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because of the development of many-body theories of nuclear matter, the long-standing, open problem of the equation of state (eos) of dense matter may be understood in the near future through the confrontation of theoretical calculations with laboratory measurements of nuclear properties & reactions and increasingly accurate observations in astronomy. in this review, we focus on the following six aspects: 1) providing a survey of the quark mean-field (qmf) model, which consistently describes a nucleon and many-body nucleonic system from a quark potential; 2) applying qmf to both nuclear matter and neutron stars; 3) extending qmf formalism to the description of hypernuclei and hyperon matter, as well as hyperon stars; 4) exploring the hadron-quark phase transition and hybrid stars by combining the qmf model with the quark matter model characterized by the sound speed; 5) constraining interquark interactions through both the gravitational wave signals and electromagnetic signals of binary merger event gw170817; and 6) discussing further opportunities to study dense matter eos from compact objects, such as neutron star cooling and pulsar glitches. | neutron star equation of state: quark mean-field (qmf) modeling and applications |
primordial black holes (pbhs) can be produced by the perturbations that exit the horizon during the inflationary phase. while inflation models predict the power spectrum of the perturbations in fourier space, the pbh abundance depends on the probability distribution function of density perturbations in real space. to estimate the pbh abundance in a given inflation model, we must relate the power spectrum in fourier space to the probability density function in real space by coarse graining the perturbations with a window function. however, there are uncertainties on what window function should be used, which could change the relation between the pbh abundance and the power spectrum. this is particularly important in considering pbhs with mass 30 m⊙, which account for the ligo events because the required power spectrum is severely constrained by the observations. in this paper, we investigate how large an influence the uncertainties on the choice of a window function has over the power spectrum required for ligo pbhs. as a result, it is found that the uncertainties significantly affect the prediction for the stochastic gravitational waves induced by the second-order effect of the perturbations. in particular, the pulsar timing array constraints on the produced gravitational waves could disappear for the real-space top-hat window function. | primordial black holes and uncertainties in the choice of the window function |
the interpretation of pulsar rotational glitches, the sudden increase in spin frequency of neutron stars, is a half-century-old challenge. the common view is that glitches are driven by the dynamics of the stellar interior, and connect in particular to the interactions between a large-scale neutron superfluid and the other stellar components. this thesis is corroborated by observational data of glitches and the post-glitch response seen in pulsars' rotation, which often involves very long timescales, from months to years. as such, glitch observables combined with consistent models incorporating the rich physics of neutron stars-from the lattice structure of their crust to the equation of state for matter beyond nuclear densities-can be very powerful at placing limits on, and reduce uncertainties of, the internal properties. this review summarises glitch observations, current data, and recent analyses, and connects them to the underlying mechanisms and microphysical parameters in the context of the most advanced theoretical glitch models to date. | pulsar glitches: observations and physical interpretation |
we examine the possibility that the light companion in the highly asymmetric binary compact object coalescence event gw190814 is a hypernuclear star. we use density functional theory with functionals that have been tuned to the properties of λ hypernuclei as well as astrophysical constraints placed by the masses of the most massive millisecond pulsars, the mass-radius range inferred from the nicer experiment, and the binary neutron star merger event gw170817. we compute general-relativistic static and maximally rotating keplerian configurations of purely nucleonic and hypernuclear stars. we find that while nucleonic stars are broadly consistent with a neutron star being involved in gw190814, this would imply no new degrees of freedom in the dense matter up to 6.5 times the nuclear saturation density. allowing for hyperonization of dense matter, we find that the maximal masses of hypernuclear stars, even for maximal rapidly rotating configurations, are inconsistent with a stellar nature interpretation of the light companion in gw190814, implying that this event involved two black holes rather than a neutron star and a black hole. | confronting gw190814 with hyperonization in dense matter and hypernuclear compact stars |
current models of gamma-ray light curves in pulsars suffer from large uncertainties on the precise location of particle acceleration and radiation. here, we present an attempt to alleviate these difficulties by solving for the electromagnetic structure of the oblique magnetosphere, particle acceleration, and the emission of radiation self-consistently, using 3d spherical particle-in-cell simulations. we find that the low-energy radiation is synchro-curvature radiation from the polar-cap regions within the light cylinder. in contrast, the high-energy emission is synchrotron radiation that originates exclusively from the y-point and the equatorial current sheet where relativistic magnetic reconnection accelerates particles. in most cases, synthetic high-energy light curves contain two peaks that form when the current sheet sweeps across the observer's line of sight. we find clear evidence of caustics in the emission pattern from the current sheet. high-obliquity solutions can present up to two additional secondary peaks from energetic particles in the wind region accelerated by the reconnection-induced flow near the current sheet. the high-energy radiative efficiency depends sensitively on the viewing angle, and decreases with increasing pulsar inclination. the high-energy emission is concentrated in the equatorial regions where most of the pulsar spin-down is released and dissipated. these results have important implications for the interpretation of gamma-ray pulsar data. | modelling high-energy pulsar light curves from first principles |
the recently developed resummation technique known as renormalization group optimized perturbation theory (rgopt) is employed in the evaluation of the equation of state (eos) describing nonstrange cold quark matter at next-to leading order. inspired by recent investigations, which suggest that stable quark matter can be made only of up and down quarks, the mass-radius relation for two flavor pure quark stars is evaluated and compared with the predictions from perturbative qcd (pqcd) at next-to-next-to leading order. this comparison explicitly shows that by being imbued with renormalization group properties, and a variational optimization procedure, the method allows for an efficient resummation of the perturbative series. remarkably, when the renormalization scale is chosen so as to reproduce maximum mass stars with m =2 -2.3 m⊙ , one obtains a mass-radius curve compatible with the masses and radii of the pulsars psr j 0740 +6620 , psr j 0030 +0451 , and the compact object hess j1731-347. moreover, the scale dependence of the eos (and mass-radius relation) obtained with the rgopt is greatly improved when compared to that of pqcd. this seminal application to the description of quark stars shows that the rgopt represents a robust alternative to pqcd when describing compressed quark matter. | nonstrange quark stars within resummed qcd |
the 30-hz rotation rate of the crab pulsar has been monitored at jodrell bank observatory since 1984 and by other observatories before then. since 1968, the rotation rate has decreased by about 0.5 hz, interrupted only by sporadic and small spin-up events (glitches). 24 of these events have been observed, including a significant concentration of 15 occurring over an interval of 11 yr following mjd 50000. the monotonic decrease of the slowdown rate is partially reversed at glitches. this reversal comprises a step and an asymptotic exponential with a 320-d time constant, as determined in the three best-isolated glitches. the cumulative effect of all glitches is to reduce the decrease in slowdown rate by about 6 per cent. overall, a low mean braking index of 2.342(1) is measured for the whole period, compared with values close to 2.5 in intervals between glitches. removing the effects of individual glitches reveals an underlying power-law slowdown with the same braking index of 2.5. we interpret this value in terms of a braking torque due to a dipolar magnetic field in which the inclination angle between the dipole and rotation axes is increasing. there may also be further effects due to a monopolar particle wind or infalling supernova debris. | 45 years of rotation of the crab pulsar |
studies of fermi data indicate an excess of gev gamma rays around the galactic center (gc), possibly due to dark matter. we show that young gamma-ray pulsars can yield a similar signal. first, a high concentration of gc supernovae naturally leads to a population of kicked pulsars symmetric about the gc. second, while very-young pulsars with soft spectra reside near the galactic plane, pulsars with spectra that have hardened with age accumulate at larger angles. this combination, including unresolved foreground pulsars, traces the morphology and spectrum of the excess. | young pulsars and the galactic center gev gamma-ray excess |
pulsar timing arrays (ptas) provide a way to detect gravitational waves at nanohertz frequencies. in this band, the most likely signals are stochastic, with a power spectrum that rises steeply at lower frequencies. indeed, the observation of a common red noise process in pulsar-timing data suggests that the first credible detection of nanohertz-frequency gravitational waves could take place within the next few years. the detection process is complicated by the nature of the signals and the noise: the first observational claims will be statistical inferences drawn at the threshold of detectability. to demonstrate that gravitational waves are creating some of the noise in the pulsar-timing data sets, observations must exhibit the hellings and downs curve -- the angular correlation function associated with gravitational waves -- as well as demonstrating that there are no other reasonable explanations. to ensure that detection claims are credible, the international pulsar timing array (ipta) has a formal process to vet results prior to publication. this includes internal sharing of data and processing pipelines between different ptas, enabling independent cross-checks and validation of results. to oversee and validate any detection claim, the ipta has also created an eight-member detection committee (dc) which includes four independent external members. ipta members will only publish their results after a formal review process has concluded. this document is the initial dc checklist, describing some of the conditions that should be fulfilled by a credible detection. at the present time none of the ptas have a detection claim; therefore this document serves as a road map for the future. | the international pulsar timing array checklist for the detection of nanohertz gravitational waves |
a repeating source of fast radio bursts (frbs) is recently discovered from a globular cluster of m81. association with a globular cluster (or other old stellar systems) suggests that strongly magnetized neutron stars, which are the most likely objects responsible for frbs, are born not only when young massive stars undergo core-collapse, but also by mergers of old white dwarfs. we find that the fractional contribution to the total frb rate by old stellar populations is at least a few per cent, and the precise fraction can be constrained by frb searches in the directions of nearby galaxies, both star-forming and elliptical ones. using very general arguments, we show that the activity time of the m81-frb source is between 104 and 106 yr, and more likely of the order of 105 yr. the energetics of radio outbursts put a lower limit on the magnetic field strength of 10$^{13}\,$g, and the spin period $\gtrsim 0.2\,$s, thereby ruling out the source being a milli-second pulsar. the upper limit on the persistent x-ray luminosity (provided by chandra), together with the high frb luminosity and frequent repetitions, severely constrains (or rules out) the possibility that the m81-frb is a scaled-up version of giant pulses from galactic pulsars. finally, the 50-ns variability time of the frb light curve suggests that the emission is produced in a compact region inside the neutron star magnetosphere, as it cannot be accounted for when the emission is at distances $\gtrsim 10^{10}\rm \, cm$. | implications of a rapidly varying frb in a globular cluster of m81 |
supermassive black hole binary systems form in galaxy mergers and reside in galactic nuclei with large and poorly constrained concentrations of gas and stars. these systems emit nanohertz gravitational waves that will be detectable by pulsar timing arrays. here we estimate the properties of the local nanohertz gravitational-wave landscape that includes individual supermassive black hole binaries emitting continuous gravitational waves and the gravitational-wave background that they generate. using the 2 micron all-sky survey, together with galaxy merger rates from the illustris simulation project, we find that there are on average 91 ± 7 continuous nanohertz gravitational-wave sources, and 7 ± 2 binaries that will never merge, within 225 mpc. these local unresolved gravitational-wave sources can generate a departure from an isotropic gravitational-wave background at a level of about 20 per cent, and if the cosmic gravitational-wave background can be successfully isolated, gravitational waves from at least one local supermassive black hole binary could be detected in 10 years with pulsar timing arrays. | the local nanohertz gravitational-wave landscape from supermassive black hole binaries |
we report the real-time discovery of a fast radio burst (frb 131104) with the parkes radio telescope in a targeted observation of the carina dwarf spheroidal galaxy. the dispersion measure of the burst is 779 cm-3 pc, exceeding predictions for the maximum line-of-sight galactic contribution by a factor of 11. the temporal structure of the burst is characterized by an exponential scattering tail with a timescale of 2.0+0.8-0.5 ms at 1582 mhz that scales as frequency to the power -4.4+1.6-1.8 (all uncertainties represent 95% confidence intervals). we bound the intrinsic pulse width to be <0.64 ms due to dispersion smearing across a single spectrometer channel. searches in 78 hr of follow-up observations with the parkes telescope reveal no additional sporadic emission and no evidence for associated periodic radio emission. we hypothesize that the burst is associated with the carina dwarf galaxy. follow-up observations at other wavelengths are necessary to test this hypothesis. | a fast radio burst in the direction of the carina dwarf spheroidal galaxy |
we present “first-principles” relativistic particle-in-cell simulations of the oblique pulsar magnetosphere with pair formation. the magnetosphere starts to form with particles extracted from the surface of the neutron star. these particles are accelerated by surface electric fields and emit photons capable of producing electron-positron pairs. we inject secondary pairs at the locations of primary energetic particles whose energy exceeds the threshold for pair formation. we find solutions that are close to the ideal force-free magnetosphere with the y-point and current sheet. solutions with obliquities ≤40° do not show pair production in the open field line region because the local current density along the magnetic field is below the goldreich-julian value. the bulk outflow in these solutions is charge-separated, and pair formation happens in the current sheet and return current layer only. solutions with higher inclinations show pair production in the open field line region, with high multiplicity of the bulk flow and the size of the pair-producing region increasing with inclination. we observe the spin-down of the star to be comparable to mhd model predictions. the magnetic dissipation in the current sheet ranges between 20% for the aligned rotator and 3% for the orthogonal rotator. our results suggest that for low obliquity neutron stars with suppressed pair formation at the light cylinder, the presence of phenomena related to pair activity in the bulk of the polar region, e.g., radio emission, may crucially depend on the physics beyond our simplified model, such as the effects of curved spacetime or multipolar surface fields. | ab initio pulsar magnetosphere: three-dimensional particle-in-cell simulations of oblique pulsars |
