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fast radio burst frb 20180916b in its host galaxy sdss j015800.28+654253.0 at 149 mpc is by far the closest-known frb with a robust host galaxy association. the source also exhibits a 16.35 day period in its bursting. here we present optical and infrared imaging as well as integral field spectroscopy observations of frb 20180916b with the wfc3 camera on the hubble space telescope and the megara spectrograph on the 10.4 m gran telescopio canarias. the 60-90 milliarcsecond (mas) resolution of the hubble imaging, along with the previous 2.3 mas localization of frb 20180916b, allows us to probe its environment with a 30-60 pc resolution. we constrain any point-like star formation or h ii region at the location of frb 20180916b to have an hα luminosity lhα ≲ 1037 erg s-1, and we correspondingly constrain the local star formation rate to be ≲10-4 m⊙ yr-1. the constraint on hα suggests that possible stellar companions to frb 20180916b should be of a cooler, less massive spectral type than o6v. frb 20180916b is 250 pc away (in projected distance) from the brightest pixel of the nearest young stellar clump, which is ∼380 pc in size (fwhm). with the typical projected velocities of pulsars, magnetars, or neutron stars in binaries (60-750 km s-1), frb 20180916b would need 800 kyr to 7 myr to traverse the observed distance from its presumed birth site. this timescale is inconsistent with the active ages of magnetars (≲10 kyr). rather, the inferred age and observed separation are compatible with the ages of high-mass x-ray binaries and gamma-ray binaries, and their separations from the nearest ob associations. | the 60 pc environment of frb 20180916b |
several pulsar timing array (pta) experiments such as nanograv and ppta reported evidence of a gravitational wave background at the nano-hz frequency band recently. this signal can originate from scalar-induced gravitational waves (sigw) generated by the enhanced curvature perturbation. here we perform a joint likelihood inference on pta datasets, and our results show that if the pta signals were indeed of sigw origin, the curvature perturbations amplitude required will produce primordial black holes (pbhs) in $[2 \times 10^{-5}, 2 \times 10^{-2}]\ m_\odot$ mass range. mergers of these pbhs can leave a strong gravitational wave signature in the $[10^{-3}, 10^8]$ hz frequency range, to be detectable at upcoming interferometers such as the einstein telescope, decigo and bbo, etc. this offers a multi-frequency opportunity to further scrutinize the source of the observed pta signal. | high frequency gravitational waves from pulsar timing arrays |
pulsar timing arrays (ptas) have recently found evidence for a nanohertz-frequency stochastic gravitational-wave background (sgwb). constraining its spectral characteristics will reveal its origins. in order to achieve this, we must understand how data and modeling conditions in each pulsar influence the precision and accuracy of sgwb spectral recovery. these goals typically require many bayesian analyses on real data sets and large-scale simulations that are slow and computationally taxing. to combat this, we have developed several new rapid approaches that instead operate on intermediate sgwb analysis products. these techniques refit sgwb spectral models to previously-computed bayesian posterior estimates of the timing power spectra. we test our new techniques on simulated pta data sets and the nanograv 12.5-year data set, where in the latter our refit posterior achieves a hellinger distance—bounded between 0 for identical distributions and 1 for zero overlap—from the current full production-level pipeline that is ≲0.1 . our techniques are ∼102- 104 times faster than the production-level likelihood, and scale much more favorably (sub-linearly) as a pta is expanded with new pulsars or observations. our techniques also allow us to demonstrate conclusively that sgwb spectral characterization in pta data sets is driven by the longest-timed pulsars and the best-measured power spectral densities, which is not necessarily the case for sgwb detection that is predicated on correlating many pulsars. indeed, the common-process spectral properties found in the nanograv 12.5-year data set are given by analyzing only the ∼14 longest-timed pulsars out of the full 45 pulsar array, and we find that the "shallowing" of the common-process power-law model occurs when gravitational-wave frequencies higher than ∼50 nanohertz are included. the implementation of our techniques is openly available as a software suite to allow fast and flexible pta sgwb spectral characterization and model selection. | rapid refitting techniques for bayesian spectral characterization of the gravitational wave background using pulsar timing arrays |
we report the detection of 72 new pulses from the repeating fast radio burst frb 121102 in breakthrough listen c-band (4-8 ghz) observations at the green bank telescope. the new pulses were found with a convolutional neural network in data taken on 2017 august 26, where 21 bursts have been previously detected. our technique combines neural network detection with dedispersion verification. for the current application, we demonstrate its advantage over a traditional brute-force dedispersion algorithm in terms of higher sensitivity, lower false-positive rates, and faster computational speed. together with the 21 previously reported pulses, this observation marks the highest number of frb 121102 pulses from a single observation, totaling 93 pulses in five hours, including 45 pulses within the first 30 minutes. the number of data points reveals trends in pulse fluence, pulse detection rate, and pulse frequency structure. we introduce a new periodicity search technique, based on the rayleigh test, to analyze the time of arrivals (toas), with which we exclude with 99% confidence periodicity in toas with periods larger than 5.1 times the model-dependent timestamp uncertainty. in particular, we rule out constant periods ≳10 ms in the barycentric arrival times, though intrinsic periodicity in the time of emission remains plausible. | fast radio burst 121102 pulse detection and periodicity: a machine learning approach |
locating the critical endpoint of qcd and the region of a first-order phase transition at finite baryon chemical potential is an active research area for qcd matter. we provide a gravitational dual description of qcd matter at finite baryon chemical potential μb and finite temperature using the nonperturbative approach from gauge/gravity duality. after fixing all model parameters using state-of-the-art lattice qcd data at zero chemical potential, the predicted equations of state and qcd trace anomaly relation are in quantitative agreement with the latest lattice results. we then give the exact location of the critical endpoint as well as the first-order transition line, which is within the coverage of many upcoming experimental measurements. moreover, using the data from our model at finite μb, we calculate the spectrum of the stochastic gravitational wave background associated with the first-order qcd transition in the early universe, which could be observable via pulsar timing in the future. | probing qcd critical point and induced gravitational wave by black hole physics |
we present a new analysis of the profile data from the 47 millisecond pulsars comprising the 12.5 yr data set of the north american nanohertz observatory for gravitational waves, which is presented in a parallel paper (alam et al., hereafter ng12.5). our reprocessing is performed using "wideband" timing methods, which use frequency-dependent template profiles, simultaneous time-of-arrival (toa) and dispersion measure (dm) measurements from broadband observations, and novel analysis techniques. in particular, the wideband dm measurements are used to constrain the dm portion of the timing model. we compare the ensemble timing results to those in ng12.5 by examining the timing residuals, timing models, and noise-model components. there is a remarkable level of agreement across all metrics considered. our best-timed pulsars produce encouragingly similar results to those from ng12.5. in certain cases, such as high-dm pulsars with profile broadening or sources that are weak and scintillating, wideband timing techniques prove to be beneficial, leading to more precise timing model parameters by 10%-15%. the high-precision, multiband measurements of several pulsars indicate frequency-dependent dms. compared to the narrowband analysis in ng12.5, the toa volume is reduced by a factor of 33, which may ultimately facilitate computational speed-ups for complex pulsar timing array analyses. this first wideband pulsar timing data set is a stepping stone, and its consistent results with ng12.5 assure us that such data sets are appropriate for gravitational wave analyses. | the nanograv 12.5 yr data set: wideband timing of 47 millisecond pulsars |
we report the discovery of four fast radio bursts (frbs) in the ongoing survey for pulsars and extragalactic radio bursts at the parkes radio telescope: frbs 150610, 151206, 151230 and 160102. our real-time discoveries have enabled us to conduct extensive, rapid multimessenger follow-up at 12 major facilities sensitive to radio, optical, x-ray, gamma-ray photons and neutrinos on time-scales ranging from an hour to a few months post-burst. no counterparts to the frbs were found and we provide upper limits on afterglow luminosities. none of the frbs were seen to repeat. formal fits to all frbs show hints of scattering while their intrinsic widths are unresolved in time. frb 151206 is at low galactic latitude, frb 151230 shows a sharp spectral cut-off, and frb 160102 has the highest dispersion measure (dm = 2596.1 ± 0.3 pc cm-3) detected to date. three of the frbs have high dispersion measures (dm > 1500 pc cm-3), favouring a scenario where the dm is dominated by contributions from the intergalactic medium. the slope of the parkes frb source counts distribution with fluences >2 jy ms is α =-2.2^{+0.6}_{-1.2} and still consistent with a euclidean distribution (α = -3/2). we also find that the all-sky rate is 1.7^{+1.5}_{-0.9}× 10^3frbs/(4π sr)/day above {∼ }2{ }{jy}{ }{ms} and there is currently no strong evidence for a latitude-dependent frb sky rate. | the survey for pulsars and extragalactic radio bursts - ii. new frb discoveries and their follow-up |
we obtain a new anisotropic solution for spherically symmetric spacetimes by analyzing the karmarkar embedding condition. for this purpose we construct a suitable form of one of the gravitational potentials to obtain a closed form solution. this form of the remaining gravitational potential allows us to solve the embedding equation and integrate the field equations. the resulting new anisotropic solution is well behaved, which can be utilized to construct realistic static fluid spheres. also we estimated the masses and radii of fluid spheres for lmc x-4, exo 1785-248, psr j1903+327 and 4u 1820-30 by using observational data set values. the masses and radii obtained show that our anisotropic solution can represent fluid spheres to a very good degree of accuracy. the physical validity of the solution depends on the parameter values of a, b and c. the solution is well behaved for the wide range of parameters values 0.00393≤ a ≤ 0.0055, 0.0002 ≤ b ≤ 0.0025 and 0.0107 ≤ c ≤ 0.0155. the range of corresponding physical parameters for the different compact stars are 0.3266≤ v_{r0} ≤ 0.3708, 0.1583≤ v_{t0} ≤ 0.2558, 0.3256≤ zs ≤ 0.4450 and 4.3587≤ γ 0 ≤ 5.6462. | anisotropic fluid spheres of embedding class one using karmarkar condition |
the next generation of space-borne gravitational wave detectors may detect gravitational waves from extreme mass-ratio inspirals with primordial black holes. to produce primordial black holes which contribute a non-negligible abundance of dark matter and are consistent with the observations, a large enhancement in the primordial curvature power spectrum is needed. for a single field slow-roll inflation, the enhancement requires a very flat potential for the inflaton, and this will increase the number of e-folds. to avoid the problem, an ultra-slow-roll inflation at the near inflection point is required. we elaborate the conditions to successfully produce primordial black hole dark matter from single field inflation and propose a toy model with polynomial potential to realize the big enhancement of the curvature power spectrum at small scales while maintaining the consistency with the observations at large scales. the power spectrum for the second order gravitational waves generated by the large density perturbations at small scales is consistent with the current pulsar timing array observations. | primordial black holes and second order gravitational waves from ultra-slow-roll inflation |
the nanograv collaboration recently reported a strong evidence for a stochastic common-spectrum process in the pulsar-timing data. we evaluate the evidence of interpreting this process as mergers of super massive black hole binaries and/or various stochastic gravitational-wave background sources in the early universe, including first-order phase transitions, cosmic strings, domain walls, and large amplitude curvature perturbations. we discuss the implications of the constraints on these possible sources. it is found that the data shows a positive evidence in favor of the cosmic strings against other gravitational-wave sources based on the bayes factor analysis. | evidence for different gravitational-wave sources in the nanograv dataset |
the canadian hydrogen intensity mapping experiment (chime) is a drift scan radio telescope operating across the 400-800 mhz band. chime is located at the dominion radio astrophysical observatory near penticton, bc, canada. the instrument is designed to map neutral hydrogen over the redshift range 0.8-2.5 to constrain the expansion history of the universe. this goal drives the design features of the instrument. chime consists of four parallel cylindrical reflectors, oriented north-south, each 100 m × 20 m and outfitted with a 256-element dual-polarization linear feed array. chime observes a two-degree-wide stripe covering the entire meridian at any given moment, observing three-quarters of the sky every day owing to earth's rotation. an fx correlator utilizes field-programmable gate arrays and graphics processing units to digitize and correlate the signals, with different correlation products generated for cosmological, fast radio burst, pulsar, very long baseline interferometry, and 21 cm absorber back ends. for the cosmology back end, the ${n}_{\mathrm{feed}}^{2}$ correlation matrix is formed for 1024 frequency channels across the band every 31 ms. a data receiver system applies calibration and flagging and, for our primary cosmological data product, stacks redundant baselines and integrates for 10 s. we present an overview of the instrument, its performance metrics based on the first 3 yr of science data, and we describe the current progress in characterizing chime's primary beam response. we also present maps of the sky derived from chime data; we are using versions of these maps for a cosmological stacking analysis, as well as for investigation of galactic foregrounds. | an overview of chime, the canadian hydrogen intensity mapping experiment |
using full 3+1 dimensional general-relativistic hydrodynamic simulations of equal- and unequal-mass neutron-star binaries with properties that are consistent with those inferred from the inspiral of gw170817, we perform a detailed study of the quark-formation processes that could take place after merger. we use three equations of state consistent with current pulsar observations derived from a novel finite-temperature framework based on v-qcd, a non-perturbative gauge/gravity model for quantum chromodynamics. in this way, we identify three different post-merger stages at which mixed baryonic and quark matter, as well as pure quark matter, are generated. a phase transition triggered collapse already \lesssim 10 ms≲10ms after the merger reveals that the softest version of our equations of state is actually inconsistent with the expected second-long post-merger lifetime of gw170817. our results underline the impact that gravitational wave observations of binary neutron-star mergers can have in constraining the equation of state of nuclear matter, especially in its most extreme regimes. | quark formation and phenomenology in binary neutron-star mergers using v-qcd |
we present the set of deep neutron star interior composition explorer (nicer) x-ray timing observations of the nearby rotation-powered millisecond pulsars psrs j0437-4715, j0030+0451, j1231-1411, and j2124-3358, selected as targets for constraining the mass-radius relation of neutron stars and the dense matter equation of state (eos) via modeling of their pulsed thermal x-ray emission. we describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass-radius relation and the dense matter eos. we find that the long-term timing and flux behavior and the fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. from phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of psr j0437-4715, for which multiple temperatures are required. | constraining the neutron star mass-radius relation and dense matter equation of state with nicer. i. the millisecond pulsar x-ray data set |
