region surrounding a middle-aged pulsar maybe a common phenomenon that can be tested by future observation. the implied diffusion coefficient in the ism is smaller than the one suggested by the standard cr propagation model, but it is fully consistent with the predictions of the spiral arm model. | positron flux and γ-ray emission from geminga pulsar and pulsar wind nebula |
the on-going arecibo pulsar-alfa (palfa) survey began in 2004 and is searching for radio pulsars in the galactic plane at 1.4 ghz. here we present a comprehensive description of one of its main data reduction pipelines that is based on the presto software and includes new interference-excision algorithms and candidate selection heuristics. this pipeline has been used to discover 40 pulsars, bringing the survey’s discovery total to 144 pulsars. of the new discoveries, eight are millisecond pulsars (msps; p\lt 10 ms) and one is a fast radio burst (frb). this pipeline has also re-detected 188 previously known pulsars, 60 of them previously discovered by the other palfa pipelines. we present a novel method for determining the survey sensitivity that accurately takes into account the effects of interference and red noise: we inject synthetic pulsar signals with various parameters into real survey observations and then attempt to recover them with our pipeline. we find that the palfa survey achieves the sensitivity to msps predicted by theoretical models but suffers a degradation for p≳ 100 ms that gradually becomes up to ∼10 times worse for p\gt 4 {{s}} at {dm}\lt 150 pc cm-3. we estimate 33 ± 3% of the slower pulsars are missed, largely due to red noise. a population synthesis analysis using the sensitivity limits we measured suggests the palfa survey should have found 224 ± 16 un-recycled pulsars in the data set analyzed, in agreement with the 241 actually detected. the reduced sensitivity could have implications on estimates of the number of long-period pulsars in the galaxy. | arecibo pulsar survey using alfa. iv. mock spectrometer data analysis, survey sensitivity, and the discovery of 40 pulsars |
in this paper a description is given of the sfxc software correlator, developed and maintained at the joint institute for vlbi in europe (jive). the software is designed to run on generic linux-based computing clusters. the correlation algorithm is explained in detail, as are some of the novel modes that software correlation has enabled, such as wide-field vlbi imaging through the use of multiple phase centres and pulsar gating and binning. this is followed by an overview of the software architecture. finally, the performance of the correlator as a function of number of cpu cores, telescopes and spectral channels is shown. | the sfxc software correlator for very long baseline interferometry: algorithms and implementation |
the highest-energy known gamma-ray sources are all located within 0°5 of extremely powerful pulsars. this raises the question of whether ultra-high-energy (uhe; >56 tev) gamma-ray emission is a universal feature expected near pulsars with a high spin-down power. using four years of data from the high altitude water cherenkov gamma-ray observatory, we present a joint-likelihood analysis of 10 extremely powerful pulsars to search for subthreshold uhe gamma-ray emission correlated with these locations. we report a significant detection (>3σ), indicating that uhe gamma-ray emission is a generic feature of powerful pulsars. we discuss the emission mechanisms of the gamma rays and the implications of this result. the individual environment, such as the magnetic field and particle density in the surrounding area, appears to play a role in the amount of emission. | evidence that ultra-high-energy gamma rays are a universal feature near powerful pulsars |
fast radio bursts (frbs) can be scattered by ionized gas in their local environments, host galaxies, intervening galaxies along their lines of sight (los), the intergalactic medium, and the milky way. the relative contributions of these different media depend on their geometric configuration and the internal properties of the gas. when these relative contributions are well understood, frb scattering is a powerful probe of density fluctuations along the los. the precise scattering measurements for frb 121102 and frb 180916 allow us to place an upper limit on the amount of scattering contributed by the milky way halo to these frbs. the scattering time $\tau \propto (\widetilde{f}\times {\mathrm{dm}}^{2}){a}_{\tau }$ , where dm is the dispersion measure, $\widetilde{f}$ quantifies electron density variations with $\widetilde{f}=0$ for a smooth medium, and the dimensionless constant aτ quantifies the difference between the mean scattering delay and the 1/e scattering time typically measured. a likelihood analysis of the observed scattering and halo dm constraints finds that $\widetilde{f}$ is at least an order of magnitude smaller in the halo than in the galactic disk. the maximum pulse broadening from the halo is τ ≲ 12 μs at 1 ghz. we compare our analysis of the milky way halo with other galaxy halos by placing limits on the scattering contributions from halos intersecting the los to frb 181112 and frb 191108. our results are consistent with halos making negligible or very small contributions to the scattering times of these frbs. | constraining galaxy halos from the dispersion and scattering of fast radio bursts and pulsars |
radio pulsars provide some of the most important constraints for our understanding of matter at supranuclear densities. so far, these constraints are mostly given by precision mass measurements of neutron stars (ns). by combining single measurements of the two most massive pulsars, j0348+0432 and j0740+6620, the resulting lower limit of 1.98 m⊙ (99 per cent confidence) of the maximum ns mass, excludes a large number of equations of state (eoss). further eos constraints, complementary to other methods, are likely to come from the measurement of the moment of inertia (moi) of binary pulsars in relativistic orbits. the double pulsar, psr j0737-3039a/b, is the most promising system for the first measurement of the moi via pulsar timing. reviewing this method, based in particular on the first meerkat observations of the double pulsar, we provide well-founded projections into the future by simulating timing observations with meerkat and the ska. for the first time, we account for the spin-down mass-loss in the analysis. our results suggest that an moi measurement with 11 per cent accuracy (68 per cent confidence) is possible by 2030. if by 2030 the eos is sufficiently well known, however, we find that the double pulsar will allow for a 7 per cent test of lense-thirring precession, or alternatively provide a ∼3σ-measurement of the next-to-leading order gravitational wave damping in gr. finally, we demonstrate that potential new discoveries of double ns systems with orbital periods shorter than that of the double pulsar promise significant improvements in these measurements and the constraints on ns matter. | constraining the dense matter equation-of-state with radio pulsars |
we study an impact of asymmetric dark matter on properties of the neutron stars and their ability to reach the two solar masses limit, which allows us to present a new range of masses of dark matter particles and their fractions inside the star. our analysis is based on the observational fact of the existence of two pulsars reaching this limit and on the theoretically predicted reduction of the neutron star maximal mass caused by the accumulation of dark matter in its interior. we also demonstrate that light dark matter particles with masses below 0.2 gev can create an extended halo around the neutron star leading not to decrease, but to increase of its visible gravitational mass. by using recent results on the spatial distribution of dark matter in the milky way, we present an estimate of its fraction inside the neutron stars located in the galaxy center. we show how the detection of a 2 m⊙ neutron star in the most central region of the galaxy will impose an upper constraint on the mass of dark matter particles of ∼60 gev . future high precision measurements of the neutron stars maximal mass near the galactic center, will put a more stringent constraint on the mass of the dark matter particle. this last result is particularly important to prepare ongoing, and future radio and x-ray surveys. | neutron stars: new constraints on asymmetric dark matter |
