Binaries comprised of a neutron star (NS) and a black hole (BH) have so far eluded observations as pulsars and with gravitational waves (GWs). We model the formation and evolution of these NS+BH binaries - including pulsar evolution - using the binary population synthesis code COMPAS. We predict the presence of a total of 50-2000 binaries containing a pulsar and a BH (PSR+BHs) in the Galactic field. We find the population observable by the next generation of radio telescopes, represented by the SKA and MeerKAT, current (LIGO/Virgo) and future (LISA) GW detectors. We conclude that the SKA will observe 1-80 PSR+BHs, with 0-4 binaries containing millisecond pulsars. MeerKAT is expected to observe 0-40 PSR+BH systems. Future radio detections of NS+BHs will constrain uncertain binary evolution processes such as BH natal kicks. We show that systems in which the NS formed first (NSBH) can be distinguished from those where the BH formed first (BHNS) by their pulsar and binary properties. We find 40% of the LIGO/Virgo observed NS+BHs from a Milky-Way like field population will have a chirp mass $\geq 3.0$ M$_\odot$. We estimate the spin distributions of NS+BHs with two models for the spins of BHs. The remnants of BHNS mergers will have a spin of $\sim$0.4, whilst NSBH merger remnants can have a spin of $\sim$0.6 or $\sim$0.9 depending on the model for BH spins. We estimate that approximately 25-1400 PSR+BHs will be radio alive whilst emitting GWs in the LISA frequency band, raising the possibility of joint observation by the SKA and LISA.
Searching for gravitational waves in pulsar timing array data is computationally intensive. The data is unevenly sampled, and the noise is heteroscedastic, necessitating the use of a time-domain likelihood function with attendant expensive matrix operations. The computational cost is exacerbated when searching for individual supermassive black hole binaries, which have a large parameter space due to the additional pulsar distance, phase offset and noise model parameters needed for each pulsar. We introduce a new formulation of the likelihood function which can be used to make the Bayesian anal…
Pulsar Timing Arrays are a prime tool to study unexplored astrophysical regimes with gravitational waves. Here we show that the detection of gravitational radiation from individually resolvable super-massive black hole binary systems can yield direct information about the masses and spins of the black holes, provided that the gravitational-wave induced timing fluctuations both at the pulsar and at the Earth are detected. This in turn provides a map of the non-linear dynamics of the gravitational field and a new avenue to tackle open problems in astrophysics connected to the formation and evolu…
I present a review of the main phenomenological properties at high energies related to massive gamma-ray binaries and I discuss some aspects of pulsar models for these objects.
Pulsar timing is a promising technique for detecting low frequency sources of gravitational waves. Historically the focus has been on the detection of diffuse stochastic backgrounds, such as those formed from the superposition of weak signals from a population of binary black holes. More recently, attention has turned to members of the binary population that are nearer and brighter, which stand out from the crowd and can be individually resolved. Here we show that the timing data from an array of pulsars can be used to recover the physical parameters describing an individual black hole binary …
The pulsar timing residuals induced by gravitational waves from non-evolving single binary sources with general elliptical orbits will be analyzed. For different orbital eccentricities, the timing residuals present different properties. The standard deviations of the timing residuals induced by a fixed gravitational wave source will be calculated for different values of the eccentricity. We will also analyze the timing residuals of PSR J0437-4715 induced by one of the best known single gravitational wave sources, the supermassive black hole binary in the blazar OJ287.
We analyze the model of gamma-ray binaries, consisting of a massive star and a pulsar with ultrarelativistic wind. We consider radiation from energetic particles, accelerated at the pulsar wind termination shock, and feedback of this radiation on the wind through production of secondary electron-positron pairs. We show that the pair feedback limits the Lorentz factor of the pulsar wind and creates a population of very energetic pairs, whose radiation may be responsible for the observed gamma-ray signal.