the present work is focused on the investigation of the existence of compact structures describing anisotropic matter distributions within the framework of modified gravity theories, specifically f (r ,t ) gravity theory. additionally, we have taken f (r ,t ) as a linear function of the ricci scalar r and the trace of the energy-momentum tensor t as f (r ,t )=r +2 χ t , where χ is a dimensionless coupling parameter, and the lagrangian matter lm=-1/3 (2 pt+pr) , to describe the complete set of field equations for the anisotropic matter distribution. we follow the embedding class i procedure using the eisland condition to obtain a full space-time description inside the stellar configuration. once the space-time geometry is specified, we determine the complete solution of modified einstein equations by using the mit bag model equation of state pr=1/3 (ρ -4 b ) that describes the strange quark matter (sqm) distribution inside the stellar system, where b denotes a bag constant. the physical validity of our anisotropic solution is confirmed by executing several physical tests. it is worth mentioning that with the help of the observed mass values for the various strange star candidates, we have predicted the exact radii by taking different values for χ and b . these predicted radii show a monotonic decreasing nature as the parameter χ is moved from -0.8 to 0.8 progressively. in this case, our anisotropic stellar system becomes more massive and transforms into more dense compact stars. we also perform a detailed graphical analysis of the compact star. as a result, for χ <0 , the current modified f (r ,t ) gravity seems promising to explain the observed massive compact astrophysical objects, similar to magnetars, massive pulsars, and chandrasekhar super white dwarfs, which are not justified in the framework of general relativity. finally, we note that when χ =0 , general relativity results for anisotropic matter distributions are recovered. | study of anisotropic strange stars in f (r ,t ) gravity: an embedding approach under the simplest linear functional of the matter-geometry coupling |
a cosmological first-order phase transition is expected to produce a stochastic gravitational wave background. if the phase transition temperature is on the mev scale, the power spectrum of the induced stochastic gravitational waves peaks around nanohertz frequencies, and can thus be probed with high-precision pulsar timing observations. we search for such a stochastic gravitational wave background with the latest data set of the parkes pulsar timing array. we find no evidence for a hellings-downs spatial correlation as expected for a stochastic gravitational wave background. therefore, we present constraints on first-order phase transition model parameters. our analysis shows that pulsar timing is particularly sensitive to the low-temperature (t ∼1 - 100 mev ) phase transition with a duration (β /h*)-1∼10-2−10-1 and therefore can be used to constrain the dark and qcd phase transitions. | constraining cosmological phase transitions with the parkes pulsar timing array |
the hydrogen intensity and real-time analysis experiment (hirax) is a new 400{800mhz radio interferometer under development for deployment in south africa. hirax will comprise 1024 six meter parabolic dishes on a compact grid and will map most of the southern sky over the course of four years. hirax has two primary science goals: to constrain dark energy and measure structure at high redshift, and to study radio transients and pulsars. hirax will observe unresolved sources of neutral hydrogen via their redshifted 21-cm emission line (`hydrogen intensity mapping'). the resulting maps of large-scale structure at redshifts 0.8{2.5 will be used to measure baryon acoustic oscillations (bao). bao are a preferential length scale in the matter distribution that can be used to characterize the expansion history of the universe and thus understand the properties of dark energy. hirax will improve upon current bao measurements from galaxy surveys by observing a larger cosmological volume (larger in both survey area and redshift range) and by measuring bao at higher redshift when the expansion of the universe transitioned to dark energy domination. hirax will complement chime, a hydrogen intensity mapping experiment in the northern hemisphere, by completing the sky coverage in the same redshift range. hirax's location in the southern hemisphere also allows a variety of cross-correlation measurements with large-scale structure surveys at many wavelengths. daily maps of a few thousand square degrees of the southern hemisphere, encompassing much of the milky way galaxy, will also open new opportunities for discovering and monitoring radio transients. the hirax correlator will have the ability to rapidly and efficiently detect transient events. this new data will shed light on the poorly understood nature of fast radio bursts (frbs), enable pulsar monitoring to enhance long-wavelength gravitational wave searches, and provide a rich data set for new radio transient phenomena searches. this paper discusses the hirax instrument, science goals, and current status. | hirax: a probe of dark energy and radio transients |
strong evidence for the helling-downs correlation curves have been reported by multiple pulsar timing array (pta) collaborations in middle 2023. in this work, we investigate the graviton mass bounds by analyzing the observational data of the overlap ruduction functions from the nanograv 15-year data release and cpta first data release. the results from our data analysis display the state-of-the-art upper limits on the graviton mass at 90\% confidence level, namely, $m_{g}\lesssim8.6\times10^{-24}\mathrm{ev}$ from nanograv and $m_{g}\lesssim3.8\times10^{-23}\mathrm{ev}$ from cpta. we also study the cosmic-variance limit on the graviton mass bounds, i.e., $\sigma_{m_{g}}^{\mathrm{cv}}=4.8\times10^{-24}\mathrm{ev}\times f/(10~\mathrm{year})^{-1}$, with $f$ being a typical frequency band of pta observations. this is equivalent to the cosmic-variance limit on the speed of gravitational waves, i.e., $\sigma_{v_{g}}^{\mathrm{cv}}=0.07c$, with $c$ being the speed of light. moreover, we discuss potential implications of these results for scenarios of ultralight tensor dark matter. | unveiling the graviton mass bounds through analysis of 2023 pulsar timing array data releases |
tenuous wind bubbles, which are formed by the spin-down activity of central compact remnants, are relevant in some models of fast radio bursts (frbs) and superluminous supernovae (sne). we study their high-energy signatures, focusing on the role of pair-enriched bubbles produced by young magnetars, rapidly rotating neutron stars, and magnetized white dwarfs. (i) first, we study the nebular properties and the conditions allowing for escape of high-energy gamma-rays and radio waves, showing that their escape is possible for nebulae with ages of ≳10-100 yr. in the rapidly rotating neutron star scenario, we find that radio emission from the quasi-steady nebula itself may be bright enough to be detected especially at sub-mm frequencies, which is relevant as a possible counterpart of pulsar-driven sne and frbs. (ii) secondly, we consider the fate of bursting emission in the nebulae. we suggest that an impulsive burst may lead to a highly relativistic flow, which would interact with the nebula. if the shocked nebula is still relativistic, pre-existing non-thermal particles in the nebula can be significantly boosted by the forward shock, leading to short-duration (maybe millisecond or longer) high-energy gamma-ray flashes. possible dissipation at the reverse shock may also lead to gamma-ray emission. (iii) after such flares, interactions with the baryonic ejecta may lead to afterglow emission with a duration of days to weeks. in the magnetar scenario, this burst-in-bubble model leads to the expectation that nearby (≲10-100 mpc) high-energy gamma-ray flashes may be detected by the high-altitude water cherenkov observatory and the cherenkov telescope array, and the subsequent afterglow emission may be seen by radio telescopes such as the very large array. (iv) finally, we discuss several implications specific to frbs, including constraints on the emission regions and limits on soft gamma-ray counterparts. | a burst in a wind bubble and the impact on baryonic ejecta: high-energy gamma-ray flashes and afterglows from fast radio bursts and pulsar-driven supernova remnants |
radio pulsars provide us with some of the most stable clocks in the universe. nevertheless several pulsars exhibit sudden spin-up events, known as glitches. more than forty years after their first discovery, the exact origin of these phenomena is still open to debate. it is generally thought that they are observational manifestation of a superfluid component in the stellar interior and provide an insight into the dynamics of matter at extreme densities. in recent years, there have been several advances on both the theoretical and observational side, that have provided significant steps forward in our understanding of neutron star interior dynamics and possible glitch mechanisms. in this article we review the main glitch models that have been proposed and discuss our understanding, in the light of current observations. | models of pulsar glitches |
we show that the periodic frb 180916.j0158+65 can be interpreted by invoking an interacting neutron star binary system with an orbital period of ∼16 days. the frbs are produced by a highly magnetized pulsar, whose magnetic field is "combed" by the strong wind from a companion star, either a massive star or a millisecond pulsar. the frb pulsar wind retains a clear funnel in the companion's wind that is otherwise opaque to induced compton or raman scatterings for repeating frb emission. the 4 day active window corresponds to the time when the funnel points toward earth. the interaction also perturbs the magnetosphere of the frb pulsar and may trigger emission of frbs. we derive the physical constraints on the comb and the frb pulsar from the observations and estimate the event rate of frbs. in this scenario, a lower limit on the period of observable frbs is predicted. we speculate that both the intrinsic factors (strong magnetic field and young age) and the extrinsic factor (interaction) may be needed to generate frbs in neutron star binary systems. | a binary comb model for periodic fast radio bursts |
the discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. in this paper, we present follow-up observations of the gravitational wave event gw170817 and its electromagnetic counterpart sss17a/dlt17ck (iau label at2017gfo) by 14 australian telescopes and partner observatories as part of australian-based and australian-led research programs. we report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. our optical spectra reveal that the transient source emission cooled from approximately 6 400 k to 2 100 k over a 7-d period and produced no significant optical emission lines. the spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. star formation in the host galaxy probably ceased at least a gyr ago, although there is evidence for a galaxy merger. binary pulsars with short (100 myr) decay times are therefore unlikely progenitors, but pulsars like psr b1534+12 with its 2.7 gyr coalescence time could produce such a merger. the displacement ( 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor. | follow up of gw170817 and its electromagnetic counterpart by australian-led observing programmes |
we review the current understanding of the diffuse gamma-ray background (dgrb). the dgrb is what remains of the total measured gamma-ray emission after the subtraction of the resolved sources and of the diffuse galactic foregrounds. it is interpreted as the cumulative emission of sources that are not bright enough to be detected individually. yet, its exact composition remains unveiled. well-established astrophysical source populations (e.g. blazars, misaligned agns, star-forming galaxies and millisecond pulsars) all represent guaranteed contributors to the dgrb. more exotic scenarios, such as dark matter annihilation or decay, may contribute as well. in this review, we describe how these components have been modeled in the literature and how the dgrb can be used to provide valuable information on each of them. we summarize the observational information currently available on the dgrb, paying particular attention to the most recent measurement of its intensity energy spectrum by the fermi lat collaboration. we also discuss the novel analyses of the auto-correlation angular power spectrum of the dgrb and of its cross-correlation with tracers of the large-scale structure of the universe. new data sets already (or soon) available are expected to provide further insight on the nature of this emission. by summarizing where we stand on the current knowledge of the dgrb, this review is intended both as a useful reference for those interested in the topic and as a means to trigger new ideas for further research. | the nature of the diffuse gamma-ray background |
we present results of the coordinated observing campaign that made the first subarcsecond localization of a fast radio burst, frb 121102. during this campaign, we made the first simultaneous detection of an frb burst using multiple telescopes: the vla at 3 ghz and the arecibo observatory at 1.4 ghz. of the nine bursts detected by the very large array at 3 ghz, four had simultaneous observing coverage at other observatories at frequencies from 70 mhz to 15 ghz. the one multi-observatory detection and three non-detections of bursts seen at 3 ghz confirm earlier results showing that burst spectra are not well modeled by a power law. we find that burst spectra are characterized by a ∼500 mhz envelope and apparent radio energy as high as 1040 erg. we measure significant changes in the apparent dispersion between bursts that can be attributed to frequency-dependent profiles or some other intrinsic burst structure that adds a systematic error to the estimate of dispersion measure by up to 1%. we use frb 121102 as a prototype of the frb class to estimate a volumetric birth rate of frb sources {r}{frb}≈ 5× {10}-5/{n}r mpc-3 yr-1, where nris the number of bursts per source over its lifetime. this rate is broadly consistent with models of frbs from young pulsars or magnetars born in superluminous supernovae or long gamma-ray bursts if the typical frb repeats on the order of thousands of times during its lifetime. | a multi-telescope campaign on frb 121102: implications for the frb population |
we analyze the polarization content of gravitational waves in horndeski theory. besides the familiar plus and cross polarizations in einstein's general relativity, there is one more polarization state which is the mixture of the transverse breathing and longitudinal polarizations. the additional mode is excited by the massive scalar field. in the massless limit, the longitudinal polarization disappears, while the breathing one persists. the upper bound on the graviton mass severely constrains the amplitude of the longitudinal polarization, which makes its detection highly unlikely by the ground-based or space-borne interferometers in the near future. however, pulsar timing arrays might be able to detect the polarization excited by the massive scalar field. since additional polarization states appear in alternative theories of gravity, the measurement of the polarizations of gravitational waves can be used to probe the nature of gravity. in addition to the plus and cross states, the detection of the breathing polarization means that gravitation is mediated by massless spin 2 and spin 0 fields, and the detection of both the breathing and longitudinal states means that gravitation is propagated by the massless spin 2 and massive spin 0 fields. | polarizations of gravitational waves in horndeski theory |
in this paper we study online reinforcement learning (rl) in partially observable dynamical systems. we focus on the predictive state representations (psrs) model, which is an expressive model that captures other well-known models such as partially observable markov decision processes (pomdp). psr represents the states using a set of predictions of future observations and is defined entirely using observable quantities. we develop a novel model-based algorithm for psrs that can learn a near optimal policy in sample complexity scaling polynomially with respect to all the relevant parameters of the systems. our algorithm naturally works with function approximation to extend to systems with potentially large state and observation spaces. we show that given a realizable model class, the sample complexity of learning the near optimal policy only scales polynomially with respect to the statistical complexity of the model class, without any explicit polynomial dependence on the size of the state and observation spaces. notably, our work is the first work that shows polynomial sample complexities to compete with the globally optimal policy in psrs. finally, we demonstrate how our general theorem can be directly used to derive sample complexity bounds for special models including $m$-step weakly revealing and $m$-step decodable tabular pomdps, pomdps with low-rank latent transition, and pomdps with linear emission and latent transition. | pac reinforcement learning for predictive state representations |
