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we present deep polarimetric observations at 154 mhz with the murchison widefield array (mwa), covering 625 deg2 centered on α = 0hand δ = -27°. the sensitivity available in our deep observations allows an in-band, frequency-dependent analysis of polarized structure for the first time at long wavelengths. our analysis suggests that the polarized structures are dominated by intrinsic emission but may also have a foreground faraday screen component. at these wavelengths, the compactness of the mwa baseline distribution provides excellent snapshot sensitivity to large-scale structure. the observations are sensitive to diffuse polarized emission at ∼54‧ resolution with a sensitivity of 5.9 mjy beam-1 and compact polarized sources at ∼2.‧4 resolution with a sensitivity of 2.3 mjy beam-1 for a subset (400 deg2) of this field. the sensitivity allows the effect of ionospheric faraday rotation to be spatially and temporally measured directly from the diffuse polarized background. our observations reveal large-scale structures (∼1°-8° in extent) in linear polarization clearly detectable in ∼2 minute snapshots, which would remain undetectable by interferometers with minimum baseline lengths of >110 m at 154 mhz. the brightness temperature of these structures is on average 4 k in polarized intensity, peaking at 11 k. rotation measure synthesis reveals that the structures have faraday depths ranging from -2 to 10 rad m-2 with a large fraction peaking at approximately +1 rad m-2. we estimate a distance of 51 ± 20 pc to the polarized emission based on measurements of the in-field pulsar j2330-2005. we detect four extragalactic linearly polarized point sources within the field in our compact source survey. based on the known polarized source population at 1.4 ghz and non-detections at 154 mhz, we estimate an upper limit on the depolarization ratio of 0.08 from 1.4 ghz to 154 mhz. | low-frequency observations of linearly polarized structures in the interstellar medium near the south galactic pole |
inspiraling massive black hole binaries (mbhbs) forming in the aftermath of galaxy mergers are expected to be the loudest gravitational-wave (gw) sources relevant for pulsar-timing arrays (ptas) at nhz frequencies. the incoherent overlap of signals from a cosmic population of mbhbs gives rise to a stochastic gw background (gwb) with characteristic strain around hc ∼ 10-15 at a reference frequency of 1 yr-1, although uncertainties around this value are large. current ptas are piercing into the gw amplitude range predicted by mbhb-population models, but no detection has been reported so far. to assess the future success prospects of pta experiments, it is therefore important to estimate the minimum gwb level consistent with our current understanding of the formation and evolution of galaxies and massive black holes (mbhs). to this purpose, we couple a semi-analytic model of galaxy evolution and an extensive study of the statistical outcome of triple mbh interactions. we show that even in the most pessimistic scenario where all mbhbs stall before entering the gw-dominated regime, triple interactions resulting from subsequent galaxy mergers inevitably drive a considerable fraction of the mbhb population to coalescence. at frequencies relevant for pta, the resulting gwb is only a factor of 2-3 suppressed compared to a fiducial model where binaries are allowed to merge over gyr time-scales . coupled with current estimates of the expected gwb amplitude range, our findings suggest that the minimum gwb from cosmic mbhbs is unlikely to be lower than hc ∼ 10-16 (at f = 1 yr-1), well within the expected sensitivity of projected ptas based on future observations with fast, meerkat, and ska. | post-newtonian evolution of massive black hole triplets in galactic nuclei - iii. a robust lower limit to the nhz stochastic background of gravitational waves |
despite considerable experimental progress large parts of the axionlike particle (alp) parameter space remain difficult to probe in terrestrial experiments. in some cases, however, small-scale structure of the alp dark matter (dm) distribution is strongly enhanced, offering opportunities for astrophysical tests. such an enhancement can be produced by a period of prenucleosynthesis early matter domination (emd). this cosmology arises in many ultraviolet completions and generates the correct relic abundance for weak coupling fa∼1016 gev , alp masses in the range 10-13 ev <ma<1 ev , and without fine-tuning of the initial misalignment angle. this range includes the qcd axion around 10-9-10-8 ev . emd enhances the growth of alp small-scale structure, leading to the formation of dense alp miniclusters which can contain nearly all of dm (depending on alp mass and reheating temperature). we study the interplay between the initial alp oscillation, reheating temperature, and effective pressure to provide analytic estimates of the minicluster abundance and properties. alp miniclusters in the emd cosmology are denser and more abundant than in λ cdm . while enhanced substructure generically reduces the prospects of direct detection experiments, we show that pulsar timing and lensing observations can discover these minihalos over a large range of alp masses and reheating temperatures. | imprints of the early universe on axion dark matter substructure |
in this paper, we use the equation of state based on a modification of the 2 +1 flavors nambu-jona-lasinio (njl) model to study the quark matter of hybrid stars. for comparison, we utilize five eoss of the relativistic mean-field (rmf) model to describe the hadronic phase. with the three-window crossover interpolation approach, we try to construct relatively soft hybrid eoss but find the maximum masses of hybrid stars do not differ much. the results are quite close to the 2 solar mass, which is consistent with the mass constraint of psr j 0348 +0432 . furthermore, it is noteworthy that the heaviest stable stars have central densities higher than that of the deconfinement transition, suggesting a pure quark core in the hybrid star. | studies of the structure of massive hybrid stars within a modified njl model |
the recent detection of pulsations from the ultraluminous x-ray source (ulx) nustar j095551+6940.8 in m82 by bachetti et al. indicates that the object is an accreting neutron star in a high-mass x-ray binary (hmxb) system. the super-eddington luminosity of the object implies that the magnetic field is sufficiently strong to suppress the scattering cross-section unless its beam is viewed at a favourable angle. we show that the torque equilibrium condition for the pulsar indicates that the dipole magnetic field of the neutron star is 6.7 × 1013 g, two orders of magnitude higher than that estimated by bachetti et al., and further point to the possibility that even stronger magnetic fields could well be in the higher multipoles. this supports the recent view that magnetars descent from hmxbs if the magnetic field decays an order of magnitude during the process of transition. | the ultraluminous x-ray source nustar j095551+6940.8: a magnetar in a high-mass x-ray binary. |
in this letter, we present a new method that quantitatively identifies the occurrence probability of equations of state (eos) beyond "standard" eos models that disfavor sharp and strong phase-transitions, based on neutron star mass and radius observations. the radii of two neutron stars with different masses are naturally correlated, in part because both of them are sensitive to the symmetry energy of the eos. we show the radii of two neutron stars observed by nicer (psr j0740+6620 and psr 0030+0451) are correlated if these two neutron stars are built upon eoss with no sharp and strong first-order phase transitions. we further show that the linear correlation of the neutron star radii can be significantly weakened, when strong and sharp first-order phase transitions occur. we propose a new quantity, ${d}_{\mathrm{l}}$, which measures the extent to which the linear correlation of the radii of two neutron stars is weakened. our method gives a 48% identification probability (with a 5% false alarm rate) that the nicer observations indicate the necessity for a sharp and strong phase transition. future observations can confirm or rule out this identification. our method is generalizable to any pair of neutron star masses and can be employed with other sets of observations in the future. | indication of sharp and strong phase-transitions from nicer observations |
context. extended gamma-ray tev emission (tev halos) around middle-aged pulsars has been detected. a proposed model to explain these tev halos is that electrons from a degree-wide pulsar wind nebula (pwn) get up-scattered by cosmic microwave background photons through inverse compton processes. however, no x-ray degree-wide faint diffuse pwne have been found around these middle-aged pulsars in previous x-ray observations. aims. we have performed a search for degree wide pwne around geminga, psr b0656+14, b0540+23, j0633+0632, and j0631+1036, using data from the first four consecutive spectrum roentgen gamma/erosita all-sky surveys. in order to better understand the mechanisms underlying the formation of tev halos, we investigated the magnetic field strength in the degree wide neighbourhood of those pulsars. results. we did not detect degree-wide diffuse emission around geminga, psr b0656+14, b0540+23, j0633+0632, and j0631+1036, which can be attributed to being powered by the rotation-powered pulsars. indeed, a close inspection of the data shows that the pulsars of interest are all embedded in diffuse emission from supernova remnants like the monogem ring or the rosetta nebula, while psr b0540+23 is located ~2.5 degrees away from the bright crab pulsar, which shines out the erosita point-spread function up to the position of psr b0540+23 and thus reduced the sensitivity to search for degree wide bright diffuse x-ray emission strongly. conclusions. despite the non-detection of any degree-wide pwn surrounding the analysed pulsars, we set flux upper limits to provide useful information on magnetic field strength and its spatial distribution around those pulsars, providing additional constraints to the proposed theory for the formation of tev halos around pulsars. | searching for x-ray counterparts of degree wide tev halos around middle-aged pulsars with srg/erosita |
in this paper, we investigate the scalar-induced gravitational waves in single-field non-attractor inflation for the pulsar timing arrays data. our model comprises three phases of inflation: the first and third phases are slow-roll inflation, while the second phase is a period of non-attractor inflation. we analyze the model's predictions for various values of the sound speed $c_s$ and examine the sharp transitions to the final attractor phase. furthermore, we study the model's predictions for nanograv observations and future gravitational wave observations. we also calculate the non-gaussianity parameter $f_{nl}$ for the non-attractor setup with a general sound speed and the sharpness parameter. | induced gravitational waves from non-attractor inflation and nanograv data |
most black holes (bhs) will absorb a neutron star (ns) companion fully intact without tidal disruption, suggesting the pair will remain dark to telescopes. even without tidal disruption, electromagnetic (em) luminosity is generated from the battery phase of the binary when the bh interacts with the ns magnetic field. originally, the luminosity was expected to be in high-energy x-rays or gamma-rays, however, we conjecture that some of the battery power is emitted in the radio bandwidth. while the luminosity and timescale are suggestive of fast radio bursts (frbs; millisecond-scale radio transients) ns-bh coalescence rates are too low to make these a primary frb source. instead, we propose that the transients form a frb sub-population, distinguishable by a double peak with a precursor. the rapid ramp-up in luminosity manifests as a precursor to the burst which is 20%-80% as luminous given 0.5 ms timing resolution. the main burst arises from the peak luminosity before the merger. the post-merger burst follows from the ns magnetic field migration to the bh, causing a shock. ns-bh pairs are especially desirable for ground-based gravitational wave (gw) observatories since the pair might not otherwise be detected, with em counterparts greatly augmenting the scientific leverage beyond the gw signal. the em signal’s ability to break degeneracies in the parameters encoded in the gw and probe the ns magnetic field strength is quite valuable, yielding insights into open problems in ns magnetic field decay. | fast radio bursts and radio transients from black hole batteries |
neutron stars are among the densest known objects in the universe and an ideal laboratory for the strange physics of supercondensed matter. while the simultaneous measurements of mass and radius of nonrotating neutron stars may impose constraints on the properties of the dense nuclear matter, the observation and study of maximally rotating ones, close to the mass-shedding limit, may lead to significantly further constraints. theoretical predictions allow neutron stars to rotate extremely fast (even more than 2000 hz ). however, until this moment, the fastest observed rotating pulsar has a frequency of 716 hz , much lower compared to the theoretical predictions. there are many suggestions for the mechanism which lead to this situation. in any case, the theoretical study of uniformly rotating neutron stars, along with accurate measurements, may offer rich information concerning the high-density part of the equation of state. in addition, neutron stars through their evolution may provide us with a criterion to determine the final fate of a rotating compact star. sensitivity of bulk neutron stars properties on the equation of state at the mass-shedding limit are the main subject of the present study. | effects of the equation of state on the bulk properties of maximally rotating neutron stars |
we present broadband, multi-epoch x-ray spectroscopy of the pulsating ultra-luminous x-ray source (ulx) in ngc 5907. simultaneous xmm-newton and nustar data from 2014 are best described by a multicolor blackbody model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk (t(r)\propto {r}-p, with p={0.608}-0.012+0.014). additionally, we detect a hard power-law tail at energies above 10 kev, which we interpret as being due to comptonization. we compare this observation to archival xmm-newton, chandra, and nustar data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78-day super-orbital period of this source. we find that observations taken around phases 0.3-0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. we discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena. | spectral changes in the hyperluminous pulsar in ngc 5907 as a function of super-orbital phase |
we report on a search for fast radio bursts (frbs) with the green bank northern celestial cap (gbncc) pulsar survey at 350 mhz. pointings amounting to a total on-sky time of 61 days were searched to a dispersion measure (dm) of 3000 pc cm-3, while the rest (23 days; 29% of the total time) were searched to a dm of 500 pc cm-3. no frbs were detected in the pointings observed through 2016 may. we estimate a 95% confidence upper limit on the frb rate of 3.6× {10}3 frbs sky-1 day-1 above a peak flux density of 0.63 jy at 350 mhz for an intrinsic pulse width of 5 ms. we place constraints on the spectral index α by running simulations for different astrophysical scenarios and cumulative flux density distributions. the nondetection with gbncc is consistent with the 1.4 ghz rate reported for the parkes surveys for α > +0.35 in the absence of scattering and free-free absorption and α > -0.3 in the presence of scattering, for a euclidean flux distribution. the constraints imply that frbs exhibit either a flat spectrum or a spectral turnover at frequencies above 400 mhz. these constraints also allow estimation of the number of bursts that can be detected with current and upcoming surveys. we predict that chime may detect anywhere from several to ∼50 frbs per day (depending on model assumptions), making it well suited for interesting constraints on spectral index, the log n-log s slope, and pulse profile evolution across its bandwidth (400-800 mhz). | a search for fast radio bursts with the gbncc pulsar survey |
we study the dynamics and evolution of the milky way nuclear star cluster performing a high-resolution direct one-million-body simulation. focusing on the interactions between such stellar systems and the central supermassive black hole, we find that different stellar components adapt their overall distribution differently. after 5 gyr, stellar mass black holes are characterized by a spatial distribution with power-slope -1.75, fully consistent with the prediction of bahcall-wolf pioneering work. using the vast amount of data available, we infer the rate for tidal disruption events, being 4 × 10-6 per yr, and estimate the number of objects that emit gravitational waves during the phases preceding the accretion on to the super-massive black hole, ∼270 per gyr. we show that some of these sources could form extreme mass-ratio inspirals. we follow the evolution of binary stars population, showing that the initial binary fraction of 5 per cent drops down to 2.5 per cent inside the inner parsec. also, we explored the possible formation of binary systems containing a compact object, discussing the implications for millisecond pulsars formation and the development of ia supernovae. | direct n-body simulation of the galactic centre |