the tibet asγ experiment provided the first measurement of the total diffuse gamma-ray emission from the galactic disk in the sub-pev energy range. based on the analysis of tev sources included in the h.e.s.s. galactic plane survey catalog, we predict the expected contribution of unresolved pulsar-powered sources in the two angular windows of the galactic plane observed by tibet asγ. we show that the sum of this additional diffuse component due to unresolved sources and the truly diffuse emission, due to cosmic-ray interaction with the interstellar medium, well saturates the tibet data, without the need to introduce a progressive hardening of the cosmic-ray spectrum toward the galactic center. | unresolved sources naturally contribute to pev gamma-ray diffuse emission observed by tibet asγ |
two recent discoveries, namely psr j0901-4046 and gleam-x j162759.5-523504.3 (hereafter gleam-x j1627), have corroborated an extant population of radio-loud periodic sources with long periods (76 and 1091 s, respectively) whose emission can hardly be explained by rotation losses. we argue that gleam-x j1627 is a highly magnetized object consistent with a magnetar (an ultra-long period magnetar, ulpm), and demonstrate it is unlikely to be either a magnetically or a rotationally powered white dwarf. by studying these sources together with previously detected objects, we find there are at least a handful of promising candidates for galactic ulpms. the detections of these objects imply a substantial number, n ≳ 13 000 and n ≳ 500 for psr j0901-4046 like and gleam-x j1627 like objects, respectively, within our galaxy. these source densities, as well as cooling age limits from non-detection of thermal x-rays, galactic offsets, timing stability and dipole spin-down limits, all imply the ulpm candidates are substantially older than confirmed galactic magnetars and that their formation channel is a common one. their existence implies widespread survival of magnetar-like fields for several myr, distinct from the inferred behaviour in confirmed galactic magnetars. ulpms may also constitute a second class of frb progenitors which could naturally exhibit very long periodic activity windows. finally, we show that existing radio campaigns are biased against detecting objects like these and discuss strategies for future radio and x-ray surveys to identify more such objects. we estimate that ${\cal o}(100)$ more such objects should be detected with ska-mid and dsa-2000. | evidence for an abundant old population of galactic ultra-long period magnetars and implications for fast radio bursts |
some repeating fast radio burst (frb) sources exhibit complex polarization behaviors, including frequency-dependent depolarization, variation of rotation measure (rm), and oscillating spectral structures of polarized components. very recently, feng et al. reported that active repeaters exhibit conspicuous frequency-dependent depolarization and a strong correlation between rm scatter (σ rm) and the temporal scattering time (τ s), ${\sigma }_{\mathrm{rm}}\propto {\tau }_{{\rm{s}}}^{1.0\pm 0.2}$ , both of which can be well described by multipath propagation through a magnetized inhomogeneous plasma screen. this observation strongly suggests that the temporal scattering and rm scatter originate from the same region. besides, a particular finding of note in feng et al. is that the frbs with compact persistent radio sources (prss) tend to have extreme σ rm. in this work, we focus on some theoretical predictions of the relations among temporal scattering, depolarization by rm scatter, and prss contributed by the magnetized plasma environment close to a repeating frb source. the behaviors of the rm scatter imply that the magnetized plasma environment is consistent with a supernova remnant or pulsar wind nebula, and the predicted σ rm-τ s relation is ${\sigma }_{\mathrm{rm}}\propto {\tau }_{{\rm{s}}}^{(0.54-0.83)}$ for different astrophysical scenarios. we further make a general discussion of prss that does not depend on specific astrophysical scenarios. we show that the specific luminosity of a prs should have a positive correlation with the rm contributed by the plasma screen. this is consistent with the observations of frb 121102 and frb 190520b. | temporal scattering, depolarization, and persistent radio emission from magnetized inhomogeneous environments near repeating fast radio burst sources |
in a search for short-time-scale astrophysical transients in time-domain data, radio-frequency interference (rfi) causes both large quantities of false positive candidates and a significant reduction in sensitivity if not correctly mitigated. here, we propose an algorithm that infers a time-variable frequency channel mask directly from short-duration (~1 s) data blocks: the method consists of computing a spectral statistic that correlates well with the presence of rfi, and then finding high outliers among the resulting values. for the latter task, we propose an outlier detection algorithm called inter-quartile range mitigation (iqrm), which is both non-parametric and robust to the presence of a trend in sequential data. the method requires no training and can, in principle, adapt to any telescope and rfi environment; its efficiency is shown on data from both the meerkat and lovell 76-m radio telescopes. iqrm is fast enough to be used in a streaming search and has been integrated into the meertrap real-time transient search pipeline. open-source python and c++ implementations are also provided. | iqrm: real-time adaptive rfi masking for radio transient and pulsar searches |
recent studies of fast radio bursts (frbs) have led to many theories associating them with young neutron stars. if this is the case, then the presence of supernova ejecta and stellar winds provides a changing dispersion measure (dm) and rotation measure (rm) that can potentially be probes of the environments of frb progenitors. here we summarize the scalings for the dm and rm in the cases of a constant density ambient medium and of a progenitor stellar wind. since the amount of ionized material is controlled by the dynamics of the reverse shock, we find that the dm changes more slowly than in previous simpler work, which assumed a constant ionization fraction. furthermore, the dm can be constant or even increasing as the supernova remnant sweeps up material, arguing that a young neutron star hypothesis for frbs is not ruled out if the dm is not decreasing over repeated bursts. the combined dm and rm measurements for the repeating frb 121102 are consistent with supernova ejecta with an age of ∼102-103 years expanding into a high density (∼100 cm-3) interstellar medium. this naturally explains its relatively constant dm over many years as well. other frbs with much lower rms may indicate that they are especially young supernovae in wind environments or that their dms are largely from the intergalactic medium. we therefore caution about inferring magnetic fields simply by dividing an rm by dm, because these quantities could originate from distinct regions along the path an frb propagates. | the dispersion and rotation measure of supernova remnants and magnetized stellar winds: application to fast radio bursts |
we present an overview and the first results from a large-scale pulsar timing programme that is part of the utmost project at the refurbished molonglo observatory synthesis radio telescope (most) near canberra, australia. we currently observe more than 400 mainly bright southern radio pulsars with up to daily cadences. for 205 (8 in binaries, 4 millisecond pulsars), we publish updated timing models, together with their flux densities, flux density variability, and pulse widths at 843 mhz, derived from observations spanning between 1.4 and 3 yr. in comparison with the atnf pulsar catalogue, we improve the precision of the rotational and astrometric parameters for 123 pulsars, for 47 by at least an order of magnitude. the time spans between our measurements and those in the literature are up to 48 yr, which allow us to investigate their long-term spin-down history and to estimate proper motions for 60 pulsars, of which 24 are newly determined and most are major improvements. the results are consistent with interferometric measurements from the literature. a model with two gaussian components centred at 139 and 463 km s-1 fits the transverse velocity distribution best. the pulse duty cycle distributions at 50 and 10 per cent maximum are best described by lognormal distributions with medians of 2.3 and 4.4 per cent, respectively. we discuss two pulsars that exhibit spin-down rate changes and drifting subpulses. finally, we describe the autonomous observing system and the dynamic scheduler that has increased the observing efficiency by a factor of 2-3 in comparison with static scheduling. | the utmost pulsar timing programme i: overview and first results |