astrophysical observations of neutron stars probe the structure of dense nuclear matter and have the potential to reveal phase transitions at high densities. most recent analyses are based on parametrized models of the equation of state with a finite number of parameters and occasionally include extra parameters intended to capture phase-transition phenomenology. however, such models restrict the types of behavior allowed and may not match the true equation of state. we introduce a complementary approach that extracts phase transitions directly from the equation of state without relying on, and thus being restricted by, an underlying parametrization. we then constrain the presence of phase transitions in neutron stars with astrophysical data. current pulsar mass, tidal deformability, and mass-radius measurements disfavor only the strongest of possible phase transitions (latent energy per particle ≳100 mev ). weaker phase transitions are consistent with observations. we further investigate the prospects for measuring phase transitions with future gravitational-wave observations and find that catalogs of o (100 ) events will (at best) yield bayes factors of ∼10 :1 in favor of phase transitions even when the true equation of state contains very strong phase transitions. our results reinforce the idea that neutron star observations will primarily constrain trends in macroscopic properties rather than detailed microscopic behavior. fine-tuned equation of state models will likely remain unconstrained in the near future. | phase transition phenomenology with nonparametric representations of the neutron star equation of state |
we study the prospects of detection at terrestrial and space interferometers, as well as at pulsar timing array experiments, of a stochastic gravitational wave background which can be produced in models of axion inflation. this potential signal, and the development of these experiments, open a new window on inflation on scales much smaller than those currently probed with cosmic microwave background and large scale structure measurements. the sourced signal generated in axion inflation is an ideal candidate for such searches, since it naturally grows at small scales, and it has specific properties (chirality and non-gaussianity) that can distinguish it from an astrophysical background. we study under which conditions such a signal can be produced at an observable level, without the simultaneous overproduction of scalar perturbations in excess of what is allowed by the primordial black hole limits. we also explore the possibility that scalar perturbations generated in a modified version of this model may provide a distribution of primordial black holes compatible with the current bounds, that can act as a seeds of the present black holes in the universe. | gravitational waves at interferometer scales and primordial black holes in axion inflation |
the symmetry energy and its density dependence are pivotal for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. recently, prex-ii reported a value of 0.283 ±0.071 fm for the neutron-skin thickness of 208pb, rskin208pb, implying a symmetry-energy slope parameter l of 106 ±37 mev, larger than most ranges obtained from microscopic calculations and other nuclear experiments. we use a nonparametric equation of state representation based on gaussian processes to constrain the symmetry energy s0, l , and rskin208pb directly from observations of neutron stars with minimal modeling assumptions. the resulting astrophysical constraints from heavy pulsar masses, ligo/virgo, and nicer favor smaller values of the neutron skin and l , as well as negative symmetry incompressibilities. combining astrophysical data with chiral effective field theory (χ eft ) and prex-ii constraints yields s0=33 .0−1.8+2.0 mev, l =53−15+14 mev, and rskin208pb=0 .17−0.04+0.04 fm. we also examine the consistency of several individual χ eft calculations with astrophysical observations and terrestrial experiments. we find that there is only mild tension between χ eft , astrophysical data, and prex-ii's rskin208pb measurement (p value =12.3 % ) and that there is excellent agreement between χ eft , astrophysical data, and other nuclear experiments. | detailed examination of astrophysical constraints on the symmetry energy and the neutron skin of 208pb with minimal modeling assumptions |
ultralight primordial black holes (pbhs)(≲109g) completely evaporate via hawking radiation (hr) and produce all the particles in a given theory regardless of their other interactions. if the right handed (rh) neutrinos are produced from pbh evaporation, successful baryogenesis via leptogenesis predicts mass scale of rh neutrinos as well as black holes. we show that, given the lepton number violation (generation of rh neutrino masses) in the theory is a consequence of a gauged u(1) breaking which is then followed by the formation of pbhs, a network of cosmic strings naturally gives rise to strong stochastic gravitational wave (gw) signal at the sensitivity level of pulsar timing arrays (pta) and ligo5. besides, due to a transient period of black hole domination in the early universe, for which baryon asymmetry is independent of initial pbh density, a break in the gw spectra occurs around mhz frequency. therefore, to observe the break along with the usual gw signal by the emission of gravitons via hr, gw detectors at higher frequencies are called for. the recent finding by the nanograv pta of a stochastic common spectrum process (interpreted as gws) across many pulsars is in tension with pbh baryogenesis for large cosmic string loops (α ≃ 0.1). | baryogenesis from ultralight primordial black holes and strong gravitational waves from cosmic strings |
the nasa telescope nicer has recently measured x-ray emissions from the heaviest of the precisely known two-solar mass neutron stars, psr j0740 + 6620. analysis of the data [astrophys. j. lett. 918, l28 (2021), 10.3847/2041-8213/ac089b, astrophys. j. lett. 918, l27 (2021), 10.3847/2041-8213/ac0a81] suggests that psr j0740 + 6620 has a radius in the range of r2.0≈(11.4 -16.1 ) km at the 68 % credibility level. in this article, we study the implications of this analysis for the sound speed in the high-density inner cores by using recent chiral effective field theory (χ eft ) calculations of the equation of state at next-to-next-to-next-to-leading order to describe outer regions of the star at modest density. we find that the lower bound on the maximum speed of sound in the inner core, min{cs,max 2} , increases rapidly with the radius of massive neutron stars. if χ eft remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, r2.0⩾13 km requires min{cs,max 2}⩾0.562 and 0.442 at the 68% and 95% credibility level, respectively. | large and massive neutron stars: implications for the sound speed within qcd of dense matter |
strongly magnetized, rapidly rotating neutron stars are contenders for the central engines of both long gamma-ray bursts (lgrbs) and hydrogen-poor superluminous supernovae (slsne-i). models for typical (minute long) lgrbs invoke magnetars with high dipole magnetic fields (bd ≳ 1015 g) and short spin-down times, slsne-i require neutron stars with weaker fields and longer spin-down times of weeks. here, we identify a transition region in the space of bd and birth period for which a magnetar can power both a lgrb and a luminous supernova. in particular, a 2 ms period magnetar with a spin-down time of ∼104 s can explain both the ultralong grb 111209 and its associated luminous sn2011kl. for magnetars with longer spin-down times, we predict even longer duration (∼105 - 6 s) grbs and brighter supernovae, a correlation that extends to swift j2058+05 (commonly interpreted as a tidal disruption event). we further show that previous estimates of the maximum rotational energy of a protomagnetar were too conservative and energies up to emax ∼ 1-2 × 1053 ergs are possible. a magnetar can therefore comfortably accommodate the extreme energy requirements recently posed by the most luminous supernova asassn-15lh. the luminous pulsar wind nebula powering asassn-15lh may lead to an `ionization breakout' x-ray burst over the coming months, accompanied by a change in the optical spectrum. | the diversity of transients from magnetar birth in core collapse supernovae |
we use stergioulas's rns code for investigating fast pulsars with equations of state (eoss) on the causality surface (where the speed of sound is equal to that of light) of the high-density eos parameter space satisfying all known constraints from both nuclear physics and astrophysics. we show that one possible explanation for gw190814's secondary component, which has mass 2.50-2.67 m⊙, is that it is a superfast pulsar spinning faster than 971 hz, about 42% below its kepler frequency. if confirmed, it would be the fastest pulsar with the highest mass yet observed. there is a large and physically allowed eos parameter space below the causality surface where pulsars heavier than 2.50 m⊙ are supported if they can rotate even faster with critical frequencies that depend strongly on the high-density behavior of nuclear symmetry energy. | gw190814's secondary component with mass 2.50-2.67 m⊙ as a superfast pulsar |
young and rotation-powered neutron stars (nss) are commonly observed as rapidly-spinning pulsars. they dissipate their rotational energy by emitting pulsar wind with electromagnetic radiation and spin down at a steady rate, according to the simple steadily-rotating magnetic dipole model. in reality, however, multiwavelength observations of radiation from the ns surface and magnetosphere have revealed that the evolution and properties of nss are highly diverse, often dubbed as 'ns zoo'. in particular, many of young and highly magnetized nss show a high degree of activities, such as sporadic electromagnetic outbursts and irregular changes in pulse arrival times. importantly, their magnetic field, which are the strongest in the universe, makes them ideal laboratories for fundamental physics. a class of highly-magnetized isolated nss is empirically divided into several subclasses. in a broad classification, they are, in the order of the magnetic field strength (b) from the highest, 'magnetars' (historically recognized as soft gamma-ray repeaters and/or anomalous x-ray pulsars), 'high-b pulsars', and (nearby) x-ray isolated nss. this article presents an introductory review for non-astrophysicists about the observational properties of highly-magnetized nss, and their implications. the observed dynamic nature of nss must be interpreted in conjunction with transient magnetic activities triggered during magnetic-energy dissipation process. in particular, we focus on how the five fundamental quantities of nss, i.e. mass, radius, spin period, surface temperature, and magnetic fields, as observed with modern instruments, change with evolution of, and vary depending on the class of, the nss. they are the foundation for a future unified theory of nss. | observational diversity of magnetized neutron stars |
a metastable cosmic-string network is a generic consequence of many grand unified theories (guts) when combined with cosmic inflation. metastable cosmic strings are not topologically stable, but decay on cosmic time scales due to pair production of gut monopoles. this leads to a network consisting of metastable long strings on superhorizon scales as well as of string loops and segments on subhorizon scales. we compute for the first time the complete stochastic gravitational-wave background (sgwb) arising from all these network constituents, including several technical improvements to both the derivation of the loop and segment contributions. we find that the gravitational waves emitted by string loops provide the main contribution to the gravitational-wave spectrum in the relevant parameter space. the resulting spectrum is consistent with the tentative signal observed by the nanograv and parkes pulsar timing collaborations for a string tension of g μ ~ 10-11…-7 and has ample discovery space for ground- and space-based detectors. for gut-scale string tensions, g μ ~ 10-8…-7, metastable strings predict a sgwb in the ligo-virgo-kagra band that could be discovered in the near future. | stochastic gravitational-wave background from metastable cosmic strings |
in this paper, we discuss the impact of the following laboratory experiments and astrophysical observation of neutron stars (nss) on their equation of state (eos): (a) the new measurement of neutron skin thickness of 208pb, ${r}_{\mathrm{skin}}^{208}=0.29\pm 0.07$ fm by the prex-ii experiment; (b) the mass measurement of psr j0740+6620 has been slightly revised down by including additional ~1.5 yr of pulsar timing data. as well as the radius measurement of psr j0740+6620 by joint nicer/xmm-newton collaboration, which has a similar size to psr j0030+0451. we combine this information using bayesian statistics along with the previous ligo/virgo and nicer observations of ns using a hybrid nuclear+piecewise-polytrope eos parameterization. our findings are as follows. (a). adding prex-ii result yields the value of empirical parameter $l={69}_{-19}^{+21}$ mev, ${r}_{\mathrm{skin}}^{208}={0.20}_{-0.05}^{+0.05}$ fm, and radius of a 1.4 m⊙ ( ${r}_{1.4})={12.75}_{-0.54}^{+0.42}$ km at 1σ confidence interval. we find these inferred values are mostly dominated by the combined astrophysical observations as the measurement uncertainty in ${r}_{\mathrm{skin}}^{208}$ by prex-ii is much broader. also, a better measurement of ${r}_{\mathrm{skin}}^{208}$ might have a small effect on the radius of low-mass nss but, for the high masses, there will be almost no effect. (b) after adding the revised mass and radius measurement of psr j0740+6620, we find the inferred radii of nss are slightly pushed toward the larger values and the uncertainty on the radius of a 2.08 m⊙ ns is moderately improved. | impact of prex-ii and combined radio/nicer/xmm-newton's mass-radius measurement of psr j0740+6620 on the dense-matter equation of state |
we present the first 2.5 yr of data from the meerkat pulsar timing array (mpta), part of meertime, a meerkat large survey project. the mpta aims to precisely measure pulse arrival times from an ensemble of 88 pulsars visible from the southern hemisphere, with the goal of contributing to the search, detection, and study of nanohertz-frequency gravitational waves as part of the international pulsar timing array. this project makes use of the meerkat telescope and operates with a typical observing cadence of 2 weeks using the l-band receiver that records data from 856 to 1712 mhz. we provide a comprehensive description of the observing system, software, and pipelines used and developed for the meertime project. the data products made available as part of this data release are from the 78 pulsars that had at least 30 observations between the start of the meertime programme in february 2019 and october 2021. these include both sub-banded and band-averaged arrival times and the initial timing ephemerides, noise models, and the frequency-dependent standard templates (portraits) used to derive pulse arrival times. after accounting for detected noise processes in the data, the frequency-averaged residuals of 67 of the pulsars achieved a root-mean-square residual precision of $\lt 1 \, \mu \rm {s}$. we also present a novel recovery of the clock correction waveform solely from pulsar timing residuals and an exploration into preliminary findings of interest to the international pulsar timing community. the arrival times, standards, and full stokes parameter-calibrated pulsar timing archives are publicly available. | the meerkat pulsar timing array: first data release |
the nicer collaboration recently reported the measurement of the mass and radius of a pulsar psr j0030+0451. we here use this new measurement to constrain one of the higher-order nuclear matter parameters $k_{\mathrm{sym,0}}$. we further combine the tidal measurement of the binary neutron star merger gw170817 by ligo/virgo to derive a joint 1-$\sigma$ constraint as $k_{\mathrm{sym,0}} = -102^{+71}_{-72}$ mev. we believe this is the most reliable bound on the parameter to date under the assumption that there is no new physics above the saturation density which impacts neutron star observations. | measuring nuclear matter parameters with nicer and ligo/virgo |