although fifty years have passed since the discovery of radio pulsars, there is still no satisfactory understanding of how these amazing objects operate. while there has been significant progress in understanding the basic properties of radio pulsars, there is as yet no consensus on key issues, such as the nature of coherent radio emission or the conversion mechanism of the electromagnetic energy of the pulsar wind into particle energy. in this review, we present the main theoretical results on the magnetosphere of neutron stars. we formulate a number of apparently simple questions, which nevertheless remain unanswered since the very beginning of the field and which must be resolved before any further progress can be made. | radio pulsars: already fifty years! |
we review the role and properties of hyperons in finite and infinite nuclear systems. in particular, we present different production mechanisms of hypernuclei, as well as several aspects of hypernuclear γ-ray spectroscopy, and the weak decay modes of hypernuclei. then we discuss the construction of hyperon-nucleon and hyperon-hyperon interactions on the basis of the meson-exchange and chiral effective field theories. recent developments based on the so-called vlow k approach and lattice quantum chromodynamics will also be addressed. finally, we go over some of the effects of hyperons on the properties of neutron and proto-neutron stars with an emphasis on the so-called `hyperon puzzle', i.e. the problem of the strong softening of the equation of state, and the consequent reduction of the maximum mass, induced by the presence of hyperons, a problem which has become more intriguing and difficult to solve due the recent measurements of approximately 2m⊙ millisecond pulsars. we discuss some of the solutions proposed to tackle this problem. we also re-examine the role of hyperons on the cooling properties of newly born neutron stars and on the development of the so-called r-mode instability. | hyperons: the strange ingredients of the nuclear equation of state |
we construct a realistic supersymmetric model for superheavy metastable cosmic strings (css) that can be investigated in the current pulsar timing array (pta) experiments. we consider shifted $\mu$ hybrid inflation in which the symmetry breaking $su(4)_c \times su(2)_l \times u(1)_r\rightarrow su(3)_c\times su(2)_l \times u(1)_{b-l}\times u(1)_r$ proceeds along an inflationary trajectory such that the topologically unstable primordial monopoles are inflated away. the breaking of $u(1)_{b-l} \times u(1)_r \rightarrow u(1)_y$ after inflation ends yields the metastable css that generate the stochastic gravitational wave background (sgwb) which is consistent with the current pta data set. the scalar spectral index $n_s$ and the tensor to scalar ratio $r$ are also compatible with planck 2018. we briefly discuss both reheating and leptogenesis in this model. | supersymmetric hybrid inflation and metastable cosmic strings in $su(4)_c \\times su(2)_l \\times u(1)_r$ |
we describe how the observed polarization properties of an astronomical object are related to its intrinsic polarization properties and the finite temporal and spectral resolutions of the observing device. moreover, we discuss the effect that a scattering screen, with non-zero magnetic field, between the source and observer has on the observed polarization properties. we show that the polarization properties are determined by the ratio of observing bandwidth and coherence bandwidth of the scattering screen and the ratio of temporal resolution of the instrument and the variability time of screen, as long as the length over which the faraday rotation induced by the screen changes by ~π is smaller than the size of the screen visible to the observer. we describe the conditions under which a source that is 100 per cent linearly polarized intrinsically might be observed as partially depolarized, and how the source's temporal variability can be distinguished from the temporal variability induced by the scattering screen. in general, linearly polarized waves passing through a magnetized scattering screen can develop a significant circular polarization. we apply the work to the observed polarization properties of a few fast radio bursts (frbs), and outline potential applications to pulsars. | faraday depolarization and induced circular polarization by multipath propagation with application to frbs |
since fast radio bursts (frbs) were discovered, their precise origins have remained a mystery. multiwavelength observations of nearby frb sources provide one of the best ways to make rapid progress in our understanding of the enigmatic frb phenomenon. we present results from a sensitive, broadband multiwavelength x-ray and radio observational campaign of frb 20200120e, the closest known extragalactic repeating frb source. at a distance of 3.63 mpc, frb 20200120e resides in an exceptional location, within a ~10 gyr-old globular cluster in the m81 galactic system. we place deep limits on both the persistent x-ray luminosity and prompt x-ray emission at the time of radio bursts from frb 20200120e, which we use to constrain possible progenitors for the source. we compare our results to various classes of x-ray sources and transients. in particular, we find that frb 20200120e is unlikely to be associated with: ultraluminous x-ray bursts (ulxbs), similar to those observed from objects of unknown origin in other extragalactic globular clusters; giant flares, like those observed from galactic and extragalactic magnetars; or most intermediate flares and very bright short x-ray bursts, similar to those seen from magnetars in the milky way. we show that frb 20200120e is also unlikely to be powered by a persistent or transient ultraluminous x-ray (ulx) source or a young, extragalactic pulsar embedded in a crab-like nebula. we also provide new constraints on the compatibility of frb 20200120e with accretion-based frb models involving x-ray binaries and models that require a synchrotron maser process from relativistic shocks to generate frb emission. these results highlight the power that multiwavelength observations of nearby frbs can provide for discriminating between potential frb progenitor models. | multiwavelength constraints on the origin of a nearby repeating fast radio burst source in a globular cluster |
this paper investigates the concept of cracking and overturning to analyze the impact of local density perturbations on the stability of self-gravitating compact objects in the framework of f (r ,ϕ ,x ) theory of gravity, where r, ϕ , and x denote the ricci scalar, scalar potential, and kinetic term, respectively. in this context, we developed the hydrostatic equilibrium equation for spherically symmetric spacetime with anisotropic matter configuration and subsequently employed the krori barua technique. we then perturb the hydrostatic equilibrium state of the configuration by employing the local density perturbation technique, while taking into account the barotropic equation of state. to validate this technique, we employed it on different compact stars namely, her x-1, sax j1808.4-3658, 4u 1820-30, psr j1614-2230, vela x-1, and cen x-3, and found that all stars exhibit cracking or overturning for a specific range of model parameters. conclusively, this study emphasizes that the proposed cracking technique provides significant insights into the stability analysis of self-gravitating compact objects. | a comprehensive discussion for the identification of cracking points in f(r) theories of gravity |
an anomalous emission component at energies of a few gigaelectronvolts and located towards the inner galaxy is present in the fermi-lat data. at present, the two most promising explanations are the annihilation of dark matter particles or the presence of a large population of unresolved point sources, most probably millisecond pulsars, at the galactic centre. here, we report an analysis of the excess characteristics using almost eight years of pass 8 ultraclean fermi-lat data with skyfact—a tool that combines image reconstruction with template-fitting techniques. we find that an emission profile that traces stellar mass in the boxy and nuclear bulge is preferred over conventional dark matter profiles. a model including the bulge is favoured over a model with dark matter at 16σ. | the fermi-lat gev excess as a tracer of stellar mass in the galactic bulge |
we perform numerical simulations of gravitational waves (gws) induced by hydrodynamic and hydromagnetic turbulent sources that might have been present at cosmological quantum chromodynamic (qcd) phase transitions. for turbulent energies of about 4% of the radiation energy density, the typical scale of such motions may have been a sizable fraction of the hubble scale at that time. the resulting gws are found to have an energy fraction of about 10-9 of the critical energy density in the nhz range today and may already have been observed by the nanograv collaboration. this is further made possible by our findings of shallower spectra proportional to the square root of the frequency for nonhelical hydromagnetic turbulence. this implies more power at low frequencies than for the steeper spectra previously anticipated. the behavior toward higher frequencies depends strongly on the nature of the turbulence. for vortical hydrodynamic and hydromagnetic turbulence, there is a sharp drop of spectral gw energy by up to five orders of magnitude in the presence of helicity, and somewhat less in the absence of helicity. for acoustic hydrodynamic turbulence, the sharp drop is replaced by a power law decay, albeit with a rather steep slope. our study supports earlier findings of a quadratic scaling of the gw energy with the magnetic energy of the turbulence and inverse quadratic scaling with the peak frequency, which leads to larger gw energies under qcd conditions. | can we observe the qcd phase transition-generated gravitational waves through pulsar timing arrays? |
we perform a bayesian analysis of the maximum mass mtov of neutron stars with a quark core, incorporating the observational data from tidal deformability of the gw170817 binary neutron star merger as detected by ligo/virgo and the mass and radius of psr j0030+0451 as detected by the neutron star interior composition explorer. the analysis is performed under the assumption that the hadron-quark phase transition is of first order, where the low-density hadronic matter described in a unified manner by the soft qmf or the stiff dd2 equation of state (eos) transforms into a high-density phase of quark matter modeled by the generic "constant-sound-speed" parameterization. the mass distribution measured for the 2.14 m⊙ pulsar msp j0740+6620 is used as the lower limit on mtov. we find the most probable values of the hybrid star maximum mass are ${m}_{\mathrm{tov}}={2.36}_{-0.26}^{+0.49}\,{\text{}}{m}_{\odot }$ ( ${2.39}_{-0.28}^{+0.47}\,{\text{}}{m}_{\odot }$ ) for qmf (dd2), with an absolute upper bound around 2.85 m⊙, to the 90% posterior credible level. such results appear robust with respect to the uncertainties in the hadronic eos. we also discuss astrophysical implications of this result, especially on the postmerger product of gw170817, short gamma-ray bursts, and other likely binary neutron star mergers. | constraints on the maximum mass of neutron stars with a quark core from gw170817 and nicer psr j0030+0451 data |
fast radio burst (frb) source frb 20180916b exhibits a 16.33-day periodicity in its burst activity. it is as of yet unclear what proposed mechanism produces the activity, but polarization information is a key diagnostic. here we report on the polarization properties of 44 bursts from frb 20180916b detected between 2018 december and 2021 december by chime/frb, the frb project on the canadian hydrogen intensity mapping experiment. in contrast to previous observations, we find significant variations in the faraday rotation measure (rm) of frb 20180916b. over the 9-month period 2021 april and 2021 december we observe an apparent secular increase in rm of ~50 rad m-2 (a fractional change of over 40%) that is accompanied by a possible drift of the emitting band to lower frequencies. this interval displays very little variation in the dispersion measure (δdm ≲ 0.8 pc cm-3), which indicates that the observed rm evolution is likely produced from coherent changes in the faraday-active medium's magnetic field. burst-to-burst rm variations appear unrelated to the activity cycle phase. the degree of linear polarization of our burst sample (≳80%) is consistent with the negligible depolarization expected for this source in the 400-800 mhz bandpass of chime. frb 20180916b joins other repeating frbs in displaying substantial rm evolution. this is consistent with the notion that repeater progenitors may be associated with young stellar populations by their preferential occupation of dynamic magnetized environments commonly found in supernova remnants, in pulsar wind nebulae, or near high-mass stellar companions. | a large-scale magneto-ionic fluctuation in the local environment of periodic fast radio burst source frb 20180916b |
we report the properties of more than 800 bursts detected from the repeating fast radio burst (frb) source frb 20201124a with the five-hundred-meter aperture spherical radio telescope (fast) during an extremely active episode on utc 2021 september 25-28 in a series of four papers. in this second paper of the series, we study the energy distribution of 881 bursts (defined as significant signals separated by dips down to the noise level) detected in the first four days of our 19 hr observational campaign spanning 17 days. the event rate initially increased exponentially but the source activity stopped within 24 hr after the 4th day. the detection of 542 bursts in one hour during the fourth day marked the highest event rate detected from one single frb source so far. the bursts have complex structures in the time-frequency space. we find a double-peak distribution of the waiting time, which can be modeled with two log-normal functions peaking at 51.22 ms and 10.05 s, respectively. compared with the emission from a previous active episode of the source detected with fast, the second distribution peak time is smaller, suggesting that this peak is defined by the activity level of the source. we calculate the isotropic energy of the bursts using both a partial bandwidth and a full bandwidth and find that the energy distribution is not significantly changed. we find that an exponentially connected broken-power law function can fit the cumulative burst energy distribution well, with the lower and higher-energy indices being -1.22 ± 0.01 and -4.27 ± 0.23, respectively. assuming a radio radiative efficiency of ηr= 10-4, the total isotropic energy of the bursts released during the four days when the source was active is already 3.9 × 1046 erg, exceeding ~23% of the available magnetar dipolar magnetic energy. this challenges the magnetar models which invoke an inefficient radio emission (e.g., synchrotron maser models). | fast observations of an extremely active episode of frb 20201124a. ii. energy distribution |
we develop a model of the generation of coherent radio emission in the crab pulsar, magnetars, and fast radio bursts (frbs). emission is produced by a reconnection-generated beam of particles via a variant of the free electron laser mechanism, operating in a weakly turbulent, guide field-dominated plasma. we first consider nonlinear thomson scattering in a guide field-dominated regime, and apply it to explain emission bands observed in crab pulsar and in frbs. we consider particle motion in a combined field of the electromagnetic wave and the electromagnetic (alfvénic) wiggler. charge bunches, created via a ponderomotive force, compton/raman scatter the wiggler field coherently. the model is both robust to the underlying plasma parameters and succeeds in reproducing a number of subtle observed features: (i) emission frequencies depend mostly on the scale λtof turbulent fluctuations and the lorentz factor of the reconnection-generated beam, $\omega \sim {\gamma }_{b}^{2}(c/{\lambda }_{t})> -it is independent of the absolute value of the underlying magnetic field. (ii) the model explains both broadband emission and the presence of emission stripes, including multiple stripes observed in the high frequency interpulse of the crab pulsar. (iii) the model reproduces correlated polarization properties: the presence of narrow emission bands in the spectrum favors linear polarization, while broadband emission can have an arbitrary polarization. (iv) the mechanism is robust to the momentum spread of the particle in the beam. we also discuss a model of wigglers as nonlinear force-free alfvén solitons (light darts). | coherent emission in pulsars, magnetars, and fast radio bursts: reconnection-driven free electron laser |