current models of magnetars require extremely strong magnetic fields to explain their observed quiescent and bursting emission, implying that the field strength within the star’s outer crust is orders of magnitude larger than the dipole component inferred from spin-down measurements. this presents a serious challenge to theories of magnetic field generation in a proto-neutron star. here, we present detailed modeling of the evolution of the magnetic field in the crust of a neutron star through 3d simulations. we find that, in the plausible scenario of equipartition of energy between global-scale poloidal and toroidal magnetic components, magnetic instabilities transfer energy to nonaxisymmetric, kilometer-sized magnetic features, in which the local field strength can greatly exceed that of the global-scale field. these intense small-scale magnetic features can induce high-energy bursts through local crust yielding, and the localized enhancement of ohmic heating can power the star’s persistent emission. thus, the observed diversity in magnetar behavior can be explained with mixed poloidal-toroidal fields of comparable energies. | magnetic field evolution in magnetar crusts through three-dimensional simulations |
neutron stars harbor extremely strong magnetic fields within their solid outer crust. the topology of this field strongly influences the surface temperature distribution and, hence, the star's observational properties. in this work, we present the first realistic simulations of the coupled crustal magnetothermal evolution of isolated neutron stars in three dimensions accounting for neutrino emission, obtained with the pseudo-spectral code parody. we investigate both the secular evolution, especially in connection with the onset of instabilities during the hall phase, and the short-term evolution following episodes of localized energy injection. simulations show that a resistive tearing instability develops in about a hall time if the initial toroidal field exceeds $\approx {10}^{15}$ g. this leads to crustal failures because of the huge magnetic stresses coupled with the local temperature enhancement produced by dissipation. localized heat deposition in the crust results in the appearance of hot spots on the star surface, which can exhibit a variety of patterns. because the transport properties are strongly influenced by the magnetic field, the hot regions tend to drift away and get deformed following the magnetic field lines while cooling. the shapes obtained with our simulations are reminiscent of those recently derived from nicer x-ray observations of the millisecond pulsar psr j0030+0451. | three-dimensional modeling of the magnetothermal evolution of neutron stars: method and test cases |
solving field equations exactly in $f(r,t)-$ gravity is a challenging task. to do so, many authors have adopted different methods such as assuming both the metric functions and an equation of state (eos) and a metric function. however, such methods may not always lead to well-behaved solutions, and the solutions may even be rejected after complete calculations. nevertheless, very recent studies on embedding class-one methods suggest that the chances of arriving at a well-behaved solution are very high, which is inspiring. in the class-one approach, one of the metric potentials is estimated and the other can be obtained using the karmarkar condition. in this study, a new class-one solution is proposed that is well-behaved from all physical points of view. the nature of the solution is analyzed by tuning the $f(r,t)-$ coupling parameter $\chi$ , and it is found that the solution leads to a stiffer eos for $\chi=-1$ than that for $\chi=1$ . this is because for small values of $\chi$ , the velocity of sound is higher, leading to higher values of $m_{\rm max}$ in the $m-r$ curve and the eos parameter $\omega$ . the solution satisfies the causality condition and energy conditions and remains stable and static under radial perturbations (static stability criterion) and in equilibrium (modified tov equation). the resulting $m-r$ diagram is well-fitted with observed values from a few compact stars such as psr j1614-2230, vela x-1, cen x-3, and sax j1808.4-3658. therefore, for different values of $\chi$ , the corresponding radii and their respective moments of inertia have been predicted from the $m-i$ curve. | exploring physical properties of compact stars in f(r,t)-gravity: an embedding approach |
we construct parameter sets of the relativistic mean-field model fitted to the recent constraints on the asymmetry energy j and the slope parameter l for pure neutron matter. we find cases of unphysical behavior, i.e., the appearance of negative pressures, for stiff parameter sets with low values of the effective mass m*/m . in some cases the equation of state of pure neutron matter turns out to be outside the allowed band given by chiral effective field theory. the mass-radius relations of neutron stars for all acceptable parameter sets shows a maximum mass in excess of 2 m⊙ being compatible with pulsar mass measurements. given the constraints on the model in the low-density regime coming from chiral effective theory, we find that the radius of a 1.4 m⊙ neutron star is nearly independent of the value of l . this is in contrast to some previous claims for a strong connection of the slope parameter with the radius of a neutron star. in fact, the mass-radius relation turns out to depend only on the isoscalar parameters of symmetric matter. the constraints of gw170817 on the tidal deformability and on the radius are also discussed. | relativistic parameterizations of neutron matter and implications for neutron stars |
the recent detection of gravitational waves and electromagnetic counterparts from the double neutron star merger event gw+em170817 supports the standard paradigm of short gamma-ray bursts (sgrbs) and kilonovae/macronovae. it is important to reveal the nature of the compact remnant left after the merger, either a black hole or neutron star, and their physical link to the origin of the long-lasting emission observed in sgrbs. the diversity of the merger remnants may also lead to different kinds of transients that can be detected in future. here we study the high-energy emission from the long-lasting central engine left after the coalescence, under certain assumptions. in particular, we consider the x-ray emission from a remnant disk and the nonthermal nebular emission from disk-driven outflows or pulsar winds. we demonstrate that late-time x-ray and high-frequency radio emission can provide useful constraints on properties of the hidden compact remnants and their connections to long-lasting sgrb emission, and we discuss the detectability of nearby merger events through late-time observations at ∼30-100 days after the coalescence. we also investigate the gev-tev gamma-ray emission that occurs in the presence of long-lasting central engines and show the importance of external inverse compton radiation due to upscattering of x-ray photons by relativistic electrons in the jet. we also search for high-energy gamma rays from gw170817 in the fermi-lat data and report upper limits on such long-lasting emission. finally, we consider the implications of gw+em170817 and discuss the constraints placed by x-ray and high-frequency radio observations. | double neutron star mergers and short gamma-ray bursts: long-lasting high-energy signatures and remnant dichotomy |