primordial black holes (pbh) from peaks in the curvature power spectrum could constitute today an important fraction of the dark matter in the universe. at horizon reentry, during the radiation era, order one fluctuations collapse gravitationally to form black holes and, at the same time, generate a stochastic background of gravitational waves coming from second order anisotropic stresses in matter. we study the amplitude and shape of this background for several phenomenological models of the curvature power spectrum that can be embedded in waterfall hybrid inflation, axion, domain wall, and boosts of pbh formation at the qcd transition. for a broad peak or a nearly scale invariant spectrum, this stochastic background is generically enhanced by about one order of magnitude, compared to a sharp feature. as a result, stellar-mass pbh from gaussian fluctuations with a wide mass distribution are already in strong tension with the limits from pulsar timing arrays, if they constitute a non negligible fraction of the dark matter. but this result is mitigated by the uncertainties on the curvature threshold leading to pbh formation. lisa will have the sensitivity to detect or rule out light pbh down to $10^{-14} m_{\odot}$. upcoming runs of ligo/virgo and future interferometers such as the einstein telescope will increase the frequency lever arm to constrain pbh from the qcd transition. ultimately, the future ska pulsar timing arrays could probe the existence of even a single stellar-mass pbh in our observable universe. | detecting the stochastic gravitational wave background from primordial black hole formation |
in this work we obtain an anisotropic neutron star solution by gravitational decoupling starting from a perfect fluid configuration which has been used to model the compact object psr j0348+0432. additionally, we consider the same solution to model the binary pulsar sax j1808.4-3658 and x-ray binaries her x-1 and cen x-3 ones. we study the acceptability conditions and obtain that the mgd-deformed solution obey the same physical requirements as its isotropic counterpart. finally, we conclude that the most stable solutions, according to the adiabatic index and gravitational cracking criterion, are those with the smallest compactness parameters, namely sax j1808.4-3658 and her x-1. | anisotropic neutron stars by gravitational decoupling |
observational results of interstellar and intergalactic magnetic fields are reviewed, including the fields in supernova remnants and loops, interstellar filaments and clouds, hii regions and bubbles, the milky way and nearby galaxies, galaxy clusters, and the cosmic web. a variety of approaches are used to investigate these fields. the orientations of magnetic fields in interstellar filaments and molecular clouds are traced by polarized thermal dust emission and starlight polarization. the field strengths and directions along the line of sight in dense clouds and cores are measured by zeeman splitting of emission or absorption lines. the large-scale magnetic fields in the milky way have been best probed by faraday rotation measures of a large number of pulsars and extragalactic radio sources. the coherent galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. the azimuthal fields in the halo reverse their directions below and above the galactic plane. the orientations of organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and faraday rotations of embedded radio galaxies and background sources. sparse evidence for very weak magnetic fields in the cosmic web is the detection of the faint radio bridge between the coma cluster and a1367. future observations should aim at the 3d tomography of the large-scale coherent magnetic fields in our galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web. | observing interstellar and intergalactic magnetic fields |
we report the properties of more than 600 bursts (including cluster-bursts) detected from the repeating fast radio burst (frb) source frb 20201124a with the five-hundred-meter aperture spherical radio telescope during an extremely active episode on utc 2021 september 25-28, in a series of four papers. the observations were carried out in the band of 1.0-1.5 ghz by using the center beam of the l-band 19-beam receiver. we monitored the source in sixteen 1 hr sessions and one 3 hr session spanning 23 days. all the bursts were detected during the first four days. in this first paper of the series, we perform a detailed morphological study of 624 bursts using the two-dimensional frequency-time "waterfall" plots, with a burst (or cluster-burst) defined as an emission episode during which the adjacent emission peaks have a separation shorter than 400 ms. the duration of a burst is therefore always longer than 1 ms, with the longest up to more than 120 ms. the emission spectra of the sub-bursts are typically narrow within the observing band with a characteristic width of ~277 mhz. the center frequency distribution has a dominant peak at about 1091.9 mhz and a secondary weak peak around 1327.9 mhz. most bursts show a frequencydownward-drifting pattern. based on the drifting patterns, we classify the bursts into five main categories: downward drifting (263) bursts, upward drifting (3) bursts, complex (203), no drifting (35) bursts, and no evidence for drifting (121) bursts. subtypes are introduced based on the emission frequency range in the band (low, middle, high and wide) as well as the number of components in one burst (1, 2, or multiple). we measured a varying scintillation bandwidth from about 0.5 mhz at 1.0 ghz to 1.4 mhz at 1.5 ghz with a spectral index of 3.0. | fast observations of an extremely active episode of frb 20201124a: i. burst morphology |
the collection of individually resolvable gravitational wave (gw) events makes up a tiny fraction of all gw signals that reach our detectors, while most lie below the confusion limit and are undetected. similarly to voices in a crowded room, the collection of unresolved signals gives rise to a background that is well-described via stochastic variables and, hence, referred to as the stochastic gw background (sgwb). in this review, we provide an overview of stochastic gw signals and characterise them based on features of interest such as generation processes and observational properties. we then review the current detection strategies for stochastic backgrounds, offering a ready-to-use manual for stochastic gw searches in real data. in the process, we distinguish between interferometric measurements of gws, either by ground-based or space-based laser interferometers, and timing-residuals analyses with pulsar timing arrays (ptas). these detection methods have been applied to real data both by large gw collaborations and smaller research groups, and the most recent and instructive results are reported here. we close this review with an outlook on future observations with third generation detectors, space-based interferometers, and potential noninterferometric detection methods proposed in the literature. | stochastic gravitational-wave backgrounds: current detection efforts and future prospects |
the dark matter particle explorer (dampe), a high energy cosmic ray and $\gamma$-ray detector in space, has recently reported the new measurement of the total electron plus positron flux between 25 gev and 4.6 tev. a spectral softening at $\sim0.9$ tev and a tentative peak at $\sim1.4$ tev have been reported. we study the physical implications of the dampe data in this work. the presence of the spectral break significantly tightens the constraints on the model parameters to explain the electron/positron excesses. the spectral softening can either be explained by the maximum acceleration limits of electrons by astrophysical sources, or a breakdown of the common assumption of continuous distribution of electron sources at tev energies in space and time. the tentive peak at $\sim1.4$ tev implies local sources of electrons/positrons with quasi-monochromatic injection spectrum. we find that the cold, ultra-relativistic $e^+e^-$ winds from pulsars may give rise to such a structure. the pulsar is requird to be middle-aged, relatively slowly-rotated, mildly magnetized, and isolated in a density cavity. the annihilation of dm particles ($m_{\chi}\sim1.5$ tev) into $e^+e^-$ pairs in a nearby clump or an over-density region may also explain the data. in the dm scenario, the inferred clump mass (or density enhancement) is about $10^7-10^8$ m$_\odot$ (or $17-35$ times of the canonical local density) assuming a thermal production cross section, which is relatively extreme compared with the expectation from numerical simulations. a moderate enhancement of the annihilation cross section via, e.g., the sommerfeld mechanism or non-thermal production, is thus needed. | interpretations of the dampe electron data |
the mass of the dark matter particle is unknown, and may be as low as ∼1 0-22 ev . the lighter part of this range, below ∼ev , is relatively unexplored both theoretically and experimentally but contains an array of natural dark matter candidates. an example is the relaxion, a light boson predicted by cosmological solutions to the hierarchy problem. one of the few generic signals such light dark matter can produce is a time-oscillating, equivalence-principle-violating force. we propose searches for this using accelerometers, and consider in detail the examples of torsion balances, atom interferometry, and pulsar timing. these approaches have the potential to probe large parts of unexplored parameter space in the next several years. thus such accelerometers provide radically new avenues for the direct detection of dark matter. | dark matter direct detection with accelerometers |
using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron-positron plasmas, for a wide range of upstream magnetizations σ and system sizes l. the particle spectra are well-represented by a power law {γ }-α , with a combination of exponential and super-exponential high-energy cutoffs, proportional to σ and l, respectively. for large l and σ, the power-law index α approaches about 1.2. | the extent of power-law energy spectra in collisionless relativistic magnetic reconnection in pair plasmas |
primordial black hole (pbh) mergers have been proposed as an explanation for the gravitational wave events detected by the ligo collaboration. such pbhs may be formed in the early universe as a result of the collapse of extremely rare high-sigma peaks of primordial fluctuations on small scales, as long as the amplitude of primordial perturbations on small scales is enhanced significantly relative to the amplitude of perturbations observed on large scales. one consequence of these small-scale perturbations is generation of stochastic gravitational waves that arise at second order in scalar perturbations, mostly before the formation of the pbhs. these induced gravitational waves have been shown, assuming gaussian initial conditions, to be comparable to the current limits from the european pulsar timing array, severely restricting this scenario. we show, however, that models with enhanced fluctuation amplitudes typically involve non-gaussian initial conditions. with such initial conditions, the current limits from pulsar timing can be evaded. the amplitude of the induced gravitational-wave background can be larger or smaller than the stochastic gravitational-wave background from supermassive black hole binaries. | stochastic gravitational waves associated with the formation of primordial black holes |
einstein’s general theory of relativity (gr) successfully describes gravity. although gr has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. one of the most fundamental predictions of gr is the existence of black holes (bhs). after the recent direct detection of gravitational waves by ligo, there is now near conclusive evidence for the existence of stellar-mass bhs. in spite of this exciting discovery, there is not yet direct evidence of the existence of bhs using astronomical observations in the electromagnetic spectrum. are bhs observable astrophysical objects? does gr hold in its most extreme limit or are alternatives needed? the prime target to address these fundamental questions is in the center of our own milky way, which hosts the closest and best-constrained supermassive bh candidate in the universe, sagittarius a* (sgr a*). three different types of experiments hold the promise to test gr in a strong-field regime using observations of sgr a* with new-generation instruments. the first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto sgr a*. this emission forms a “shadow” around the event horizon cast against the background, whose predicted size (∼50μas) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (eht). the second experiment aims to monitor stars orbiting sgr a* with the next-generation near-infrared (nir) interferometer gravity at the very large telescope (vlt). the third experiment aims to detect and study a radio pulsar in tight orbit about sgr a* using radio telescopes (including the atacama large millimeter array or alma). the blackholecam project exploits the synergy between these three different techniques and contributes directly to them at different levels. these efforts will eventually enable us to measure fundamental bh parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of gr (e.g. testing the no-hair theorem) and probe the spacetime around a bh in any metric theory of gravity. here, we review our current knowledge of the physical properties of sgr a* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around sgr a*. we conclude that the galactic center provides a unique fundamental-physics laboratory for experimental tests of bh accretion and theories of gravity in their most extreme limits. | blackholecam: fundamental physics of the galactic center |
we report the discovery of an extended very-high-energy (vhe) gamma-ray source around the location of the middle-aged (207.8 kyr) pulsar psr j 0622 +3749 with the large high-altitude air shower observatory (lhaaso). the source is detected with a significance of 8.2 σ for e >25 tev assuming a gaussian template. the best-fit location is (right ascension, declination) =(95.47 ° ±0.11 ° ,37.92 ° ±0.09 ° ) , and the extension is 0.40 ° ±0.07 ° . the energy spectrum can be described by a power-law spectrum with an index of -2.92 ±0.17stat±0.02sys . no clear extended multiwavelength counterpart of the lhaaso source has been found from the radio to sub-tev bands. the lhaaso observations are consistent with the scenario that vhe electrons escaped from the pulsar, diffused in the interstellar medium, and scattered the interstellar radiation field. if interpreted as the pulsar halo scenario, the diffusion coefficient, inferred for electrons with median energies of ∼160 tev , is consistent with those obtained from the extended halos around geminga and monogem and much smaller than that derived from cosmic ray secondaries. the lhaaso discovery of this source thus likely enriches the class of so-called pulsar halos and confirms that high-energy particles generally diffuse very slowly in the disturbed medium around pulsars. | extended very-high-energy gamma-ray emission surrounding psr j 0622 +3749 observed by lhaaso-km2a |
we search for stochastic gravitational wave background emitted from cosmic strings using the parkes pulsar timing array data over 15 years. while the common power-law excess revealed by several pulsar timing array experiments might be accounted for by the gravitational wave background from cosmic strings, the lack of the characteristic hellings-downs correlation cannot establish its physical origin yet. the constraints on the cosmic string model parameters are thus derived with the conservative assumption that the common power-law excess is due to unknown background. two representative cosmic string models with different loop distribution functions are considered. we obtain constraints on the dimensionless string tension parameter g μ <10-11- 10-10, which is more stringent by two orders of magnitude than that obtained by the high-frequency ligo-virgo experiment for one model, and less stringent for the other. the results provide the chance to test the grand unified theories, with the spontaneous symmetry breaking scale of u (1 ) being two-to-three orders of magnitude below 1016 gev . the pulsar timing array experiments are thus entirely complementary to the ligo-virgo experiment in probing the cosmic strings and the underlying beyond standard model physics in the early universe. | searching for cosmic string induced stochastic gravitational wave background with the parkes pulsar timing array |