we propose that chirally asymmetric plasma can be produced in the gap regions of the magnetospheres of pulsars and black holes. we show that, in the case of supermassive black holes situated in active galactic nuclei, the chiral charge density and the chiral chemical potential are very small and unlikely to have any observable effects. in contrast, the chiral asymmetry produced in the magnetospheres of magnetars can be substantial. it can trigger the chiral plasma instability that, in turn, can lead to observable phenomena in magnetars. in particular, the instability should trigger circularly polarized electromagnetic radiation in a wide window of frequencies, spanning from radio to near-infrared. as such, the produced chiral charge has the potential to affect some features of fast radio bursts. | chiral anomalous processes in magnetospheres of pulsars and black holes |
the optical study of the heated substellar companions of black widow (bw) millisecond pulsars (msps) provides unique information on the msp particle and radiation output and on the neutron star mass. here we present an analysis of optical photometry and spectroscopy of a set of relatively bright bws, many newly discovered in association with fermi γ-ray sources. interpreting the optical data requires sophisticated models of the companion heating. we provide a uniform analysis, selecting the preferred heating model and reporting on the companion masses and radii, the pulsar heating power, and neutron star mass. the substellar companions are substantially degenerate, with average densities 15-30× solar, but are inflated above their zero temperature radii. we find evidence that the most extreme recycled bw pulsars have both large >0.8m ⊙ accreted mass and low <108g magnetic fields. examining a set of heavy bws, we infer that neutron star masses larger than 2.19m ⊙ (1σ confidence) or 2.08m ⊙ (3σ confidence) are required; these bounds exclude all but the stiffest equations of state in standard tabulations. | an optical study of the black widow population |
it was suggested that the γ-ray halo around geminga might not be interpreted by slow diffusion. if the ballistic regime of electron/positron propagation is considered, the geminga halo may be explained even with a large diffusion coefficient. in this work, we examine this effect by taking the generalized jüttner propagator as the approximate relativistic green's function for diffusion and find that the morphology of the geminga halo can be marginally fitted in the fast-diffusion scenario. however, the recently discovered γ-ray halo around psr j0622+3749 at the large high-altitude air shower observatory cannot be explained by the same effect and slow diffusion is the only solution. furthermore, both the two pulsar halos require a conversion efficiency from the pulsar spin-down energy to the high-energy electrons/positrons much larger than 100%, if they are interpreted by this ballistic transport effect. therefore, we conclude that slow diffusion is necessary to account for the γ-ray halos around pulsars. | slow diffusion is necessary to explain the γ-ray pulsar halos |
mixed fermion-boson stars are stable, horizonless, everywhere-regular solutions of the coupled einstein-(complex, massive) klein-gordon-euler system. while isolated neutron stars and boson stars are uniquely determined by their central energy density, mixed configurations conform to an extended parameter space that depends on the combination of the number of fermions and (ultralight) bosons. the wider possibilities offered by fermion-boson stars could help to explain the tension in the measurements of neutron star masses and radii reported in recent multimessenger observations and nuclear physics experiments. in this work, we construct equilibrium configurations of mixed fermion-boson stars with realistic equations of state for the fermionic component and different percentages of bosonic matter. we show that our solutions are in excellent agreement with multimessenger data, including gravitational-wave events gw170817 and gw190814 and x-ray pulsars psr j 0030 +0451 and psr j 0740 +6620 , as well as with nuclear physics constraints from the prex-2 experiment. | can fermion-boson stars reconcile multimessenger observations of compact stars? |
we model here the merger histories of the supermassive black hole (smbh) population in the late stages of a cosmological simulation of a ~ 2 × 1013 m ⊙ galaxy group. the gravitational dynamics around the several tens of smbhs (m • > 7.5 × 107 m ⊙) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the ketju code. the 11 smbhs that form binaries and a hierarchical triplet eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (gw) emission. the binaries form at eccentricities of e ~ 0.3-0.9, with one system evolving to a very high eccentricity of e = 0.998, and merge on timescales of a few tens to several hundred megayears. during the simulation, the merger-induced gw recoil kicks eject one smbh remnant from the central host galaxy. this temporarily drives the galaxy off the m •-σ ⋆ relation; however, the galaxy returns to the relation due to subsequent galaxy mergers, which bring in new smbhs. this showcases a possible mechanism contributing to the observed scatter of the m •-σ ⋆ relation. finally, we show that pulsar timing arrays and lisa would be able to detect parts of the gw signals from the smbh mergers that occur during the ~4 gyr time span simulated with ketju. | signatures of the many supermassive black hole mergers in a cosmologically forming massive early-type galaxy |
pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. the vela pulsar wind nebula is powered by a young pulsar (b0833-45, aged 11,000 years)1 and located inside an extended structure called vela x, which is itself inside the supernova remnant2. previous x-ray observations revealed two prominent arcs that are bisected by a jet and counter jet3,4. radio maps have shown high linear polarization of 60% in the outer regions of the nebula5. here we report an x-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge—approaching the theoretical limit of what can be produced by synchrotron emission. we infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field. | vela pulsar wind nebula x-rays are polarized to near the synchrotron limit |
in spite of the rich phenomenology of the polarization properties of radio pulsars, the rotating vector model (rvm) created 50 years ago remains the best method to determine the beam geometry of a pulsar. we apply the rvm to a sample of 854 radio pulsars observed with the meerkat telescope in order to draw conclusions about the population of pulsars as a whole. the main results are that (i) the geometrical interpretation of the position angle (pa) traverse is valid in the majority of the population, (ii) the pulsars for which the rvm fails tend to have a high fraction of circular polarization compared to linear polarization, (iii) emission heights obtained through both geometrical and relativistic methods show that the majority of pulsars must have emission heights less than 1000 km independent of the spin period, (iv) orthogonal mode jumps are seen in the pa traverse in about one third of the population. all these results are weakly dependent on the pulsar spin-down energy. | the thousand-pulsar-array programme on meerkat - xi. application of the rotating vector model |
in order to address the generation of neutron star magnetic fields, with particular focus on the dichotomy between magnetars and radio pulsars, we consider the properties of dynamos as inferred from other astrophysical systems. with sufficiently low (modified) rossby number, convective dynamos are known to produce dipole-dominated fields whose strength scales with convective flux, and we argue that these expectations should apply to the convective protoneutron stars (pnss) at the centers of core-collapse supernovae. we analyze a suite of three-dimensional simulations of core collapse, featuring a realistic equation of state and full neutrino transport, in this context. all our progenitor models, ranging from 9 m ⊙ to 25 m ⊙, including one with initial rotation, have sufficiently vigorous pns convection to generate dipole fields of order ~1015 gauss, if the modified rossby number resides in the critical range. thus, the magnetar/radio pulsar dichotomy may arise naturally in part from the distribution of core rotation rates in massive stars. | on the origin of pulsar and magnetar magnetic fields |
the origin of fast radio bursts (frbs), the brightest cosmic explosion in radio bands, remains unknown. magnetar-related mechanisms are currently favored. the searches for short-term periodicity that is naturally expected for such fast-spinning compact objects, however, have failed. we introduce here a novel method for a comprehensive analysis of active frbs' behaviors in the time-energy domain. using ``pincus index'' and ``maximum lyapunov exponent'', we were able to quantify the stochasticity and chaos, respectively, of the bursting events and put frbs in the context of common transient physical phenomena, such as pulsars, earthquakes, and solar flares. in the bivariate time-energy domain, repeated frb bursts' behaviors deviate significantly (more random, less chaotic) from pulsars, earthquakes, and solar flares. frb bursts wander in time-energy space stochastically, akin to brownian motions. the high degree of stochasticity suggests complex and even multi-origins for frbs. | frbs' brownian motion on time-energy bivariate space |
pulsar timing arrays (ptas) and the laser interferometer space antenna (lisa) will open complementary observational windows on massive black hole binaries (mbhbs), i.e. with masses in the range ${\sim} 10^6\!-\!10^{10}\, \rm m_{\odot }$. while ptas may detect a stochastic gravitational wave background from a population of mbhbs, during operation lisa will detect individual merging mbhbs. to demonstrate the profound interplay between lisa and ptas, we estimate the number of mbhb mergers that one can expect to observe with lisa by extrapolating direct observational constraints on the mbhb merger rate inferred from pta data. for this, we postulate that the common signal observed by ptas (and consistent with the increased evidence recently reported) is an astrophysical background sourced by a single mbhb population. we then constrain the lisa detection rate, $\mathcal {r}$, in the mass-redshift space by combining our bayesian-inferred merger rate with lisa's sensitivity to spin-aligned, inspiral-merger-ringdown waveforms. using an astrophysically informed formation model, we predict a $95{{\, \rm per\, cent}}$ upper limit on the detection rate of $\mathcal {r} \lt 134\, {\rm yr}^{-1}$ for binaries with total masses in the range $10^7\!-\!10^8\, \rm m_{\odot }$. for higher masses, i.e. ${\gt} 10^8\, \rm m_{\odot }$, we find $\mathcal {r} \lt 2\, (1)\, \mathrm{yr}^{-1}$ using an astrophysically informed (agnostic) formation model, rising to $11 \, (6)\, \mathrm{yr}^{-1}$ if the lisa sensitivity bandwidth extends down to 10-5 hz. forecasts of lisa science potential with pta background measurements should improve as ptas continue their search. | implications of pulsar timing array observations for lisa detections of massive black hole binaries |
context. gas expulsion is a central concept in some of the models for multiple populations and the light-element anti-correlations in globular clusters. if the star formation efficiency was around 30 per cent and the gas expulsion happened on the crossing timescale, this process could preferentially expel stars born with the chemical composition of the proto-cluster gas, while stars with special composition born in the centre would remain bound. recently, a sample of extragalactic, gas-free, young massive clusters has been identified that has the potential to test the conditions for gas expulsion.aims: we investigate the conditions required for residual gas expulsion on the crossing timescale. we consider a standard initial mass function and different models for the energy production in the cluster: metallicity-dependent stellar winds, radiation, supernovae and more energetic events, such as hypernovae, which are related to gamma ray bursts. the latter may be more energetic than supernovae by up to two orders of magnitude.methods: we computed a large number of thin-shell models for the gas dynamics, and calculated whether the rayleigh-taylor instability is able to disrupt the shell before it reaches the escape speed.results: we show that the success of gas expulsion depends on the compactness index of a star cluster c5 ≡ (m∗/ 105 m⊙)/(rh/ pc), with initial stellar mass m∗ and half-mass radius rh. for given c5, a certain critical, local star formation efficiency is required to remove the rest of the gas. common stellar feedback processes may not lead to gas expulsion with significant loss of stars above c5 ≈ 1. considering pulsar winds and hypernovae, the limit increases to c5 ≈ 30. if successful, gas expulsion generally takes place on the crossing timescale. some observed young massive clusters have 1 <c5< 10 and are gas-free at ≈10 myr. this suggests that gas expulsion does not affect their stellar mass significantly, unless powerful pulsar winds and hypernovae are common in such objects. by comparison to observations, we show that c5 is a better predictor for the expression of multiple populations than stellar mass. the best separation between star clusters with and without multiple populations is achieved by a stellar winds-based gas expulsion model, where gas expulsion would occur exclusively in star clusters without multiple populations. single and multiple population clusters also have little overlap in metallicity and age.conclusions: globular clusters should initially have c5 ≲ 100, if the gas expulsion paradigm was correct. early gas expulsion, which is suggested by the young massive cluster observations, hence would require special circumstances, and is excluded for several objects. most likely, the stellar masses did not change significantly at the removal of the primordial gas. instead, the predictive power of the c5 index for the expression of multiple populations is consistent with the idea that gas expulsion may prevent the expression of multiple populations. on this basis, compact young massive clusters should also have multiple populations. | gas expulsion in massive star clusters?. constraints from observations of young and gas-free objects |
gravitational waves (gw) sourced by second-order primordial curvature fluctuations are among the favoured models fitting the recent pulsar timing array (pta) measurement of a stochastic gw background (sgwb). we study how spectral distortions (sds) and anisotropies of the cosmic microwave background (cmb) can constrain such scalar fluctuations. whereas cobe firas data have no sufficient sensitivity to probe the pta lognormal hypothesis, we show how future pixie-like experiments can detect the cmb sds from the scalar-induced interpretation of the sgwb in pta data. we finally show how the transformative synergy between pta data and future cmb sd measurements is important for reconstructing primordial fluctuations at these small scales. | the quest of cmb spectral distortions to probe the scalar-induced gravitational wave background interpretation in pta data |
neutron star-black hole (nsbh) mergers detected in gravitational waves have the potential to shed light on supernova physics, the dense matter equation of state, and the astrophysical processes that power their potential electromagnetic counterparts. we use the population of four candidate nsbh events detected in gravitational waves so far with a false alarm rate ≤1 yr-1 to constrain the mass and spin distributions and multimessenger prospects of these systems. we find that the black holes in nsbhs are both less massive and have smaller dimensionless spins than those in black hole binaries. we also find evidence for a mass gap between the most massive neutron stars and least massive black holes in nsbhs at 98.6-per cent credibility. using an approach driven by gravitational-wave data rather than binary simulations, we find that fewer than 14 per cent of nsbh mergers detectable in gravitational waves will have an electromagnetic counterpart. while the inferred presence of a mass gap and fraction of sources with a counterpart depend on the event selection and prior knowledge of source classification, the conclusion that the black holes in nsbhs have lower masses and smaller spin parameters than those in black hole binaries is robust. finally, we propose a method for the multimessenger analysis of nsbh mergers based on the non-detection of an electromagnetic counterpart and conclude that, even in the most optimistic case, the constraints on the neutron star equation of state that can be obtained with multimessenger nsbh detections are not competitive with those from gravitational-wave measurements of tides in binary neutron star mergers and radio and x-ray pulsar observations. | population properties and multimessenger prospects of neutron star-black hole mergers following gwtc-3 |