within a confining quark matter model which considers phenomenologically the quark confinement and asymptotic freedom as well as the chiral symmetry restoration and quark deconfinement at high baryon density, we find that if the up-down quark matter (u d qm ) is more stable than nuclear matter and strange quark matter (sqm), the maximum mass of static quark stars with u d qm is 2.87 m⊙ under agreement with both the constraints on star tidal deformability from gravitational wave signal gw170817 and the mass-radius of psr j 0030 +0451 and psr j 0740 +6620 measured by nicer. in contrast, the conventional strange quark star with the sqm that is more stable than nuclear matter while the nuclear matter is more stable than u d qm , has a maximum static mass of only 1.87 m⊙ and its radius significantly deviates from nicer's constraint. our results thus provide circumstantial evidence suggesting the recently reported gw190814's secondary component with a mass of 2.59-0.09+0.08 m⊙ could be an up-down quark star. | gw190814: circumstantial evidence for up-down quark star |
we investigate the possibility that gw170817 was not the merger of two conventional neutron stars (ns), but involved at least one if not two hybrid stars with a quark matter core that might even belong to a third family of compact stars. to this end, we develop a bayesian analysis method for selecting the most probable equation of state (eos) under a set of constraints from compact star physics, which now also include the tidal deformability from gw170817 and the first result for the mass and radius determination for psr j0030+0451 by the nicer collaboration. we apply this method for the first time to a two-parameter family of hybrid eos based on the dd2 model with nucleonic excluded volume for hadronic matter and the color superconducting generalized nlnjl model for quark matter. the model has a variable onset density for deconfinement and can mimic the effects of pasta phases with the possibility of producing a third family of hybrid stars in the mass-radius diagram. the main findings of this study are that: (1) the presence of multiple configurations for a given mass (twins or even triples) corresponds to a set of disconnected lines in the λ1–λ2 diagram of tidal deformabilities for binary mergers, so that merger events from the same mass range may result in a probability landscape with different peak positions; (2) the bayesian analysis with the above observational constraints favors an early onset of the deconfinement transition, at masses of monset≤0.8m⊙ with an m–r relationship that in the range of observed neutron star masses is almost indistinguishable from that of a soft hadronic akmal, pandharipande, and ravenhall (apr) eos; (3) a few, yet fictitious measurements of the nicer experiment two times more accurate than the present value and a different mass and radius that would change the posterior likelihood so that hybrid eos with a phase transition onset in the range monset = 1.1–1.6 m⊙ would be favored. | was gw170817 a canonical neutron star merger? bayesian analysis with a third family of compact stars |
diffusive tev gamma-ray emissions have been recently discovered extending beyond the pulsar wind nebulae of a few middle-aged pulsars, implying that energetic electron/ positron pairs are escaping from the pulsar wind nebulae and radiating in the ambient interstellar medium. it has been suggested that these extended emissions constitute a distinct class of nonthermal sources, termed “pulsar halos”. in this paper, i will review the research progress on pulsar halos and discuss our current understanding on their physics, including the multiwavelength observations, different theoretical models, as well as implications for the origin of cosmic-ray positron excess and galactic diffuse gamma-ray emission. | the physics of pulsar halos: research progress and prospect |
gravitational-wave (gw) astronomy is transforming our understanding of the universe by probing phenomena invisible to electromagnetic observatories. a comprehensive exploration of the gw frequency spectrum is essential to fully harness this potential. remarkably, current methods have left the μ hz frequency band almost untouched. here, we show that this μ hz gap can be filled by searching for deviations in the orbits of binary systems caused by their resonant interaction with gws. in particular, we show that laser ranging of the moon and artificial satellites around the earth, as well as timing of binary pulsars, may discover the first gw signals in this band, or otherwise set stringent new constraints. to illustrate the discovery potential of these binary resonance searches, we consider the gw signal from a cosmological first-order phase transition, showing that our methods will probe models of the early universe that are inaccessible to any other near-future gw mission. we also discuss how our methods can shed light on the possible gw signal detected by nanograv, either constraining its spectral properties or even giving an independent confirmation. | bridging the μ hz gap in the gravitational-wave landscape with binary resonances |
gamma-ray data from the fermi large area telescope reveal an unexplained, apparently diffuse, signal from the galactic bulge1-3 that peaks near ~2 gev with an approximately spherical4 intensity profile ∝ r−2.4 (refs. 3,5), where r is the radial distance to the galactic centre, that extends to angular radial scales of at least ~10° and possibly to ~20° (refs. 6,7). the origin of this `galactic centre excess' (gce) has been debated, with proposed sources prominently including self-annihilating dark matter1,4 and a hitherto undetected population of millisecond pulsars (msps)8. however, the conventional channel for the generation of msps has been found to predict too many low-mass x-ray binary (lmxb) systems9 and, because of the expected large natal kicks, may not accommodate10 the close spatial correspondence11-13 between the gce signal and stars in the bulge. here we report a binary population synthesis (bps) forward model that demonstrates that an msp population arising from the accretion-induced collapse (aic) of o-ne white dwarfs in galactic bulge binaries can naturally reproduce the morphology, spectral shape and intensity of the gce signal while also obeying lmxb constraints. synchrotron emission from msp-launched cosmic ray electrons and positrons may simultaneously explain the mysterious microwave `haze'14 from the inner galaxy. | millisecond pulsars from accretion-induced collapse as the origin of the galactic centre gamma-ray excess signal |
in this work we study static neutron stars in the context of several inflationary models which are popular in cosmology. these inflationary models are nonminimally coupled scalar theories which yield a viable inflationary phenomenology in both jordan and einstein frames. by considering the constraints from inflationary theories, which basically determine the values of the potential strength, usually considered as a free parameter in astrophysical neutron star works, we construct and solve the tolman-oppenheimer-volkoff equations using a solid python-3 lsoda integrator. for our study we consider several popular inflationary models, such as the universal attractors, the rp attractors (three distinct model values), the induced inflation, the quadratic inflation, the higgs inflation and the a -attractors (two distinct model values) and for the following popular equations of state the wff1, the sly, the apr, the ms1, the ap3, the ap4, the eng, the mpa1 and the ms1b. we construct the m -r diagram and we confront the resulting theory with theoretical and observational constraints. as we demonstrate, remarkably, all the neutron stars produced by all the inflationary models we considered are compatible with all the constraints for the mpa1 equation of state. it is notable that for this particular equation of state, the maximum masses of the neutron stars are in the mass-gap region with m >2.5 m⊙, but lower than the three solar masses causal limit. another important feature of our work is that it may be possible to discriminate inflationary attractors which at the cosmological level are indistinguishable using the m -r graphs of static neutron stars, however we point out the limitations in discriminating the inflationary attractors. also we show that the wff1, ms1 and ms1b seem to be entirely ruled out, regarding a viable description of static neutron stars. we also make the observation that as the nicer constraints are pushed towards larger radii, as for example in the case of the black widow pulsar psr j0952-0607, it seems that equations of state that produce neutron stars with maximum masses in the mass gap region, with m >2.5 m⊙, but lower than the three solar masses causal limit, are favored and are compatible with the modified nicer constraints. finally we question the ability of the mpa1 equation of state to pass all the theoretical and observational constraints and we impose the question whether this equation of state plays any fundamental role in static neutron star physics. | inflationary attractors predictions for static neutron stars in the mass-gap region |
screened modified gravity (smg) is a kind of scalar-tensor theory with screening mechanisms, which can suppress the fifth force in dense regions and allow theories to evade the solar system and laboratory tests. in this paper, we investigate how the screening mechanisms in smg affect the gravitational radiation damping effects, calculate in detail the rate of the energy loss due to the emission of tensor and scalar gravitational radiations, and derive their contributions to the change in the orbital period of the binary system. we find that the scalar radiation depends on the screened parameters and the propagation speed of scalar waves, and the scalar dipole radiation dominates the orbital decay of the binary system. for strongly self-gravitating bodies, all effects of scalar sector are strongly suppressed by the screening mechanisms in smg. by comparing our results to observations of binary system psr j 1738 +0333 , we place the stringent constraints on the screening mechanisms in smg. as an application of these results, we focus on three specific models of smg (chameleon, symmetron, and dilaton), and derive the constraints on the model parameters, respectively. | gravitational radiation from compact binary systems in screened modified gravity |
the hawc collaboration has discovered a γ -ray emission extended about 2 degrees around the geminga and monogem pulsar wind nebulae (pwne) at γ -ray energies eγ>5 tev . we analyze, for the first time, almost 10 years of γ -ray data obtained with the fermi large area telescope at eγ>8 gev in the direction of geminga and monogem. since these two pulsars are close to the galactic plane we run our analysis with ten different interstellar emission models (iems) to study the systematics due to the modeling of this component. we detect a γ -ray halo around geminga with a significance in the range 7.8 - 11.8 σ depending on the iem considered. this measurement is compatible with e+ and e- emitted by the pwn, which inverse compton scatter (ics) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around 1.1 ×1027 cm2/s at 100 gev. we include in our analysis the proper motion of the geminga pulsar which is relevant for γ rays produced for ics in the fermi-lat energy range. we find that an efficiency of about 1% for the conversion of the spin-down energy of the pulsar into e+ and e- is required to be consistent with γ -ray data from fermi-lat and hawc. the inferred contribution of geminga to the e+ flux is at most 20% at the highest-energy ams-02 data. our results are compatible with the interpretation that the cumulative emission from galactic pulsars explains the positron excess. | detection of a γ -ray halo around geminga with the fermi-lat data and implications for the positron flux |
we present the first optical spectroscopy of five confirmed (or strong candidate) redback millisecond pulsar binaries, obtaining complete radial velocity curves for each companion star. the properties of these millisecond pulsar binaries with low-mass, hydrogen-rich companions are discussed in the context of the 14 confirmed and 10 candidate field redbacks. we find that the neutron stars in redbacks have a median mass of 1.78 ± 0.09 m ⊙ with a dispersion of σ = 0.21 ± 0.09. neutron stars with masses in excess of 2 m ⊙ are consistent with, but not firmly demanded by, current observations. redback companions have median masses of 0.36 ± 0.04 m ⊙ with a scatter of σ = 0.15 ± 0.04 m ⊙, and a tail possibly extending up to 0.7-0.9 m ⊙. candidate redbacks tend to have higher companion masses than confirmed redbacks, suggesting a possible selection bias against the detection of radio pulsations in these more massive candidate systems. the distribution of companion masses between redbacks and the less massive black widows continues to be strongly bimodal, which is an important constraint on evolutionary models for these systems. among redbacks, the median efficiency of converting the pulsar spin-down energy to γ-ray luminosity is ∼10%. | optical spectroscopy and demographics of redback millisecond pulsar binaries |
meertime is a five-year large survey project to time pulsars with meerkat, the 64-dish south african precursor to the square kilometre array. the science goals for the programme include timing millisecond pulsar (msps) to high precision ( <1 μs ) to study the galactic msp population and to contribute to global efforts to detect nanohertz gravitational waves with the international pulsar timing array (ipta). in order to plan for the remainder of the programme and to use the allocated time most efficiently, we have conducted an initial census with the meerkat `l-band' receiver of 189 msps visible to meerkat and here present their dispersion measures, polarisation profiles, polarisation fractions, rotation measures, flux density measurements, spectral indices, and timing potential. as all of these observations are taken with the same instrument (which uses coherent dedispersion, interferometric polarisation calibration techniques, and a uniform flux scale), they present an excellent resource for population studies. we used wideband pulse portraits as timing standards for each msp and demonstrated that the meertime pulsar timing array (mpta) can already contribute significantly to the ipta as it currently achieves better than 1 μs timing accuracy on 89 msps (observed with fortnightly cadence). by the conclusion of the initial five-year meertime programme in 2024 july, the mpta will be extremely significant in global efforts to detect the gravitational wave background with a contribution to the detection statistic comparable to other long-standing timing programmes. | the meertime pulsar timing array: a census of emission properties and timing potential |
binary pulsar observations and gravitational wave detections seriously constrained scalar-tensor theories with massless scalar field allowing only small deviations from general relativity. if we consider a nonzero mass of the scalar field, though, significant deviations from general relativity are allowed for values of the parameters that are in agreement with the observations. in the present paper we extend this idea and we study scalar-tensor theory with massive field with self-interaction term in the potential. the additional term suppresses the scalar field in the neutron star models in addition to the effect of the mass of the scalar field but still, large deviations from pure gr can be observed for values of the parameters that are in agreement with the observations. | static and slowly rotating neutron stars in scalar-tensor theory with self-interacting massive scalar field |
in this white paper we present the potential of the enhanced x-ray timing and polarimetry (extp) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. the tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. under development by an international consortium led by the institute of high energy physics of the chinese academy of sciences, the extp mission is expected to be launched in the mid 2020s. | dense matter with extp |