extending our earlier work, a new family of three hartree-fock-bogoliubov (hfb) mass models, labeled hfb-30, hfb-31, and hfb-32, is presented, along with their underlying interactions, bsk30, bsk31, and bsk32, respectively. the principle new feature is a purely phenomenological pairing term that depends on the density gradient. this enables us to have a bulk pairing term that is fitted to realistic nuclear-matter calculations in which for the first time the self-energy corrections are included, while the behavior of the nucleon effective masses in asymmetric homogeneous nuclear matter is significantly improved. furthermore, in the particle-hole channel all the highly realistic constraints of our earlier work are retained. in particular, the unconventional skyrme forces containing t4 and t5 terms are still constrained to fit realistic equations of state of neutron matter stiff enough to support the massive neutron stars psr j1614-2230 and psr j0348+0432. all unphysical long-wavelength spin and spin-isospin instabilities of nuclear matter, including the unphysical transition to a polarized state in neutron-star matter, are eliminated. our three interactions are characterized by values of the symmetry coefficient j of 30, 31, and 32 mev, respectively. the best fit to the database of 2353 nuclear masses is found for model hfb-31 (j =31 mev ) with a model error of 0.561 mev. this model also fits the charge-radius data with an root-mean-square error of 0.027 fm. | further explorations of skyrme-hartree-fock-bogoliubov mass formulas. xvi. inclusion of self-energy effects in pairing |
we investigate the properties of a stochastic gravitational wave background produced by a first-order electroweak phase transition in the regime of extreme supercooling. we study a scenario whereby the percolation temperature that signifies the completion of the transition, t_p, is as low as a few mev (nucleosynthesis temperature), while most of the true vacuum bubbles are formed much earlier at the nucleation temperature, t_n∼ 50 gev. this implies that the gravitational wave spectrum is mainly produced by the collisions of large bubbles and characterised by a large amplitude and a peak frequency as low as f ∼ 10^{-9}{-}10^{-7} hz. we show that such a scenario can occur in (but not limited to) a model based on a non-linear realisation of the electroweak gauge group, so that the higgs vacuum configuration is altered by a cubic coupling. in order to carefully quantify the evolution of the phase transition of this model over such a wide temperature range we go beyond the usual fast transition approximation, taking into account the expansion of the universe as well as the behaviour of the nucleation probability at low temperatures. our computation shows that there exists a range of parameters for which the gravitational wave spectrum lies at the edge between the exclusion limits of current pulsar timing array experiments and the detection band of the future square kilometre array observatory. | gravitational waves from a supercooled electroweak phase transition and their detection with pulsar timing arrays |
periodicities observed in two fast radio burst (frb) sources (16 days in frb 180916.j0158+65 and 160 days in frb 121102) are consistent with that of tight, stellar-mass binary systems. in the case of frb 180916.j0158+65 the primary is an early ob-type star with the mass-loss rate $\dot{m}\sim {10}^{-8}\mbox{--}{10}^{-7}{m}_{\odot }$ yr-1, and the secondary is a neutron star. the observed periodicity is not intrinsic to the frb's source, but is due to the orbital phase-dependent modulation of the absorption conditions in the massive star's wind. the observed relatively narrow frb activity window implies that the primary's wind dynamically dominates that of the pulsar, $\eta ={l}_{\mathrm{sd}}/(\dot{m}{v}_{w}c)\leqslant 1$ , where lsd is the pulsar spin-down, $\dot{m}$ is the primary's wind mass-loss rate, and vw is its velocity. the condition η ≤ 1 requires a mildly powerful pulsar with lsd ≲ 1037 erg s-1. the observations are consistent with magnetically powered radio emission originating in the magnetospheres of young, strongly magnetized neutron stars, the classical magnetars. | frb periodicity: mild pulsars in tight o/b-star binaries |
conventional approaches to probing axions and axion-like particles (alps) typically rely on a coupling to photons. however, if this coupling is extremely weak, alps become invisible and are effectively decoupled from the standard model. here we show that such invisible axions, which are viable candidates for dark matter, can produce a stochastic gravitational wave background in the early universe. this signal is generated in models where the invisible axion couples to a dark gauge boson that experiences a tachyonic instability when the axion begins to oscillate. incidentally, the same mechanism also widens the viable parameter space for axion dark matter. quantum fluctuations amplified by the exponentially growing gauge boson modes source chiral gravitational waves. for axion decay constants f ≳ 1017 gev, this signal is detectable by either pulsar timing arrays or space/ground-based gravitational wave detectors for a broad range of axion masses, thus providing a new window to probe invisible axion models. | audible axions |
aims: we present a new microscopic hadron-quark hybrid equation of state model for astrophysical applications, from which compact hybrid star configurations are constructed. these are composed of a quark core and a hadronic shell with a first-order phase transition at their interface. the resulting mass-radius relations are in accordance with the latest astrophysical constraints.methods: the quark matter description is based on a quantum chromodynamics (qcd) motivated chiral approach with higher-order quark interactions in the dirac scalar and vector coupling channels. for hadronic matter we select a relativistic mean-field equation of state with density-dependent couplings. since the nucleons are treated in the quasi-particle framework, an excluded volume correction has been included for the nuclear equation of state at suprasaturation density which takes into account the finite size of the nucleons.results: these novel aspects, excluded volume in the hadronic phase and the higher-order repulsive interactions in the quark phase, lead to a strong first-order phase transition with large latent heat, i.e. the energy-density jump at the phase transition, which fulfils a criterion for a disconnected third-family branch of compact stars in the mass-radius relationship. these twin stars appear at high masses (~2 m⊙) that are relevant for current observations of high-mass pulsars.conclusions: this analysis offers a unique possibility by radius observations of compact stars to probe the qcd phase diagram at zero temperature and large chemical potential and even to support the existence of a critical point in the qcd phase diagram. | a new quark-hadron hybrid equation of state for astrophysics. i. high-mass twin compact stars |
recent modeling of neutron star interior composition explorer observations of thermal x-ray pulsations from the surface of the isolated millisecond pulsar psr j0030+0451 suggests that the hot emitting regions on the pulsar’s surface are far from antipodal, which is at odds with the classical assumption that the magnetic field in the pulsar magnetosphere is predominantly that of a centered dipole. here, we review these results and examine previous attempts to constrain the magnetospheric configuration of psr j0030+0451. to the best of our knowledge, there is in fact no direct observational evidence that psr j0030+0451’s magnetic field is a centered dipole. developing models of physically motivated, non-canonical magnetic field configurations and the currents that they can support poses a challenging task. however, such models may have profound implications for many aspects of pulsar research, including pulsar braking, estimates of birth velocities, and interpretations of multi-wavelength magnetospheric emission. | a nicer view of psr j0030+0451: evidence for a global-scale multipolar magnetic field |
over the past few decades, the measurement precision of some pulsar timing experiments has advanced from ∼10 μs to ∼10 ns, revealing many subtle phenomena. such high precision demands both careful data handling and sophisticated timing models to avoid systematic error. to achieve these goals, we present pint (pint is not tempo3), a high-precision python pulsar timing data analysis package, which is hosted on github and available on the python package index (pypi) as pint-pulsar. pint is well tested, validated, object oriented, and modular, enabling interactive data analysis and providing an extensible and flexible development platform for timing applications. it utilizes well-debugged public python packages (e.g., the numpy and astropy libraries) and modern software development schemes (e.g., version control and efficient development with git and github) and a continually expanding test suite for improved reliability, accuracy, and reproducibility. pint is developed and implemented without referring to, copying, or transcribing the code from other traditional pulsar timing software packages (e.g., tempo/tempo2) and therefore provides a robust tool for cross-checking timing analyses and simulating pulse arrival times. in this paper, we describe the design, use, and validation of pint, and we compare timing results between it and tempo and tempo2. | pint: a modern software package for pulsar timing |
we present the pulse arrival times and high-precision dispersion measure estimates for 14 millisecond pulsars observed simultaneously in the 300 $-$ 500 mhz and 1260 $-$ 1460 mhz frequency bands using the upgraded giant metrewave radio telescope. the data spans over a baseline of 3.5 years (2018-2021), and is the first official data release made available by the indian pulsar timing array collaboration. this data release presents a unique opportunity for investigating the interstellar medium effects at low radio frequencies and their impact on the timing precision of pulsar timing array experiments. in addition to the dispersion measure time series and pulse arrival times obtained using both narrowband and wideband timing techniques, we also present the dispersion measure structure function analysis for selected pulsars. our ongoing investigations regarding the frequency dependence of dispersion measures have been discussed. based on the preliminary analysis for five millisecond pulsars, we do not find any conclusive evidence of chromaticity in dispersion measures. data from regular simultaneous two-frequency observations are presented for the first time in this work. this distinctive feature leads us to the highest precision dispersion measure estimates obtained so far for a subset of our sample. simultaneous multi-band upgraded giant metrewave radio telescope observations in 300 $-$ 500 mhz and 1260 $-$ 1460 mhz are crucial for high-precision dispersion measure estimation and for the prospect of expanding the overall frequency coverage upon the combination of data from the various pulsar timing array consortia in the near future. parts of the data presented in this work are expected to be incorporated into the upcoming third data release of the international pulsar timing array. | the indian pulsar timing array: first data release |
in this work, we have adopted gravitational decoupling by minimal geometric deformation (mgd) approach and have developed an anisotropic version of well-known tolman vii isotropic solution in the framework of f(r, t) gravity, where r is ricci scalar and t is trace of energy momentum tensor. the set of field equations has been developed with respect to total energy momentum tensor, which combines effective energy momentum tensor in f(r, t) gravity and additional source ϕij. following mgd approach, the set of field equations has been separated into two sections. one section represents f(r, t) field equations, while the other is related to the source ϕij. the matching conditions for inner and outer geometry have also been discussed, and an anisotropic solution has been developed using mimic constraint for radial pressure. in order to check viability of the solution, we have considered observation data of three different compact star models, named psr j1614-2230, psr 1937+21 and sax j1808.4-3658, and have discussed thermodynamical properties analytically and graphically. the energy conditions are found to be satisfied for the three compact stars. the stability analysis has been presented through causality condition and herrera's cracking concept, which ensures physical acceptability of the solution. | an anisotropic version of tolman vii solution in f(r, t) gravity via gravitational decoupling mgd approach |
we present a family of new exact solutions for relativistic anisotropic stellar objects by considering a four-dimensional spacetime embedded in a five-dimensional pseudo euclidean space, known as class i solutions. these solutions are well behaved in all respects, satisfy all energy conditions, and the resulting compactness parameter is also within buchdahl limit. the well-behaved nature of the solutions for a particular star solely depends on the index n. we have discussed the solutions in detail for the neutron star xte j1739-285 (m=1.51m_⊙, ~r=10.9 km). for this particular star, the solution is well behaved in all respects for 8 ≤ n ≤ 20. however, the solutions with n<8 possess an increasing trend of the sound speed and the solutions belonging to n>20 disobey the causality condition. further, the well-behaved nature of the solutions for psr j0348+0432 (2.01m_⊙, ~11 km), exo 1785-248 (1.3m_⊙, 8.85 km), and her x-1 (0.85m_⊙, 8.1 km) are specified by the index n with limits 24 ≤ n ≤ 54, 1.5 ≤ n ≤ 4, and 0.8 ≤ n ≤ 2.7, respectively. | a family of well-behaved karmarkar spacetimes describing interior of relativistic stars |
the extremely high brightness temperatures of pulsars and fast radio bursts (frbs) require their radiation mechanisms to be coherent. coherent curvature radiation from bunches has been long discussed as the mechanism for radio pulsars and recently for frbs. assuming that bunches are already generated in pulsar magnetospheres, we calculate the spectrum of coherent curvature radiation under a three-dimensional magnetic field geometry. different from previous works assuming parallel trajectories and a monoenergetic energy distribution of electrons, we consider a bunch characterized by its length, curvature radius of the trajectory family, bunch opening angle, and electron energy distribution. we find that the curvature radiation spectra of the bunches are characterized by a multisegment broken power law, with the break frequencies depending on bunch properties and trajectory configuration. we also emphasize that in a pulsar magnetosphere, only the fluctuation of net charges with respect to the background (goldreich-julian) outflow can make a contribution to coherent radiation. we apply this model to constrain the observed spectra of pulsars and frbs. for a typical pulsar ({b}p={10}12 {{g}}, p = 0.1 s), a small fluctuation of the net charge δn gj ∼ 0.1n gj can provide the observable flux. for frbs, the fluctuating net charge may be larger due to its abrupt nature. for δn gj ∼ n gj, a neutron star with a strong magnetic field and fast rotation is required to power an frb in the spindown-powered model. the requirement is less stringent in the cosmic comb model thanks to the larger cross section and compressed charge density of the bunch made by the external astrophysical stream that combs the magnetosphere. | bunching coherent curvature radiation in three-dimensional magnetic field geometry: application to pulsars and fast radio bursts |
with the upcoming commensal surveys for fast radio bursts (frbs), and their high candidate rate, usage of machine learning algorithms for candidate classification is a necessity. such algorithms will also play a pivotal role in sending real-time triggers for prompt follow-ups with other instruments. in this paper, we have used the technique of transfer learning to train the state-of-the-art deep neural networks for classification of frb and radio frequency interference (rfi) candidates. these are convolutional neural networks which work on radio frequency-time and dispersion measure-time images as the inputs. we trained these networks using simulated frbs and real rfi candidates from telescopes at the green bank observatory. we present 11 deep learning models, each with an accuracy and recall above 99.5 per cent on our test data set comprising of real rfi and pulsar candidates. as we demonstrate, these algorithms are telescope and frequency agnostic and are able to detect all frbs with signal-to-noise ratios above 10 in askap and parkes data. we also provide an open-source python package fetch (fast extragalactic transient candidate hunter) for classification of candidates, using our models. using fetch, these models can be deployed along with any commensal search pipeline for real-time candidate classification. | fetch: a deep-learning based classifier for fast transient classification |