Binary pulsars allow us to carry out precision tests of gravity and have placed stringent bounds on a broad class of theories beyond general relativity. Current and future radio telescopes, such as FAST, SKA, and MeerKAT, may find a new astrophysical system, a pulsar orbiting around a black hole, which will provide us a new source for probing gravity. In this paper, we systematically study the prospects of testing general relativity with such black hole-pulsar binaries. We begin by finding a mapping between generic non-Einsteinian parameters in the orbital decay rate and theoretical constants …
We propose to use pulsar-black hole binaries as a probe of gravitational collider physics. Induced by the gravitation of the pulsar, the atomic transitions of the boson cloud around the black hole back-react on the orbital motion. This leads to the deviation of binary period decrease from that predicted by general relativity, which can be directly probed by the Rømer delay of pulsar time-of-arrivals. The sensitivity and accuracy of this approach is estimated for two typical atomic transitions. It is shown that once the transitions happen within the observable window, the pulsar-timing accuracy…
Pulsar binaries, in particular redback systems, provide good sources to study the pulsar wind flow and its interaction with the companion stars. {\it Fermi}-LAT have proposed probable pulsar binary candidates in its catalogs. To identify pulsar binary sources from the catalog, orbital modulation search of binary candidates is an effective way. TESS observes in survey mode for a large part of the sky and thus provide an excellent data set to periodicity search of pulsar binary candidates by observing the flux variation, thought to mainly come from the stellar companion. Using TESS data we look …
Pulsar timing arrays are sensitive to gravitational wave perturbations produced by individual supermassive black hole binaries during their early inspiral phase. Modified gravity theories allow for the emission of gravitational dipole radiation, which is enhanced relative to the quadrupole contribution for low orbital velocities, making the early inspiral an ideal regime to test for the presence of modified gravity effects. Using a theory-agnostic description of modified gravity theories based on the parametrized post-Einsteinian framework, we explore the possibility of detecting deviations fr…
Redback millisecond pulsars (hereafter redbacks) are a sub-population of eclipsing millisecond pulsars in close binaries. The formation processes of these systems are not clear. The three pulsars showing transitions between rotation- and accretion-powered states belong to both redbacks and transient low-mass X-ray binaries (LMXBs), suggesting a possible evolutionary link between the them. Through binary evolution calculations, we show that the accretion disks in almost all LMXBs are subject to the thermal-viscous instability during certain evolutionary stages, and the parameter space for the d…
(abridged) Gamma-ray binaries are massive stars with compact object companions that are observed to emit most of their energy in the gamma-ray range. One of these binaries is known to contain a radio pulsar, PSR B1259-63. Synchrotron and inverse Compton emission from particles accelerated beyond the light cylinder in striped pulsar winds has been proposed to explain the X-ray to high energy (HE, $>$ 100 MeV) gamma-ray emission from isolated pulsars. This pulsar model extends naturally to binary environments, where seed photons for inverse Compton scattering are provided by the companion star. …
Gamma-ray loud X-ray binaries are binary systems that show non-thermal broadband emission from radio to gamma rays. If the system comprises a massive star and a young non-accreting pulsar, their winds will collide producing broadband non-thermal emission, most likely originated in the shocked pulsar wind. Thermal X-ray emission is expected from the shocked stellar wind, but until now it has neither been detected nor studied in the context of gamma-ray binaries. We present a semi-analytic model of the thermal X-ray emission from the shocked stellar wind in pulsar gamma-ray binaries, and find th…
A rotating black hole can be clouded by light bosons via superradiance, and thus acquire an atom-like structure. If such a gravitational atom system is companioned with a pulsar, the pulsar can trigger transitions between energy levels of the gravitational atom, and these transitions can be detected by pulsar timing. We show that in such pulsar-black hole systems, fine and hyperfine structure transitions are more likely to be probed than the Bohr transition. Also, the calculation of these fine and hyperfine structure transitions are under better analytic control. Thus, these fine and hyperfine…
We have performed population synthesis calculation on the formation of binaries containing a black hole (BH) and a neutron star (NS) in the Galactic disk. Some of important input parameters, especially for the treatment of common envelope evolution, are updated in the calculation. We have discussed the uncertainties from the star formation rate of the Galaxy and the velocity distribution of NS kicks on the birthrate ($ \sim 0.6-13 \rm\, Myr^{-1}$) of BH/NS binaries. From incident BH/NS binaries, by modelling the orbital evolution duo to gravitational wave radiation and the NS evolution as radi…