fast radio bursts (frbs) are short pulses observed in radio frequencies usually originating from cosmological distances. the discovery of frb 200428 and its x-ray counterpart from the galactic magnetar sgr j1935+2154 suggests that at least some frbs can be generated by magnetars. however, the majority of x-ray bursts from magnetars are not associated with radio emission. the fact that only in rare cases can an frb be generated raises the question regarding the special triggering mechanism of frbs. here we report a giant glitch from sgr j1935+2154, which occurred approximately $3.1\pm2.5$\,day before frb 200428, with $\delta\nu=19.8\pm1.4$ {\rm $\mu$hz} and $\delta\dot{\nu}=6.3\pm1.1$\,phz s$^{-1}$. the corresponding spin-down power change rate $\delta\dot\nu/\dot\nu$ is among the largest in all the detected pulsar glitches. the glitch contains a delayed spin-up process that is only detected in the crab pulsar and the magnetar 1e 2259+586, a large persistent offset of the spin-down rate, and a recovery component which is about one order of magnitude smaller than the persistent one. the temporal coincidence between the glitch and frb 200428 suggests a physical connection between the two. the internally triggered giant glitch of the magnetar likely altered the magnetosphere structure dramatically in favour of frb generation, which subsequently triggered many x-ray bursts and eventually frb 200428 through additional crustal cracking and alfvén wave excitation and propagation. | a giant glitch from the magnetar sgr j1935+2154 before frb 200428 |
in this contribution, we discuss the cosmological scenario where unstable domain walls are formed in the early universe and their late-time annihilation produces a significant amount of gravitational waves. after describing cosmological constraints on long-lived domain walls, we estimate the typical amplitude and frequency of gravitational waves observed today. we also review possible extensions of the standard model of particle physics that predict the formation of unstable domain walls and can be probed by observation of relic gravitational waves. it is shown that recent results of pulser timing arrays and direct detection experiments partially exclude the relevant parameter space, and that a much wider parameter space can be covered by the next generation of gravitational wave observatories. | a review of gravitational waves from cosmic domain walls |
in this paper, we attempt to investigate the nature of the first gravitational wave (gw) signal to be detected by pulsar timing arrays (ptas): will it be an individual, resolved supermassive black hole binary (sbhb), or a stochastic background made by the superposition of gws produced by an ensemble of sbhbs? to address this issue, we analyse a broad set of simulations of the cosmological population of sbhbs that cover the entire parameter space allowed by current electromagnetic observations in an unbiased way. for each simulation, we construct the expected gw signal and identify the loudest individual sources. we then employ appropriate detection statistics to evaluate the relative probability of detecting each type of source as a function of time for a variety of ptas; we consider the current international pta, and speculate into the era of the square kilometre array. the main properties of the first detectable individual sbhbs are also investigated. contrary to previous work, we cast our results in terms of the detection probability (dp), since the commonly adopted criterion based on a signal-to-noise ratio threshold is statistic-dependent and may result in misleading conclusions for the statistics adopted here. our results confirm quantitatively that a stochastic signal is more likely to be detected first (with between 75 and 93 per cent probability, depending on the array), but the dp of single-sources is not negligible. our framework is very flexible and can be easily extended to more realistic arrays and to signal models including environmental coupling and sbhb eccentricity. | expected properties of the first gravitational wave signal detected with pulsar timing arrays |
the accretion flow around x-ray pulsars with a strong magnetic field is funnelled by the field to relatively small regions close to the magnetic poles of the neutron star (ns), the hotspots. during strong outbursts regularly observed from some x-ray pulsars, the x-ray luminosity can be so high that the emerging radiation is able to stop the accreting matter above the surface via radiation-dominated shock, and the accretion column begins to rise. this border luminosity is usually called the `critical luminosity'. here we calculate the critical luminosity as a function of the ns magnetic field strength b using the exact compton scattering cross-section in a strong magnetic field. influence of the resonant scattering and photon polarization is taken into account for the first time. we show that the critical luminosity is not a monotonic function of the b-field. it reaches a minimum of a few 1036 erg s-1 when the cyclotron energy is about 10 kev and a considerable amount of photons from a hotspot have energy close to the cyclotron resonance. for small b, this luminosity is about 1037 erg s-1, nearly independent of the parameters. it grows for the b-field in excess of 1012 g because of the drop in the effective cross-section of interaction below the cyclotron energy. we investigate how different types of the accretion flow and geometries of the accretion channel affect the results and demonstrate that the general behaviour of the critical luminosity on the b-field is very robust. the obtained results are shown to be in good agreement with the available observational data and provide a necessary ground for the interpretation of upcoming high-quality data from the currently operating and planned x-ray telescopes. | the critical accretion luminosity for magnetized neutron stars |
gaseous circumbinary accretion discs provide a promising mechanism to facilitate the mergers of supermassive black holes (smbhs) in galactic nuclei. we measure the torques exerted on accreting smbh binaries, using 2d, isothermal, moving-mesh, viscous hydrodynamical simulations of circumbinary accretion discs. our computational domain includes the entire inner region of the circumbinary disc, with the individual black holes (bhs) treated as point masses on the grid. a sink prescription is used to account for accretion on to each bh through well-resolved minidiscs. we explore a range of mass-removal rates for the sinks. we find that the torque exerted on the binary is primarily gravitational, and dominated by the gas orbiting close behind and ahead of the individual bhs. the torques are sensitive to the sink prescription: slower sinks result in more gas accumulating near the bhs and more negative torques, driving more rapid binary merger. for faster sinks, the torques are less negative, and eventually turn positive (for unphysically fast sinks). when the minidiscs are modelled as standard α discs, our results are insensitive to the chosen sink radius. when scaled to \dot{m}/\dot{m}_edd=0.3, the implied residence time-scale is ≈3 × 106 yr, independent of the smbh masses and orbital separation. for binaries with total mass ≲ 107 m⊙, this is shorter than the inspiral time due to gravitational wave (gw) emission alone, implying that gas discs will have a significant impact on the smbh binary population and can affect the gw signal for pulsar timing arrays. | on the orbital evolution of supermassive black hole binaries with circumbinary accretion discs |
as pulsars lose energy, primarily in the form of magnetic dipole radiation, their rotation slows down accordingly. for some pulsars, this spin-down is interrupted by occasional abrupt spin-up events known as glitches1. a glitch is hypothesized to be a catastrophic release of pinned vorticity2 that provides an exchange of angular momentum between the superfluid outer core and the crust. this is manifested by a minute alteration in the rotation rate of the neutron star and its co-rotating magnetosphere, which is revealed by an abrupt change in the timing of observed radio pulses. measurement of the flux density, polarization and single-pulse arrival times of the glitch with high time resolution may reveal the equation of state of the crustal superfluid, its drag-to-lift ratio and the parameters that describe its friction with the crust3. this has not hitherto been possible because glitch events happen unpredictably. here we report single-pulse radio observations of a glitch in the vela pulsar, which has a rotation frequency of 11.2 hertz. the glitch was detected on 2016 december 12 at 11:36 universal time, during continuous observations of the pulsar over a period of three years. we detected sudden changes in the pulse shape coincident with the glitch event: one pulse was unusually broad, the next pulse was missing (a `null') and the following two pulses had unexpectedly low linear polarization. this sequence was followed by a 2.6-second interval during which pulses arrived later than usual, indicating that the glitch affects the magnetosphere. | alteration of the magnetosphere of the vela pulsar during a glitch |
in this work, a physically reasonable metric potential grr and a specific choice of the anisotropy has been utilized to obtain closed-form solutions of the einstein field equation for a spherically symmetric anisotropic matter distribution. this class of solution has been used to develop viable models for observed pulsars. smooth matching of interior spacetime metric with the exterior schwarzschild metric and utilizing the condition that radial pressure is zero across the boundary leads us to determine the model parameters. a particular pulsar 4 u 1820 -30 having current estimated mass and radius (m a s s =1.58 m⊙ and r a d i u s =9.1 km) has been allowed for testing the physical acceptability of the developed model. the gross physical nature of the observed pulsar has been analyzed graphically. the stability of the model is also discussed given causality conditions, adiabatic index and generalized tolman-oppenheimer-volkov (tov) equation under the forces acting on the system. to show that this model is compatible with observational data, few more pulsars have been considered, and all the requirements of a realistic star are highlighted. additionally, the mass-radius (m-r) relationship of compact stellar objects analyzed for this model. the maximum mass for the presented model is ≈4 m⊙ which is compared with the realization of rhoades and ruffini (phys rev lett 32:324, 1974). | a new class of compact stellar model compatible with observational data |
over the past 13 yr, the parkes radio telescope has observed a large number of pulsars using digital filter bank backends with high time and frequency resolution and the capability for stokes recording. here, we use archival data to present polarimetry data at an observing frequency of 1.4 ghz for 600 pulsars with spin-periods ranging from 0.036 to 8.5 s. we comment briefly on some of the statistical implications from the data and highlight the differences between pulsars with high and low spin-down energy. the data set, images and table of properties for all 600 pulsars are made available in a public data archive maintained by the csiro. | polarimetry of 600 pulsars from observations at 1.4 ghz with the parkes radio telescope |
the equatorial current sheet in pulsar magnetospheres is often regarded as an ideal site for particle acceleration via relativistic reconnection. using 2d spherical particle-in-cell simulations, we investigate particle acceleration in the axisymmetric pulsar magnetosphere as a function of the injected plasma multiplicity and magnetization. we observe a clear transition from a highly charge-separated magnetosphere for low plasma injection with little current and spin-down power, to a nearly force-free solution for high plasma multiplicity characterized by a prominent equatorial current sheet and high spin-down power. we find significant magnetic dissipation in the current sheet, up to 30 per cent within 5 light-cylinder radii in the high-multiplicity regime. the simulations unambiguously demonstrate that the dissipated poynting flux is efficiently channelled to the particles in the sheet, close to the y-point within about 1-2 light-cylinder radii from the star. the mean particle energy in the sheet is given by the upstream plasma magnetization at the light cylinder. the study of particle orbits shows that all energetic particles originate from the boundary layer between the open and the closed field lines. energetic positrons always stream outwards, while high-energy electrons precipitate back towards the star through the sheet and along the separatrices, which may result in auroral-like emission. our results suggest that the current sheet and the separatrices may be the main source of high-energy radiation in young pulsars. | particle acceleration in axisymmetric pulsar current sheets |
future searches for gravitational waves from space will be sensitive to double compact objects in our milky way. we present new simulations of the populations of double black holes (bhbhs), bh neutron stars (bhnss), and double neutron stars (nsnss) that will be detectable by the planned space-based gravitational-wave detector called laser interferometer space antenna (lisa). for our estimates, we use an empirically informed model of the metallicity-dependent star formation history of the milky way. we populate it using an extensive suite of binary population-synthesis predictions for varying assumptions relating to mass transfer, common-envelope, supernova kicks, remnant masses, and wind mass-loss physics. for a 4(10) yr lisa mission, we predict between 30-370(50-550) detections over these variations, out of which 6-154 (9-238) are bhbhs, 2-198 (3-289) are bhnss, and 3-35 (4-57) are nsnss. we expect that about 50% (60%) can be distinguished from double white dwarf sources based on their mass or eccentricity and localization. specifically, for about 10% (15%), we expect to be able to determine chirp masses better than 10%. for 13% (13%), we expect sky-localizations better than 1°. we discuss how the variations in the physics assumptions alter the distribution of properties of the detectable systems, even when the detection rates are unchanged. we further discuss the possibility of multimessenger observations of pulsar populations with the square kilometre array and assess the benefits of extending the lisa mission. | gravitational wave sources in our galactic backyard: predictions for bhbh, bhns, and nsns binaries detectable with lisa |
we present here the first convincing observational manifestation of a magnetar-like magnetic field in an accreting neutron star in binary system - the first pulsating ultraluminous x-ray source x-2 in the galaxy m82. using the chandra x-ray observatory data, we show that the source exhibit the bimodal distribution of the luminosity with two well-defined peaks separated by a factor of 40. this behaviour can be interpreted as the action of the `propeller regime' of accretion. the onset of the propeller in a 1.37 s pulsar at luminosity of ∼1040 erg s-1 implies the dipole component of the neutron star magnetic field of ∼1014 g. | propeller effect in action in the ultraluminous accreting magnetar m82 x-2 |
ultraluminous pulsars are a definite proof that persistent super-eddington accretion occurs in nature. they support the scenario according to which most ultraluminous x-ray sources (ulxs) are super-eddington accretors of stellar mass rather than sub-eddington intermediate mass black holes. an important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at mildly relativistic speeds. in practice, the spectral resolution of x-ray gratings such as rgs onboard xmm-newton is required to resolve their observational signatures in ulxs. using rgs, outflows have been discovered in the spectra of three ulxs (none of which are currently known to be pulsars). most recently, the fourth ultraluminous pulsar was discovered in ngc 300. here we report detection of an ultrafast outflow (ufo) in the x-ray spectrum of the object, with a significance of more than 3σ, during one of the two simultaneous observations of the source by xmm-newton and nustar in december 2016. the outflow has a projected velocity of 65 000 km s-1 (0.22c) and a high ionization factor with a log value of 3.9. this is the first direct evidence for a ufo in a neutron star ulx and also the first time that this evidence in a ulx spectrum is seen in both soft and hard x-ray data simultaneously. we find no evidence of the ufo during the other observation of the object, which could be explained by either clumpy nature of the absorber or a slight change in our viewing angle of the accretion flow. | evidence for a variable ultrafast outflow in the newly discovered ultraluminous pulsar ngc 300 ulx-1 |