mirror sectors have been proposed to address the problems of dark matter, baryogenesis, and the neutron lifetime anomaly. in this work we study a new, powerful probe of mirror neutrons: neutron star temperatures. when neutrons in the neutron star core convert to mirror neutrons during collisions, the vacancies left behind in the nucleon fermi seas are refilled by more energetic nucleons, releasing immense amounts of heat in the process. we derive a new constraint on the allowed strength of neutron-mirror-neutron mixing from observations of the coldest (sub-40 000 kelvin) neutron star, psr 2144 −3933 . our limits compete with laboratory searches for neutron-mirror-neutron transitions but apply to a range of mass splittings between the neutron and mirror neutron that is 19 orders of magnitude larger. this heating mechanism, also pertinent to other neutron disappearance channels such as exotic neutron decay, provides a compelling physics target for upcoming ultraviolet, optical, and infrared telescopes to study thermal emissions of cold neutron stars. | neutron star internal heating constraints on mirror matter |
we report the discovery of 13 new pulsars in the globular cluster ngc 1851 by the trapum large survey project using the meerkat radio telescope. the discoveries consist of six isolated millisecond pulsars (msps) and seven binary pulsars, of which six are msps and one is mildly recycled. for all the pulsars, we present the basic kinematic, astrometric, and orbital parameters, where applicable, as well as their polarimetric properties, when these are measurable. two of the binary msps (psr j0514−4002d and psr j0514−4002e) are in wide and extremely eccentric (e > 0.7) orbits with a heavy white dwarf and a neutron star as their companion, respectively. with these discoveries, ngc 1851 is now tied with m28 as the cluster with the third largest number of known pulsars (14). its pulsar population shows remarkable similarities with that of m28, terzan 5, and other clusters with comparable structural parameters. the newly found pulsars are all located in the innermost regions of ngc 1851 and will likely enable, among other things, detailed studies of the cluster structure and dynamics. the data used for the pulse profiles and dynamical spectra are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/664/a27 | trapum discovery of 13 new pulsars in ngc 1851 using meerkat |
pulsar polarization profiles form a very basic database for understanding the emission processes in a pulsar magnetosphere. after careful polarization calibration of the 19-beam l-band receiver and verification of beam-offset observation results, we obtain polarization profiles of 682 pulsars from observations by the five-hundred-meter aperture spherical radio telescope (fast) during the galactic plane pulsar snapshot survey and other normal fast projects. among them, polarization profiles of about 460 pulsars are observed for the first time. the profiles exhibit diverse features. some pulsars have a polarization position angle curve with a good s-shaped swing, some with orthogonal modes; some have components with highly linearly polarized components or strong circularly polarized components; some have a very wide profile, coming from an aligned rotator, and some have an interpulse from a perpendicular rotator; some wide profiles are caused by interstellar scattering. we derive geometric parameters for 190 pulsars from the s-shaped position angle curves or with orthogonal modes. we find that the linear and circular polarization or the widths of pulse profiles have various frequency dependencies. pulsars with a large fraction of linear polarization are more likely to have a large edot. | fast pulsar database. i. polarization profiles of 682 pulsars |
the recent observation of the object hess j1731-347 suggests the existence of a very light and very compact neutron star, which is a challenge for commonly used equations of state for dense matter. in this work we present a relativistic mean-field model enriched with isovector and isoscalar meson crossing terms. these interactions dominate the behavior of the symmetry energy and account for the small radius. the proposed model fulfills the recent constraints concerning the symmetry energy slope and the state-of-the-art compact star constraints derived from the nicer measurements of the pulsars psr j 0030 +0451 and psr j 0740 +6620 , as well as from the gw170817 event and its associated electromagnetic counterparts at2017gfo/grb170817a. | relativistic mean-field model for the ultracompact low-mass neutron star hess j1731-347 |
the most energetic neutron stars, powered by their rotation, are capable of producing pulsed radiation from the radio up to γ rays with nearly tev energies. these pulsars are part of the universe of energetic and powerful particle accelerators, using their uniquely fast rotation and formidable magnetic fields to accelerate particles to ultra-relativistic speed. the extreme properties of these stars provide an excellent testing ground, beyond earth experience, for nuclear, gravitational, and quantum-electrodynamical physics. a wealth of γ-ray pulsars has recently been discovered with the fermi gamma-ray space telescope. the energetic γ rays enable us to probe the magnetospheres of neutron stars and particle acceleration in this exotic environment. we review the latest developments in this field, beginning with a brief overview of the properties and mysteries of rotation-powered pulsars, and then discussing γ-ray observations and magnetospheric models in more detail. | gamma-ray pulsars: a gold mine |
calculations of the effects of band structure on the neutron superfluid density in the crust of neutron stars made under the assumption that the effects of pairing are small [n. chamel, phys. rev. c 85, 035801 (2012)] lead to moments of inertia of superfluid neutrons so small that the crust alone is insufficient to account for the magnitude of neutron star glitches. inspired by earlier work on ultracold atomic gases in an optical lattice, we investigate fermions with attractive interactions in a periodic lattice in the mean-field approximation. the effects of band structure are suppressed when the pairing gap is of order or greater than the strength of the lattice potential. by applying the results to the inner crust of neutron stars, we conclude that the reduction of the neutron superfluid density is considerably less than previously estimated and, consequently, it is premature to rule out models of glitches based on neutron superfluidity in the crust. | superfluid density of neutrons in the inner crust of neutron stars: new life for pulsar glitch models |
we present the galactic merger rate for double neutron star (dns) binaries using an observed sample of eight dns systems merging within a hubble time. this sample includes the recently discovered, highly relativistic dns systems j1757-1854 and j1946+2052, and is approximately three times the sample size used in previous estimates of the galactic merger rate by kim et al. using this sample, we calculate the vertical scale height for dns systems in the galaxy to be z 0 = 0.4 ± 0.1 kpc. we calculate a galactic dns merger rate of {{ \mathcal r }}mw}={42}-14+30 myr-1 at the 90% confidence level. the corresponding dns merger detection rate for advanced ligo (laser interferometer gravitational-wave observatory) is {{ \mathcal r }}ligo}={0.18}-0.06+0.13× {≤ft({d}{{r}}/100{mpc}\right)}3 {yr}}-1, where d r is the range distance. using this merger detection rate and the predicted range distance of 120-170 mpc for the third observing run of ligo, we predict, accounting for 90% confidence intervals, that ligo-virgo will detect anywhere between zero and two dns mergers. we explore the effects of the underlying pulsar population properties on the merger rate and compare our merger detection rate with those estimated using different formation and evolutionary scenario of dns systems. as we demonstrate, reconciling the rates is sensitive to assumptions about the dns population, including its radio pulsar luminosity function. future constraints from further gravitational wave dns detections and pulsar surveys anticipated in the near future should permit tighter constraints on these assumptions. | future prospects for ground-based gravitational-wave detectors: the galactic double neutron star merger rate revisited |