We present a systematic study of the evolution of intermediate- and low-mass X-ray binaries consisting of an accreting neutron star of mass $1.0-1.8 M_{\odot}$ and a donor star of mass $1.0-6.0 M_{\odot}$. In our calculations we take into account physical processes such as unstable disk accretion, radio ejection, bump-induced detachment, and outflow from the $L_{2}$ point. Comparing the calculated results with the observations of binary radio pulsars, we report the following results. (1) The allowed parameter space for forming binary pulsars in the initial orbital period - donor mass plane inc…
There is accumulating evidence for mass ejection in low-mass X-ray binaries (LMXBs) driven by radio pulsar activity during X-ray quiescence. In this paper we consider the condition for mass ejection by comparing the radiation pressure from a millisecond pulsar, and the gas pressure at the inner Lagrange point or at the surrounding accretion disk. We calculate the critical spin period of the pulsar below which mass ejection is allowed. Combining with the evolution of the mass transfer rate, we present constraints on the orbital periods of the systems. We show that mass ejection could happen in …
The couplings between supermassive black-hole binaries and their environments within galactic nuclei have been well studied as part of the search for solutions to the final parsec problem. The scattering of stars by the binary or the interaction with a circumbinary disk may efficiently drive the system to sub-parsec separations, allowing the binary to enter a regime where the emission of gravitational waves can drive it to merger within a Hubble time. However, these interactions can also affect the orbital parameters of the binary. In particular, they may drive an increase in binary eccentrici…
The Lense-Thirring effect from spinning neutron stars in double neutron star binaries contribute to the periastron advance of the orbit. This extra term involves the moment of inertia of the neutron stars. Moment of inertia, on the other hand, depends on the mass and spin of the neutron star as well as the equation of state of the matter. If at least one member of the double neutron star binary (better the faster one) is a radio pulsar, then accurate timing analysis might lead to the estimation of the contribution of the Lense-Thirring effect to the periastron advance, which will lead to the m…
We study sky maps and light curves of gamma-ray emission from neutron stars in compact binaries, and in isolation. We briefly review some gamma-ray emission models, and reproduce sky maps from a standard isolated pulsar in the Separatrix Layer model. We consider isolated pulsars with several variations of a dipole magnetic field, including superpositions, and predict their gamma-ray emission. Our results provide new heuristics on what can and cannot be inferred about the magnetic field configuration of pulsars from high-energy observations. We find that typical double-peak light curves can be …
Pulsars are some of the most accurate clocks found in nature, while black holes offer a unique arena for the study of quantum gravity. As such, pulsar-black hole (PSR-BH) binaries provide ideal astrophysical systems for detecting the effects of quantum gravity. With the success of aLIGO and the advent of instruments like the SKA and eLISA, the prospects for the discovery of such PSR-BH binaries are very promising. We argue that PSR-BH binaries can serve as ready-made testing grounds for proposed resolutions to the black hole information paradox. We propose using timing signals from a pulsar be…
Several binary systems have been detected at High Energy (HE, E > 100 MeV) and/or Very High Energy (VHE, E > 100 GeV) gamma rays. Some of them are X-ray binaries in which accretion feeds relativistic radio jets and powers the non-thermal emission (i.e., microquasars), whereas in others the power comes from the wind of a young pulsar instead of accretion. Although the power mechanism in these systems is different (accretion vs pulsar wind), all of them are radio, X-ray and gamma-ray emitters, and have a high-mass bright companion (O or B) star that is a source of seed photons for IC scattering …
The stellar encounter rate Gamma has been shown to be strongly correlated with the number of X-ray binaries in clusters and also to the number of radio pulsars. However, the pulsar populations in different clusters show remarkably different characteristics: in some GCs the population is dominated by binary systems, in others by single pulsars and exotic systems that result from exchange encounters. In this paper, we describe a second dynamical parameter for globular clusters, the encounter rate for a single binary, gamma. We find that this parameter provides a good characterization of the diff…
Binaries consisting of a pulsar and a black hole (BH) are a holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date. Clues as to where such systems might exist are therefore highly desirable. We show that if the central parsec around Sgr A* harbors a cluster of ~25,000 stellar BHs (as predicted by mass segregation arguments) and if it is also rich in recycled pulsar bin…
Several gamma-ray binaries show extended X-ray emission that may be associated to interactions of an outflow with the medium. Some of these systems are, or may be, high-mass binaries harboring young nonaccreting pulsars, in which the stellar and the pulsar winds collide, generating a powerful outflow that should terminate at some point in the ambient medium. This work studies the evolution and termination, as well as the related radiation, of the shocked-wind flow generated in high-mass binaries hosting powerful pulsars. A characterization, based on previous numerical work, is given for the st…