the fast radio burst frb121102 has been observed to repeat in an irregular fashion. using published timing data of the observed bursts, we show that poissonian statistics are not a good description of this random process. as an alternative, we suggest to describe the intervals between bursts with a weibull distribution with a shape parameter smaller than one, which allows for the clustered nature of the bursts. we quantify the amount of clustering using the parameters of the weibull distribution and discuss the consequences that it has for the detection probabilities of future observations and for the optimization of observing strategies. allowing for this generalization, we find a mean repetition rate of r=5.7^{+3.0}_{-2.0} per day and index k=0.34^{+0.06}_{-0.05} for a correlation function ξ(t) = (t/t0)k - 1. | on the non-poissonian repetition pattern of frb121102 |
pulsar glitches are rapid spin-up events that occur in the rotation of neutron stars, providing a valuable probe into the physics of the interiors of these objects. long-term monitoring of a large number of pulsars facilitates the detection of glitches and the robust measurements of their parameters. the jodrell bank pulsar timing programme regularly monitors more than 800 radio pulsars and has accrued, in some cases, over 50 yr of timing history on individual objects. in this paper, we present 106 new glitches in 70 radio pulsars as observed up to the end of 2018. for 70 per cent of these pulsars, the event we report is its only known glitch. for each new glitch, we provide measurements of its epoch, amplitude, and any detected changes to the spin-down rate of the star. combining these new glitches with those listed in the jodrell bank glitch catalogue, we analyse a total sample of 543 glitches in 178 pulsars. we model the distribution of glitch amplitudes and spin-down rate changes using a mixture of two gaussian components. we corroborate the known dependence of glitch rate and activity on pulsar spin-down rates and characteristic ages, and show that younger pulsars tend to exhibit larger glitches. pulsars with spin-down rates between 10-14 and 10-10.5 hz s-1 show a mean reversal of 1.8 per cent of their spin-down as a consequence of glitches. our results are qualitatively consistent with the superfluid vortex unpinning models of pulsar glitches. | the jodrell bank glitch catalogue: 106 new rotational glitches in 70 pulsars |
we review primordial black hole (pbh) formation in the axionlike curvaton model and investigate whether pbhs formed in this model can be the origin of the gravtitational wave (gw) signals detected by the advanced ligo. in this model, small-scale curvature perturbations with large amplitude are generated, which is essential for pbh formation. on the other hand, large curvature perturbations also become a source of primordial gws by their second-order effects. severe constraints are imposed on such gws by pulsar timing array (pta) experiments. we also check the consistency of the model with these constraints. in this analysis, it is important to take into account the effect of non-gaussianity, which is generated easily in the curvaton model. we see that, if there are non-gaussianities, the fixed amount of pbhs can be produced with a smaller amplitude of the primordial power spectrum. | primordial black holes for the ligo events in the axionlike curvaton model |
we discuss possible association of fast radio bursts (frbs) with supergiant pulses emitted by young pulsars (ages ∼ tens to hundreds of years) born with regular magnetic field but very short - few milliseconds - spin periods. we assume that frbs are extra-galactic events coming from distances d ≲ 100 mpc and that most of the dispersion measure (dm) comes from the material in the freshly ejected snr shell. we then predict that for a given burst the dm should decrease with time and that frbs are not expected to be seen below ∼300 mhz due to free-free absorption in the expanding ejecta. a supernova might have been detected years before the burst; frbs are mostly associated with star-forming galaxies. the model requires that some pulsars are born with very fast spins, of the order of few milliseconds. the observed distribution of spin-down powers dot{e} in young energetic pulsars is consistent with equal birth rate per decade of dot{e}. accepting this injection distribution and scaling the intrinsic brightness of frbs with dot{e}, we predict the following properties of a large sample of frbs: (i) the brightest observed events come from a broad distribution in distances; (ii) for repeating bursts brightness either remains nearly constant (if the spin-down time is longer than the age of the pulsar) or decreases with time otherwise; in the latter case dm ∝ dot{e}. | fast radio bursts as giant pulses from young rapidly rotating pulsars |
we update the ephemeris of the eclipsing high-mass x-ray binary (hmxb) systems lmc x-4, cen x-3, 4u 1700-377, 4u 1538-522, smc x-1, igr j18027-2016, vela x-1,igr j17252-3616, xte j1855-026, and oao 1657-415 with the help of more than ten years of monitoring these sources with the all sky monitor onboard rxte and with the integral soft gamma-ray imager onboard integral. these results are used to refine previous measurements of the orbital period decay of all sources (where available) and provide the first accurate values of the apsidal advance in vela x-1 and 4u 1538-522. updated values for the masses of the neutron stars hosted in the ten hmxbs are also provided, as well as the long-term light curves folded on the best determined orbital parameters of the sources. these light curves reveal complex eclipse ingresses and egresses that are understood mostly as being caused by accretion wakes. our results constitute a database to be used for population and evolutionary studies of hmxbs and for theoretical modeling of long-term accretion in wind-fed x-ray binaries. appendix a is available in electronic form at http://www.aanda.org | ephemeris, orbital decay, and masses of ten eclipsing high-mass x-ray binaries |
binary interactions lead to the formation of intriguing objects, such as compact binaries, supernovae, gamma ray bursts, x-ray binaries, pulsars, novae, cataclysmic variables, hot subdwarf stars, barium stars and blue stragglers. to study the evolution of binary populations and the consequent formation of these objects, many methods have been developed over the years, for which a robust approach named binary population synthesis (bps) warrants special attention. this approach has seen widespread application in many areas of astrophysics, including but not limited to analyses of the stellar content of galaxies, research on galactic chemical evolution and studies concerning star formation and cosmic re-ionization. in this review, we discuss the role of bps, its general picture and the various components that comprise it. we pay special attention to the stability criteria for mass transfer in binaries, as this stability largely determines the fate of binary systems. we conclude with our perspectives regarding the future of this field. | binary population synthesis |
metastable cosmic strings appear in models of new physics with a two-step symmetry breaking $g\to h\to 1$, where $\pi_1(h)\neq 0$ and $\pi_1(g)=0$. they decay via the monopole-antimonopole pair creation inside. conventionally, the breaking rate has been estimated by an infinitely thin string approximation, which requires a large hierarchy between the symmetry breaking scales. in this paper, we reexamine it by taking into account the finite sizes of both the cosmic string and the monopole. we obtain a robust lower limit on the tunneling factor $e^{-s_b}$ even for regimes the conventional estimate is unreliable. in particular, it is relevant to the cosmic string interpretation of the gravitational wave signals recently reported by pulsar timing array experiments. | revisiting metastable cosmic string breaking |
we search nanograv's 12.5 yr data set for evidence of a gravitational-wave background (gwb) with all the spatial correlations allowed by general metric theories of gravity. we find no substantial evidence in favor of the existence of such correlations in our data. we find that scalar-transverse (st) correlations yield signal-to-noise ratios and bayes factors that are higher than quadrupolar (tensor-transverse, tt) correlations. specifically, we find st correlations with a signal-to-noise ratio of 2.8 that are preferred over tt correlations (hellings and downs correlations) with bayesian odds of about 20:1. however, the significance of st correlations is reduced dramatically when we include modeling of the solar system ephemeris systematics and/or remove pulsar j0030+0451 entirely from consideration. even taking the nominal signal-to-noise ratios at face value, analyses of simulated data sets show that such values are not extremely unlikely to be observed in cases where only the usual tt modes are present in the gwb. in the absence of a detection of any polarization mode of gravity, we place upper limits on their amplitudes for a spectral index of γ = 5 and a reference frequency of f yr = 1 yr-1. among the upper limits for eight general families of metric theories of gravity, we find the values of ${a}_{\mathrm{tt}}^{95 \% }=(9.7\pm 0.4)\times {10}^{-16}$ and ${a}_{\mathrm{st}}^{95 \% }=(1.4\pm 0.03)\times {10}^{-15}$ for the family of metric spacetime theories that contain both tt and st modes. | the nanograv 12.5-year data set: search for non-einsteinian polarization modes in the gravitational-wave background |
the maximum mass of a non-rotating neutron star, mtov, plays a very important role in deciphering the structure and composition of neutron stars and in revealing the equation of state (eos) of nuclear matter. although with a large-error bar, the recent mass estimate for the black-widow binary pulsar psr j0952-0607, i.e. m = 2.35 ± 0.17 m⊙, provides the strongest lower bound on mtov and suggests that neutron stars with very large masses can, in principle, be observed. adopting an agnostic modelling of the eos, we study the impact that large masses have on the neutron-star properties. in particular, we show that assuming $m_{\rm tov}\gtrsim 2.35\, {\rm m_\odot}$ constrains tightly the behaviour of the pressure as a function of the energy density and moves the lower bounds for the stellar radii to values that are significantly larger than those constrained by the nicer measurements, rendering the latter ineffective in constraining the eos. we also provide updated analytic expressions for the lower bound on the binary tidal deformability in terms of the chirp mass and show how larger bounds on mtov lead to tighter constraints for this quantity. in addition, we point out a novel quasi-universal relation for the pressure profile inside neutron stars that is only weakly dependent on the eos and the maximum-mass constraint. finally, we study how the sound speed and the conformal anomaly are distributed inside neutron stars and show how these quantities depend on the imposed maximum-mass constraints. | impact of large-mass constraints on the properties of neutron stars |
the puzzling mechanism of coherent radio emission remains unknown, but fortunately, repeating fast radio bursts (frbs) provide a precious opportunity, with extremely bright subpulses created in a clear and vacuum-like pulsar magnetosphere. frbs are millisecond-duration signals that are highly dispersed at distant galaxies but with uncertain physical origin(s). coherent curvature radiation by bunches has already been proposed for repeating frbs. the charged particles are created during central star's quakes, which can form bunches streaming out along curved magnetic field lines, so as to trigger frbs. the nature of narrow-band radiation with time-frequency drifting can be a natural consequence that bunches could be observed at different times with different curvatures. additionally, high linear-polarization can be seen if the line of sight is confined to the beam angle, whereas the emission could be highly circular-polarized if off-beam. it is also discussed that pulsar surface may be full of small hills (i.e., zits) which would help producing bulk of energetic bunches for repeating frbs as well as for rotation-powered pulsars. | repeating frbs reveal the secret of pulsar magnetospheric activity |
dark photon dark matter may be produced by the cosmic strings in association with the dark u(1) gauge symmetry breaking. we perform three-dimensional lattice simulations of the abelian-higgs model and follow the evolution of cosmic strings. in particular, we simulate the case of (very) light vector boson and find that such vector bosons are efficiently produced by the collapse of small loops while the production is inefficient in the case of heavy vector boson. we calculate the spectrum of the gravitational wave background produced by the cosmic string loops for the light vector boson case and find characteristic features in the spectrum, which can serve as a probe of the dark photon dark matter scenario. in particular, we find that the current ground-based detectors may be sensitive to such gravitational wave signals and also on-going/future pulsar timing observations give stringent constraint on the dark photon dark matter scenario. | dark photon dark matter from cosmic strings and gravitational wave background |
pulsar timing arrays (ptas) are sensitive to oscillations in the gravitational potential along the line-of-sight due to ultralight particle pressure. we calculate the probing power of ptas for ultralight bosons across all frequencies, from those larger than the inverse observation time to those smaller than the inverse distance to the pulsar. we show that since the signal amplitude grows comparably to the degradation in pta sensitivity at frequencies smaller than inverse observation time, the discovery potential can be extended towards lower masses by over three decades, maintaining high precision. we demonstrate that, in the mass range $10^{-26} -10^{-23}$ ev, existing 15-year pta data can robustly detect or rule out an ultralight component down to $o(1 - 10)\%$ of the total dark matter. non-detection, together with other bounds in different mass ranges, will imply that ultralight scalar/axion can comprise at most $1-10\%$ of dark matter in the $10^{-30}\!-\!10^{-17}$ ev range. with 30 years of observation, current ptas can extend the reach down to $0.1-1 \%$, while next-generation ptas such as ska can attain the $0.01-0.1\%$ precision. we generalize the analysis and derive predictions for ultralight spin-1 vector (i.e. dark photon) and spin-2 tensor dark components. | probing ultralight scalar, vector and tensor dark matter with pulsar timing arrays |
using observations of x-ray pulsar hercules x-1 by the imaging x-ray polarimetry explorer we report a highly significant (>17σ) detection of the polarization signal from an accreting neutron star. the observed degree of linear polarization of ~10% is far below theoretical expectations for this object, and stays low throughout the spin cycle of the pulsar. both the degree and angle of polarization exhibit variability with the pulse phase, allowing us to measure the pulsar spin position angle 57(2) deg and the magnetic obliquity 12(4) deg, which is an essential step towards detailed modelling of the intrinsic emission of x-ray pulsars. combining our results with the optical polarimetric data, we find that the spin axis of the neutron star and the angular momentum of the binary orbit are misaligned by at least ~20 deg, which is a strong argument in support of the models explaining the stability of the observed superorbital variability with the precession of the neutron star. | determination of x-ray pulsar geometry with ixpe polarimetry |
pulsar timing arrays have found evidence for a low-frequency gravitational wave background (gwb). assuming the gwb is produced by supermassive black hole binaries (smbhbs), the next gravitational wave (gw) signals astronomers anticipate are continuous waves (cws) from single smbhbs and their associated gwb anisotropy. the prospects for detecting cws and anisotropy are highly dependent on the astrophysics of smbhb populations. thus, information from single sources can break degeneracies in astrophysical models and place much more stringent constraints than the gwb alone. we simulate and evolve smbhb populations, model their gws, and calculate their anisotropy and detectability. we investigate how varying components of our semi-analytic model, including the galaxy stellar mass function, the smbh--host galaxy relation ($m_\mathrm{bh}$--$m_\mathrm{bulge}$), and the binary evolution prescription impact the expected detections. the cw occurrence rate is greatest for few total binaries, high smbhb masses, large scatter in $m_\mathrm{bh}$--$m_\mathrm{bulge}$, and long hardening times. the occurrence rate depends most on the binary evolution parameters, implying that cws offer a novel avenue to probe binary evolution. the most detectable cw sources are in the lowest frequency bin for a 16.03-year pta, have masses from $\sim\!\!10^9-10^{10}\mathrm{m}_\odot$, and are $\sim\!\!1$ gpc away. the level of anisotropy increases with frequency, with the angular power spectrum over multipole modes $\ell$ varying in low-frequency $c_{\ell>0}/c_0$ from $\sim\!\!5\times 10^{-3}$ to $\sim\!\!2\times10^{-1}$, depending on the model; typical values are near current upper limits. observing this anisotropy would support smbhb models for the gwb over cosmological models, which tend to be isotropic. | beyond the background: gravitational wave anisotropy and continuous waves from supermassive black hole binaries |