we report 21-year timing of one of the most precise pulsars: psr j1713+0747. its pulse times of arrival are well modeled by a comprehensive pulsar binary model including its three-dimensional orbit and a noise model that incorporates short- and long-timescale correlated noise such as jitter and red noise. its timing residuals have weighted root mean square ∼92 ns. the new data set allows us to update and improve previous measurements of the system properties, including the masses of the neutron star (1.31 ± 0.11 m⊙) and the companion white dwarf (0.286 ± 0.012 m⊙) as well as their parallax distance 1.15 ± 0.03 kpc. we measured the intrinsic change in orbital period, {\dot{p}}{{b}}{int}, is -0.20 ± 0.17 ps s-1, which is not distinguishable from zero. this result, combined with the measured {\dot{p}}{{b}}{int} of other pulsars, can place a generic limit on potential changes in the gravitational constant g. we found that \dot{g}/g is consistent with zero [(-0.6 ± 1.1) × 10-12 yr-1, 95% confidence] and changes at least a factor of 31 (99.7% confidence) more slowly than the average expansion rate of the universe. this is the best \dot{g}/g limit from pulsar binary systems. the {\dot{p}}{{b}}{int} of pulsar binaries can also place limits on the putative coupling constant for dipole gravitational radiation {κ }{{d}}=(-0.9+/- 3.3)× {10}-4 (95% confidence). finally, the nearly circular orbit of this pulsar binary allows us to constrain statistically the strong-field post-newtonian parameters δ, which describes the violation of strong equivalence principle, and {\hat{α }}3, which describes a breaking of both lorentz invariance in gravitation and conservation of momentum. we found, at 95% confidence, {{δ }}\lt 0.01 and {\hat{α }}3\lt 2× {10}-20 based on psr j1713+0747. | testing theories of gravitation using 21-year timing of pulsar binary j1713+0747 |
in this paper, we have introduced a new f(r) gravity model as an attempt to have a model with more parametric control, so that the model can be used to explain the existing problems as well as to explore new directions in physics of gravity, by properly constraining it with recent observational data. here basic aim is to study the properties of gravitational waves (gws) in this new model. in f(r) gravity metric formalism, the model shows the existence of scalar degree of freedom as like other f(r) gravity models. due to this reason, there is a scalar mode of polarization of gws present in the theory. this polarization mode exists in a mixed state, of which one is transverse massless breathing mode with non-vanishing trace and the other is massive longitudinal mode. the longitudinal mode being massive, travels at speed less than the usual tensor modes found in general relativity (gr). moreover, for a better understanding of the model, we have studied the potential and mass of scalar graviton in both jordan frame and einstein frame. this model can pass the solar system tests and can explain primordial and present dark energy. also, we have put constraints on the model. it is found that the correlation function for the third mode of polarization under certain mass scale predicted by the model agrees well with the recent data of pulsar timing arrays. it seems that this new model would be useful in dealing with different existing issues in the areas of astrophysics and cosmology. | a new f(r) gravity model and properties of gravitational waves in it |
a few years after its discovery as a magnetar, sgr j1935+2154 started a new burst-active phase on 2020 april 27, accompanied by a large enhancement of its x-ray persistent emission. radio single bursts were detected during this activation, strengthening the connection between magnetars and fast radio bursts. we report on the x-ray monitoring of sgr j1935+2154 from ∼3 days prior to ∼3 weeks after its reactivation, using swift, the nuclear spectroscopic telescope array (nustar), and the neutron star interior composition explorer (nicer). we detected x-ray pulsations in the nicer and nustar observations, and constrained the spin period derivative to $| \dot{p}| \lt 3\times {10}^{-11}$ s s-1 (3σ c.l.). the pulse profile showed a variable shape switching between single and double-peaked as a function of time and energy. the pulsed fraction decreased from ∼34% to ∼11% (5-10 kev) over ∼10 days. the x-ray spectrum was well fit by an absorbed blackbody model with temperature decreasing from ktbb ∼ 1.6 to 0.45-0.6 kev, plus a nonthermal power-law component (γ ∼ 1.2) observed up to ∼25 kev with nustar. the 0.3-10 kev x-ray luminosity increased in less than 4 days from $\sim 6\times {10}^{33}{d}_{6.6}^{2}$ erg s-1 to about $3\times {10}^{35}{d}_{6.6}^{2}$ erg s-1 and then decreased again to $2.5\times {10}^{34}{d}_{6.6}^{2}$ erg s-1 over the following 3 weeks of the outburst, where d6.6 is the source distance in units of 6.6 kpc. we also detected several x-ray bursts, with properties typical of short magnetar bursts. | the x-ray reactivation of the radio bursting magnetar sgr j1935+2154 |
improving survey specifications are causing an exponential rise in pulsar candidate numbers and data volumes. we study the candidate filters used to mitigate these problems during the past fifty years. we find that some existing methods such as applying constraints on the total number of candidates collected per observation, may have detrimental effects on the success of pulsar searches. those methods immune to such effects are found to be ill-equipped to deal with the problems associated with increasing data volumes and candidate numbers, motivating the development of new approaches. we therefore present a new method designed for on-line operation. it selects promising candidates using a purpose-built tree-based machine learning classifier, the gaussian hellinger very fast decision tree (gh-vfdt), and a new set of features for describing candidates. the features have been chosen so as to i) maximise the separation between candidates arising from noise and those of probable astrophysical origin, and ii) be as survey-independent as possible. using these features our new approach can process millions of candidates in seconds (∼1 million every 15 seconds), with high levels of pulsar recall (90%+). this technique is therefore applicable to the large volumes of data expected to be produced by the square kilometre array (ska). use of this approach has assisted in the discovery of 20 new pulsars in data obtained during the lofar tied-array all-sky survey (lotaas). | fifty years of pulsar candidate selection: from simple filters to a new principled real-time classification approach |
the molonglo observatory synthesis telescope (most) is an 18000 m2 radio telescope located 40 km from canberra, australia. its operating band (820-851 mhz) is partly allocated to telecommunications, making radio astronomy challenging. we describe how the deployment of new digital receivers, field programmable gate array-based filterbanks, and server-class computers equipped with 43 graphics processing units, has transformed the telescope into a versatile new instrument (utmost) for studying the radio sky on millisecond timescales. utmost has 10 times the bandwidth and double the field of view compared to the most, and voltage record and playback capability has facilitated rapid implementaton of many new observing modes, most of which operate commensally. utmost can simultaneously excise interference, make maps, coherently dedisperse pulsars, and perform real-time searches of coherent fan-beams for dispersed single pulses. utmost operates as a robotic facility, deciding how to efficiently target pulsars and how long to stay on source via real-time pulsar folding, while searching for single pulse events. regular timing of over 300 pulsars has yielded seven pulsar glitches and three fast radio bursts during commissioning. utmost demonstrates that if sufficient signal processing is applied to voltage streams, innovative science remains possible even in hostile radio frequency environments. | the utmost: a hybrid digital signal processor transforms the molonglo observatory synthesis telescope |