the frequency dependence of radio pulse arrival times provides a probe of structures in the intervening media. demorest et al. was the first to show a short-term (∼100-200 days) reduction in the electron content along the line of sight to psr j1713+0747 in data from 2008 (approximately mjd 54750) based on an apparent dip in the dispersion measure of the pulsar. we report on a similar event in 2016 (approximately mjd 57510), with average residual pulse-arrival times ≈-3.0, -1.3, and -0.7 μs at 820, 1400, and 2300 mhz, respectively. timing analyses indicate possible departures from the standard ν -2 dispersive-delay dependence. we discuss and rule out a wide variety of potential interpretations. we find the likeliest scenario to be lensing of the radio emission by some structure in the interstellar medium, which causes multiple frequency-dependent pulse arrival-time delays. | a second chromatic timing event of interstellar origin toward psr j1713+0747 |
this python module provides an interface for querying the australia telescope national facility (atnf) pulsar catalogue (manchester et al. 2005). the intended users are astronomers wanting to extract data from the catalogue through a script rather than having to download and parse text tables output using the standard web interface. it allows users to access information, such as pulsar frequencies and sky locations, on all pulsars in the catalogue. querying of the catalogue can easily be incorporated into python scripts. | psrqpy: a python interface for querying the atnf pulsar catalogue |
supermassive black holes (bhs) are believed to be the central powerhouse of active galactic nuclei. applying the pulsar outer-magnetospheric particle accelerator theory to bh magnetospheres, we demonstrate that an electric field is exerted along the magnetic field lines near the event horizon of a rotating bh. in this particle accelerator (or a gap), electrons and positrons are created by photon-photon collisions and accelerated in the opposite directions by this electric field, efficiently emitting gamma-rays via curvature and inverse-compton processes. it is shown that a gap arises around the null-charge surface formed by the frame-dragging effect, provided that there is no current injection across the gap boundaries. the gap is dissipating a part of the hole’s rotational energy, and the resultant gamma-ray luminosity increases with decreasing plasma accretion from the surroundings. considering an extremely rotating supermassive bh, we show that such a gap reproduces the significant very-high-energy (vhe) gamma-ray flux observed from the radio galaxy ic 310, provided that the accretion rate becomes much less than the eddington rate particularly during its flare phase. it is found that the curvature process dominates the inverse-compton process in the magnetosphere of ic 310, and that the observed power-law-like spectrum in vhe gamma-rays can be explained to some extent by a superposition of the curvature emissions with varying curvature radius. it is predicted that the vhe spectrum extends into higher energies with increasing vhe photon flux. | energetic gamma radiation from rapidly rotating black holes |
we introduce simplified models for enhancements in the matter power spectrum at small scales and study their implications for dark matter substructure and gravitational observables. these models capture the salient aspects of a variety of early universe scenarios that predict enhanced small-scale structure, such as axionlike particle dark matter, light vector dark matter, and epochs of early matter domination. we use a model-independent, semianalytic treatment to map bumps in the matter power spectrum to early forming subsolar mass dark matter halos and estimate their evolution, disruption, and contribution to the substructure of clusters and galaxies at late times. we discuss the sensitivity of gravitational observables, including pulsar timing arrays and caustic microlensing, to both the presence of bumps in the power spectrum and variations in their basic properties. | dark matter microhalos from simplified models |
a bright radio burst was newly discovered in sgr 1935+2154, which exhibits some fast radio burst (frb)-like temporal and frequency properties, suggesting a neutron star (ns)/magnetar magnetospheric origin of frbs. we propose an explanation of the temporal and frequency properties of sub-pulses of repeating frbs based on the generic geometry within the framework of charged-bunching coherent curvature radiation in the magnetosphere. the sub-pulses in a radio burst come from bunches of charged particles moving along different magnetic field lines. their radiation beams sweep across the line of sight at different times, and those radiating at the more curved part tend to be seen earlier and at higher frequency. however, by considering bunches generated at slightly different times, we find there is also a small probability that the emission from the less curved part can be seen earlier. we simulate the time-frequency structures by deriving various forms of the electric acceleration field in the magnetosphere. this structure of sub-pulses is a natural consequence of coherent curvature radiation from an ns/magnetar magnetosphere with suddenly and violently triggered sparks. we apply this model to explain the time-frequency structure within a specific dipolar configuration by invoking the transient pulsar-like sparking from the inner gap of a slowly rotating ns, and we have also applied it to more generic configurations. | on the magnetospheric origin of repeating fast radio bursts |
observations of binary pulsars and pulsars in globular clusters suggest that at least some pulsars must receive weak natal kicks at birth. if all pulsars received strong natal kicks above 50 km s-1, those born in globular clusters would predominantly escape, while wide binaries would be disrupted. on the other hand, observations of transverse velocities of isolated radio pulsars indicate that only 5 ± 2% have velocities below 50 km s-1. we explore this apparent tension with rapid binary population synthesis modeling. we propose a model in which supernovae with characteristically low natal kicks (e.g., electron-capture supernovae) only occur if the progenitor star has been stripped via binary interaction with a companion. we show that this model naturally reproduces the observed pulsar speed distribution and without reducing the predicted merging double neutron star yield. we estimate that the zero-age main-sequence mass range for noninteracting progenitors of electron-capture supernovae should be no wider than ≈0.2 m⊙. | constraints on weak supernova kicks from observed pulsar velocities |
cosmic rays (crs) leave their sources mainly along the local magnetic field; in doing so they excite both resonant and nonresonant modes through streaming instabilities. the excitation of these modes leads to enhanced scattering and in turn to a large pressure gradient that causes the formation of bubbles of gas, crs, and self-generated magnetic fields expanding into the interstellar medium. by means of hybrid particle-in-cell simulations, we show that, by exciting the nonresonant instability, crs excavate a cavity around their source where the diffusivity is strongly suppressed. this finding invalidates the so far largely adopted flux-tube assumption, under which particles move along magnetic lines even in the nonlinear regime. this phenomenon is general and is expected to occur around any sufficiently powerful cr source in the galaxy. our results might provide a physical explanation of the numerous claims of suppressed cr diffusion around galactic sources such as supernova remnants, pulsar wind nebulae, and stellar clusters. | dynamical effects of cosmic rays on the medium surrounding their sources |
we perform general relativistic, magnetohydrodynamic simulations of merging binary neutron stars incorporating neutrino transport and magnetic fields. our new radiative transport module for neutrinos adopts a general relativistic, truncated-moment formalism. the binaries consist of two identical, irrotational stars modeled by the skyrme lyon (sly) nuclear equation of state. they are initially in quasicircular orbit and threaded with a poloidal magnetic field that extends from the stellar interior into the exterior, as in typical pulsars. we insert neutrino processes shortly after the merger and focus on the role of neutrinos in launching a jet following the collapse of the hypermassive neutron star (hmns) remnant to a spinning black hole (bh). we treat two microphysical versions: one (a "warm-up") evolving a single neutrino species and considering only charged-current processes and the other evolving three species (νe,ν¯e,νx) and related processes. we trace the evolution until the system reaches a quasiequilibrium state after bh formation. we find that the bh +disk remnant eventually launches an incipient jet. the electromagnetic poynting luminosity is ∼1053 erg s-1, consistent with that of typical short gamma-ray bursts. the effect of neutrino cooling shortens the lifetime of the hmns and lowers the amplitude of the major peak of the gravitational wave power spectrum somewhat. after bh formation, neutrinos help clear out the matter near the bh poles, resulting in lower baryon-loaded surrounding debris. the neutrino luminosity resides in the range ∼1052 -53 erg s-1 once quasiequilibrium is achieved. comparing with the neutrino-free models, we observe that the inclusion of neutrinos yields similar ejecta masses and is inefficient in carrying off additional angular momentum. | jet launching from binary neutron star mergers: incorporating neutrino transport and magnetic fields |
einstein's theory of general relativity is one of the pillars of modern physics. for decades, the theory has been mainly tested in the weak field regime with experiments in the solar system and observations of binary pulsars. thanks to a new generation of observational facilities, the past 5 years have seen remarkable changes in this field and there are now numerous efforts for testing general relativity in the strong field regime with black holes and neutron stars using different techniques. here i will review the work of my group at fudan university devoted to test general relativity with black hole x-ray data. | testing general relativity with black hole x-ray data: a progress report |
recently, some fast radio burst (frb) repeaters were reported to exhibit complex, diverse variations of faraday rotation measures (rms), which implies that they are surrounded by an inhomogeneous, dynamically evolving, magnetized environment. we systematically investigate some possible astrophysical processes that may cause rm variations of an frb repeater. the processes include (1) a supernova remnant (snr) with a fluctuating medium; (2) a binary system with stellar winds from a massive/giant star companion or stellar flares from a low-mass star companion; (3) a pair plasma medium from a neutron star (including pulsar winds, pulsar wind nebulae, and magnetar flares); (4) outflows from a massive black hole. for the snr scenario, a large relative rm variation within a few years requires that the snr is young with a thin and local anisotropic shell, or the size of dense gas clouds in interstellar/circumstellar medium around the snr is extremely small. if the rm variation is caused by the companion medium in a binary system, it is more likely from the stellar winds of a massive/giant star companion. the rm variation contributed by stellar flares from a low-mass star is disfavored, because this scenario predicts an extremely large relative rm variation during a short period of time. the scenarios invoking a pair plasma from a neutron star can be ruled out due to their extremely low rm contributions. outflows from a massive black hole could provide a large rm variation if the frb source is in the vicinity of the black hole. | faraday rotation measure variations of repeating fast radio burst sources |
we perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (wd-ns binaries), and measure the specific angular momentum of material lost from the binary in disc winds. by incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. we find that only binaries containing helium white dwarfs (wds) with masses less than a critical mass of mwd, crit = 0.2 m⊙ undergo stable mass transfer and evolve into ultracompact x-ray binaries. systems with higher mass wds experience unstable mass transfer, which leads to tidal disruption of the wd. our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-eddington mass-transfer rates. we find that the eccentricities expected for wd-ns binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. our results are broadly consistent with the observed numbers of both ultracompact x-ray binaries and radio pulsars with wd companions. the observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass wds. | mass transfer in white dwarf-neutron star binaries |
we investigate constraints on the spectral index of primordial gravitational waves (gws), paying particular attention to a blue-tilted spectrum. such constraints can be used to test a certain class of models of the early universe. we investigate observational bounds from ligo+virgo, pulsar timing and big bang nucleosynthesis, taking into account the suppression of the amplitude at high frequencies due to reheating after inflation and also late-time entropy production. constraints on the spectral index are presented by changing values of parameters such as reheating temperatures and the amount of entropy produced at late time. we also consider constraints under the general modeling approach which can approximately describe various scenarios of the early universe. we show that the constraints on the blue spectral tilt strongly depend on the underlying assumption and, in some cases, a highly blue-tilted spectrum can still be allowed. | blue-tilted tensor spectrum and thermal history of the universe |
we report on the analysis of nustar observations of the be-transient x-ray pulsar v 0332+53 during the giant outburst in 2015 and another minor outburst in 2016. we confirm the cyclotron-line energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. we discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-induced decay of the neutron stars magnetic field. at lower luminosities, we find for the first time a hint of departure from the anticorrelation of line energy with flux, which we interpret as a transition from super- to sub-critical accretion associated with the disappearance of the accretion column. finally, we confirm and briefly discuss the orbital modulation observed in the outburst light curve of the source. | luminosity dependence of the cyclotron line and evidence for the accretion regime transition in v 0332+53 |
we present the orbital solution for the newly discovered transient be x-ray binary swift j0243.6+6124 based on the data from the gamma-ray burst monitor on board fermi obtained during the october 2017 outburst. we model the doppler induced and intrinsic spin variations of the neutron star assuming that the latter is driven by accretion torque, and we discuss the implications of the observed spin variations for the parameters of the neutron star and the binary. in particular, we conclude that the neutron star must be strongly magnetized, and estimate the distance to the source at 5 kpc. | orbit and intrinsic spin-up of the newly discovered transient x-ray pulsar swift j0243.6+6124 |
we have observed a large glitch in the crab pulsar (psr b0531+21). the glitch occurred around mjd 58064 (2017 november 8) when the pulsar underwent an increase in the rotation rate of δν = 1.530 × 10-5 hz, corresponding to a fractional increase of δν/ν = 0.516 × 10-6, making this event the largest glitch ever observed in this source. due to our high-cadence and long-dwell time observations of the crab pulsar, we are able to partially resolve a fraction of the total spin-up of the star. this delayed spin-up occurred over a time-scale of ∼1.7 d and is similar to the behaviour seen in the 1989 and 1996 large crab pulsar glitches. the spin-down rate also increased at the glitch epoch by δ \dot{ν } / \dot{ν } = 7 × 10^{-3}. in addition to being the largest such event observed in the crab, the glitch occurred after the longest period of glitch inactivity since at least 1984 and we discuss a possible relationship between glitch size and waiting time. no changes to the shape of the pulse profile were observed near the glitch epoch at 610 or 1520 mhz, nor did we identify any changes in the x-ray flux from the pulsar. the long-term recovery from the glitch continues to progress as \dot{ν } slowly rises towards pre-glitch values. in line with other large crab glitches, we expect there to be a persistent change to \dot{ν }. we continue to monitor the long-term recovery with frequent, high-quality observations. | the largest glitch observed in the crab pulsar |