context. recently, doroshenko and collaborators reported a very low-mass compact star, a central compact object named xmmu j173203.3−344518 inside the supernova remnant hess j1731−347. its tiny mass is at odds with all calculations of minimum masses of neutron stars generated by iron cores, therefore (and even if not compellingly) it has been suggested to be a "strange star". in addition to the mass, the radius and surface temperature were extracted from the data, and the whole body of information should ultimately reveal whether this object is truly consistent with an exotic composition.aims: our aim is to understand the status of the compact object xmmu j173203.3−344518 in hess j1731−347 within the existing models of strange stars, including its prompt formation.methods: the information obtained on the mass, radius and surface temperature are compared to theoretical calculations performed within usual theoretical models using general relativity as the assumed theory of gravitation and a handful of cooling scenarios. a qualitative discussion showing the consistency of the strange-matter driven supernova scenario with a low-mass compact star is provided.results: we found that the object hess j1731−347 fits within the same quark star models recently employed to explain the masses and radii of the nicer objects psr j040+6620 and psr j0030+0451, in which both quantities were simultaneously determined. it is also remarkable to find that a simple cooling scenario devised 30 yr ago with superconducting quarks provides an overall good explanation of the surface temperature.conclusions: we conclude that xmmu j173203.3−344518 in the remnant hess j1731−347 fits into a strange star scenario that is also consistent with heavier compact stars, which can also belong to the same class and constitute an homogeneous type of self-bound objects produced in nature. | a light strange star in the remnant hess j1731−347: minimal consistency checks |
the european pulsar timing array (epta) collaboration has recently released an extended data set for six pulsars (dr2) and reported evidence for a common red noise signal. here we present a noise analysis for each of the six pulsars. we consider several types of noise: (i) radio frequency independent, 'achromatic', and time-correlated red noise; (ii) variations of dispersion measure and scattering; (iii) system and band noise; and (iv) deterministic signals (other than gravitational waves) that could be present in the pta data. we perform bayesian model selection to find the optimal combination of noise components for each pulsar. using these custom models we revisit the presence of the common uncorrelated red noise signal previously reported in the epta dr2 and show that the data still supports it with a high statistical significance. next, we confirm that there is no preference for or against the hellings-downs spatial correlations expected for the stochastic gravitational-wave background. the main conclusion of the epta dr2 paper remains unchanged despite a very significant change in the noise model of each pulsar. however, modelling the noise is essential for the robust detection of gravitational waves and its impact could be significant when analysing the next epta data release, which will include a larger number of pulsars and more precise measurements. | noise analysis in the european pulsar timing array data release 2 and its implications on the gravitational-wave background search |
the recent compelling observation of the nanohertz stochastic gravitational wave background has brought to light a new galactic arena to test gravity. in this paper, we derive a formula for the most general expression of the stochastic gravitational wave background correlation that could be tested with pulsar timing and future square kilometer arrays. our expressions extends the harmonic space analysis, also often referred to as the power spectrum approach, to predict the correlation signatures of an anisotropic polarized stochastic gravitational wave background with subluminal tensor, vector, and scalar gravitational degrees of freedom. we present the first few nontrivial anisotropy and polarization signatures in the correlation and discuss their dependence on the gravitational wave speed and pulsar distances. our results set up tests that could potentially be used to rigorously examine the isotropy of the stochastic gravitational wave background and strengthen the existing constraints on possible non-einsteinian polarizations in the nanohertz gravitational wave regime. | correlations for an anisotropic polarized stochastic gravitational wave background in pulsar timing arrays |
we explore the inflationary dynamics leading to formation of closed domain walls in course of evolution of an axion like particle (alp) field whose peccei-quinn-like phase transition occurred well before inflationary epoch. evolving after inflation, the domain walls may leave their imprint in stochastic gravitational waves background, in the frequency range accessible for the pulsar timing array measurements. we derive the characteristic strain power spectrum produced by the distribution of the closed domain walls and relate it with the recently reported nanograv signal excess. we found that the slope of the frequency dependence of the characteristic strain spectrum generated by the domain walls is very well centered inside the range of the slopes in the signal reported by the nanograv. analyzing the inflationary dynamic of the alp field, in consistency with the isocurvature constraint, we reveal those combinations of the parameters where the signal from the inflationary induced alps domain walls could saturate the amplitude of the nanograv excess. the evolution of big enough closed domain walls may end up in wormholes with the walls escaping into baby universes. we study the conditions when closed walls escape into baby universes and could leave a detectable imprint in the stochastic gravitational waves background. | looking at the nanograv signal through the anthropic window of axionlike particles |
we derive a new interior solution for stellar compact objects in f (r ) gravity assuming a differential relation to constrain the ricci curvature scalar. to this aim, we consider specific forms for the radial component of the metric and the first derivative of f (r ) . after, the time component of the metric potential and the form of f (r ) function are derived. from these results, it is possible to obtain the radial and tangential components of pressure and the density. the resulting interior solution represents a physically motivated anisotropic neutron star model. it is possible to match it with a boundary exterior solution. from this matching, the components of metric potentials can be rewritten in terms of a compactness parameter c which has to be c =2 g m /r c2<<0.5 for physical consistency. other physical conditions for real stellar objects are taken into account according to the solution. we show that the model accurately bypasses conditions like the finiteness of radial and tangential pressures, and energy density at the center of the star, the positivity of these components through the stellar structure, and the negativity of the gradients. these conditions are satisfied if the energy-conditions hold. moreover, we study the stability of the model by showing that tolman-oppenheimer-volkoff equation is at hydrostatic equilibrium. the solution is matched with observational data of millisecond pulsars with a withe dwarf companion and pulsars presenting thermonuclear bursts. | anisotropic compact stars in f(r) gravity |
symmetries play a fundamental role in modern theories of gravity. the strong equivalence principle (sep) constitutes a collection of gravitational symmetries which are all implemented by general relativity. alternative theories, however, are generally expected to violate some aspects of sep. we test three aspects of sep using observed change rates in the orbital period and eccentricity of binary pulsar j1713+0747: (1) the gravitational constant's constancy as part of locational invariance of gravitation; (2) the universality of free fall (uff) for strongly self-gravitating bodies; (3) the post-newtonian parameter \hat{α }_3 in gravitational lorentz invariance. based on the pulsar timing result of the combined data set from the north american nanohertz gravitational observatory and the european pulsar timing array, we find \dot{g}/g = (-0.1 ± 0.9) × 10^{-12} yr^{-1}, which is weaker than solar system limits, but applies for strongly self-gravitating objects. furthermore, we obtain an improved test for a uff violation by a strongly self-gravitating mass falling in the gravitational field of our galaxy, with a limit of |δ| < 0.002 (95 per cent c.l.). finally, we derive an improved limit on the self-acceleration of a gravitationally bound rotating body, to a preferred reference frame in the universe, with -3× 10^{-20} < \hat{α }_3 < 4× 10^{-20} (95 per cent c.l.). these results are based on direct uff and \hat{α }_3 tests using pulsar binaries, and they overcome various limitations of previous tests of this kind. | tests of gravitational symmetries with pulsar binary j1713+0747 |
we present three-dimensional (3d) global kinetic pulsar magnetosphere models, where the charged particle trajectories and the corresponding electromagnetic fields are treated self-consistently. for our study, we have developed a cartesian 3d relativistic particle-in-cell code that incorporates radiation reaction forces. we describe our code and discuss the related technical issues, treatments, and assumptions. injecting particles up to large distances in the magnetosphere, we apply arbitrarily low to high particle injection rates, and obtain an entire spectrum of solutions from close to the vacuum-retarded dipole to close to the force-free (ff) solution, respectively. for high particle injection rates (close to ff solutions), significant accelerating electric field components are confined only near the equatorial current sheet outside the light cylinder. a judicious interpretation of our models allows the particle emission to be calculated, and consequently, the corresponding realistic high-energy sky maps and spectra to be derived. using model parameters that cover the entire range of spin-down powers of fermi young and millisecond pulsars, we compare the corresponding model γ-ray light curves, cutoff energies, and total γ-ray luminosities with those observed by fermi to discover a dependence of the particle injection rate, { \mathcal f }, on the spin-down power, \dot{{ \mathcal e }}, indicating an increase of { \mathcal f } with \dot{{ \mathcal e }}. our models, guided by fermi observations, provide field structures and particle distributions that are not only consistent with each other but also able to reproduce a broad range of the observed γ-ray phenomenologies of both young and millisecond pulsars. | three-dimensional kinetic pulsar magnetosphere models: connecting to gamma-ray observations |
we report on a timing programme of 74 young pulsars that have been observed by the parkes 64-m radio telescope over the past decade. using modern bayesian timing techniques, we have measured the properties of 124 glitches in 52 of these pulsars, of which 74 are new. we demonstrate that the glitch sample is complete to fractional increases in spin frequency greater than $\delta \nu ^{90{{\ \rm per\ cent}}}_{\mathrm{ g}}/\nu \approx 8.1 \times 10^{-9}$ . we measure values of the braking index, n, in 33 pulsars. in most of these pulsars, their rotational evolution is dominated by episodes of spin-down with n > 10, punctuated by step changes in the spin-down rate at the time of a large glitch. the step changes are such that, averaged over the glitches, the long-term n is small. we find a near one-to-one relationship between the interglitch value of n and the change in spin-down of the previous glitch divided by the interglitch time interval. we discuss the results in the context of a range of physical models. | the impact of glitches on young pulsar rotational evolution |
ultra-low-frequency gravitational waves (gws) generated by individual inspiraling supermassive black hole binaries (smbhbs) in the centers of galaxies may be detected by pulsar timing arrays (ptas) in the future. these gw signals encoding absolute cosmic distances can serve as bright and dark sirens, having potential to be developed into a precise cosmological probe. here we show that an ska-era pta consisting of 100 millisecond pulsars may observe about 25 bright sirens and 41 dark sirens during a 10-year observation. the bright sirens, together with the cmb data, have comparable capabilities to current mainstream data for measuring the equation of state of dark energy. the dark sirens could make the measurement precision of the hubble constant close to that of current distance-ladder observation. our results indicate that ultra-low-frequency gws from individual smbhbs are of great significance in exploring the nature of dark energy and measuring the hubble constant. | ultra-low-frequency gravitational waves from individual supermassive black hole binaries as standard sirens |
it is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our universe. the nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magnitude. the ultralight scalar field dark matter, consisting of extremely light bosons with m ∼10-22 ev and often called "fuzzy" dark matter, provides intriguing solutions to some challenges at sub-galactic scales for the standard cold dark matter model. as shown by khmelnitsky and rubakov, such a scalar field in the galaxy would produce an oscillating gravitational potential with nanohertz frequencies, resulting in periodic variations in the times of arrival of radio pulses from pulsars. the parkes pulsar timing array (ppta) has been monitoring 20 millisecond pulsars at two- to three-week intervals for more than a decade. in addition to the detection of nanohertz gravitational waves, ppta offers the opportunity for direct searches for fuzzy dark matter in an astrophysically feasible range of masses. we analyze the latest ppta data set which includes timing observations for 26 pulsars made between 2004 and 2016. we perform a search in this data set for evidence of ultralight dark matter in the galaxy using bayesian and frequentist methods. no statistically significant detection has been made. we, therefore, place upper limits on the local dark matter density. our limits, improving on previous searches by a factor of 2 to 5, constrain the dark matter density of ultralight bosons with m ≤10-23 ev to be below 6 gev cm-3 with 95% confidence in the earth neighborhood. finally, we discuss the prospect of probing the astrophysically favored mass range m ≳10-22 ev with next-generation pulsar timing facilities. | parkes pulsar timing array constraints on ultralight scalar-field dark matter |
the fermi satellite has recently detected gamma-ray emission from the central regions of our galaxy. this may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. we show that the observed signal may instead be generated by millisecond pulsars that formed in dense star clusters in the galactic halo. most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. the gamma-ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and are in remarkable agreement with the observations. these gamma-rays are from fossil remains of dispersed clusters, telling the history of the galactic bulge. | disrupted globular clusters can explain the galactic center gamma-ray excess |
we study the radio signals generated when an axion star enters the magnetosphere of a neutron star. as the axion star moves through the resonant region where the plasma-induced photon mass becomes equal to the axion mass, the axions can efficiently convert into photons, giving rise to an intense, transient radio signal. we show that a dense axion star with a mass ∼10-13 m⊙ composed of ∼10 μ ev axions can account for most of the mysterious fast radio bursts. | fast radio bursts from axion stars moving through pulsar magnetospheres |