over 40 years of research suggests that the common envelope phase, in which an evolved star engulfs its companion upon expansion, is the critical evolutionary stage forming short-period, compact-object binary systems, such as coalescing double compact objects, x-ray binaries, and cataclysmic variables. in this work, we adapt the one-dimensional hydrodynamic stellar evolution code, mesa, to model the inspiral of a 1.4 m ⊙ neutron star (ns) inside the envelope of a 12 m ⊙ red supergiant star. we self-consistently calculate the drag force experienced by the ns and the back-reaction onto the expanding envelope as the ns spirals in. nearly all of the hydrogen envelope escapes, expanding to large radii (∼102 au) where it forms an optically thick envelope with temperatures low enough that dust formation occurs. we simulate the ns orbit until only 0.8 m ⊙ of the hydrogen envelope remains around the giant star’s core. our results suggest that the inspiral will continue until another ≈0.3 m ⊙ are removed, at which point the remaining envelope will retract. upon separation, a phase of dynamically stable mass transfer onto the ns accretor is likely to ensue, which may be observable as an ultraluminous x-ray source. the resulting binary, comprised of a detached 2.6 m ⊙ helium star and an ns with a separation of 3.3-5.7 r ⊙, is expected to evolve into a merging double neutron-star, analogous to those recently detected by ligo/virgo. for our chosen combination of binary parameters, our estimated final separation (including the phase of stable mass transfer) suggests a very high α ce-equivalent efficiency of ≈5. | the complete evolution of a neutron-star binary through a common envelope phase using 1d hydrodynamic simulations |
mode and polarization-division multiplexing technologies (mdm and pdm) can offer considerable parallelism for optical multiplexing biosensors, complex optical neural networks, and high-capacity optical interconnects, while requiring only a single-wavelength laser source. thanks to the mature fabrication processes of silicon nitride and superior material properties of lithium niobate, the silicon nitride loaded lithium niobate on insulator (lnoi) platform allows the integration of high-speed optical modulators and optical (de)multiplexing devices to achieve high-capacity and low-cost photonic integrated circuits suitable for data communication applications. in this contribution, mdm and pdm are investigated in a silicon nitride loaded lnoi (x-cut) platform. as a proof of concept, an asymmetrical directional coupler-based mode (de)multiplexer (mmux) and polarization splitter-rotator (psr) are designed, fabricated, and experimentally demonstrated. the measured insertion losses are lower than 1.46 and 1.49 db, while the inter-modal crosstalk is lower than −13.03 and −17.75 db for the mmux and psr, respectively, for a wavelength range of 1525-1565 nm. a 40 gbps data transmission experiment demonstrates the data transmission capabilities of the fabricated devices. the measured eye diagrams are clear and wide-open, and the bit error rate measurements show reasonable power penalties, indicating good device performance. | mode and polarization-division multiplexing based on silicon nitride loaded lithium niobate on insulator platform |
we investigated rastall gravity, for an anisotropic star with a static spherical symmetry, whereas the matter-geometry coupling as assumed in rastall theory (rt) is expected to play a crucial role differentiating rt from general relativity (gr). indeed, all the obtained results confirm that rt is not equivalent to gr, however, it produces same amount of anisotropy as gr for static spherically symmetric stellar models. we used the observational constraints on the mass and the radius of the pulsar her x-1 to determine the model parameters confirming the physical viability of the model. we found that the matter-geometry coupling in rt allows slightly less size than gr for a given mass. we confirmed the model viability via other twenty pulsars' observations. utilizing the strong energy condition we determined an upper bound on compactness umax∼0.603 , in agreement with buchdahl limit, whereas rastall parameter ϵ =-0.1 . for a surface density compatible with a neutron core at nuclear saturation density the mass-radius curve allows masses up to 3.53 m⊙ . we note that there is no equation of state is assumed, however the model fits well with linear behaviour. we split the twenty pulsars into four groups according to the boundary densities. three groups are compatible with neutron cores while one group fits perfectly with higher boundary density 8 ×1014 g/cm3 which suggests that those pulsars may have quark-gluon cores. | non-trivial class of anisotropic compact stellar model in rastall gravity |
we present a comprehensive analysis of a supersymmetric $so(10)$ grand unified theory, which is broken to the standard model via the breaking of two intermediate symmetries. the spontaneous breaking of the first intermediate symmetry, $b-l$, leads to the generation of cosmic strings and right-handed neutrino masses and further to an observable cosmological background of gravitational waves and generation of light neutrino masses via type-i seesaw mechanism. supersymmetry breaking manifests as sparticle masses below the $b-l$ breaking but far above the electroweak scale due to proton decay limits. this naturally pushes the $b-l$ breaking scale close to the gut scale, leading to the formation of metastable cosmic strings, which can provide a gravitational wave spectrum consistent with the recent pulsar timing arrays observation. we perform a detailed analysis of this model using two-loop renormalisation group equations, including threshold corrections, to determine the symmetry-breaking scale consistent with the recent pulsar timing arrays signals such as nanograv 15-year data and testable by the next-generation limits on proton decay from hyper-k and juno. simultaneously, we find the regions of the model parameter space that can predict the measured quark and lepton masses and mixing, baryon asymmetry of our universe, a viable dark matter candidate and can be tested by a combination of neutrinoless double beta decay searches and limits on the sum of neutrinos masses. | testing realistic $so(10)$ susy guts with proton decay and gravitational waves |
we answer frequently asked questions (faqs) about the hellings and downs correlation curve -- the "smoking-gun" signature that pulsar timing arrays (ptas) have detected gravitational waves (gws). many of these questions arise from inadvertently applying intuition about the effects of gws on ligo-like detectors to the case of pulsar timing, where not all of that intuition applies. this is because earth-based detectors, like ligo and virgo, have arms that are short (km scale) compared to the wavelengths of the gws that they detect (approximately 100-10,000 km). in contrast, ptas respond to gws whose wavelengths (tens of light-years) are much shorter than their arms (a typical pta pulsar is hundreds to thousands of light-years from earth). to demonstrate this, we calculate the time delay induced by a passing gw along an earth-pulsar baseline (a "one-arm, one-way" detector) and compare it in the "short-arm" (ligo-like) and "long-arm" (pta) limits. this provides qualitative and quantitative answers to many questions about the hellings and downs curve. the resulting faq sheet should help in understanding the "evidence for gws" recently announced by several pta collaborations. | answers to frequently asked questions about the pulsar timing array hellings and downs curve |
we combine pulsar population synthesis with simulation-based inference to constrain the magneto-rotational properties of isolated galactic radio pulsars. we first develop a flexible framework to model neutron-star birth properties and evolution, focusing on their dynamical, rotational and magnetic characteristics. in particular, we sample initial magnetic-field strengths, $b$, and spin periods, $p$, from log-normal distributions and capture the late-time magnetic-field decay with a power law. each log-normal is described by a mean, $\mu_{\log b}, \mu_{\log p}$, and standard deviation, $\sigma_{\log b}, \sigma_{\log p}$, while the power law is characterized by the index, $a_{\rm late}$, resulting in five free parameters. we subsequently model the stars' radio emission and observational biases to mimic detections with three radio surveys, and produce a large database of synthetic $p$-$\dot{p}$ diagrams by varying our input parameters. we then follow a simulation-based inference approach that focuses on neural posterior estimation and employ this database to train deep neural networks to directly infer the posterior distributions of the five model parameters. after successfully validating these individual neural density estimators on simulated data, we use an ensemble of networks to infer the posterior distributions for the observed pulsar population. we obtain $\mu_{\log b} = 13.10^{+0.08}_{-0.10}$, $\sigma_{\log b} = 0.45^{+0.05}_{-0.05}$ and $\mu_{\log p} = -1.00^{+0.26}_{-0.21}$, $\sigma_{\log p} = 0.38^{+0.33}_{-0.18}$ for the log-normal distributions, and $a_{\rm late} = -1.80^{+0.65}_{-0.61}$ for the power law at $95\%$ credible interval. our approach represents a crucial step towards robust statistical inference for complex population-synthesis frameworks and forms the basis for future multi-wavelength analyses of galactic pulsars. | isolated pulsar population synthesis with simulation-based inference |
the standard model, extended with three right-handed (rh) neutrinos, is the simplest model that can explain light neutrino masses, the baryon asymmetry of the universe, and dark matter (dm). models in which rh neutrinos are light are generally easier to test in experiments. in this work, we show that, even if the rh neutrinos are super-heavy (mi=1,2,3> 109 gev)—close to the grand unification scale—the model can be tested thanks to its distinct features on the stochastic gravitational wave (gw) background. we consider an early universe filled with ultralight primordial black holes (pbh) that produce a super-heavy rh neutrino dm via hawking radiation. the other pair of rh neutrinos generates the baryon asymmetry via thermal leptogenesis, much before the pbhs evaporate. gw interferometers can test this novel spectrum of masses thanks to the gws induced by the pbh density fluctuations. in a more refined version, wherein a u(1) gauge symmetry breaking dynamically generates the seesaw scale, the pbhs also cause observable spectral distortions on the gws from the u(1)-breaking cosmic strings. thence, a low-frequency gw feature related to dm genesis and detectable with a pulsar-timing array must correspond to a mid- or high-frequency gw signature related to baryogenesis at interferometer scales. | pbh-infused seesaw origin of matter and unique gravitational waves |
motivated by the recent release of new results from five different pulsar timing array (pta) experiments claiming to have found compelling evidence for primordial gravitational waves (gw) at nano-hz frequencies, we study the consequences for two popular beyond the standard model (sm) frameworks, where such nano-hz gw can arise due to annihilating domain walls (dw). minimal framework of dirac leptogenesis, as well as left-right symmetric model (lrsm) can lead to formation of dw due to spontaneous breaking of z 2 symmetry. considering the nanograv 15 yr data, we show that the scale of dirac leptogenesis should be above 107 gev for conservative choices of dirac yukawa couplings with fine-tuning at the level of the sm. the scale of minimal lrsm is found to be more constrained m lr ~ 106 gev in order to fit the nanograv 15 yr data. on the other hand, the non-minimal lrsm can be compatible with the nanograv data for 102 tev ≲ m lr ≲ 103 tev but with the corresponding b - l breaking scale violating collider bounds. | scale of dirac leptogenesis and left-right symmetry in the light of recent pta results |
we study the electrodynamics of a kinetically mixed dark photon cloud that forms through superradiance around a spinning black hole, and design strategies to search for the resulting multimessenger signals. a dark photon superradiance cloud sources a rotating dark electromagnetic field which, through kinetic mixing, induces a rotating visible electromagnetic field. standard model charged particles entering this field initiate a transient phase of particle production that populates a plasma inside the cloud and leads to a system which shares qualitative features with a pulsar magnetosphere. we study the electrodynamics of the dark photon cloud with resistive magnetohydrodynamics methods applicable to highly magnetized plasma, adapting techniques from simulations of pulsar magnetospheres. we identify turbulent magnetic field reconnection as the main source of dissipation and electromagnetic emission, and compute the peak luminosity from clouds around solar-mass black holes to be as large as 1043 erg /s for observationally allowed dark photon parameter space. the emission is expected to have a significant x-ray component and to potentially be periodic, with period set by the dark photon mass. the luminosity is comparable to the brightest x-ray sources in the universe, allowing for searches at distances of up to hundreds of mpc with existing telescopes. we discuss observational strategies, including targeted electromagnetic follow-ups of solar-mass black hole mergers and targeted continuous gravitational wave searches of anomalous pulsars. | dark photon superradiance: electrodynamics and multimessenger signals |
this paper investigate the impacts of local density perturbations on the stability of self-gravitating compact objects by utilizing cracking technique within the context of f(r, t) gravity, where r and t represent the ricci scalar, and the trace of energy-momentum, respectively. to achieve this, we developed the hydrostatic equilibrium equation for spherically symmetric spacetime with anisotropic matter configuration and subsequently applied the krori-barua spacetime coefficient. subsequently, the hydrostatic equilibrium equation of the configuration is perturbed by employing the local density perturbations to the system, while considering a barotropic equation of state. to ascertain the validity of the proposed technique, we applied it to several compact stars, including, her x-1, sax j1808.4-3658, 4u 1820-30, psr j1614-2230, vela x-1, cen x-3, and rxj1856-37 and found that all the considered stars exhibit cracking or overturning. this study conclusively highlights the significance of the cracking technique in providing valuable insights into the stability analysis of self-gravitating compact objects. | development of local density perturbation technique to identify cracking points in f(r, t) gravity |
the spontaneous breaking of u(1) b - l around the scale of grand unification can simultaneously account for hybrid inflation, leptogenesis, and neutralino dark matter, thus resolving three major puzzles of particle physics and cosmology in a single predictive framework. the b - l phase transition also results in a network of cosmic strings. if strong and electroweak interactions are unified in an so (10) gauge group, containing u(1) b - l as a subgroup, these strings are metastable. in this case, they produce a stochastic background of gravitational waves that evades current pulsar timing bounds, but features a flat spectrum with amplitude h2ωgw ∼10-8 at interferometer frequencies. ongoing and future ligo observations will hence probe the scale of b - l breaking. | probing the scale of grand unification with gravitational waves |
the north american nanohertz observatory for gravitational waves (nanograv) recently reported evidence for the presence of a common stochastic signal across their array of pulsars. the origin of this signal is still unclear. one possibility is that it is due to a stochastic gravitational-wave background (sgwb) in the ∼1-10 nhz frequency region. taking the nanograv observational result at face value, we show that this signal would be fully consistent with an sgwb produced by an unresolved population of in-spiralling massive black hole binaries (mbhbs) predicted by current theoretical models. considering an astrophysically agnostic model, the mbhb merger rate is loosely constrained. including additional constraints from galaxy pairing fraction and mbh-bulge scaling relations, we find that the mbhb merger rate is ${1.2\times 10^{-5}}{\rm -}{4.5\times 10^{-4}}\, \mathrm{mpc}^{-3}\, \mathrm{gyr}^{-1}$ , the mbhb merger time-scale is $\le 2.7\, \mathrm{gyr}$ , and the norm of the mbh-mbulge relation is $\ge 1.2\times 10^{8}\, {\rm m}_\odot$ (all quoted at 90 per cent credible intervals). regardless of the astrophysical details of mbhb assembly, the nanograv result would imply that a sufficiently large population of massive black holes pair up, form binaries and merge within a hubble time. | massive black hole binary systems and the nanograv 12.5 yr results |
in the present article, we have constructed static anisotropic compact star models of einstein field equations for the spherical symmetric metric of embedding class one. by assuming the particular form of the metric function ν, we have solved the einstein field equations for anisotropic matter distribution. the anisotropic models represent the realistic compact objects such as sax j 1808.4-3658 (ss1), her x-1, vela x-12, psr j1614-2230 and cen x-3. we have reported our results in details for the compact star her x-1 on the ground of physical properties such as pressure, density, velocity of sound, energy conditions, tov equation and red-shift etc. along with these, we have also discussed about the stability of the compact star models. finally we made a comparison between our anisotropic stars with the realistic objects on the key aspects as central density, central pressure, compactness and surface red-shift. | modelling of anisotropic compact stars of embedding class one |