we present an algorithm to generalize a plethora of well-known solutions to einstein field equations describing spherically symmetric relativistic fluid spheres by relaxing the pressure isotropy condition on the system. by suitably fixing the model parameters in our formulation, we generate closed-form solutions which may be treated as an anisotropic generalization of a large class of solutions describing isotropic fluid spheres. from the resultant solutions, a particular solution is taken up to show its physical acceptability. making use of the current estimate of mass and radius of a known pulsar, the effects of anisotropic stress on the gross physical behaviour of a relativistic compact star is also highlighted. | anisotropic generalization of well-known solutions describing relativistic self-gravitating fluid systems: an algorithm |
in paper i of this series, we detected a significant value of the braking index (n) for 19 young, high- $\dot{e}$ radio pulsars using ∼10 yr of timing observations from the 64-m parkes radio telescope. here, we investigate this result in more detail using a bayesian pulsar timing framework to model timing noise and to perform selection to distinguish between models containing exponential glitch recovery and braking index signatures. we show that consistent values of n are maintained with the addition of substantial archival data, even in the presence of glitches. we provide strong arguments that our measurements are unlikely due to exponential recovery signals from unseen glitches even though glitches play a key role in the evolution of a pulsar's spin frequency. we conclude that, at least over decadal time-scales, the value of n can be significantly larger than the canonical 3 and discuss the implications for the evolution of pulsars. | timing of young radio pulsars - ii. braking indices and their interpretation |
wave effects are often neglected in microlensing studies; however, for coherent point-like sources, such as pulsars and fast radio bursts (frbs), wave effects will become important in their gravitational lensing. in this paper, we describe the wave-optics formalism, its various limits, and the conditions for which these limits hold. using the simple point lens as an example, we show that the frequency dependence of wave effects breaks degeneracies that are present in the usual geometric optics limit, and constructive interference results in larger magnifications further from the lens. this latter fact leads to a generic increase in cross-section for microlensing events in the wave-optics regime compared to the geometric optics regime. for realistic per cent-level spectral sensitivities, this leads to a relative boost in lensing cross-section of more than an order of magnitude. we apply the point-lens model to the lensing of frbs and pulsars and find that these radio sources will be lensed in the full wave-optics regime by isolated masses in the range of $0.1\!-\!100\,{\rm m}_\oplus$, which includes free-floating planets (ffps), whose einstein radius is smaller than the fresnel scale. more generally, the interference pattern allows an instantaneous determination of lens masses, unlike traditional microlensing techniques that only yield a mass inference from the event time-scale. | wave effects in the microlensing of pulsars and frbs by point masses |
fast radio bursts (frbs) are short-duration radio transients of unknown origin. thus far, they have been blindly detected at millisecond time-scales with dispersion measures (dms) between 110 and 2600 pc cm-3. however, the observed pulse width, dm, and even brightness distributions depend strongly on the time and frequency resolution of the detection instrument. spectral and temporal resolution also significantly affect frb detection rates, similar to beam size and system-equivalent flux density. i discuss the interplay between underlying frb properties and instrumental response, and provide a generic formalism for calculating the observed distributions of parameters given an intrinsic frb distribution, focusing on pulse width and dm. i argue that if there exist many frbs of duration <<1 ms (as with giant pulses from galactic pulsars) or events with high dm, they are being missed due to the deleterious effects of smearing. i outline how to optimize the spectral and temporal resolution for frb surveys that are throughput-limited. i also investigate how such effects may have been imprinted on the distributions of frbs at real telescopes, like the different observed dms at askap and parkes. finally, i discuss the impact of intrinsic correlations between frb parameters on detection statistics. | interpreting the distributions of frb observables |
we present a detailed study of the complex time-frequency structure of a sample of previously reported bursts of frb 121102 detected with the meerkat telescope in september 2019. the wide contiguous bandwidth of these observations have revealed a complex bifurcating structure in some bursts at 1250 mhz. when de-dispersed to their structure-optimized dispersion measures (dms), two of the bursts show a clear deviation from the cold plasma dispersion relationship below 1250 mhz. we find a differential dm of ${\sim }1{-}2~{\rm pc \, cm^{-3}}$ between the lower and higher frequency regions of each burst. we investigate the possibility of plasma lensing by gaussian lenses of ~10 au in the host galaxy, and demonstrate that they can qualitatively produce some of the observed burst morphologies. other possible causes for the observed frequency dependence, such as faraday delay, are also discussed. unresolved sub-components in the bursts, however, may have led to an incorrect dm determination. we hence advise exercising caution when considering bursts in isolation. we analyse the presence of two apparent burst pairs. one of these pairs is a potential example of upward frequency drift. the possibility that burst pairs are echoes is also discussed. the average structure-optimized dm is found to be $563.5\pm 0.2 (\text{sys}) \pm 0.8 (\text{stat})\, {\rm pc \, cm^{-3}}$ - consistent with the values reported in 2018. we use two independent methods to determine the structure-optimized dm of the bursts: the dm_phase algorithm and autocorrelation functions. the latter - originally developed for pulsar analysis - is applied to fast radio bursts for the first time in this paper. | an analysis of the time-frequency structure of several bursts from frb 121102 detected with meerkat |
psr j1906+0746 is a young pulsar in the relativistic binary with the second-shortest known orbital period, of 3.98 hr. we here present a timing study based on five years of observations, conducted with the five largest radio telescopes in the world, aimed at determining the companion nature. through the measurement of three post-keplerian orbital parameters, we find the pulsar mass to be 1.291(11) m ⊙, and the companion mass 1.322(11) m ⊙, respectively. these masses fit well in the observed collection of double neutron stars (dnss), but are also compatible with other systems where a young pulsar such as j1906+0746 is orbited by a white dwarf (wd). neither radio pulsations nor dispersion-inducing outflows that could have further established the companion nature were detected. we derive an h i-absorption distance, which indicates that an optical confirmation of a wd companion is very challenging. the pulsar is fading fast due to geodetic precession, limiting future timing improvements. we conclude that the young pulsar j1906+0746 is likely part of a dns, or is otherwise orbited by an older wd, in an exotic system formed through two stages of mass transfer. | the binary companion of young, relativistic pulsar j1906+0746 |
in a search with the parkes radio telescope of 56 unidentified fermi-large area telescope (lat) gamma-ray sources, we have detected 11 millisecond pulsars (msps), 10 of them discoveries, of which five were reported by kerr et al. we did not detect radio pulsations from six other pulsars now known in these sources. we describe the completed survey, which included multiple observations of many targets conducted to minimize the impact of interstellar scintillation, acceleration effects in binary systems, and eclipses. we consider that 23 of the 39 remaining sources may still be viable pulsar candidates. we present timing solutions and polarimetry for five of the msps and gamma-ray pulsations for psr j1903-7051 (pulsations for five others were reported in the second fermi-lat catalog of gamma-ray pulsars). two of the new msps are isolated and five are in \gt 1 day circular orbits with 0.2-0.3 {m}⊙presumed white dwarf companions. psr j0955-6150, in a 24 day orbit with a ≈ 0.25 {m}⊙companion but eccentricity of 0.11, belongs to a recently identified class of eccentric msps. psr j1036-8317 is in an 8 hr binary with a \gt 0.14 {m}⊙companion that is probably a white dwarf. psr j1946-5403 is in a 3 hr orbit with a \gt 0.02 {m}⊙companion with no evidence of radio eclipses. | parkes radio searches of fermi gamma-ray sources and millisecond pulsar discoveries |
extreme scattering events (eses) in the interstellar medium (ism) were first observed in regular flux measurements of compact extragalactic sources. they are characterized by a flux variation over a period of weeks, suggesting the passage of a “diverging plasma lens” across the line of sight (los). modeling the refraction of such a lens indicates that the structure size must be of the order of au and the electron density of the order of 10s of cm-3. similar structures have been observed in measurements of pulsar intensity scintillation and group delay. here we report observations of two eses, showing increases in both intensity scintillation and dispersion made with the parkes pulsar timing array. these allow us to make more complete models of the ese, including an estimate of the “outer-scale” of the turbulence in the plasma lens. these observations clearly show that the ese structure is fully turbulent on an au scale. they provide some support for the idea that the structures are extended along the los, such as would be the case for a scattering shell. the dispersion measurements also show a variety of au scale structures that would not be called eses, yet involve electron density variations typical of eses and likely have the same origin. | pulsar observations of extreme scattering events |
anisotropic bursts of gravitational radiation produced by events such as supermassive black hole mergers leave permanent imprints on space. such gravitational wave `memory' (gwm) signals are, in principle, detectable through pulsar timing as sudden changes in the apparent pulse frequency of a pulsar. if an array of pulsars is monitored as a gwm signal passes over the earth, the pulsars would simultaneously appear to change pulse frequency by an amount that varies with their sky position in a quadrupolar fashion. here, we describe a search algorithm for such events and apply the algorithm to approximately six years of data from the parkes pulsar timing array. we find no gwm events and set an upper bound on the rate for events which could have been detected. we show, using simple models of black hole coalescence rates, that this non-detection is not unexpected. | searching for gravitational wave memory bursts with the parkes pulsar timing array |
binary neutron star (bns) mergers are the leading model to explain the phenomenology of short gamma-ray bursts (sgrbs). recent observations of long-lasting x-ray afterglows of sgrbs challenge standard paradigms and indicate that in a large fraction of events a long-lived neutron star (ns) may be formed rather than a black hole. understanding the mechanisms underlying these afterglows is necessary in order to address the open questions concerning the nature of sgrb central engines. however, recent theoretical progress has been hampered by the fact that the timescales of interest for the afterglow emission are inaccessible to numerical relativity simulations. here we present a detailed model to bridge the gap between numerical simulations of the merger process and the relevant timescales for the afterglows, assuming that the merger results in a long-lived ns. this model is formulated in terms of a set of coupled differential equations that follow the evolution of the post-merger system and predict its electromagnetic (em) emission in a self-consistent way, starting from initial data that can be extracted from bns merger simulations. the model presented here also allows us to search for suitable em counterparts for multimessenger astronomy, which is expected to become reality within the next few years thanks to ground-based gw detectors such as advanced ligo and virgo. this paper discusses the formulation and implementation of the model. in a companion paper, we employ this model to predict the em emission from ∼ {10}-2 to ∼ {10}7 {{s}} after a bns merger and discuss the implications in the context of sgrbs and multimessenger astronomy. | electromagnetic emission from long-lived binary neutron star merger remnants. i. formulation of the problem |
cosmological observations precisely measure primordial variations in the density of the universe at megaparsec and larger scales, but much smaller scales remain poorly constrained. however, sufficiently large initial perturbations at small scales can lead to an abundance of ultradense dark matter minihalos that form during the radiation epoch and survive into the late-time universe. because of their early formation, these objects can be compact enough to produce detectable microlensing signatures. we investigate whether the eros, ogle, and hsc surveys can probe these halos by fully accounting for finite source size and extended lens effects. we find that current data may already constrain the amplitudes of primordial curvature perturbations in a new region of parameter space, but this conclusion is strongly sensitive to yet undetermined details about the internal structures of these ultradense halos. under optimistic assumptions, current and future hsc data would constrain a power spectrum that features an enhancement at scales k ∼107/mpc , and an amplitude as low as pζ≃10-4 may be accessible. this is a particularly interesting regime because it connects to primordial black hole formation in a portion of the ligo/virgo/kagra mass range and the production of scalar-induced gravitational waves in the nanohertz frequency range reachable by pulsar timing arrays. these prospects motivate further study of the ultradense halo formation scenario to clarify their internal structures. | lensing constraints on ultradense dark matter halos |
this paper is devoted to evaluating exact anisotropic spherical solutions for static self-gravitating systems through extended geometric deformation decoupling technique. for this purpose, we consider an isotropic tolman iv solution and extend it to anisotropic domain by transforming both temporal as well as radial metric potentials. to examine the physical viability and stability of interior anisotropic solutions, we plot energy bounds, tov equation, causality condition and adiabatic index for the stars her x-i and psr j 1416-2230. it is found that both obtained models show realistic behavior as they fulfill all physical constraints as well as stability criterion. we conclude that the extended gravitational decoupling approach provides more proficient results to discuss the interior configuration of stellar structures. | anisotropic spherical solutions through extended gravitational decoupling approach |
nasa's nicer telescope has recently provided evidence for non-dipolar magnetic field structures in rotation-powered millisecond pulsars. these stars are assumed to have gone through a prolonged accretion spin-up phase, begging the question of what accretion flows on to stars with complex magnetic fields would look like. we present results from a suite of general relativistic magnetohydrodynamic simulations of accreting neutron stars for dipole, quadrupole, and quadrudipolar stellar field geometries. this is a first step towards simulating realistic hotspot shapes in a general relativistic framework to understand hotspot variability in accreting millisecond pulsars. we find that the location and size of the accretion columns resulting in hotspots changes significantly depending on initial stellar field strength and geometry. we also find that the strongest contributions to the stellar torque are from disc-connected field lines and the pulsar wind, leading to spin-down in almost the entire parameter regime explored here. we further analyse angular momentum transport in the accretion disc due to large-scale magnetic stresses, turbulent stresses, and wind and compressible effects which we identify with convective motions. the disc collimates the initial open stellar flux forming jets. for dipoles, the disc-magnetosphere interaction can either enhance or reduce jet power compared to the isolated case. however for quadrupoles, the disc always leads to an enhanced net open flux making the jet power comparable to the dipolar case. we discuss our results in the context of observed neutron star jets and provide a viable mechanism to explain radio power both in the low- and high-magnetic field case. | grmhd simulations of accreting neutron stars with non-dipole fields |