models of dark matter (dm) can leave unique imprints on the universe's small scale structure by boosting density perturbations on small scales. we study the capability of pulsar timing arrays to search for, and constrain, subhalos from such models. the models of dm we consider are ordinary adiabatic perturbations in λcdm, qcd axion miniclusters, models with early matter domination, and vector dm produced during inflation. we show that λcdm, largely due to tidal stripping effects in the milky way, is out of reach for ptas. axion miniclusters may be within reach, although this depends crucially on whether the axion relic density is dominated by the misalignment or string contribution. models where there is matter domination with a reheat temperature below 1 gev may be observed with future ptas. lastly, vector dm produced during inflation can be detected if it is lighter than 10−16 gev. we also make publicly available a python monte carlo tool for generating the pta time delay signal from any model of dm substructure. | probing small-scale power spectra with pulsar timing arrays |
milagro and the high altitude water cherenkov (hawc) observatory have detected extended tev gamma-ray emission around nearby pulsar wind nebulae. building on these discoveries, t. linden et al., phys. rev. d 96, 103016 (2017)., 10.1103/physrevd.96.103016 identified a new source class—tev halos—powered by the interactions of high-energy electrons and positrons that have escaped from the pwn, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. many theoretical properties of tev halos remain mysterious, but empirical arguments suggest that they are ubiquitous. the key to progress is finding more halos. we outline prospects for new discoveries and calculate their expectations and uncertainties. we predict, using models normalized to current data, that future hawc and cherenkov telescope array observations will detect in total ∼50-240tev halos, though we note that multiple systematic uncertainties still exist. further, the existing high energy stereoscopic system source catalog could contain ∼10-50tev halos that are presently classified as unidentified sources or pwn candidates. we quantify the importance of these detections for new probes of the evolution of tev halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays. | tev halos are everywhere: prospects for new discoveries |
we have recently shown that axions and axionlike particles (alps) may emit an observable stochastic gravitational wave (gw) background when they begin to oscillate in the early universe. in this note, we identify the regions of alp parameter space, which may be probed by future gw detectors, including ground- and space-based interferometers, and pulsar timing arrays. interestingly, these experiments have the ability to probe axions from the bottom up, i.e., in the very weakly coupled regime, which is otherwise unconstrained. furthermore, we discuss the effects of finite dark photon mass and kinetic mixing on the mechanism, as well as the (in)sensitivity to couplings of the axion to standard model fields. we conclude that realistic axion and alp scenarios may indeed be probed by gw experiments in the future and provide signal templates for further studies. | gravitational wave probes of axionlike particles |
as the third paper in the multiple-part series, we report the statistical properties of radio bursts detected from the repeating fast radio burst (frb) source frb 20201124a with the five-hundred-meter aperture spherical radio telescope during an extremely active episode between the 25th and 28th of september 2021 (ut). we focus on the polarization properties of 536 bright bursts with s/n > 50. we found that the faraday rotation measures (rms) monotonically dropped from -579 to -605 rad m-2 in the 4 day window. the rm values were compatible with the values (-300 to -900 rad m-2) reported 4 months ago. however, the rm evolution rate in the current observation window was at least an order of magnitude smaller than the one (~500 rad m-2 day-1) previously reported during the rapid rm-variation phase, but is still higher than the one (≤1 rad m-2 day-1) during the later rm no-evolution phase. the bursts of frb 20201124a were highly polarized with the total degree of polarization (circular plus linear) greater than 90% for more than 90% of all bursts. the distribution of linear polarization position angles (pas), degree of linear polarization (l/i) and degree of circular polarization (v/i) can be characterized with unimodal distribution functions. during the observation window, the distributions became wider with time, i.e., with larger scatter, but the centroids of the distribution functions remained nearly constant. for individual bursts, significant pa variations (confidence level 5σ) were observed in 33% of all bursts. the polarization of single pulses seems to follow certain complex trajectories on the poincaré sphere, which may shed light on the radiation mechanism at the source or the plasma properties along the path of frb propagation. | fast observations of an extremely active episode of frb 20201124a. iii. polarimetry |
we present the discovery of psr j0250+5854, a radio pulsar with a spin period of 23.5 s. this is the slowest-spinning radio pulsar known. psr j0250+5854 was discovered by the lofar tied-array all-sky survey (lotaas), an all-northern-sky survey for pulsars and fast transients at a central observing frequency of 135 mhz. we subsequently detected pulsations from the pulsar in the interferometric images of the lofar two-meter sky survey, allowing for subarcsecond localization. this, along with a pre-discovery detection 2 years prior, allowed us to measure the spin-period derivative to be \dot{p}=2.7× {10}-14 s s-1. the observed spin period derivative of psr j0250+5854 indicates a surface magnetic field strength, characteristic age, and spindown luminosity of 2.6× {10}13 g, 13.7 myr, and 8.2× {10}28 erg s-1, respectively, for a dipolar magnetic field configuration. this also places the pulsar beyond the conventional pulsar death line, where radio emission is expected to cease. the spin period of psr j0250+5854 is similar to those of the high-energy-emitting magnetars and x-ray dim isolated neutron stars (xdinss). however, the pulsar was not detected by the swift/x-ray telescope in the energy band of 0.3-10 kev, placing a bolometric luminosity limit of 1.5× {10}32 erg s-1 for an assumed {n}{{h}}=1.35× {10}21 cm-2 and a temperature of 85 ev (typical of xdinss). we discuss the implications of the discovery for models of the pulsar death line as well as the prospect of finding more similarly long-period pulsars, including the advantages provided by lotaas for this. | lofar discovery of a 23.5 s radio pulsar |
the hypernuclear matter is studied within the relativistic hartree-fock theory employing several parametrizations of the hypernuclear density functional with density-dependent couplings. the equations of state and compositions of hypernuclear matter are determined for each parametrization and compact stars are constructed by solving their structure equations in spherical symmetry. we quantify the softening effect of fock terms on the equation of state, as well as discuss the impact of tensor interactions, which are absent in the hartree theories. starting from models of density functionals which are fixed in the nuclear sector to the nuclear phenomenology, we vary the couplings in the hyperonic sector around the central values which are fitted to the hyperon potentials in nuclear matter. we use the su(6) spin-flavor and su(3) flavor symmetric quark models to relate the hyperonic couplings to the nucleonic ones. we find, consistent with previous hartree studies, that for the su(6) model the maximal masses of compact stars are below the two-solar mass limit. in the su(3) model we find sufficiently massive compact stars with cores composed predominantly of λ and ξ hyperons and a low fraction of leptons (mostly electrons). the parameter space of the su(3) model is identified where simultaneously hypernuclear compact stars obey the astrophysical limits on pulsar masses and the empirical hypernuclear potentials in nuclear matter are reproduced. | hypernuclear stars from relativistic hartree-fock density functional theory |
close double neutron stars (dnss) have been observed as galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational wave sources. they are believed to have experienced at least one common envelope episode (cee) during their evolution prior to dns formation. in the last decades, there have been numerous efforts to understand the details of the common envelope (ce) phase, but its computational modelling remains challenging. we present and discuss the properties of the donor and the binary at the onset of the roche lobe overflow (rlof) leading to these cees as predicted by rapid binary population synthesis models. these properties can be used as initial conditions for detailed simulations of the ce phase. there are three distinctive populations, classified by the evolutionary stage of the donor at the moment of the onset of the rlof: giant donors with fully convective envelopes, cool donors with partially convective envelopes, and hot donors with radiative envelopes. we also estimate that, for standard assumptions, tides would not circularise a large fraction of these systems by the onset of rlof. this makes the study and understanding of eccentric mass-transferring systems relevant for dns populations. | common envelope episodes that lead to double neutron star formation |
pulsars are factories of relativistic electrons and positrons that propagate away from the pulsar, eventually permeating our galaxy. the acceleration and propagation of these cosmic particles are a matter of intense debate. in the last few years, we have had the opportunity to directly observe the injection of these particles into the interstellar medium through the discovery of gamma-ray haloes around pulsars. this new type of gamma-ray source is produced by electrons and positrons diffusing out of the pulsar wind nebula and scattering ambient photon fields to produce gamma rays. this correspondingly new field of study comes with a number of observations and constraints at different wavelengths and a variety of theoretical models that can explain the properties of these haloes. we examine the characteristics of the propagation of cosmic rays inferred from the observations of gamma-ray haloes and their local and global implications for particle transport within the galaxy. we also discuss the prospects for observations of these sources with facilities such as the large high altitude air shower observatory, the cherenkov telescope array or the southern wide-field gamma-ray observatory in the near future. | gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the galaxy |
general relativity has been widely tested in weak gravitational fields but still stands largely untested in the strong-field regime. according to the no-hair theorem, black holes in general relativity depend only on their masses and spins and are described by the kerr metric. mass and spin are the first two multipole moments of the kerr spacetime and completely determine all higher-order moments. the no-hair theorem and, hence, general relativity can be tested by measuring potential deviations from the kerr metric affecting such higher-order moments. sagittarius a* (sgr a*), the supermassive black hole at the center of the milky way, is a prime target for precision tests of general relativity with several experiments across the electromagnetic spectrum. first, near-infrared (nir) monitoring of stars orbiting around sgr a* with current and new instruments is expected to resolve their orbital precessions. second, timing observations of radio pulsars near the galactic center may detect characteristic residuals induced by the spin and quadrupole moment of sgr a*. third, the event horizon telescope, a global network of mm and sub-mm telescopes, aims to study sgr a* on horizon scales and to image the silhouette of its shadow cast against the surrounding accretion flow using very-long baseline interferometric (vlbi) techniques. both nir and vlbi observations may also detect quasiperiodic variability of the emission from the accretion flow of sgr a*. in this review, i discuss our current understanding of the spacetime of sgr a* and the prospects of nir, timing, and vlbi observations to test its kerr nature in the near future. | sgr a* and general relativity |
much of the parameter space relevant to the evolution of astrophysical circumbinary accretion discs remains unexplored. we have carried out a suite of circumbinary disc simulations surveying both disc thickness and kinematic viscosity, using both constant-ν and constant-α prescriptions. we focus primarily on disc aspect ratios between 0.1 and 0.033, and on viscosities between ν = 0.0005 and ν = 0.008 (in units of binary semimajor axis and orbital frequency), and specialize to circular equal-mass binaries. both factors strongly influence the evolution of the binary semimajor axis: at ν = 0.0005, inspirals occur at aspect ratios ≲ 0.059, while at ν = 0.004 inspirals occur only at aspect ratios ≲ 0.04. inspirals occur largely because of the increasingly strong negative torque on the binary by streams of material which lag the binary, with negligible contributions from resonant torques excited in the circumbinary disc. we find that reductions in accretion rate occur when simulations are initialized too far from the eventual quasi-steady state driven by interaction with the binary, rather than being intrinsically linked to the disc aspect ratio. we find not only that the cavity size increases as viscosity is decreased, but that thinner circumbinary discs become more eccentric. our results suggest that supermassive black hole binaries should be driven, more rapidly than previous estimates, from ~parsec separations to distances where gravitational waves drive their inspiral, potentially reducing the number of binaries observable by pulsar timing arrays. | a survey of disc thickness and viscosity in circumbinary accretion: binary evolution, variability, and disc morphology |
dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condensation. therefore an anisotropic neutron star core seems more realistic than an ideally isotropic one. we model anisotropic neutron stars working in the krori-barua (kb) ansatz without preassuming an equation of state. we show that the physics of general kb solutions is encapsulated in the compactness. imposing physical and stability requirements yields a maximum allowed compactness 2 g m /r c2<0.71 for a kb-spacetime. we further input observational data from numerous pulsars and calculate the boundary density. we focus especially on data from the ligo/virgo collaboration as well as recent independent measurements of mass and radius of miilisecond pulsars with white dwarf companions by the neutron star interior composition explorer (nicer). for these data the kb-spacetime gives the same boundary density which surprisingly equals the nuclear saturation density within the data precision. since this value designates the boundary of a neutron core, the kb-spacetime applies naturally to neutron stars. for this boundary condition we calculate a maximum mass of 4.1 solar masses. | anisotropic neutron stars modelling: constraints in krori-barua spacetime |
background: the recent accurate measurement of the mass of two pulsars close to or above 2 m⊙ has raised the question of whether such large pulsar masses allow for the existence of exotic degrees of freedom, such as hyperons, inside neutron stars. purpose: in the present work, we will investigate, within a phenomenological relativistic mean field approach, how the existing hypernuclei properties may constrain the neutron star equation of state and confront the neutron star maximum masses obtained with equations of state calibrated to hypernuclei properties with the astrophysical 2 m⊙ constraint. method: the study is performed using a relativistic mean field approach to describe both the hypernuclei and the neutron star equations of state. unified equations of state are obtained. a set of five models that describe 2 m⊙ when only nucleonic degrees of freedom are employed. some of these models also satisfy other well-established laboratory or theoretical constraints. results: the λ -meson couplings are determined for all the models considered, and the λ potential in symmetric nuclear matter and λ matter at saturation are calculated. maximum neutron star masses are determined for two values of the λ -ω meson coupling, gω λ=2 gω n/3 and gω λ=gω n , and a wide range of values for gϕ λ. hyperonic stars with the complete baryonic octet are studied, restricting the coupling of the σ and ξ hyperons to the ω ,ρ , and σ mesons due to the lack of experimental data, and maximum star masses calculated. conclusions: we conclude that, within a phenomenological relativistic mean field approach, the currently available hypernuclei experimental data and the lack of constraints on the asymmetric equation of state of nuclear matter at high densities set only a limited number of constraints on the neutron star matter equation of state using the recent 2 m⊙ observations. it is shown that the λ potential in symmetric nuclear matter takes a value of ∼30 -32 mev at saturation for the gω λ coupling given by the su(6) symmetry, being of the order of the values generally used in the literature. on the other hand, the λ potential in λ matter varies between -14 and -8 mev, taking for vector mesons couplings the su(6) values, at variance with generally employed values between -1 and -5 mev. if the su(6) constraint is relaxed and the vector meson couplings to hyperons are kept to values not larger than those of nucleons, then values between -13 and +9 mev are obtained. | hypernuclei and massive neutron stars |