aims: we demonstrate the high multiplex advantage of crowded field 3d spectroscopy with the new integral field spectrograph muse by means of a spectroscopic analysis of more than 12 000 individual stars in the globular cluster ngc 6397.methods: the stars are deblended with a point spread function fitting technique, using a photometric reference catalogue from hst as prior, including relative positions and brightnesses. this catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis via a full-spectrum fitting method based on a large grid of phoenix spectra.results: we analysed the largest sample so far available for a single globular cluster of 18 932 spectra from 12 307 stars in ngc 6397. we derived a mean radial velocity of vrad = 17.84 ± 0.07 km s-1 and a mean metallicity of [fe/h] = -2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the red giant branch (rgb). we determine teff and [fe/h] from the spectra, and log g from hst photometry. this is the first very comprehensive hertzsprung-russell diagram (hrd) for a globular cluster based on the analysis of several thousands of stellar spectra, ranging from the main sequence to the tip of the rgb. furthermore, two interesting objects were identified; one is a post-agb star and the other is a possible millisecond-pulsar companion. data products are available at http://muse-vlt.eu/sciencebased on observations obtained at the very large telescope (vlt) of the european southern observatory, paranal, chile (eso programme id 60.a-9100(c)). | muse crowded field 3d spectroscopy of over 12 000 stars in the globular cluster ngc 6397. i. the first comprehensive hrd of a globular cluster |
we study current bounds on strong first-order phase transitions (pts) along the equation of state (eos) of dense strongly interacting matter in neutron stars, under the simplifying assumption that on either side of the pt, the eos can be approximated by a simple polytropic form. we construct a large ensemble of possible eoss of this form, anchor them to chiral effective field theory calculations at nuclear density and perturbative quantum chromodynamics at high densities, and subject them to astrophysical constraints from high-mass pulsars and gravitational-wave observations. within this setup, we find that a pt permits neutron-star solutions with larger radii, but only if the transition begins below twice nuclear saturation density. we also identify a large parameter space of allowed pts currently unexplored by numerical-relativity studies. additionally, we locate a small region of parameter space allowing twin-star solutions, though we find them to only marginally pass the current astrophysical constraints. finally, we find that sizeable cores of high-density matter beyond the pt may be located in the centers of some stable neutron stars, primarily those with larger masses. | constraints on strong phase transitions in neutron stars |
it is a well-known fact that compact gravitating objects admit bound state configurations for massive bosonic fields. in this work we describe a new class of superradiant instabilities of axion bound states in neutron star magnetospheres. the instability arises from the mixing of axion and photon modes in the magnetic field of the neutron star which extract energy from the rotating magnetosphere. unlike for black holes, where the dissipation required for superradiance is provided by an absorptive horizon, the non-hermitian dynamics in this paper come from the resistivity in the stellar magnetosphere arising from a finite bulk conductivity. the axion field mixes with photon modes which superradiantly scatter off the magnetosphere, extracting rotational energy which is then deposited back into the axion sector leading to an instability. we derive the superradiant eigenfrequencies for the axion-photon system using quantum mechanical perturbation theory on the axion boundstate, drawing an analogy with atomic selection rules. we then compare the characteristic time scale of the instability to the spin-down measurements of pulsars which limit the allowed rate of angular momentum extraction from neutron stars. | axion superradiance in rotating neutron stars |
the measurement of ultra-high energy (uhe) neutrinos (e > 1016 ev) opens a new field of astronomy with the potential to reveal the sources of ultra-high energy cosmic rays especially if combined with observations in the electromagnetic spectrum and gravitational waves. the arianna pilot detector explores the detection of uhe neutrinos with a surface array of independent radio detector stations in antarctica which allows for a cost-effective instrumentation of large volumes. twelve stations are currently operating successfully at the moore's bay site (ross ice shelf) in antarctica and at the south pole. we will review the current state of arianna and its main results. we report on a newly developed wind generator that successfully operates in the harsh antarctic conditions and powers the station for a substantial time during the dark winter months. the robust arianna surface architecture, combined with environmentally friendly solar and wind power generators, can be installed at any deep ice location on the planet and operated autonomously. we report on the detector capabilities to determine the neutrino direction by reconstructing the signal arrival direction of a 800 m deep calibration pulser, and the reconstruction of the signal polarization using the more abundant cosmic-ray air showers. finally, we describe a large-scale design - aria - that capitalizes on the successful experience of the arianna operation and is designed sensitive enough to discover the first uhe neutrino. | targeting ultra-high energy neutrinos with the arianna experiment |
light axionic dark matter, motivated by string theory, is increasingly favored for the "no-wimp era". galaxy formation is suppressed below a jeans scale, of ≃ 10^8 m_⊙ by setting the axion mass to, m_b ∼ 10^{-22}ev, and the large dark cores of dwarf galaxies are explained as solitons on the de-broglie scale. this is persuasive, but detection of the inherent scalar field oscillation at the compton frequency, ω_b= (2.5 months)^{-1}(m_b/10^{-22}ev), would be definitive. by evolving the coupled schrödinger-poisson equation for a bose-einstein condensate, we predict the dark matter is fully modulated by de-broglie interference, with a dense soliton core of size ≃ 150pc, at the galactic center. the oscillating field pressure induces general relativistic time dilation in proportion to the local dark matter density and pulsars within this dense core have detectably large timing residuals, of ≃ 400nsec/(m_b/10^{-22}ev). this is encouraging as many new pulsars should be discovered near the galactic center with planned radio surveys. more generally, over the whole galaxy, differences in dark matter density between pairs of pulsars imprints a pairwise galactocentric signature that can be distinguished from an isotropic gravitational wave background. | recognising axionic dark matter by compton and de-broglie scale modulation of pulsar timing |
psr j1023+0038 is the first millisecond pulsar discovered to pulsate in the visible band; such a detection took place when the pulsar was surrounded by an accretion disk and also showed x-ray pulsations. we report on the first high time resolution observational campaign of this transitional pulsar in the disk state, using simultaneous observations in the optical (telescopio nazionale galileo, nordic optical telescope, telescopi joan oró), x-ray (xmm-newton, nustar, nicer), infrared (gran telescopio canarias), and uv (swift) bands. optical and x-ray pulsations were detected simultaneously in the x-ray high-intensity mode in which the source spends ∼70% of the time, and both disappeared in the low mode, indicating a common underlying physical mechanism. in addition, optical and x-ray pulses were emitted within a few kilometers and had similar pulse shapes and distributions of the pulsed flux density compatible with a power-law relation fν∝ ν -0.7 connecting the optical and the 0.3-45 kev x-ray band. optical pulses were also detected during flares with a pulsed flux reduced by one-third with respect to the high mode; the lack of a simultaneous detection of x-ray pulses is compatible with the lower photon statistics. we show that magnetically channeled accretion of plasma onto the surface of the neutron star cannot account for the optical pulsed luminosity (∼1031 erg s-1). on the other hand, magnetospheric rotation-powered pulsar emission would require an extremely efficient conversion of spin-down power into pulsed optical and x-ray emission. we then propose that optical and x-ray pulses are instead produced by synchrotron emission from the intrabinary shock that forms where a striped pulsar wind meets the accretion disk, within a few light cylinder radii away, ∼100 km, from the pulsar. | pulsating in unison at optical and x-ray energies: simultaneous high time resolution observations of the transitional millisecond pulsar psr j1023+0038 |