with the ever-increasing sensitivity and timing baselines of modern radio telescopes, a growing number of pulsars are being shown to exhibit transitions in their rotational and radio emission properties. in many of these cases, the two are correlated with pulsars assuming a unique spin-down rate ($\dot{\nu }$) for each of their specific emission states. in this work, we revisit 17 radio pulsars previously shown to exhibit spin-down rate variations. using a gaussian process regression (gpr) method to model the timing residuals and the evolution of the profile shape, we confirm the transitions already observed and reveal new transitions in 8 yr of extended monitoring with greater time resolution and enhanced observing bandwidth. we confirm that seven of these sources show emission-correlated $\dot{\nu }$ transitions ($\delta \dot{\nu }$), and we characterize this correlation for one additional pulsar, psr b1642-03. we demonstrate that gpr is able to reveal extremely subtle profile variations given sufficient data quality. we also corroborate the dependence of $\delta \dot{\nu }$ amplitude on $\dot{\nu }$ and pulsar characteristic age. linking $\delta \dot{\nu }$ to changes in the global magnetospheric charge density δρ, we speculate that $\dot{\nu }$ transitions associated with large δρ values may be exhibiting detectable profile changes with improved data quality, in cases where they have not previously been observed. | long-term rotational and emission variability of 17 radio pulsars |
previous studies in low pressure magnetized capacitively coupled radio frequency (rf) plasmas operated in argon with optimized geometric reactor symmetry have shown that the magnetic asymmetry effect (mae) allows to control the particle flux energy distributions at the electrodes, the plasma symmetry, and the dc self-bias voltage by tuning the magnetron-like magnetic field adjacent to one electrode (oberberg et al 2019 plasma sources sci. technol. 28 115021; oberberg et al 2018 plasma sources sci. technol. 27 105018). in this way non-linear electron resonance heating (nerh) induced via the self-excitation of the plasma series resonance (psr) was also found to be controllable. such plasma sources are frequently used for reactive rf magnetron sputtering, but the discharge conditions used for such applications are significantly different compared to those studied previously. a high dc self-bias voltage (generated via a geometric reactor asymmetry) is required to realize a sufficiently high ion bombardment energy at the target electrode and a reactive gas must be added to deposit ceramic compound layers. thus in this work, the mae is investigated experimentally in a geometrically asymmetric capacitively coupled rf discharge driven at 13.56 mhz and operated in mixtures of argon and oxygen. the dc self-bias, the symmetry parameter, the time resolved rf current, the plasma density, and the mean ion energy at the grounded electrode are measured as a function of the driving voltage amplitude and the magnetic field at the powered electrode. results obtained in pure argon discharges are compared to measurements performed in argon with reactive gas admixture. the results reveal a dominance of the geometrical over the magnetic asymmetry. the dc self-bias voltage as well as the symmetry parameter are found to be only weakly influenced by a change of the magnetic field compared to previous results obtained in a geometrically more symmetric reactor. nevertheless, the magnetic field is found to provide the opportunity to control nerh magnetically also in geometrically asymmetric reactors. adding oxygen does not alter these discharge properties significantly compared to a pure argon discharge. | the magnetic asymmetry effect in geometrically asymmetric capacitively coupled radio frequency discharges operated in ar/o2 |
ecological environment quality assessment (eeq) is an important parameter for sustainable development and sometimes it's more valuable during industrialization and urbanization progress of the region. this study compares remote sensing ecological index (rsei) and ecological index (ei) and assesses eeq with the most popular pressure-state-response (psr) method based on a set of remote sensing and statistical indexes through a weight system in samara region russia. ei and rsei framed from 15 toand 4 indicators respectively and in the psr framework natural-human pressure and health state assessed based on 12 and 3 indicators respectively and in the last response index developed from pressure and state indicators. results indicate that rsei and ei showed very strong comparability in the ecological sense but ei reflect more effectively eeq changes than rsei. finally this study explores all important effective indicators developed from remote sensing for eco-environment quality assessment, which provide a strong decision making base for sustainable development as well as provide a new and latest technological support for long term comprehensive mapping, monitoring and assessment of environment. | eco-environmental quality assessment based on pressure-state-response framework by remote sensing and gis |
we evaluate the performance of four different machine learning (ml) algorithms: an artificial neural network multi-layer perceptron (ann mlp), adaboost, gradient boosting classifier (gbc), and xgboost, for the separation of pulsars from radio frequency interference (rfi) and other sources of noise, using a dataset obtained from the post-processing of a pulsar search pipeline. this dataset was previously used for the cross-validation of the spinn-based machine learning engine, obtained from the reprocessing of the htru-s survey data (morello et al., 2014). we have used the synthetic minority over-sampling technique (smote) to deal with high-class imbalance in the dataset. we report a variety of quality scores from all four of these algorithms on both the non-smote and smote datasets. for all the above ml methods, we report high accuracy and g-mean for both the non-smote and smote cases. we study the feature importances using adaboost, gbc, and xgboost and also from the minimum redundancy maximum relevance approach to report algorithm-agnostic feature ranking. from these methods, we find that the signal to noise of the folded profile to be the best feature. we find that all the ml algorithms report fprs about an order of magnitude lower than the corresponding fprs obtained in morello et al. (2014), for the same recall value. | separation of pulsar signals from noise using supervised machine learning algorithms |
the soft-gamma repeater swift j1818.0-1607 is only the fifth magnetar found to exhibit pulsed radio emission. using the ultra-wideband low receiver system of the parkes radio telescope, we conducted a 3 hr observation of swift j1818.0-1607. folding the data at a rotation period of p = 1.363 s, we obtained wideband polarization profiles and flux density measurements covering radio frequencies between 704 and 4032 mhz. after measuring, and then correcting for the pulsar's rotation measure of 1442.0 ± 0.2 rad m-2, we find the radio profile is between 80% and 100% linearly polarized across the wide observing band, with a small amount of depolarization at low frequencies that we ascribe to scatter broadening. we also measure a steep spectral index of $\alpha =-{2.26}_{-0.03}^{+0.02}$ across our large frequency range, a significant deviation from the flat or inverted spectra often associated with radio-loud magnetars. the steep spectrum and temporal rise in flux density bears some resemblance to the behavior of the magnetar-like, rotation-powered pulsar psr j1119-6127. this leads us to speculate that swift j1818.0-1607 may represent an additional link between rotation-powered pulsars and magnetars. | spectropolarimetric properties of swift j1818.0-1607: a 1.4 s radio magnetar |
pulsar timing experiments require high-fidelity template profiles in order to minimize the biases in pulse time-of-arrival (toa) measurements and their uncertainties. efforts to acquire more precise toas given the fixed effective area of telescopes, finite receiver noise, and limited integration time have led pulsar astronomers to the solution of implementing ultra-wideband receivers. this solution, however, has run up against the problem that pulse profile shapes evolve with frequency, which raises the question of how to properly measure and analyze toas obtained using template-matching methods. this paper proposes a new method for one facet of this problem, that of template profile generation, and demonstrates it on the well-timed millisecond pulsar j1713+0747. specifically, we decompose pulse profile evolution into a linear combination of basis eigenvectors, the coefficients of which change slowly with frequency such that their evolution is modeled simply by a sum of low-degree piecewise polynomial spline functions. these noise-free, high-fidelity, frequency-dependent templates can be used to make measurements of so-called “wideband toas” simultaneously with an estimate of the instantaneous dispersion measure. the use of wideband toas is becoming important for pulsar timing array experiments, as the volume of data sets comprised of conventional, subbanded toas are quickly becoming unwieldy for the bayesian analyses needed to uncover latent gravitational wave signals. although motivated by high-precision timing experiments, our technique is applicable in more general pulsar observations. | frequency-dependent template profiles for high-precision pulsar timing |
galactic electron density distribution models are crucial tools for estimating the impact of the ionised interstellar medium on the impulsive signals from radio pulsars and fast radio bursts. the two prevailing galactic electron density models (gedms) are ymw16 (yao et al. <xref rid="r50" ref-type="bibr">2017</xref>, apj, 835, 29) and ne2001 (cordes & lazio <xref rid="r10" ref-type="bibr">2002</xref>, arxiv e-prints, pp astro-ph/0207156). here, we introduce a software package pygedm which provides a unified application programming interface for these models and the yt20 (yamasaki & totani <xref rid="r49" ref-type="bibr">2020</xref>, apj, 888, 105) model of the galactic halo. we use pygedm to compute all-sky maps of galactic dispersion measure (dm) for ymw16 and ne2001 and compare the large-scale differences between the two. in general, ymw16 predicts higher dm values towards the galactic anticentre. ymw16 predicts higher dms at low galactic latitudes, but ne2001 predicts higher dms in most other directions. we identify lines of sight for which the models are most discrepant, using pulsars with independent distance measurements. ymw16 performs better on average than ne2001, but both models show significant outliers. we suggest that future campaigns to determine pulsar distances should focus on targets where the models show large discrepancies, so future models can use those measurements to better estimate distances along those line of sight. we also suggest that the galactic halo should be considered as a component in future gedms, to avoid overestimating the galactic dm contribution for extragalactic sources such as frbs. | a comparison of galactic electron density models using pygedm |
scalar-tensor theories (stts) are a widely studied alternative to general relativity (gr) in which gravity is endowed with an additional scalar degree of freedom. although severely constrained by solar system and pulsar timing experiments, there remains a large set of stts which are consistent with all present day observations. in this paper, we investigate the possibility of probing a yet unconstrained region of the parameter space of stts based on the fact that stability properties of highly compact neutron stars in these theories may radically differ from those in gr. | possibility of setting a new constraint to scalar-tensor theories |
pulsars are wonderful gravitational probes. their tiny size and stellar mass give their rotation periods a stability comparable to that of atomic frequency standards. this is especially true of the rapidly rotating "millisecond pulsars" (msps). many of these rapidly rotating pulsars are in orbit with another star, allowing pulsar timing to probe relativistic perturbations to the orbital motion. pulsars have provided the most stringent tests of theories of relativistic gravitation, especially in the strong-field regime, and have shown that einstein's general theory of relativity is an accurate description of the observed motions. many other gravitational theories are effectively ruled out or at least severely constrained by these results. msps can also be used to form a "pulsar timing array" (pta). ptas are galactic-scale interferometers that have the potential to directly detect nanohertz gravitational waves from astrophysical sources. orbiting super-massive black holes in the cores of distant galaxies are the sources most likely to be detectable. although no evidence for gravitational waves has yet been found in pta data sets, the latest limits are seriously constraining current ideas on galaxy and black-hole evolution in the early universe. | pulsars and gravity |
motivated by the recent suggestions that very massive pulsar (psr j0952-0607) and very light compact object (hess j1731-347) exist, in this article, we revisit the possibility of such objects being strange stars instead of the standard hadronic neutron stars. we study the possible presence of local anisotropy and how it affects the macroscopic properties of strange stars and compare our results with the recent constraints presented in the literature. we found that the presence of anisotropy increases the maximum mass, the radius of the canonical star, and its tidal deformability for positive values of λbl and the opposite for negative values. we also show that although we cannot rule out the possibility of very compact objects being standard hadronic neutron stars, strange stars easily fulfill most of the observational constraints. | anisotropic strange stars in the spotlight: unveiling constraints through observational data |
we introduce a new method for evaluating the oscillatory integrals which describe natural interference patterns. as an illustrative example of contemporary interest, we consider astrophysical plasma lensing of coherent sources like pulsars and fast radio bursts in radioastronomy. plasma lenses are known to occur near the source, in the interstellar medium, as well as in the solar wind and the earth's ionosphere. such lensing is strongest at long wavelengths hence it is generally important to go beyond geometric optics and into the full wave optics regime. our computational method is a spinoff of new techniques two of us, and our collaborators, have developed for defining and performing lorentzian path integrals. cauchy's theorem allows one to transform a computationally fragile and expensive, highly oscillatory integral into an exactly equivalent sum of absolutely and rapidly convergent integrals which can be evaluated in polynomial time. we require only that it is possible to analytically continue the lensing phase, expressed in the integrated coordinates, into the complex domain. we give a first-principles derivation of the fresnel-kirchhoff integral, starting from feynman's path integral for a massless particle in a refractive medium. we then demonstrate the effectiveness of our method by computing the interference patterns of thom's caustic catastrophes, both in their "normal forms" and within a variety of more realistic, local lens models, over all wavelengths. our numerical method, implemented in a freely downloadable code, provides a fast, accurate tool for modeling interference patterns in radioastronomy and other fields of physics. | oscillatory path integrals for radio astronomy |
magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering high-energy flares in astrophysical sources including pulsar wind nebulae and black hole jets. reconnection is usually seen as the (essentially two-dimensional) nonlinear evolution of the tearing instability disrupting a thin current sheet. to test how this process operates in three dimensions, we conduct a comprehensive particle-in-cell simulation study comparing two- and three-dimensional evolution of long, thin current sheets in moderately magnetized, collisionless, relativistically hot electron-positron plasma, and find dramatic differences. we first systematically characterize this process in two dimensions, where classic, hierarchical plasmoid-chain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths and system sizes. we then show that three-dimensional (3-d) simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic drift-kink instability. thus, 3-d current sheet evolution is not always fundamentally classical reconnection with perturbing 3-d effects but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details and even stochastic events. it often yields slower but longer-lasting and ultimately greater magnetic energy release than in two dimensions. intriguingly, non-thermal particle acceleration is astonishingly robust, depending on the upstream magnetization and guide field, but otherwise yielding similar particle energy spectra in two and three dimensions. although the variety of underlying current sheet behaviours is interesting, the similarities in overall energy release and particle spectra may be more remarkable. | reconnection and particle acceleration in three-dimensional current sheet evolution in moderately magnetized astrophysical pair plasma |
there are several unexplored regions of the short-duration radio transient phase space. one such unexplored region is the luminosity gap between giant pulses (from pulsars) and cosmologically located fast radio bursts (frbs). the survey for transient astronomical radio emission 2 (stare2) is a search for such transients out to 7 mpc. stare2 has a field of view of 3.6 steradians and is sensitive to 1 millisecond transients above ∼300 kjy. with a two-station system we have detected and localized a solar burst, demonstrating that the pilot system is capable of detecting short duration radio transients. we found no convincing non-solar transients with duration between 65 μs and 34 ms in 200 days of observing, limiting with 95% confidence the all-sky rate of transients above ∼300 kjy to <40 sky-1 yr-1. if the luminosity function of frbs could be extrapolated down to 300 kjy for a distance of 10 kpc, then one would expect the rate to be ∼2 sky-1 yr-1. | stare2: detecting fast radio bursts in the milky way |
typically, neutron stars are discovered by observations at radio, x-ray or gamma-ray wavelengths. unlike radio pulsar surveys and x-ray observations, optical time-domain surveys can unveil and characterize exciting but less explored non-accreting and/or non-beaming neutron stars in binaries. here we report the discovery of such a neutron star candidate using the lamost spectroscopic survey. the candidate, designated lamost j112306.9 + 400736, is in a single-lined spectroscopic binary containing an optically visible m star. the star's large radial velocity variation and ellipsoidal variations indicate a relatively massive unseen companion. utilizing follow-up spectroscopy from the palomar 200 in. telescope and high-precision photometry from the transiting exoplanet survey satellite, we measure a companion mass of 1.2 4−0.03+0.03m⊙ . main-sequence stars with this mass are ruled out, leaving a neutron star or a massive white dwarf. although a massive white dwarf cannot be excluded, the lack of uv excess radiation from the companion supports the neutron star hypothesis. deep radio observations with the five-hundred-meter aperture spherical radio telescope (fast) yielded no detections of either pulsed or persistent emission. j112306.9 + 400736 is not detected in numerous x-ray and gamma-ray surveys, suggesting that the neutron star candidate is not currently accreting and pulsing. our work exemplifies the capability of discovering compact objects in non-accreting close binaries by synergizing optical time-domain spectroscopy and high-cadence photometry. | a dynamically discovered and characterized non-accreting neutron star-m dwarf binary candidate |
we have implemented prescriptions for modelling pulsars in the rapid binary population synthesis code compact object mergers: population astrophysics and statistics. we perform a detailed analysis of the double neutron star (dns) population, accounting for radio survey selection effects. the surface magnetic field decay time-scale (∼1000 myr) and mass-scale (∼0.02 m⊙) are the dominant uncertainties in our model. mass accretion during common envelope evolution plays a non-trivial role in recycling pulsars. we find a best-fitting model that is in broad agreement with the observed galactic dns population. though the pulsar parameters (period and period derivative) are strongly biased by radio selection effects, the observed orbital parameters (orbital period and eccentricity) closely represent the intrinsic distributions. the number of radio observable dnss in the milky way at present is about 2500 in our model, corresponding to approximately 10 per cent of the predicted total number of dnss in the galaxy. using our model calibrated to the galactic dns population, we make predictions for dns mergers observed in gravitational waves. the dns chirp mass distribution varies from 1.1 to 2.1 m⊙ and the median is found to be 1.14 m⊙. the expected effective spin χeff for isolated dnss is ≲0.03 from our model. we predict that 34 per cent of the current galactic isolated dnss will merge within a hubble time, and have a median total mass of 2.7 m⊙. finally, we discuss implications for fast radio bursts and post-merger remnant gravitational waves. | modelling double neutron stars: radio and gravitational waves |
we report the detection of gev γ-ray emission from the very-high-energy γ-ray source ver j2227+608 associated with the “tail” region of supernova remnant (snr) g106.3+2.7. the gev γ-ray emission is extended and spatially coincident with molecular clouds traced by co emission. the broadband gev to tev emission of ver j2227+608 can be well fitted by a single power-law function with an index of 1.90 ± 0.04, without obvious indication of spectral cutoff toward high energies. the pure leptonic model for the γ-ray emission can be marginally ruled out by the x-ray and tev data. in the hadronic model, the low energy content of crs and the hard γ-ray spectrum, in combination with the center bright source structure, suggest that ver j2227+608 may be powered by the pulsar wind nebula instead of shocks of the snr. and the cutoff energy of the proton distribution needs to be higher than ∼400 tev, which makes it an attractive pevatron candidate. future observations by the upcoming large high altitude air shower observatory and the cherenkov telescope array in the north could distinguish these models and constrain the maximum energy of cosmic rays in snrs. | ver j2227+608: a hadronic pevatron pulsar wind nebula? |
the magic stereoscopic system collected 69 hours of crab nebula data between october 2009 and april 2011. analysis of this data sample using the latest improvements in the magic stereoscopic software provided an unprecedented precision of spectral and night-by-night light curve determination at gamma rays. we derived a differential spectrum with a single instrument from 50 gev up to almost 30 tev with 5 bins per energy decade. at low energies, magic results, combined with fermi-lat data, show a flat and broad inverse compton peak. the overall fit to the data between 1 gev and 30 tev is not well described by a log-parabola function. we find that a modified log-parabola function with an exponent of 2.5 instead of 2 provides a good description of the data (χred2 = 35 / 26). using systematic uncertainties of the magic and fermi-lat measurements we determine the position of the inverse compton peak to be at (53 ±3stat +31syst -13syst) gev, which is the most precise estimation up to date and is dominated by the systematic effects. there is no hint of the integral flux variability on daily scales at energies above 300 gev when systematic uncertainties are included in the flux measurement. we consider three state-of-the-art theoretical models to describe the overall spectral energy distribution of the crab nebula. the constant b-field model cannot satisfactorily reproduce the vhe spectral measurements presented in this work, having particular difficulty reproducing the broadness of the observed ic peak. most probably this implies that the assumption of the homogeneity of the magnetic field inside the nebula is incorrect. on the other hand, the time-dependent 1d spectral model provides a good fit of the new vhe results when considering a 80 μg magnetic field. however, it fails to match the data when including the morphology of the nebula at lower wavelengths. | measurement of the crab nebula spectrum over three decades in energy with the magic telescopes |
we report on the polarization properties of two fast radio bursts (frbs): 151230 and 160102 discovered in the survey for pulsars and extragalactic radio bursts (superb) at the parkes radio telescope. frb 151230 is observed to be 6 ± 11 per cent circularly polarized and 35 ± 13 per cent linearly polarized with a rotation measure (rm) consistent with zero. conversely, frb 160102 is observed to have a circular polarization fraction of 30 ± 11 per cent, linear polarization fraction of 84 ± 15 per cent for rm = -221(6) radm-2, and the highest measured dispersion measure (2596.1 ± 0.3 pc cm-3) for an frb to date. we examine possible progenitor models for frb 160102 in extragalactic, non-cosmological and cosmological scenarios. after accounting for the galactic foreground contribution, we estimate the intrinsic rm to be -256(9) rad m-2 in the low-redshift case and ∼-2.4×102 rad m-2 in the high-redshift case. we assess the relative likeliness of these scenarios and how each can be tested. we also place constraints on the scattering measure and study the impact of scattering on the signal's polarization position angle. | the survey for pulsars and extragalactic radio bursts - iii. polarization properties of frbs 160102 and 151230 |
we explain the excess of the antiproton fraction recently reported by the ams-02 experiment by considering collisions between cosmic-ray protons accelerated by a local supernova remnant and the surrounding dense cloud. the same "pp collisions" provide the right ratio of daughter particles to fit the observed positron excess simultaneously in the natural model parameters. the supernova happened in relatively lower metallicity than the major cosmic-ray sources. the cutoff energy of electrons marks the supernova age of {∼ }105 years, while the antiproton excess may extend to higher energy. both antiproton and positron fluxes are completely consistent with our predictions in an earlier paper [y. fujita et al., phys. rev. d 80, 063003 (2009) [arxiv:0903.5298 [astro-ph.he]]]. | can we explain ams-02 antiproton and positron excesses simultaneously by nearby supernovae without pulsars or dark matter? |
resonances are among the clearest quantum mechanical signatures of scattering processes. previously, shape resonances and feshbach resonances have been observed in inelastic and reactive collisions involving atoms or diatomic molecules. structure in the integral cross section has been observed in a handful of elastic collisions involving polyatomic molecules. the present paper presents the observation of shape resonances in the reactive scattering of a polyatomic molecule, nh3. a merged-beam study of the gas phase he(3s1) + nh3 penning ionization reaction dynamics is described in the collision energy range 3.3 μev < ecoll < 10 mev. in this energy range, the reaction rate is governed by long-range attraction. peaks in the integral cross section are observed at collision energies of 1.8 mev and 7.3 mev and are assigned to ℓ = 15,16 and ℓ = 20,21 partial wave resonances, respectively. the experimental results are well reproduced by theoretical calculations with the short-range reaction probability psr = 0.035. no clear signature of the orbiting resonances is visible in the branching ratio between nh3+ and nh2+ formation. | observation of orbiting resonances in he(3s1) + nh3 penning ionization |
since its initial discovery, the fast radio burst (frb) frb 121102 has been found to be repeating with millisecond-duration pulses. very recently, 14 new bursts were detected by the green bank telescope during its continuous monitoring observations. in this paper, we show that the burst energy distribution has a power-law form which is very similar to the gutenberg-richter law of earthquakes. in addition, the distribution of burst waiting time can be described as a poissonian or gaussian distribution, which is consistent with earthquakes, while the aftershock sequence exhibits some local correlations. these findings suggest that the repeating frb pulses may originate from the starquakes of a pulsar. noting that the soft gamma-ray repeaters (sgrs) also exhibit such distributions, the frb could be powered by some starquake mechanisms associated with the sgrs, including the crustal activity of a magnetar or solidification-induced stress of a newborn strangeon star. these conjectures could be tested with more repeating samples. | frb 121102: a starquake-induced repeater? |
the alpha magnetic spectrometer (ams-02) has just published the unprecedentedly precise measurement of the cosmic electron and positron spectra. in this paper, we try to give a quantitative study on the ams-02 results by a global fitting to the electron and positron spectra, together with the updated positron fraction data. the markov chain monte carlo algorithm is adopted to do the fitting. the primary electron spectrum and the parameters for pulsars or dark matter that contribute extra positrons are determined simultaneously. we find that there is a hardening of the primary electron spectrum at ∼60 gev . with such a new feature at the background spectrum, both the pulsars and dark matter can explain the ams-02 results very well. the dark matter scenario shows a drop at the positron fraction at ∼300 gev but suffers very strong constraints from fermi γ -ray observations. the fitting results also suggest that the propagation model with convection may be more favored by the lepton data than the reacceleration model. | quantitative study of the ams-02 electron/positron spectra: implications for pulsars and dark matter properties |
aims: we present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 mhz with the lofar high-band antennas. based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium.methods: the polarisation data presented in this paper have been calibrated for the geometric-projection and beam-shape effects that distort the polarised information as detected with the lofar antennas. we have used rm synthesis to determine the amount of faraday rotation in the data at the time of the observations. the ionospheric contribution to the measured faraday rotation was estimated using a model of the ionosphere. to study the propagation effects, we have compared our low-frequency polarisation observations with archival data at 240, 400, 600, and 1400 mhz.results: the predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. the derived spectra offer only partial support for the expected effects of birefringence on the polarisation properties, with only about half of our sample being consistent with the model's predictions. it is noted that for some pulsars these measurements are contaminated by the effects of interstellar scattering. for a number of pulsars in our sample, we have observed significant variations in the amount of faraday rotation as a function of pulse phase, which is possibly an artefact of scattering. these variations are typically two orders of magnitude smaller than that observed at 1400 mhz by noutsos et al. (2009), for a different sample of southern pulsars. in this paper we present a possible explanation for the difference in magnitude of this effect between the two frequencies, based on scattering. finally, we have estimated the magnetospheric emission heights of low-frequency radiation from four pulsars, based on the phase lags between the flux-density and the pa profiles, and the theoretical framework of blaskiewicz et al. (1991, apj, 370, 643). these estimates yielded heights of a few hundred km; at least for psr b1133+16, this is consistent with emission heights derived based on radius-to-frequency mapping, but is up to a few times larger than the recent upper limit based on pulsar timing.conclusions: our work has shown that models, like magnetospheric birefringence, cannot be the sole explanation for the complex polarisation behaviour of pulsars. on the other hand, we have reinforced the claim that interstellar scattering can introduce a rotation of the pa with frequency that is indistinguishable from faraday rotation and also varies as a function of pulse phase. in one case, the derived emission heights appear to be consistent with the predictions of radius-to-frequency mapping at 150 mhz, although this interpretation is subject to a number of systematic uncertainties. | pulsar polarisation below 200 mhz: average profiles and propagation effects |