The gamma-ray binaries LS 5039 and LS I +61 303 have been detected by Cerenkov telescopes at TeV energies, exhibiting periodic behavior correlated with the orbital period. These gamma-ray binary systems have also been recently detected by the Fermi Gamma-ray Telescope at GeV energies, and combination of GeV and TeV observations are providing both, expected and surprising results. We summarize these results, also considering the multi-frequency scenario, from the perspective of pulsar systems. We discuss similarities and differences of models in which pulsar wind/star wind shocks, or pulsar win…
Magneto-Active Environments in Pulsar Binaries with the MeerKAT Telescope: I. Pulsar sample and their basic properties
University of Cape Town（开普敦大学）；Tsinghua University（清华大学）；SKA Observatory（平方公里阵列射电望远镜天文台）；INAF-Osservatorio Astronomico di Cagliari（意大利国家天体物理研究所卡利亚里天文台）；University of Botswana（博茨瓦纳大学）；McGill University（麦吉尔大学）
A large number of new "black widow" and "redback" energetic millisecond pulsars with irradiated stellar companions have been discovered through radio and optical searches of unidentified \textit{Fermi} sources. Synchrotron emission, from particles accelerated up to several TeV in the intrabinary shock, exhibits modulation at the binary orbital period. Our simulated double-peaked X-ray light curves modulated at the orbital period, produced by relativistic Doppler-boosting along the intrabinary shock, are found to qualitatively match those observed in many sources. In this model, redbacks and tr…
Recent observational and theoretical results have suggested that some of ultraluminous X-ray (ULX) sources may contain neutron star (NS) accretors. However, the formation channel and properties of donor stars of NS ULXs remain uncertain. By adopting the non-conservative and rotation-dependent mass transfer model in the primordial binary evolution, we investigate the way to form pulsar ULXs like observed pulsar ULXs in a systematic way. Our simulation results indicate that pulsar ULXs with Be stars and intermediate or/and high mass donors match observed apparent luminosities, orbital periods an…
We present evidence for a simple linear relationship between the orbital period and super-orbital period in ultra-luminous X-ray (ULX) pulsars, akin to what is seen in the population of disc-fed neutron star super-giant X-ray binary and Be/X-ray binary systems. We argue that the most likely cause of this relationship is the modulation of precessing hot spots or density waves in an accretion or circumstellar disc by the binary motion of the system, implying a physical link between ULX pulsars and high-mass X-ray binary (HMXB) pulsars. This hypothesis is supported by recent studies of Galactic a…
We have compiled a comprehensive library of all X-ray observations of Magellanic pulsars carried out by {\it XMM-Newton}, {\it Chandra}, and {\it RXTE} in the period 1997-2014. In this work, we use the data from 53 high-mass Be/X-ray binaries in the Small Magellanic Cloud to demonstrate that the distribution of spin-period derivatives vs. spin periods of spinning-down pulsars is not at all different than that of the accreting spinning-up pulsars. The inescapable conclusion is that the up and down samples were drawn from the same continuous parent population, therefore Be/X-ray pulsars that are…
Binary systems that harbor a non-accreting pulsar are efficient non-thermal emitters, from radio to gamma rays. This broadband emission is thought to come from the region where the companion star and pulsar winds collide. A paradigmatic example of this source type is PSR B1259$-$63. Whereas the high-energy radiation probes the shock structure at the binary scale, the radio emission is produced well outside the system, in regions where the shocked stellar and pulsar winds are likely mixing due to hydrodynamical instabilities. Understanding the evolution of the shocked flow depends strongly on a…
The quasi-periodic feature of 1-10 years exhibited in pulsar timing noise has not been well understood since 1980. The recently demonstrated correlation between timing noise and variation of pulse profile motivates us to further investigate its origins. We suggest that the quasi-periodicity feature of timing noise, with rapid oscillations lying on lower frequency structure, comes from the geodetic precession of an unseen binary system, which induces additional motion of the pulsar spin axis. The resultant change of azimuth and latitude at which the observer's line of sight crosses the emission…
Ultralight bosons can be abundantly produced through superradiance process by a spinning black hole and form a bound state with hydrogen-like spectrum. We show that such a "gravitational atom" typically possesses anomalously large mass quadrupole and leads to significant orbital precession when it forms an eccentric binary with a second compact object. Dynamically formed black hole binaries or pulsar-black hole binaries are typically eccentric during their early inspirals. We show that the large orbital precession can generate distinct and observable signature in their gravitational wave or pu…
In recent years, surprise discoveries of pulsed emission from the Crab and Vela pulsars above 100 GeV have drawn renewed attention to this largely unexplored region of the energy range. In this paper, we discuss example light curves due to curvature emission, with good resolution in the different energy bands. Continued light curve modelling may help to discriminate between different emission mechanisms, as well as constrain the location where emission is produced within the pulsar magnetosphere, including regions beyond the light cylinder.