context. a precise understanding of the equation of state (eos) of dense and hot matter is key to modeling relativistic astrophysical environments, including core-collapse supernovae (ccsne), protoneutron star (pnss) evolution, and compact binary mergers.aims: in this paper, we extend the microscopic zero-temperature bl (bombaci and logoteta) nuclear eos to finite temperature and arbitrary nuclear composition. we employ this new eos to describe hot β-stable nuclear matter and to compute various structural properties of nonrotating pns. we also apply the eos to perform dynamical simulations of a spherically symmetric ccsn.methods: the eos is derived using the finite temperature extension of the brueckner-bethe-goldstone quantum many-body theory in the brueckner-hartree-fock approximation. neutron star properties are computed by solving the tolman-oppenheimer-volkoff structure equations numerically. the sperically symmetric ccsn simulations are performed using the agile-idsa code.results: our eos models are able to reproduce typical features of both pns and spherically symmetric ccsn simulations. in addition, our eos model is consistent with present measured neutron star masses and particularly with the masses: m = 2.01 ± 0.04 m⊙ and m = 2.14-0.18+0.20 m⊙ of the neutron stars in psr j0348+0432 and psr j0740+6620 respectively. finally, we suggest a feasible mechanism to produce low-mass black holes (m ∼ 2 m⊙) that could have far-reaching consequences for interpreting the gravitational wave event gw190814 as a bh-bh merger. | microscopic equation of state of hot nuclear matter for numerical relativity simulations |
the magnetar swift j1818.0-1607 was discovered in 2020 march when swift detected a 9 ms hard x-ray burst and a long-lived outburst. prompt x-ray observations revealed a spin period of 1.36 s, soon confirmed by the discovery of radio pulsations. we report here on the analysis of the swift burst and follow-up x-ray and radio observations. the burst average luminosity was lburst ∼ 2 × 1039 erg s-1 (at 4.8 kpc). simultaneous observations with xmm-newton and nustar three days after the burst provided a source spectrum well fit by an absorbed blackbody ( ${n}_{{\rm{h}}}$ = (1.13 ± 0.03) × 1023 cm-2 and kt = 1.16 ± 0.03 kev) plus a power law (γ = 0.0 ± 1.3) in the 1-20 kev band, with a luminosity of ∼8 × 1034 erg s-1, dominated by the blackbody emission. from our timing analysis, we derive a dipolar magnetic field b ∼ 7 × 1014 g, spin-down luminosity ${\dot{e}}_{\mathrm{rot}}\sim 1.4\times {10}^{36}$ erg s-1, and characteristic age of 240 yr, the shortest currently known. archival observations led to an upper limit on the quiescent luminosity <5.5 × 1033 erg s-1, lower than the value expected from magnetar cooling models at the source characteristic age. a 1 hr radio observation with the sardinia radio telescope taken about 1 week after the x-ray burst detected a number of strong and short radio pulses at 1.5 ghz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 min-1 and accounted for ∼50% of the total pulsed radio fluence. we conclude that swift j1818.0-1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence. | a very young radio-loud magnetar |
we present an analytical solution of a highly magnetized jet/wind flow. the left side of the general force-free jet/wind equation (the "pulsar" equation) is separated into a rotating and a nonrotating term. the two equations with either term can be solved analytically, and the two solutions match each other very well. therefore, we obtain a general approximate solution of a magnetically dominated jet/wind, which covers from the nonrelativistic to relativistic regimes, with the drift velocity well matching the cold plasma velocity. the acceleration of a jet includes three stages. (1) the jet flow is located within the alfvén critical surface (i.e., the light cylinder), has a nonrelativistic speed, and is dominated by toroidal motion. (2) the jet is beyond the alfvén critical surface where the flow is dominated by poloidal motion and becomes relativistic. the total velocity in these two stages follows the same law vγ = ωr. (3) the evolution law is replaced by $v{\rm{\gamma }}\approx 1/\left(\theta \sqrt{2-\nu }\right)$ , where θ is the half-opening angle of the jet and 0 ≤ ν ≤ 2 is a free parameter determined by the magnetic field configuration. this is because the earlier efficient acceleration finally breaks the causality connection between different parts in the jet, preventing a global solution. the jet has to carry local charges and currents to support an electromagnetic balance. this approximate solution is consistent with known theoretical results and numerical simulations, and it is more convenient to directly compare with observations. this theory may be used to constrain the spin of black holes in astrophysical jets. | analytical solution of magnetically dominated astrophysical jets and winds: jet launching, acceleration, and collimation |
many near-term quantum computing algorithms are conceived as variational quantum algorithms, in which parameterized quantum circuits are optimized in a hybrid quantum-classical setup. examples are variational quantum eigensolvers, quantum approximate optimization algorithms as well as various algorithms in the context of quantum-assisted machine learning. a common bottleneck of any such algorithm is constituted by the optimization of the variational parameters. a popular set of optimization methods work on the estimate of the gradient, obtained by means of circuit evaluations. we will refer to the way in which one can combine these circuit evaluations as gradient rules. this work provides a comprehensive picture of the family of gradient rules that vary parameters of quantum gates individually. the most prominent known members of this family are the parameter shift rule (psr) and the finite differences method. to unite this family, we propose a generalized psr that expresses all members of the aforementioned family as special cases, and discuss how all of these can be seen as providing access to a linear combination of exact first- and second-order derivatives. we further prove that a psr with one non-shifted evaluation and only one shifted circuit evaluation does not exist, and introduce a novel perspective for approaching new gradient rules. | single-component gradient rules for variational quantum algorithms |
the distribution of the dark matter (dm) in dm-admixed neutron stars (danss) is supposed to result in either a dense dark core or an extended dark halo, subject to the dm fraction of the dans (fχ ) and the dm properties, such as the mass (mχ ) and the strength of the self-interaction (y). in this paper, we perform an in-depth analysis of the formation criterion for dark cores/dark halos, and point out that the relative distribution of these two components is essentially determined by the ratio of the central enthalpy of the dm component to that of the baryonic matter component inside the danss. for the critical case where the radii of the dm and the baryonic matter are the same, we further derive an analytical formula to describe the dependence of ${f}_{\chi }^{\mathrm{crit}}$ on mχand y for a given dans mass. the relative distribution of the two components in danss can lead to different observational effects. we here focus on the modification of the pulsar pulse profile, due to the extra light-bending effect in the case of a dark halo existence, and conduct the first investigation into the dark halo effects on the pulse profile. we find that the peak flux deviation is strongly dependent on the ratio of the halo mass to the radius of the dm component. last, we perform bayesian parameter estimation on the dm particle properties, based on the recent x-ray observations of psr j0030+0451 and psr j0740+6620 by the neutron star interior composition explorer. | dark matter admixed neutron star properties in the light of x-ray pulse profile observations |