millisecond pulsars are neutron stars that attain their very fast rotation during a 108-109-yr-long phase of disk accretion of matter from a low-mass companion star1,2. they can be detected as accretion-powered millisecond x-ray pulsars if towards the end of this phase their magnetic field is strong enough to channel the in-flowing matter towards their magnetic poles3. when mass transfer is reduced or ceases altogether, pulsed emission generated by magnetospheric particle acceleration and powered by the star rotation is observed, preferentially in the radio4 and gamma-ray5 bands. a few transitional millisecond pulsars that swing between an accretion-powered x-ray pulsar regime and a rotationally powered radio pulsar regime in response to variations of the mass in-flow rate have been recently identified6,7. here, we report the detection of optical pulsations from a transitional millisecond pulsar. the pulsations were observed when the pulsar was surrounded by an accretion disk, and originated inside the magnetosphere or within a few hundreds of kilometres from it. energy arguments rule out reprocessing of accretion-powered x-ray emission and argue against a process related to accretion onto the pulsar polar caps; synchrotron emission of electrons in a rotation-powered pulsar magnetosphere8 seems more likely. | optical pulsations from a transitional millisecond pulsar |
geminga pulsar is surrounded by a multitev γ-ray halo radiated by the high-energy electrons and positrons accelerated by the central pulsar wind nebula (pwn). the angular profile of the γ-ray emission reported by high-altitude water cherenkov observatory indicates an anomalously slow diffusion for the cosmic-ray electrons and positrons in the halo region around geminga. in the paper we study the possible mechanism for the origin of the slow diffusion. at first, we consider the self-generated alfvén waves due to the streaming instability of the electrons and positrons released by geminga. however, even considering a very optimistic scenario for the wave growth, we find this mechanism does not work to account for the extremely slow diffusion at the present day, if taking the proper motion of geminga pulsar into account. the reason is straightforward as the pwn is too weak to generate enough high-energy electrons and positrons to stimulate strong turbulence at the late time. we then propose an assumption that the strong turbulence is generated by the shock wave of the parent supernova remnant (snr) of geminga. geminga may still be inside the snr, and we find that the snr can provide enough energy to generate the slow-diffusion circumstance. the tev haloes around psr b0656+14, vela x, and psr j1826-1334 may also be explained under this assumption. | possible origin of the slow-diffusion region around geminga |
we present an analytic model to describe the supermassive black hole binary (smbhb) merger rate in the universe with astrophysical observables: galaxy stellar mass function, pair fraction, merger time-scale, and black hole-host galaxy relations. we construct observational priors and compute the allowed range of the characteristic spectrum hc of the gravitational wave background (gwb) to be 10-16 < hc < 10-15 at a frequency of f = 1 yr-1. we exploit our parametrization to tackle the problem of astrophysical inference from pulsar timing array (pta) observations. we simulate a series of upper limits and detections and use a nested sampling algorithm to explore the parameter space. corroborating previous results, we find that the current pta non-detection does not place significant constraints on any observables; however, either future upper limits or detections will significantly enhance our knowledge of the smbhb population. if a gwb is not detected at a level of hc(f = 1 yr-1) = 10-17, our current understanding of galaxy and smbhb mergers is disfavoured at a 5σ level, indicating a combination of severe binary stalling, overestimating of the smbh-host galaxy relations, and extreme dynamical properties of merging smbhbs. conversely, future detections of a square kilometre array (ska)-type instrument will allow to constrain the normalization of the smbhb merger rate in the universe, the time between galaxy pairing and smbhb merging, the normalization of the smbh-host galaxy relations and the dynamical binary properties, including their eccentricity and density of stellar environment. | constraining astrophysical observables of galaxy and supermassive black hole binary mergers using pulsar timing arrays |
we present the discovery of a new optical/x-ray source likely associated with the fermi γ-ray source 4fgl j1408.6-2917. its high-amplitude periodic optical variability, large spectroscopic radial-velocity semiamplitude, evidence for optical emission lines and flaring, and x-ray properties together imply the source is probably a new black widow millisecond pulsar binary. we compile the properties of the 41 confirmed and suspected field black widows, finding a median secondary mass of 0.027 ± 0.003 m ⊙. considered jointly with the more massive redback millisecond pulsar binaries, we find that the "spider" companion mass distribution remains strongly bimodal, with essentially zero systems having companion masses of between ~0.07 and 0.1 m ⊙. x-ray emission from black widows is typically softer and less luminous than in redbacks, consistent with less efficient particle acceleration in the intrabinary shock in black widows, excepting a few systems that appear to have more efficient "redback-like" shocks. together black widows and redbacks dominate the census of the fastest spinning field millisecond pulsars in binaries with known companion types, making up ≳80% of systems with p spin < 2 ms. similar to redbacks, the neutron star masses in black widows appear on average significantly larger than the canonical 1.4 m ⊙, and many of the highest-mass neutron stars claimed to date are black widows with m ns ≳ 2.1 m ⊙. both of these observations are consistent with an evolutionary picture where spider millisecond pulsars emerge from short orbital period progenitors that had a lengthy period of mass transfer initiated while the companion was on the main sequence, leading to fast spins and high masses. | a new flaring black widow candidate and demographics of black widow millisecond pulsars in the galactic field |
we report the discovery of psr j0952-0607, a 707 hz binary millisecond pulsar that is now the fastest-spinning neutron star known in the galactic field (i.e., outside of a globular cluster). psr j0952-0607 was found using lofar at a central observing frequency of 135 mhz, well below the 300 mhz to 3 ghz frequencies typically used in pulsar searches. the discovery is part of an ongoing lofar survey targeting unassociated fermi-large area telescope γ-ray sources. psr j0952-0607 is in a 6.42 hr orbit around a very low-mass companion ({m}{{c}}≳ 0.02 {m}⊙ ), and we identify a strongly variable optical source, modulated at the orbital period of the pulsar, as the binary companion. the light curve of the companion varies by 1.6 mag from {r}{\prime }=22.2 at maximum to {r}{\prime }> 23.8, indicating that it is irradiated by the pulsar wind. swift observations place a 3σ upper limit on the 0.3-10 {kev} x-ray luminosity of {l}x< 1.1× {10}31 erg s-1 (using the 0.97 kpc distance inferred from the dispersion measure). though no eclipses of the radio pulsar are observed, the properties of the system classify it as a black widow binary. the radio pulsed spectrum of psr j0952-0607, as determined through flux density measurements at 150 and 350 mhz, is extremely steep with α ∼ -3 (where s\propto {ν }α ). we discuss the growing evidence that the fastest-spinning radio pulsars have exceptionally steep radio spectra, as well as the prospects for finding more sources like psr j0952-0607. | lofar discovery of the fastest-spinning millisecond pulsar in the galactic field |
we present a detailed, broad-band x-ray spectral analysis of the ultraluminous x-ray source (ulx) pulsar ngc 7793 p13, a known super-eddington source, utilizing data from the xmm-newton, nustar and chandra observatories. the broad-band xmm-newton+nustar spectrum of p13 is qualitatively similar to the rest of the ulx sample with broad-band coverage, suggesting that additional ulxs in the known population may host neutron star accretors. through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures ∼0.5 and 1.5 kev are required to fit the data below 10 kev, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. the characteristic radii of the thermal components appear to be comparable, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. we suggest a scenario in which the thick inner disc expected for super-eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of b ≲ 6 × 1012 g for the dipolar component of the central neutron star's magnetic field. evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ulxs. finally, we examine the spectrum exhibited by p13 during one of its unusual 'off' states. these data require both a hard power-law component, suggesting residual accretion on to the neutron star, and emission from a thermal plasma, which we argue is likely associated with the p13 system. | super-eddington accretion on to the neutron star ngc 7793 p13: broad-band x-ray spectroscopy and ultraluminous x-ray sources |
the atacama millimeter/submillimeter array (alma) phasing project (app) has developed and deployed the hardware and software necessary to coherently sum the signals of individual alma antennas and record the aggregate sum in very long baseline interferometry (vlbi) data exchange format. these beamforming capabilities allow the alma array to collectively function as the equivalent of a single large aperture and participate in global vlbi arrays. the inclusion of phased alma in current vlbi networks operating at (sub)millimeter wavelengths provides an order of magnitude improvement in sensitivity, as well as enhancements in u-v coverage and north-south angular resolution. the availability of a phased alma enables a wide range of new ultra-high angular resolution science applications, including the resolution of supermassive black holes on event horizon scales and studies of the launch and collimation of astrophysical jets. it also provides a high-sensitivity aperture that may be used for investigations such as pulsar searches at high frequencies. this paper provides an overview of the alma phasing system design, implementation, and performance characteristics. | the alma phasing system: a beamforming capability for ultra-high-resolution science at (sub)millimeter wavelengths |
a bayesian analysis of the possible behaviors of the dense matter equation of state informed by recent ligo-virgo as well as nicer measurements reveals that all the present observations are compatible with a fully nucleonic hypothesis for the composition of dense matter, even in the core of the most massive pulsar psr j0740+6620. under the hypothesis of a nucleonic composition, we extract the most general behavior of the energy per particle of symmetric matter and density dependence of the symmetry energy, compatible with the astrophysical observations as well as our present knowledge of low-energy nuclear physics from effective field theory predictions and experimental nuclear mass data. these results can be used as a null hypothesis to be confronted with future constraints on dense matter to search for possible exotic degrees of freedom. | the nuclear matter density functional under the nucleonic hypothesis |
in this work we investigate the link between high-mass x-ray binaries (hmxbs) and star formation in the large magellanic cloud (lmc), our nearest star-forming galaxy. using optical photometric data, we identify the most likely counterpart of 44 x-ray sources. among the 40 hmxbs classified in this work, we find 33 be/x-ray binaries (be-xrbs), and 4 supergiant xrbs. using this census and the published spatially resolved star formation history map of the lmc, we find that the hmxbs (and as expected the x-ray pulsars) are present in regions with star formation bursts ∼6-25 myr ago, in contrast to the small magellanic cloud (smc), for which this population peaks at later ages (∼25-60 myr ago). we also estimate the hmxb production rate to be equal to one system per ∼43.5× 10-3 m⊙ yr-1 or one system per ∼143m⊙ of stars formed during the associated star formation episode. therefore, the formation efficiency of hmxbs in the lmc is ∼17 times lower than that in the smc. we attribute this difference primarily in the different ages and metallicity of the hmxb populations in the two galaxies. we also set limits on the kicks imparted on the neutron star during the supernova explosion. we find that the time elapsed since the supernova kick is ∼3 times shorter in the lmc than the smc. this in combination with the average offsets of the hmxbs from their nearest star clusters results in ∼4 times faster transverse velocities for hmxbs in the lmc than in the smc. | star formation history and x-ray binary populations: the case of the large magellanic cloud |
accreting neutron stars can power a wide range of astrophysical phenomena including short- and long-duration gamma-ray bursts, ultra-luminous x-ray sources, and x-ray binaries. numerical simulations are a valuable tool for studying the accretion-disk-magnetosphere interaction that is central to these problems, most clearly for the recently discovered transitional millisecond pulsars. however, magnetohydrodynamic (mhd) methods, widely used for simulating accretion, have difficulty in highly magnetized stellar magnetospheres, while force-free methods, suitable for such regions, cannot include the accreting gas. we present an mhd method that can stably evolve essentially force-free, highly magnetized regions, and describe the first time-dependent relativistic simulations of magnetized accretion onto millisecond pulsars. our axisymmetric general-relativistic mhd simulations for the first time demonstrate how the interaction of a turbulent accretion flow with a pulsar’s electromagnetic wind can lead to the transition of an isolated pulsar to the accreting state. this transition naturally leads to the formation of relativistic jets, whose power can greatly exceed the power of the isolated pulsar’s wind. if the accretion rate is below a critical value, the pulsar instead expels the accretion stream. more generally, our simulations produce for the first time the four possible accretion regimes, in order of decreasing mass accretion rate: (a) crushed magnetosphere and direct accretion; (b) magnetically channeled accretion onto the stellar poles; (c) the propeller state, where material enters through the light cylinder but is prevented from accreting by the centrifugal barrier; (d) almost perfect exclusion of the accretion flow from the light cylinder by the pulsar wind. | general-relativistic simulations of four states of accretion onto millisecond pulsars |
observations by the fermi-lat have uncovered a bright, spherically symmetric excess surrounding the center of the milky way galaxy. the spectrum of the γ -ray excess peaks sharply at an energy ∼2 gev , exhibiting a hard spectrum at lower energies, and falls off quickly above an energy ∼5 gev . the spectrum of the excess above ∼10 gev is potentially an important discriminator between different physical models for its origin. we focus our study on observations of the γ -ray excess at energies exceeding 10 gev, finding: (1) a statistically significant excess remains in the energy range 9.5-47.5 gev, which is not degenerate with known diffuse emission templates such as the fermi bubbles, (2) the radial profile of the excess at high energies remains relatively consistent with data near the spectral peak (3) the data above ∼5 gev prefer a slightly greater ellipticity with a major axis oriented perpendicular to the galactic plane. using the recently developed non-poissonian template fit, we find mild evidence for a point-source origin for the high-energy excess, although given the statistical and systematic uncertainties we show that a smooth origin of the high-energy emission cannot be ruled out. we discuss the implication of these findings for pulsar and dark matter models of the γ -ray excess. finally we provide a number of updated measurements of the γ -ray excess, utilizing novel diffuse templates and the pass 8 data set. | high-energy tail of the galactic center gamma-ray excess |
we present a comparative study of the low-frequency eclipses of spider (compact and irradiating binary) psrs b1957+20 and j1816+4510. combining these data with those of three other eclipsing systems we study the frequency dependence of the eclipse duration. psrs b1957+20 and j1816+4510 have similar orbital properties, but the companions to the pulsars have masses that differ by an order of magnitude. a dedicated campaign to simultaneously observe the pulsed and imaged continuum flux densities throughout the eclipses reveals many similarities between the excess material within the two binaries, irrespective of the companion star properties. the observations show that the pulsar fluxes are removed from the line of sight throughout the main body of the eclipses. for psr j1816 + 4510, we present the first direct evidence of an eclipse mechanism that transitions from one that removes the pulsar flux from the line of sight to one that merely smears out pulsations, and claim that this is a consequence of scattering in a tail of material flowing behind the companion. inferred mass-loss rates from the companion stars are found to be $\dot{m}_{\text{c}} \sim 10^{-12}$ and $\sim 2 \times 10^{-13}\,\mathrm{m}_\odot$ yr-1 for psr b1957+20 and psr j1816 + 4510, respectively; seemingly too low to evaporate the stars within hubble time. measurements of eclipse durations over a wide range of radio frequencies show a significant dependence of eclipse duration on frequency for all pulsars, with wider eclipses at lower frequencies. these results provide a marked improvement in the observational constraints available for theoretical studies of the eclipse mechanisms. | study of spider pulsar binary eclipses and discovery of an eclipse mechanism transition |