neutron stars are excellent emitters of gravitational waves. squeezing matter beyond nuclear densities invites exotic physical processes, many of which violently transfer large amounts of mass at relativistic velocities, disrupting spacetime and generating copious quantities of gravitational radiation. i review mechanisms for generating gravitational waves with neutron stars. this includes gravitational waves from radio and millisecond pulsars, magnetars, accreting systems, and newly born neutron stars, with mechanisms including magnetic and thermoelastic deformations, various stellar oscillation modes, and core superfluid turbulence. i also focus on what physics can be learnt from a gravitational wave detection, and where additional research is required to fully understand the dominant physical processes at play. | gravitational waves from neutron stars: a review |
the stochastic gravitational-wave background is imprinted on the times of arrival of radio pulses from millisecond pulsars. traditional pulsar timing analyses fit a timing model to each pulsar and search the residuals of the fit for a stationary time correlation. this method breaks down at gravitational-wave frequencies below the inverse observation time of the array; therefore, existing analyses restrict their searches to frequencies above 1 nhz. an effective method to overcome this challenge is to study the correlation of secular drifts of parameters in the pulsar timing model itself. in this paper, we show that timing model correlations are sensitive to sub-nhz stochastic gravitational waves and perform a search using existing measurements of pulsar spin decelerations and pulsar binary orbital decay rates. we do not observe a signal at our present sensitivity, constraining the stochastic gravitational-wave relic energy density to ωgw(f )<3.8 ×10-9 at 450 phz with sensitivity that scales as the frequency squared until approximately 10 phz. we place additional limits on the amplitude of a power-law spectrum of a⋆≲1.8 ×10-14 for a reference frequency of f*=1 yr-1 and the spectral index expected from supermassive black hole binaries, γ =13 /3 . if detection of a supermassive black hole binary signal above 1 nhz is confirmed, this search method will serve as a critical complementary probe of the dynamics of galaxy evolution. | searching for stochastic gravitational waves below a nanohertz |
recent observations have detected extended tev γ -ray emission surrounding young and middle-aged pulsars. the morphology of these "tev halos" requires cosmic-ray diffusion to be locally suppressed by a factor of ∼100 - 1000 compared to the typical interstellar medium. no model currently explains this suppression. we show that cosmic-ray self-confinement can significantly inhibit diffusion near pulsars. the steep cosmic-ray gradient generates alfvén waves that resonantly scatter the same cosmic-ray population, suppressing diffusion within ∼20 pc of young pulsars (≲100 kyr ). in this model, tev halos evolve through two phases, a growth phase where alfvén waves are resonantly generated and cosmic-ray diffusion becomes increasingly suppressed, and a subsequent relaxation phase where the diffusion coefficient returns to the standard interstellar value. intriguingly, cosmic rays are not strongly confined early in the tev halo evolution, allowing a significant fraction of injected e± to escape. if these e± also escape from the surrounding supernova remnant, they would provide a natural explanation for the positron excess observed by pamela and ams-02. recently created tev cosmic rays are confined in the tev halo, matching observations by hawc and h.e.s.s. while our default model relaxes too rapidly to explain the confinement of tev cosmic rays around mature pulsars, such as geminga, models utilizing a kraichnan turbulence spectrum experience much slower relaxation. thus, observations of tev halos around mature pulsars may provide a probe into our understanding of interstellar turbulence. | self-generated cosmic-ray confinement in tev halos: implications for tev γ -ray emission and the positron excess |
rapidly spinning neutron stars are promising sources of continuous gravitational waves. detecting such a signal would allow probing of the physical properties of matter under extreme conditions. a significant fraction of the known pulsar population belongs to binary systems. searching for unknown neutron stars in binary systems requires specialized algorithms to address unknown orbital frequency modulations. we present a search for continuous gravitational waves emitted by neutron stars in binary systems in early data from the third observing run of the advanced ligo and advanced virgo detectors using the semicoherent, gpu-accelerated, binaryskyhough pipeline. the search analyzes the most sensitive frequency band of the ligo detectors, 50-300 hz. binary orbital parameters are split into four regions, comprising orbital periods of three to 45 days and projected semimajor axes of two to 40 light seconds. no detections are reported. we estimate the sensitivity of the search using simulated continuous wave signals, achieving the most sensitive results to date across the analyzed parameter space. | all-sky search in early o3 ligo data for continuous gravitational-wave signals from unknown neutron stars in binary systems |
we investigate remnant neutron star masses (in particular, the minimum allowed mass) by performing advanced stellar evolution calculations and neutrino-radiation hydrodynamics simulations for core-collapse supernova explosions. we find that, based on standard astrophysical scenarios, low-mass carbon-oxygen cores can have sufficiently massive iron cores that eventually collapse, explode as supernovae, and give rise to remnant neutron stars that have a minimum mass of 1.17 m⊙ - compatible with the lowest mass of the neutron star precisely measured in a binary system of psr j0453+1559. | on the minimum mass of neutron stars |
the ligo/virgo collaborations recently announced the detection of a binary neutron star merger, gw190425. the mass of gw190425 is significantly larger than the masses of galactic double neutron stars known through radio astronomy. we hypothesize that gw190425 formed differently from galactic double neutron stars, via unstable 'case bb' mass transfer. according to this hypothesis, the progenitor of gw190425 was a binary consisting of a neutron star and a ∼4- $5\, {\mathrm{ m}_\odot }$ helium star, which underwent common-envelope evolution. following the supernova of the helium star, an eccentric double neutron star was formed, which merged in ${\lesssim }10\, {\rm myr}$ . the helium star progenitor may explain the unusually large mass of gw190425, while the short time to merger may explain why similar systems are not observed in radio. to test this hypothesis, we measure the eccentricity of gw190425 using publicly available ligo/virgo data. we constrain the eccentricity at $10\, {\rm hz}$ to be e ≤ 0.007 with $90{{\ \rm per\ cent}}$ confidence. this provides no evidence for or against the unstable mass transfer scenario, because the binary is likely to have circularized to e ≲ 10-4 by the time it was detected. future detectors will help to reveal the formation channel of mergers similar to gw190425 using eccentricity measurements. | on the origin of gw190425 |
the `magnificent seven' (m7) are a group of radio-quiet isolated neutron stars discovered in the soft x-rays through their purely thermal surface emission. owing to the large inferred magnetic fields (b ≈ 1013 g), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the thermal emission. a large observed polarization degree (pd) is, however, expected only if quantum-electrodynamic (qed) polarization effects are present in the magnetized vacuum around the star. the detection of a strong linearly polarized signal would therefore provide the first observational evidence of qed effects in the strong-field regime. while polarization measurements in the soft x-rays are not feasible yet, optical polarization measurements are within reach also for quite faint targets, like the m7 which have optical counterparts with magnitudes ≈26-28. here, we report on the measurement of optical linear polarization for the prototype, and brightest member, of the class, rx j1856.5-3754 (v ∼ 25.5), the first ever for one of the m7, obtained with the very large telescope. we measured a pd = 16.43 ± 5.26 per cent and a polarization position angle pa = 145.39° ± 9.44°, computed east of the north celestial meridian. the pd that we derive is large enough to support the presence of vacuum birefringence, as predicted by qed. | evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star rx j1856.5-3754 |
the matter state inside neutron stars (nss) is an exciting problem in astrophysics, nuclear physics, and particle physics. the equation of state (eos) of nss plays a crucial role in the present multimessenger astronomy, especially after the event of gw170817. we propose a new ns eos, “qmf18,” from the quark level, which describes robust observational constraints from a free-space nucleon, nuclear matter saturation, heavy pulsar measurements, and the tidal deformability of the very recent gw170817 observation. for this purpose, we employ the quark mean-field model, which allows us to tune the density dependence of the symmetry energy and effectively study its correlations with the love number and the tidal deformability. we provide tabulated data for the new eos and compare it with other recent eoss from various many-body frameworks. | neutron star equation of state from the quark level in light of gw170817 |
the smooth spin-down of young pulsars is perturbed by two non-deterministic phenomenon, glitches, and timing noise. although the timing noise provides insights into nuclear and plasma physics at extreme densities, it acts as a barrier to high-precision pulsar timing experiments. an improved methodology based on the bayesian inference is developed to simultaneously model the stochastic and deterministic parameters for a sample of 85 high-\dot{e} radio pulsars observed for ∼10 yr with the 64-m parkes radio telescope. timing noise is known to be a red process and we develop a parametrization based on the red-noise amplitude (ared) and spectral index (β). we measure the median ared to be -10.4^{+1.8}_{-1.7} yr3/2 and β to be -5.2^{+3.0}_{-3.8} and show that the strength of timing noise scales proportionally to ν 1|\dot{ν }|^{-0.6± 0.1}, where ν is the spin frequency of the pulsar and \dot{ν } is its spin-down rate. finally, we measure significant braking indices for 19 pulsars and proper motions for 2 pulsars, and discuss the presence of periodic modulation in the arrival times of 5 pulsars. | timing of young radio pulsars - i. timing noise, periodic modulation, and proper motion |
the black hole in the center of the milky way, sgr a*, has the largest mass-to-distance ratio among all known black holes in the universe. this property makes sgr a* the optimal target for testing the gravitational no-hair theorem. in the near future, major developments in instrumentation will provide the tools for high-precision studies of its spacetime via observations of relativistic effects in stellar orbits, in the timing of pulsars, and in horizon-scale images of its accretion flow. we explore here the prospect of measuring the properties of the black hole spacetime using all of these three types of observations. we show that the correlated uncertainties in the measurements of the black hole spin and quadrupole moment using the orbits of stars and pulsars are nearly orthogonal to those obtained from measuring the shape and size of the shadow the black hole casts on the surrounding emission. combining these three types of observations will therefore allow us to assess and quantify systematic biases and uncertainties in each measurement and lead to a highly accurate, quantitative test of the gravitational no-hair theorem. | a quantitative test of the no-hair theorem with sgr a* using stars, pulsars, and the event horizon telescope |
the main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionized interstellar medium. timing observations are extraordinarily sensitive to changes in path-length between the pulsar and the earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. here we present a timing analysis of 25 pulsars observed as part of the parkes pulsar timing array (ppta) project over time spans of up to 24 yr. the data are from the second data release of the ppta, which we have extended by including legacy data. we make the first detection of shapiro delay in four southern pulsars (psrs j1017-7156, j1125-6014, j1545-4550, and j1732-5049), and of parallax in six pulsars. the prominent shapiro delay of psr j1125-6014 implies a neutron star mass of mp = 1.5 ± 0.2 m⊙ (68 per cent credibility interval). measurements of both shapiro delay and relativistic periastron advance in psr j1600-3053 yield a large but uncertain pulsar mass of $m_p = 2.06^{+0.44}_{-0.41}$ m⊙ (68 per cent credibility interval). we measure the distance to psr j1909-3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments. | the parkes pulsar timing array second data release: timing analysis |
axions are well-motivated candidates for dark matter. recently, much interest has focused on the detection of photons produced by the resonant conversion of axion dark matter in neutron star magnetospheres. various groups have begun to obtain radio data to search for the signal, however, more work is needed to obtain a robust theory prediction for the corresponding radio lines. in this work we derive detailed properties for the signal, obtaining both the line shape and time-dependence. the principal physical effects are from refraction in the plasma as well as from gravitation which together lead to substantial lensing which varies over the pulse period. the time-dependence from the co-rotation of the plasma with the pulsar distorts the frequencies leading to a doppler broadened signal whose width varies in time. for our predictions, we trace curvilinear rays to the line of sight using the full set of equations from hamiltonian optics for a dispersive medium in curved spacetime. thus, for the first time, we describe the detailed shape of the line signal as well as its time dependence, which is more pronounced compared to earlier results. our prediction of the features of the signal will be essential for this kind of dark matter search. | radio line properties of axion dark matter conversion in neutron stars |
spectra of stochastic gravitational waves (gw) generated in cosmological first-order phase transitions are computed within strongly correlated theories with a dual holographic description. the theories are mostly used as models of dark sectors. in particular, we consider the so-called witten-sakai-sugimoto model, a su(n) gauge theory coupled to different matter fields in both the fundamental and the adjoint representations. the model has a well-known top-down holographic dual description which allows us to perform reliable calculations in the strongly coupled regime. we consider the gw spectra from bubble collisions and sound waves arising from two different kinds of first-order phase transitions: a confinement/deconfinement one and a chiral symmetry breaking/restoration one. depending on the model parameters, we find that the gw spectra may fall within the sensibility region of ground-based and space-based interferometers, as well as of pulsar timing arrays. in the latter case, the signal could be compatible with the recent potential observation by nanograv. when the two phase transitions happen at different critical temperatures, characteristic spectra with double frequency peaks show up. moreover, in this case we explicitly show how to correct the redshift factors appearing in the formulae for the gw power spectra to account for the fact that adiabatic expansion from the first transition to the present times cannot be assumed anymore. | dark holograms and gravitational waves |
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