spider pulsars are millisecond pulsars in short-period (≲12-h) orbits with low-mass (~0.01-0.4 m⊙) companion stars. the pulsars ablate plasma from the companion star, causing time delays and eclipses of the radio emission from the pulsar. the magnetic field of the companion has been proposed to strongly influence both the evolution of the binary system1 and the eclipse properties of the pulsar emission2. changes in the rotation measure (rm) have been seen in a spider system, implying that there is an increase in the magnetic field near the eclipse3. here we report a diverse range of evidence for a highly magnetized environment in the spider system psr b1744 - 24a4, located in the globular cluster terzan 5. we observe semi-regular profile changes to the circular polarization, v, when the pulsar emission passes close to the companion. this suggests that there is faraday conversion where the radio wave tracks a reversal in the parallel magnetic field and constrains the companion magnetic field, b (> 10 g). we also see irregular, fast changes in the rm at random orbital phases, implying that the magnetic strength of the stellar wind, b, is greater than 10 mg. there are similarities between the unusual polarization behaviour of psr b1744 - 24a and some repeating fast radio bursts (frbs)5-7. together with the possible binary-produced long-term periodicity of two active repeating frbs8,9, and the discovery of a nearby frb in a globular cluster10, where pulsar binaries are common, these similarities suggest that a proportion of frbs have binary companions. | a highly magnetized environment in a pulsar binary system |
high sensitivity radio searches of unassociated γ-ray sources have proven to be an effective way of finding new pulsars. using the five-hundred-meter aperture spherical radio telescope (fast) during its commissioning phase, we have carried out a number of targeted deep searches of fermi large area telescope (lat) γ-ray sources. on february 27, 2018 we discovered an isolated millisecond pulsar (msp), psr j0318+0253, coincident with the unassociated γ-ray source 3fgl j0318.1+0252. psr j0318+0253 has a spin period of 5.19 ms, a dispersion measure (dm) of 26 pc cm−3 corresponding to a dm distance of about 1.3 kpc, and a period-averaged flux density of (∼11±2) µjy at l-band (1.05-1.45 ghz). among all high energy msps, psr j0318+0253 is the faintest ever detected in radio bands, by a factor of at least ∼4 in terms of l-band fluxes. with the aid of the radio ephemeris, an analysis of 9.6 years of fermi-lat data revealed that psr j0318+0253 also displays strong γ-ray pulsations. follow-up observations carried out by both arecibo and fast suggest a likely spectral turn-over around 350 mhz. this is the first result from the collaboration between fast and the fermi-lat teams as well as the first confirmed new msp discovery by fast, raising hopes for the detection of many more msps. such discoveries will make a significant contribution to our understanding of the neutron star zoo while potentially contributing to the future detection of gravitational waves, via pulsar timing array (pta) experiments. | fast discovery of an extremely radio-faint millisecond pulsar from the fermi-lat unassociated source 3fgl j0318.1+0252 |
we analyse the stochastic properties of the 49 pulsars that comprise the first international pulsar timing array (ipta) data release. we use bayesian methodology, performing model selection to determine the optimal description of the stochastic signals present in each pulsar. in addition to spin-noise and dispersion-measure (dm) variations, these models can include timing noise unique to a single observing system, or frequency band. we show the improved radio-frequency coverage and presence of overlapping data from different observing systems in the ipta data set enables us to separate both system and band-dependent effects with much greater efficacy than in the individual pulsar timing array (pta) data sets. for example, we show that psr j1643-1224 has, in addition to dm variations, significant band-dependent noise that is coherent between ptas which we interpret as coming from time-variable scattering or refraction in the ionized interstellar medium. failing to model these different contributions appropriately can dramatically alter the astrophysical interpretation of the stochastic signals observed in the residuals. in some cases, the spectral exponent of the spin-noise signal can vary from 1.6 to 4 depending upon the model, which has direct implications for the long-term sensitivity of the pulsar to a stochastic gravitational-wave (gw) background. by using a more appropriate model, however, we can greatly improve a pulsar's sensitivity to gws. for example, including system and band-dependent signals in the psr j0437-4715 data set improves the upper limit on a fiducial gw background by ∼60 per cent compared to a model that includes dm variations and spin-noise only. | from spin noise to systematics: stochastic processes in the first international pulsar timing array data release |
aims: we present the results of the monitoring programmes performed with the swift/xrt telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well-known x-ray pulsars 4u 0115+63 and v 0332+53.methods: both sources form binary systems with be optical companions and undergo so-called giant outbursts every 3-4 years. the current observational campaigns were performed with the swift/xrt telescope in the soft x-ray band (0.5-10 kev) during the declining phases of the outbursts exhibited by both sources in 2015.results: the transitions to the propeller regime were detected at the threshold luminosities of (1.4 ± 0.4) × 1036 erg s-1 and (2.0 ± 0.4) × 1036 erg s-1 for 4u 0115+63 and v 0332+53, respectively. spectra of the sources are shown to be significantly softer during the low state. in both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into the propeller regime. the strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star. | propeller effect in two brightest transient x-ray pulsars: 4u 0115+63 and v 0332+53 |
the diffuse very high-energy (vhe; >100 gev) γ-ray emission observed in the central 200 pc of the milky way by h.e.s.s. was found to follow dense matter distribution in the central molecular zone (cmz) up to a longitudinal distance of about 130 pc to the galactic centre (gc), where the flux rapidly decreases. this was initially interpreted as the result of a burst-like injection of energetic particles 104 yr ago, but a recent more sensitive h.e.s.s. analysis revealed that the cosmic-ray (cr) density profile drops with the distance to the centre, making data compatible with a steady cosmic pevatron at the gc. in this paper, we extend this analysis to obtain, for the first time, a detailed characterisation of the correlation with matter and to search for additional features and individual γ-ray sources in the inner 200 pc. taking advantage of 250 h of h.e.s.s. data and improved analysis techniques, we perform a detailed morphology study of the diffuse vhe emission observed from the gc ridge and reconstruct its total spectrum. to test the various contributions to the total γ-ray emission, we used an iterative 2d maximum-likelihood approach that allows us to build a phenomenological model of the emission by summing a number of different spatial components. we show that the emission correlated with dense matter covers the full cmz and that its flux is about half the total diffuse emission flux. we also detect some emission at higher latitude that is likely produced by hadronic collisions of crs in less dense regions of the gc interstellar medium. we detect an additional emission component centred on the gc and extending over about 15 pc that is consistent with the existence of a strong cr density gradient and confirms the presence of a cr accelerator at the very centre of our galaxy. we show that the spectrum of full ridge diffuse emission is compatible with that previously derived from the central regions, suggesting that a single population of particles fills the entire cmz. finally, we report the discovery of a vhe γ-ray source near the gc radio arc and argue that it is produced by the pulsar wind nebula candidate g0.13-0.11. | characterising the vhe diffuse emission in the central 200 parsecs of our galaxy with h.e.s.s. |
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