The discovery of a binary comprising a black hole (BH) and a millisecond pulsar (MSP) would yield insights into stellar evolution and facilitate exquisitely sensitive tests of general relativity. Globular clusters (GCs) are known to harbor large MSP populations and recent studies suggest that GCs may also retain a substantial population of stellar mass BHs. We modeled the formation of BH+MSP binaries in GCs through exchange interactions between binary and single stars. We found that in dense, massive clusters most of the dynamically formed BH+MSP binaries will have orbital periods of 2 to 10 d…
High-mass binaries hosting young pulsars can be powerful gamma-ray emitters. The stellar wind of the massive star in the system is expected to be clumpy. Since the high-energy emission comes from the pulsar-star wind interaction, the presence of clumps can affect the spectrum and variability of this radiation. We look for the main effects of the clumps on the two-wind interaction region and on the non-thermal radiation. A simple analytical model for the two-wind interaction dynamics was developed accounting for the lifetime of clumps under the pulsar-wind impact. This time plays a very importa…
A spatially-independent model that can be used to calculate the temporal evolution of the electron/ positron spectrum in a spherically expanding pulsar wind nebula is presented. The model is applied to the young nebula G21.5-0.9, as well as to the unidentified TeV sources HESS J1427-608 and HESS J1507-622. The parameters derived from the model strengthens the idea that the unidentified sources can be identified as evolved pulsar wind nebulae.
Unpulsed, high-energy emission from pulsar binaries can be attributed to the interaction of a pulsar wind with that of a companion star. At the shock between the outflows, particles carried away from the pulsar magnetosphere are accelerated and radiate both in synchrotron and inverse Compton processes. This emission constitutes a significant fraction of the pulsar spin-down luminosity. It is not clear however, how the highly magnetized pulsar wind could convert its mainly electromagnetic energy into the particles with such high efficiency. Here we investigate a scenario in which a pulsar strip…
Pulsar winds interacting with sources of external pressure are well-established as efficient and prolific TeV accelerators in our Galaxy. Yet, enabled by observations from Fermi-LAT, a growing class of non-accreting pulsars in binaries has emerged and these are likely to become apparent as TeV emitters in the CTA era. This class consists of the black widows and redbacks, binaries in which a millisecond pulsar interacts with its low-mass companion. In such systems, an intrabinary shock can form as a site of particle acceleration and associated nonthermal emission. We motivate why these sources …
Gamma-ray binaries are colliding wind binaries (CWB) composed of a massive star a non-accreting pulsar with a highly relativistic wind. Particle acceleration at the shocks results in emission going from extended radio emission to the gamma-ray band. The interaction region is expected to show common features with stellar CWB. Performing numerical simulations with the hydrodynamical code RAMSES, we focus on their structure and stability and find that the Kelvin-Helmholtz instability (KHI) can lead to important mixing between the winds and destroy the large scale spiral structure. To investigate …
We report a detailed analysis of the orbital properties of binary millisecond pulsar (MSP) with a white dwarf (WD) companion. Positive correlations between the orbital period $P_{\rm b}$ and eccentricity $ε$ are found in two classes of MSP binaries with a He WD and with a CO/ONeMg WD, though their trends are different. The distribution of $P_{\rm b}$ is not uniform. Deficiency of sources at $P_{\rm b}\sim35-50$~days (Gap 1) have been mentioned in previous studies. On the other hand, another gap at $P_{\rm b}\sim2.5-4.5$~days (Gap 2) is identified for the first time. Inspection of the relation …
Reliable neutron star mass measurements are key to determining the equation-of-state of cold nuclear matter, but these are rare. "Black Widows" and "Redbacks" are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. While inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly-understood variability. Using data fro…
Binary millisecond pulsars (MSPs) are believed to have descended from low-mass X-ray binaries (LMXBs), which have experienced substantial mass transfer and tidal circularization. Therefore, they should have very circular orbits. However, the discovery of several eccentric binary MSPs (with eccentricity $e\sim 0.01-0.1$) challenges this standard picture. Three models have been proposed thus far based on accretion-induced collapse of massive white dwarfs (WDs), neutron star-strange star transition, and formation of circumbinary disks. All of them are subject to various uncertainties, and are not…
The discovery of transitional millisecond pulsars (tMSPs) provided conclusive proof that neutron star (NS) low-mass X-ray binaries (LMXBs) comprise part of the evolutionary pathway towards binary millisecond pulsars (MSPs). Redback and black widow `spider' pulsars are a sub-category of binary MSPs that `devour' their companions through ablation - the process through which material is lifted from the stellar surface by a pulsar wind. In addition to reducing the companion star's mass, ablation introduces observable characteristics like extended, energy-dependent and asymmetric eclipse profiles i…