an axion rotating in field space can produce dark photons in the early universe via tachyonic instability. this explosive particle production creates a background of stochastic gravitational waves that may be visible at pulsar timing arrays or other gravitational wave detectors. this scenario provides a novel history for dark photon dark matter. the dark photons may be warm at a level detectable in future 21-cm line surveys. for a consistent cosmology, the radial direction of the complex field containing the axion must be thermalized. we explore a concrete thermalization mechanism in detail and also demonstrate how this setup can be responsible for the generation of the observed baryon asymmetry. | gravitational waves and dark photon dark matter from axion rotations |
we describe the model of surface emission from a rapidly rotating neutron star that is applied to neutron star interior composition explorer x-ray data of millisecond pulsars in order to statistically constrain the neutron star mass-radius relation and dense matter equation of state. to ensure that the associated calculations are both accurate and precise, we conduct an extensive suite of verification tests between our numerical codes for both the schwarzschild + doppler and oblate schwarzschild approximations, and compare both approximations against exact numerical calculations. we find superb agreement between the code outputs, as well as in comparisons against a set of analytical and semi-analytical calculations, which, combined with their speed, demonstrates that the codes are well suited for large-scale statistical sampling applications. a set of verified, high-precision reference synthetic pulse profiles is provided to the community to facilitate testing of other independently developed codes. | constraining the neutron star mass-radius relation and dense matter equation of state with nicer. ii. emission from hot spots on a rapidly rotating neutron star |
the quantum chromodynamics (qcd) axion may modify the cooling rates of neutron stars (nss). the axions are produced within the ns cores from nucleon bremsstrahlung and, when the nucleons are in superfluid states, cooper pair breaking and formation processes. we show that four of the nearby isolated magnificent seven nss along with psr j0659 are prime candidates for axion cooling studies because they are coeval, with ages of a few hundred thousand years known from kinematic considerations, and they have well-measured surface luminosities. we compare these data to dedicated ns cooling simulations incorporating axions, profiling over uncertainties related to the equation of state, ns masses, surface compositions, and superfluidity. our calculations of the axion and neutrino emissivities include high-density suppression factors that also affect sn 1987a and previous ns cooling limits on axions. we find no evidence for axions in the isolated ns data, and within the context of the kim-shifman-vainshtein-zakharov qcd axion model, we constrain ma≲16 mev at 95% confidence level. an improved understanding of ns cooling and nucleon superfluidity could further improve these limits or lead to the discovery of the axion at weaker couplings. | upper limit on the qcd axion mass from isolated neutron star cooling |
collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. they are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulæ, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. it turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. it is complexified by the impact due to high-energy cosmic rays in astrophysical environments. this review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. a particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. it appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. the shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. a special section is dedicated to new laser plasma experiments probing shock physics. | the microphysics of collisionless shock waves |
we present a study of the spectral properties of 441 pulsars observed with the parkes radio telescope near the centre frequencies of 728, 1382 and 3100 mhz. the observations at 728 and 3100 mhz were conducted simultaneously using the dual-band 10-50 cm receiver. these high-sensitivity, multifrequency observations provide a systematic and uniform sample of pulsar flux densities. we combine our measurements with spectral data from the literature in order to derive the spectral properties of these pulsars. using techniques from robust regression and information theory, we classify the observed spectra in an objective, robust and unbiased way into five morphological classes: simple or broken power law, power law with either low- or high-frequency cut-off and log-parabolic spectrum. while about 79 per cent of the pulsars that could be classified have simple power-law spectra, we find significant deviations in 73 pulsars, 35 of which have curved spectra, 25 with a spectral break and 10 with a low-frequency turn-over. we identify 11 gigahertz-peaked spectrum (gps) pulsars, with 3 newly identified in this work and 8 confirmations of known gps pulsars; 3 others show tentative evidence of gps, but require further low-frequency measurements to support this classification. the weighted mean spectral index of all pulsars with simple power-law spectra is -1.60 ± 0.03. the observed spectral indices are well described by a shifted log-normal distribution. the strongest correlations of spectral index are with spin-down luminosity, magnetic field at the light-cylinder and spin-down rate. we also investigate the physical origin of the observed spectral features and determine emission altitudes for three pulsars. | spectral properties of 441 radio pulsars |
the discovery of pulsars opened a new research field that allows studying a wide range of physics under extreme conditions. more than 3,000 pulsars are currently known, including especially more than 200 of them studied at gamma-ray frequencies. by putting recent insights into the pulsar magnetosphere in a historical context and by comparing them to key observational features at radio and high-energy frequencies, we show the following: magnetospheric structure of young energetic pulsars is now understood. limitations still exist for old nonrecycled and millisecond pulsars. the observed high-energy radiation is likely produced in the magnetospheric current sheet beyond the light cylinder. there are at least two different radio emission mechanisms. one operates in the inner magnetosphere, whereas the other one works near the light cylinder and is specific to pulsars with the high magnetic field strength in that region. radio emission from the inner magnetosphere is intrinsically connected to the process of pair production, and its observed properties contain the imprint of both the geometry and propagation effects through the magnetospheric plasma.we discuss the limitations of our understanding and identify a range of observed phenomena and physical processes that still await explanation in thefuture. this includes connecting the magnetospheric processes to spin-down properties to explain braking and possible evolution of spin orientation, building a first-principles model of radio emission and quantitative connections with observations. | pulsar magnetospheres and their radiation |
gravitational waves offer a new window to probe the nature of gravity, including answering if the mediating particle, graviton, has a nonzero mass or not. pulsar timing arrays measure stochastic gravitational wave background (sgwb) at ∼1 - 100 nanohertz . recently, the north american nanohertz observatory for gravitational waves (nanograv) collaboration reported an uncorrelated common-spectrum process in their 12.5-year dataset with no substantial evidence that the process comes from the sgwb predicted by general relativity. in this work, we explore the possibility of an sgwb from massive gravity in the dataset and find that a massless graviton is preferred because of the relatively larger bayes factor. without statistically significant evidence for dispersion-related correlations predicted by massive gravity, we place upper limits on the amplitude of the sgwb for graviton mass smaller than 10-23 ev as amg<3.21 ×10-15 at 95% confidence level. | search for stochastic gravitational-wave background from massive gravity in the nanograv 12.5-year dataset |
we present several arguments which favor the scenario of two coexisting families of compact stars: hadronic stars and quark stars. besides the well-known hyperon puzzle of the physics of compact stars, a similar puzzle exists also when considering delta resonances. we show that these particles appear at densities close to twice saturation density and must be therefore included in the calculations of the hadronic equation of state. such an early appearance is strictly related to the value of the l parameter of the symmetry energy that has been found, in recent phenomenological studies, to lie in the range 40 < l < 62 mev. we discuss also the threshold for the formation of deltas and hyperons for hot and lepton-rich hadronic matter. similarly to the case of hyperons, also delta resonances cause a softening of the equation of state, which makes it difficult to obtain massive hadronic stars. quark stars, on the other hand, can reach masses up to 2.75 m_{⊙} as predicted by perturbative qcd calculations. we then discuss the observational constraints on the masses and the radii of compact stars. the tension between the precise measurements of high masses and the indications of the existence of very compact stellar objects (with radii of the order of 10km) is relieved when assuming that very massive compact stars are quark stars and very compact stars are hadronic stars. finally, we discuss recent interesting measurements of the eccentricities of the orbits of millisecond pulsars in low mass x-ray binaries. the high values of the eccentricities found in some cases could be explained by assuming that the hadronic star, initially present in the binary system, converts to a quark star due to the increase of its central density. | the scenario of two families of compact stars. part 1. equations of state, mass-radius relations and binary systems |
we perform magnetohydrodynamic simulations in full general relativity (grmhd) of quasi-circular, equal-mass, binary neutron stars that undergo merger. the initial stars are irrotational, n = 1 polytropes and are magnetized. we explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. in both cases the adopted magnetic fields are initially dynamically unimportant. the merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter a/m bh ∼ 0.74) immersed in a magnetized accretion disk. about 4000m ∼ 60(m ns/1.625 m ⊙) ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow—an incipient jet—is launched. the lifetime of the accretion disk, which likely equals the lifetime of the jet, is δ t ∼ 0.1 (m ns/1.625 m ⊙) s. in contrast to black hole-neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars. | binary neutron star mergers: a jet engine for short gamma-ray bursts |
we argue that comparison with observations of theoretical models for the velocity distribution of pulsars must be done directly with the observed quantities, that is parallax and the two components of proper motion. we have developed a formalism to do so, and applied it to pulsars with accurate vlbi measurements. for computational convenience, we model the data with maxwellians. we find that a distribution with two maxwellians improves significantly on a single maxwellian. the "mixed" model takes into account that pulsars move away from their place of birth, a narrow region around the galactic plane. the best model has 42% of the pulsars in a maxwellian with average velocity σ√{8/π=120} km s-1, and 58% in a maxwellian with average velocity 540 km s-1. about 5% of the pulsars has a velocity at birth less than 60 km s-1. for the youngest pulsars (τc < 10 myr), these numbers are 32% with 130 km s-1, 68% with 520 km s-1, and 3%, with appreciable uncertainties. our analysis shows that the velocity distribution is wider than can be described with a single maxwellian; it does not prove that two maxwellians provide a better description than other wide models. | the observed velocity distribution of young pulsars |
keck-telescope spectrophotometry of the companion of psr j1810+1744 shows a flat, but asymmetric light-curve maximum and a deep, narrow minimum. the maximum indicates strong gravity darkening (gd) near the l1 point, along with a heated pole and surface winds. the minimum indicates a low underlying temperature and substantial limb darkening. the gd is a consequence of extreme pulsar heating and the near-filling of the roche lobe. light-curve modeling gives a binary inclination i = 65°7 ± 0°4. with the keck-measured radial-velocity amplitude kc = 462.3 ± 2.2 km s-1, this gives an accurate neutron star mass mns = 2.13 ± 0.04 m⊙, with important implications for the dense-matter equation of state. a classic direct-heating model, ignoring the l1 gravitational darkening, would predict an unphysical mns > 3 m⊙. a few other "spider" pulsar binaries have similar large heating and fill factor; thus, they should be checked for such effects. | psr j1810+1744: companion darkening and a precise high neutron star mass |
to understand the nature of supernovae and neutron star (ns) formation, as well as binary stellar evolution and their interactions, it is important to probe the distribution of ns masses. until now, all double ns (dns) systems have been measured as having a mass ratio close to unity (q ≥ 0.91). here, we report the measurement of the individual masses of the 4.07-day binary pulsar j0453+1559 from measurements of the rate of advance of periastron and shapiro delay: the mass of the pulsar is mp = 1.559 ± 0.005 m⊙ and that of its companion is {m}{{c}}=1.174+/- 0.004 m⊙ q = 0.75. if this companion is also an ns, as indicated by the orbital eccentricity of the system (e = 0.11), then its mass is the smallest precisely measured for any such object. the pulsar has a spin period of 45.7 ms and a spin period derivative of \dot{{\text{}}p} = (1.8616±0.0007)×10-19 s s-1 from these, we derive a characteristic age of ∼ 4.1×109 years and a magnetic field of ∼ 2.9×109 g, i.e., this pulsar was mildly recycled by the accretion of matter from the progenitor of the companion star. this suggests that it was formed with (very approximately) its current mass. thus, nss form with a wide range of masses, which is important for understanding their formation in supernovae. it is also important for the search for gravitational waves released during an ns-ns merger: it is now evident that we should not assume that all dns systems are symmetric. | pulsar j0453+1559: a double neutron star system with a large mass asymmetry |
high-mass x-ray binaries are fundamental in the study of stellar evolution, nucleosynthesis, structure and evolution of galaxies and accretion processes. hard x-rays observations by integral and swift have broadened significantly our understanding in particular for the super-giant systems in the milky way, whose number has increased by almost a factor of three. integral played a crucial role in the discovery, study and understanding of heavily obscured systems and of fast x-ray transients. most super-giant systems can now be classified into three categories: classical/obscured, eccentric and fast transient. the classical systems feature low eccentricity and variability factor of , mostly driven by hydrodynamic phenomena occurring on scales larger than the accretion radius. among them, systems with short orbital periods and close to roche-lobe overflow or with slow winds appear highly obscured. in eccentric systems, the variability amplitude can reach even higher factors because of the contrast of the wind density along the orbit. four super-giant systems, featuring fast outbursts, very short orbital periods and anomalously low accretion rates, are not yet understood. simulations of the accretion processes on relatively large scales have progressed and reproduce parts of the observations. the combined effects of wind clumps, magnetic fields, neutron star rotation and eccentricity ought to be included in future modelling work. observations with integral in combination with other observatories were also important for detecting cyclotron resonant scattering features in spectra of x-ray pulsars, probing their variations and the geometry of the accretion column and emission regions. finally, the unique characteristics of integral and its long life time played a fundamental role for building a complete catalogue of hxmbs, to study the different populations of these systems in our galaxy and to constrain some of the time scales and processes driving their birth and evolution. | high-mass x-ray binaries in the milky way. a closer look with integral |