spinning neutron stars asymmetric with respect to their rotation axis are potential sources of continuous gravitational waves for ground-based interferometric detectors. in the case of known pulsars a fully coherent search, based on matched filtering, which uses the position and rotational parameters obtained from electromagnetic observations, can be carried out. matched filtering maximizes the signal-to-noise (snr) ratio, but a large sensitivity loss is expected in case of even a very small mismatch between the assumed and the true signal parameters. for this reason, narrow-band analysis methods have been developed, allowing a fully coherent search for gravitational waves from known pulsars over a fraction of a hertz and several spin-down values. in this paper we describe a narrow-band search of 11 pulsars using data from advanced ligo's first observing run. although we have found several initial outliers, further studies show no significant evidence for the presence of a gravitational wave signal. finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of the 11 targets over the bands searched; in the case of j1813-1749 the spin-down limit has been beaten for the first time. for an additional 3 targets, the median upper limit across the search bands is below the spin-down limit. this is the most sensitive narrow-band search for continuous gravitational waves carried out so far. | first narrow-band search for continuous gravitational waves from known pulsars in advanced detector data |
for about half a century, the radio pulsar population was observed to spin in the ~0.002-12 s range, with different pulsar classes having a spin-period evolution that differs substantially depending on their magnetic fields or past accretion history. the recent detection of several slowly rotating pulsars has reopened the long-standing question of the exact physics, and observational biases, driving the upper bound of the period range of the pulsar population. in this work, we perform a parameter study of the spin-period evolution of pulsars interacting with supernova fallback matter and specifically look at the fallback accretion disk scenario. depending on the initial conditions at formation, this evolution can differ substantially from the typical dipolar spin-down, resulting in pulsars that show spin periods longer than their coeval peers. by using general assumptions for the pulsar spin period and magnetic field at birth, initial fallback accretion rates, and including magnetic field decay, we find that very long spin periods (≳100 s) can be reached in the presence of strong, magnetar-like magnetic fields (≳1014 g) and moderate initial fallback accretion rates (~1022-1027 g s-1). in addition, we study the cases of two recently discovered periodic radio sources, the pulsar psr j0901-4046 (p = 75.9 s) and the radio transient gleam-x j162759.5-523504.3 (p = 1091 s), in light of our model. we conclude that the supernova fallback scenario could represent a viable channel to produce a population of long-period isolated pulsars that only recent observation campaigns are starting to unveil. | long-period pulsars as possible outcomes of supernova fallback accretion |
most neutron stars (nss) and black holes (bhs) are believed to be the final remnants in the evolution of massive stars. in this study, we propose a new formation channel for the formation of bhs and peculiar nss [specifically, magnetars and thorne-żytkow objects (t$\dot{\rm z}$os)], which we refer to as the core-merger-induced collapse (cmic) model. this model involves the merger during a common-envelope phase of an oxygen/neon/magnesium composition white dwarf and the core of a hydrogen-rich or helium-rich non-degenerate star, leading to the creation of peculiar new types of objects. the results of binary population synthesis simulations show that the cmic channel could make important contributions to the populations of (millisecond) pulsars, t$\dot{\rm z}$os, magnetars, and bhs. the possibility of superluminous supernovae powered by t$\dot{\rm z}$os, magnetars, and bhs formed through the cmic model is also being investigated. magnetars with immediate matter surroundings formed after the cmic might be good sources for fast radio bursts. | stellar core-merger-induced collapse: new formation pathways for black holes, thorne-żytkow objects, magnetars, and superluminous supernovae |
supermassive black hole binaries (smbhbs) should form frequently in galactic nuclei as a result of galaxy mergers. at subparsec separations, binaries become strong sources of low-frequency gravitational waves (gws), targeted by pulsar timing arrays. we used recent upper limits on continuous gws from the north american nanohertz observatory for gravitational waves (nanograv) 11 yr data set to place constraints on putative smbhbs in nearby massive galaxies. we compiled a comprehensive catalog of ~44,000 galaxies in the local universe (up to redshift ~0.05) and populated them with hypothetical binaries, assuming that the total mass of the binary is equal to the smbh mass derived from global scaling relations. assuming circular equal-mass binaries emitting at nanograv's most sensitive frequency of 8 nhz, we found that 216 galaxies are within nanograv's sensitivity volume. we ranked the potential smbhbs based on gw detectability by calculating the total signal-to-noise ratio such binaries would induce within the nanograv array. we placed constraints on the chirp mass and mass ratio of the 216 hypothetical binaries. for 19 galaxies, only very unequal-mass binaries are allowed, with the mass of the secondary less than 10% that of the primary, roughly comparable to constraints on an smbhb in the milky way. however, we demonstrated that the (typically large) uncertainties in the mass measurements can weaken the upper limits on the chirp mass. additionally, we were able to exclude binaries delivered by major mergers (mass ratio of at least 1/4) for several of these galaxies. we also derived the first limit on the density of binaries delivered by major mergers purely based on gw data. | the nanograv 11 yr data set: limits on supermassive black hole binaries in galaxies within 500 mpc |
the mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of r-process elements. discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (gw170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. but uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. in contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. we calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. for the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3d radiative transport simulations to produce the spectra. we show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. we compare our gamma spectra with the potential detection limits of next generation detectors, including the lunar occultation explorer (lox), the all-sky medium energy gamma-ray observatory (amego), and the compton spectrometer and imager (cosi). we show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra. | gamma rays from kilonova: a potential probe of r-process nucleosynthesis |
in this work, we present an anisotropic generalization of the buchdahl static stellar model by implementing the method of extended gravitational decoupling and further requirement of vanishing complexity (herrera, phys rev d 97:044010,2018). starting off with a general spherically symmetric static metric with two unknown gravitational potentials, we impose the condition of vanishing complexity which then reduces the problem to a single-generating metric function [contreras and stuchlik, eur. phys. j. c (2022) 82:706]. the buchdahl ansatz is then employed to obtain the complete gravitational behavior of the isotropic seed solution. the method of extended gravitational decoupling is thereafter utilized to obtain an anisotropic counterpart of the buchdahl perfect gravitating sphere. we subject our solution to rigorous physical tests to ensure that it serves as a viable candidate for compact objects such as neutron stars as well as pulsars. we show that the decoupling parameter plays a crucial role in determining the stability and regularity of several key physical features of the entire stellar model. a novel finding of our investigation is the energy exchange between the seed solution and the secondary source which we show is sensitive to the decoupling parameter. in addition, it has also been shown that the direction of energy flow depends on the radial distance of the shell from the center of the stellar configuration.in this work, we present an anisotropic generalization of the buchdahl static stellar model by implementing the method of extended gravitational decoupling and further requirement of vanishing complexity (herrera, phys rev d 97:044010,2018). starting off with a general spherically symmetric static metric with two unknown gravitational potentials, we impose the condition of vanishing complexity which then reduces the problem to a single-generating metric function [contreras and stuchlik, eur. phys. j. c (2022) 82:706]. the buchdahl ansatz is then employed to obtain the complete gravitational behavior of the isotropic seed solution. the method of extended gravitational decoupling is thereafter utilized to obtain an anisotropic counterpart of the buchdahl perfect gravitating sphere. we subject our solution to rigorous physical tests to ensure that it serves as a viable candidate for compact objects such as neutron stars as well as pulsars. we show that the decoupling parameter plays a crucial role in determining the stability and regularity of several key physical features of the entire stellar model. a novel finding of our investigation is the energy exchange between the seed solution and the secondary source which we show is sensitive to the decoupling parameter. in addition, it has also been shown that the direction of energy flow depends on the radial distance of the shell from the center of the stellar configuration. | complexity-free anisotropic solution of buchdahl's model and energy exchange between relativistic fluids by extended gravitational decoupling |
we present observations of 35 high spin-down energy radio pulsars using the meerkat telescope. polarization profiles and associated parameters are also presented. we derive the geometry for a selection of pulsars which show interpulse emission. we point out that, in several cases, these radio pulsars should also be seen in γ-rays but that improved radio timing is required to aid the high-energy detection. we discuss the relationship between the width of the radio profile and its high-energy detectability. finally, we reflect on the correlation between the spin-down energy and the radio polarization fraction and the implications this may have for γ-ray emission. | the thousand-pulsar-array programme on meerkat iv: polarization properties of young, energetic pulsars |
the question of what gravitational theory could supersede general relativity has been central in theoretical physics for decades. many disparate alternatives have been proposed motivated by cosmology, quantum gravity and phenomenological angles, and have been subjected to tests derived from cosmological, solar system and pulsar observations typically restricted to linearized regimes. gravitational waves from compact binaries provide new opportunities to probe these theories in the strongly gravitating/highly dynamical regimes. to this end however, a reliable understanding of the dynamics in such a regime is required. unfortunately, most of these theories fail to define well posed initial value problems, which prevents at face value from meeting such challenge. in this work, we introduce a consistent program able to remedy this situation. this program is inspired in the approach to "fixing" viscous relativistic hydrodynamics introduced by israel and stewart in the late 70's. we illustrate how to implement this approach to control undesirable effects of higher order derivatives in gravity theories and argue how the modified system still captures the true dynamics of the putative underlying theories in 3 +1 dimensions. we sketch the implementation of this idea in a couple of effective theories of gravity, one in the context of noncommutative geometry, and one in the context of chern-simons modified general relativity. | fixing extensions to general relativity in the nonlinear regime |
recent observations indicate that in a large fraction of binary neutron star (bns) mergers a long-lived neutron star (ns) may be formed rather than a black hole. unambiguous electromagnetic (em) signatures of such a scenario would strongly impact our knowledge on how short gamma-ray bursts (sgrbs) and their afterglow radiation are generated. furthermore, such em signals would have profound implications for multimessenger astronomy with joint em and gravitational-wave (gw) observations of bns mergers, which will soon become reality thanks to the ground-based advanced ligo/virgo gw detector network. here we explore such em signatures based on the model presented in a companion paper, which provides a self-consistent evolution of the post-merger system and its em emission up to ∼107 s. light curves and spectra are computed for a wide range of post-merger physical properties. we present x-ray afterglow light curves corresponding to the “standard” and the “time-reversal” scenario for sgrbs (prompt emission associated with the merger or with the collapse of the long-lived ns). the light curve morphologies include single and two-plateau features with timescales and luminosities that are in good agreement with swift observations. furthermore, we compute the x-ray signal that should precede the sgrb in the time-reversal scenario, the detection of which would represent smoking-gun evidence for this scenario. finally, we find a bright, highly isotropic em transient peaking in the x-ray band at ∼102-104 s after the bns merger with luminosities of lx ∼ 1046-1048 erg s-1. this signal represents a very promising em counterpart to the gw emission from bns mergers. | electromagnetic emission from long-lived binary neutron star merger remnants. ii. lightcurves and spectra |
we review observations of several classes of neutron-star-powered outflows: pulsar-wind nebulae (pwne) inside shell supernova remnants (snrs), pwne interacting directly with interstellar medium (ism), and magnetar-powered outflows. we describe radio, x-ray, and gamma-ray observations of pwne, focusing first on integrated spectral-energy distributions (seds) and global spectral properties. high-resolution x-ray imaging of pwne shows a bewildering array of morphologies, with jets, trails, and other structures. several of the 23 so far identified magnetars show evidence for continuous or sporadic emission of material, sometimes associated with giant flares, and a few "magnetar-wind nebula" have been recently identified. | pulsar-wind nebulae and magnetar outflows: observations at radio, x-ray, and gamma-ray wavelengths |
in this work, we report the discovery and characterization of psr j1411+2551, a new binary pulsar discovered in the arecibo 327 mhz drift pulsar survey. our timing observations of the radio pulsar in the system span a period of about 2.5 years. this timing campaign allowed a precise measurement of its spin period (62.4 ms) and its derivative (9.6 ± 0.7) × 10-20 s s-1 from these, we derive a characteristic age of >9.1 gyr and a surface magnetic field strength of <2.6 × 109 g. these numbers indicate that this pulsar was mildly recycled by accretion of matter from the progenitor of the companion star. the system has an eccentric (e = 0.17) 2.61 day orbit. this eccentricity allows a highly significant measurement of the rate of advance of periastron, \dot{ω } =0.07686+/- 0.00046^\circ {{yr}}-1. assuming general relativity accurately describes the orbital motion, this implies a total system mass m = 2.538 ± 0.022 m ⊙. the minimum companion mass is 0.92 m ⊙ and the maximum pulsar mass is 1.62 m ⊙. the large companion mass and the orbital eccentricity suggest that psr j1411+2551 is a double neutron star system; the lightest known to date including the dns merger gw170817. furthermore, the relatively low orbital eccentricity and small proper motion limits suggest that the second supernova had a relatively small associated kick; this and the low system mass suggest that it was an ultra-stripped supernova. | pulsar j1411+2551: a low-mass double neutron star system |
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