the recent theoretical advance known as the minimal geometric deformation (mgd) method has initiated renewed interest in investigating higher-curvature gravitational effects in relativistic astrophysics. in this work, we model a strange star within the context of einstein-gauss-bonnet gravity with the help of the mgd technique. starting off with the tolman metric ansatz, together with the mit bag model equation of state applicable to hadronic matter, anisotropy is introduced via the superposition of the seed source and the decoupled energy-momentum tensor. the solution of the governing systems of equations bifurcates into two distinct models, namely, the mimicking of the θ sector to the seed radial pressure and energy density and a regular fluid model. each of these models can be interpreted as self-gravitating static, compact objects with the exterior described by the vacuum boulware-deser solution. utilizing observational data for three stellar candidates, namely psr j1614-2230, psr j1903+317, and lmc x-4, we subject our solutions to rigorous viability tests based on regularity and stability. we find that the einstein-gauss-bonnet parameter and the decoupling constant compete against each other for ensuring physically realizable stellar structures. the novel feature of the work is the demonstration of stable compact objects with stellar masses in excess of m = 2 m ⊙ without appealing to exotic matter. the analysis contributes new insights and physical consequences concerning the development of ultracompact astrophysical entities. | gravitationally decoupled strange star model beyond the standard maximum mass limit in einstein-gauss-bonnet gravity |
we search for stochastic gravitational wave background (sgwb) generated by domain wall networks in the data release-2 of parkes pulsar timing array and find that the observed strong common power-law process can be explained by domain wall networks for the wall tension $\sigma_{\textrm{dw}}\sim (29-414~\textrm{tev})^3$ and the wall-decay temperature $t_d\sim 20-257~\textrm{mev}$ at $68\%$ credible level. interestingly, the same parameter region can largely alleviate the hubble tension, if the free particles generated from domain wall networks further decay into dark radiation. this coincidence that a domain wall network can simultaneously account for the nano-hertz sgwb and hubble tension is robust, independent of domain wall parameters and applicable to observations by other pulsar timing array collaborations in general. on the other hand, assuming that the common power-law process is not due to domain wall networks, we can put stringent constraints on the wall tension and decay temperature. | domain wall network: a dual solution for gravitational waves and hubble tension? |
the repeating fast radio burst (frb) source, frb 20201124a, was found to be highly active in 2021 march and april. we observed the source with the effelsberg 100-m radio telescope at 1.36 ghz on 2021 april 9 and detected 20 bursts. a downward drift in frequency over time is clearly seen from the majority of bursts in our sample. a structure-maximizing dispersion measure (dm) search on the multicomponent bursts in our sample yields a dm of 411.6 ± 0.6 pc cm-3. we find that the rotation measure (rm) of the bursts varies around their weighted mean value of -601 rad m-2 with a standard deviation of 11.1 rad m-2. this rm magnitude is 10 times larger than the expected galactic contribution along this line of sight (los). we estimate an los magnetic field strength of 4-6 µg, assuming that the entire host galaxy dm contributes to the rm. further polarization measurements will help determine frb 20201124a's rm stability. the bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating frb. their polarization position angles (pas) are flat across the burst envelopes and vary between bursts. we argue that the varying polarization fractions and pas of frb 20201124a are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the rm variability, is hard to explain with faraday conversion. the high linear polarization fractions, flat pas, and downward drift from frb 20201124a bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters. | polarization properties of frb 20201124a from detections with the effelsberg 100-m radio telescope |
gravitational waves (gws) from compact binary coalescences can be used as standard sirens to explore the cosmic expansion history. in the next decades, it is anticipated that we could obtain the multi-band gw standard siren data (from nanohertz to a few hundred hertz), which are expected to play an important role in cosmological parameter estimation. in this work, we provide, for the first time to the best of our knowledge, joint constraints on cosmological parameters using the future multi-band gw standard siren observations. we simulate the multi-band gw standard sirens based on the ska-era pulsar timing array (pta), taiji observatory, and cosmic explorer (ce) to perform cosmological analysis. in the λcdm model, we find that the joint pta+taiji+ce data could provide a tight constraint on the hubble constant with a $ 0.5\% $ precision. moreover, pta+taiji+ce could break the cosmological parameter degeneracies generated by cmb, especially in the dynamical dark energy models. when combining the pta+taiji+ce data with the cmb data, the constraint precisions of $\omega_\rm{m}$ and $ h_0 $ are $ 1.0\% $ and $ 0.3\% $ , respectively, meeting the standard of precision cosmology. the joint cmb+pta+taiji+ce data give $ \sigma(w)=0.028 $ in the wcdm model and $ \sigma(w_0)=0.11 $ and $ \sigma(w_a)=0.32 $ in the $ w_0w_a $ cdm model, which are comparable with or close to the latest constraint results by cmb+bao+sn. in conclusion, the future multi-band gw observations are expected to be used for exploring the nature of dark energy and measuring the hubble constant. *supported by the national ska program of china (2022ska0110200, 2022ska0110203) and the national natural science foundation of china (11975072, 11875102, 11835009) | joint constraints on cosmological parameters using future multi-band gravitational wave standard siren observations |
we present high-precision timing observations spanning up to nine years for 37 millisecond pulsars monitored with the green bank and arecibo radio telescopes as part of the north american nanohertz observatory for gravitational waves (nanograv) project. we describe the observational and instrumental setups used to collect the data, and methodology applied for calculating pulse times of arrival; these include novel methods for measuring instrumental offsets and characterizing low signal-to-noise ratio timing results. the time of arrival data are fit to a physical timing model for each source, including terms that characterize time-variable dispersion measure and frequency-dependent pulse shape evolution. in conjunction with the timing model fit, we have performed a bayesian analysis of a parameterized timing noise model for each source, and detect evidence for excess low-frequency, or “red,” timing noise in 10 of the pulsars. for 5 of these cases this is likely due to interstellar medium propagation effects rather than intrisic spin variations. subsequent papers in this series will present further analysis of this data set aimed at detecting or limiting the presence of nanohertz-frequency gravitational wave signals. | the nanograv nine-year data set: observations, arrival time measurements, and analysis of 37 millisecond pulsars |
we investigate the spectrum of gravitational waves arising from primordial inflation in the presence of a string-theoretical higher-curvature correction, specifically, the gauss-bonnet coupling term for the inflaton (modulus) field. we show that if the modulus field exhibits a wall-crossinglike behavior in the moduli space, there can be a period of gauss-bonnet coupling-term domination during the usual slow-roll. this phenomenon is potentially detectable as the gravitational wave spectrum exhibits a characteristic peak caused by the brief domination of the gauss-bonnet coupling term. we explore the possibility of measuring such gravitational waves with pulsar timing array experiments such as nanograv, and future space-borne interferometers such as lisa, decigo, and taiji. | probing the inflationary moduli space with gravitational waves |
we present a search for continuous gravitational waves from five radio pulsars, comprising three recycled pulsars (psr j0437-4715, psr j0711-6830, and psr j0737-3039a) and two young pulsars: the crab pulsar (j0534+2200) and the vela pulsar (j0835-4510). we use data from the third observing run of advanced ligo and virgo combined with data from their first and second observing runs. for the first time, we are able to match (for psr j0437-4715) or surpass (for psr j0711-6830) the indirect limits on gravitational-wave emission from recycled pulsars inferred from their observed spin-downs, and constrain their equatorial ellipticities to be less than 10-8. for each of the five pulsars, we perform targeted searches that assume a tight coupling between the gravitational-wave and electromagnetic signal phase evolution. we also present constraints on psr j0711-6830, the crab pulsar, and the vela pulsar from a search that relaxes this assumption, allowing the gravitational-wave signal to vary from the electromagnetic expectation within a narrow band of frequencies and frequency derivatives. | gravitational-wave constraints on the equatorial ellipticity of millisecond pulsars |
pulsar timing arrays (pta) are a promising probe to the cosmologically novel nanohertz gravitational wave (gw) regime through the stochastic gw background. in this work, we consider subluminal gw modes as a possible source of correlations in a pta, utilizing the public code ptafast and the 12.5 years correlations data by nanograv, which we hypothesize are sourced by gws. our results show no evidence in support of tensor- or vector-induced gw correlations in the data, and that vector correlations are disfavored. this places an upper bound to the graviton mass, mg≲10-22 ev , characteristic of the pta gw energy scale. | constraining gravitational wave propagation using pulsar timing array correlations |
the nine-year h.e.s.s. galactic plane survey (hgps) has yielded the most uniform observation scan of the inner milky way in the tev gamma-ray band to date. the sky maps and source catalogue of the hgps allow for a systematic study of the population of tev pulsar wind nebulae found throughout the last decade. to investigate the nature and evolution of pulsar wind nebulae, for the first time we also present several upper limits for regions around pulsars without a detected tev wind nebula. our data exhibit a correlation of tev surface brightness with pulsar spin-down power ė. this seems to be caused both by an increase of extension with decreasing ė, and hence with time, compatible with a power law rpwn(ė) ė-0.65±0.20, and by a mild decrease of tev gamma-ray luminosity with decreasing ė, compatible with l1-10 tev ė0.59±0.21. we also find that the offsets of pulsars with respect to the wind nebula centre with ages around 10 kyr are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. in the present data, it seems that a large pulsar offset is correlated with a high apparent tev efficiency l1-10 tev/ė. in addition to 14 hgps sources considered firmly identified pulsar wind nebulae and 5 additional pulsar wind nebulae taken from literature, we find 10 hgps sources that are likely tev pulsar wind nebula candidates. using a model that subsumes the present common understanding of the very high-energy radiative evolution of pulsar wind nebulae, we find that the trends and variations of the tev observables and limits can be reproduced to a good level, drawing a consistent picture of present-day tev data and theory. | the population of tev pulsar wind nebulae in the h.e.s.s. galactic plane survey |
we investigate the possibility of measuring the primordial gravitational wave (gw) signal across 21 decades in frequencies, using the cosmic microwave background (cmb), pulsar timing arrays (pta), and laser and atomic interferometers. for the cmb and pta experiments we consider the litebird mission and the square kilometer array (ska), respectively. for the interferometers we consider space mission proposals including the laser interferometer space antenna (lisa), the big bang observer (bbo), the deci-hertz interferometer gravitational wave observatory (decigo), the μares experiment, the decihertz observatory (do), and the atomic experiment for dark matter and gravity exploration in space (aedge), as well as the ground-based einstein telescope (et) proposal. we implement the mathematics needed to compute sensitivities for both cmb and interferometers, and derive the response functions for the latter from the first principles. we also evaluate the effect of the astrophysical foreground contamination in each experiment. we present binned sensitivity curves and error bars on the energy density parameter, ωgwh2, as a function of frequency for two representative classes of models for the stochastic background of primordial gw: the quantum vacuum fluctuation in the metric from single-field slow-roll inflation, and the source-induced tensor perturbation from the spectator axion-su(2) inflation models. we find excellent prospects for joint measurements of the gw spectrum by cmb and space-borne interferometers mission proposals. | measuring the spectrum of primordial gravitational waves with cmb, pta and laser interferometers |
we present an overview of future observational facilities that will significantly enhance our understanding of the fundamental nature of dark matter. these facilities span a range of observational techniques including optical/near-infrared imaging and spectroscopy, measurements of the cosmic microwave background, pulsar timing, 21-cm observations of neutral hydrogen at high redshift, and the measurement of gravitational waves. such facilities are a critical component of a multi-pronged experimental program to uncover the nature of dark matter, while often providing complementary measurements of dark energy, neutrino physics, and inflation. | snowmass2021 cosmic frontier white paper: observational facilities to study dark matter |
the possibility that in the mass range around 10-12 m⊙ most of dark matter is constituted 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 ∼1012 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 k and g inflations, we propose a new mechanism to enhance the power spectrum at small scales by introducing a noncanonical kinetic term [1 -2 g (ϕ )]x with the function -g (ϕ ) having a peak. away from the peak, g (ϕ ) is negligible and we recover the usual slow-roll inflation which is constrained by the cosmic microwave background anisotropy 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 (ϕ ) to several significant digits. the energy spectrum ωgw(f ) of secondary gravitational waves produced by the model have the characteristic power law behavior ωgw(f )∼fn and is testable by pulsar timing array and space based gravitational wave detectors. | primordial black holes and secondary gravitational waves from k and g inflation |
dark matter (dm) particles in the universe accumulate in neutron stars (nss) through their interactions with ordinary matter. it has been known that their annihilation inside the ns core causes late-time heating, with which the surface temperature becomes a constant value of ts ≃ (2 - 3) ×103 k for the ns age t ≳10 6 - 7 years. this conclusion is, however, drawn based on the assumption that the beta equilibrium is maintained in nss throughout their life, which turns out to be invalid for rotating pulsars. the slowdown in the pulsar rotation drives the ns matter out of beta equilibrium, and the resultant imbalance in chemical potentials induces late-time heating, dubbed as rotochemical heating. this effect can heat a ns up to ts ≃106 k for t ≃10 6 - 7 years. in fact, recent observations found several old nss whose surface temperature is much higher than the prediction of the standard cooling scenario and is consistent with the rotochemical heating. motivated by these observations, in this letter, we reevaluate the significance of the dm heating in nss, including the effect of the rotochemical heating. we then show that the signature of dm heating can still be detected in old ordinary pulsars, while it is concealed by the rotochemical heating for old millisecond pulsars. to confirm the evidence for the dm heating, however, it is necessary to improve our knowledge on nucleon pairing gaps as well as to evaluate the initial period of the pulsars accurately. in any cases, a discovery of a very cold ns can give a robust constraint on the dm heating, and thus on dm models. to demonstrate this, as an example, we also discuss the case that the dm is the neutral component of an electroweak multiplet, and show that an observation of a ns with ts ≲103 k imposes a stringent constraint on such a dm candidate. | dark matter heating vs. rotochemical heating in old neutron stars |
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