It has been suggested that there are $\sim 10^5$ black hole-neutron star (BH-NS) binaries in our Galaxy. However, despite the effort of intensive radio search for decades, none of these binaries has been found to date. These binaries are regarded as a holy grail of astronomy because they can greatly improve our understanding about relativistic systems of compact objects and fundamental physics. In this article, we propose the existence of exotic BH-NS binaries which can open a new way in searching the missing BH-NS binaries in our Galaxy. By considering the possible dark matter density spikes formed around the primordial black holes in the BH-NS binaries, we show that extremely high temperature ($\sim 10^6$ K) could be maintained on the surface of the neutron stars due to effective dark matter capture. This interesting feature can also help reveal the nature of dark matter and possibly further improve the upper limit of dark matter scattering cross section well below $10^{-47}$ cm$^2$.
In this chapter, I present the main X-ray observational characteristics of black-hole binaries and low magnetic field neutron-star binaries, concentrating on what can be considered similarities or differences, with particular emphasis on their fast-timing behaviour.
In this lecture, we give a first introduction to neutron stars, based on fundamental physical principles. After outlining their amazing macroscopic properties, as obtained from observations, we infer the extreme conditions of matter in their interiors. We then describe two crucial physical phenomena which characterize compact stars, gravitational stability of strongly degenerate matter and neutronization of nuclear matter with increasing density, and explain how the formation and properties of neutron stars are a consequence of the extreme compression of matter under gravity. Finally, we descr…
Neutron star X-ray binaries (NS XRBs) associated with supernova remnants (SNRs) are the youngest X-ray binaries that can provide insights into the early evolution of X-ray binaries and the formation properties of neutron stars. There are an increasing number of NS XRBs discovered to be harbored in SNRs in our and nearby galaxies. In this work, we perform binary population synthesis calculations to simulate the population of NS XRBs associated with SNRs for different types of companions, including Roche-lobe overfilling main-sequence stars, Be stars and supergiants. We estimate their birthrates…
We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From the equilibrium solutions we explore the stability of the accretion mounds using the PLUTO MHD code. We find that pressure driven modes disrupt the equilibria beyond a threshold mound mass, forming dynamic structures, as matter spreads over the neutron star surface. Our results show that local variation of magnetic field will significantly affect the shape and nature of the cyclotron features observed in the spectra of High Mass X-ray Binaries…
A substantial fraction of the known neutron stars resides in X-ray binaries -- systems in which one compact object accretes matter from a companion star. Neutron stars in X-ray binaries have magnetic fields among the highest found in the Universe, spanning at least the range from $\sim10^8$ to several 10$^{13}$ G. The magnetospheres around these neutron stars have a strong influence on the accretion process, which powers most of their emission. The magnetic field intensity and geometry, are among the main factors responsible for the large variety of spectral and timing properties observed in t…
Relativistic Lense-Thirring precession of a tilted inner accretion disk around a compact object has been proposed as a mechanism for low-frequency (~0.01-70 Hz) quasi-periodic oscillations (QPOs) in the light curves of X-ray binaries. A substantial misalignment angle (~15-20 degrees) between the inner-disk rotation axis and the compact-object spin axis is required for the effects of this precession to produce observable modulations in the X-ray light curve. A consequence of this misalignment is that in high-inclination X-ray binaries the precessing inner disk will quasi-periodically intercept …
A close high-mass binary system consisting of a neutron star (NS) and a massive OB supergiant companion is expected to lead to a TZO structure, which consists of a NS core and a stellar envelope. We use the scenario machine program to calculate the formation tracks of TZOs in close high mass NS binaries and their subsequent evolution. We propose and demonstrate that the explosion and instant contraction of a TZO structure leave its stellar remnant as a soft gamma ray repeater and an anomalous X-ray pulsar respectively.
We perform a hierarchical Bayesian inference to investigate the population properties of the coalescing compact binaries involving at least one neutron star (NS). With the current observation data, we can not rule out either of the Double Gaussian, Single Gaussian and Uniform NS mass distribution models, although the mass distribution of the Galactic NSs is slightly preferred by the gravitational wave (GW) observations. The mass distribution of black holes (BHs) in the neutron star-black hole (NSBH) population is found to be similar to that for the Galactic X-ray binaries. Additionally, the ra…
X-ray polarimetry missions like IXPE will be able to measure for the first time the polarization properties of accreting, weakly magnetized neutron stars in Low Mass X-ray Binaries. In this work we present simulations of the expected X-ray polarized signal including the coronal emission for different geometries of the corona itself, i.e. a slab above the accretion disc and a spherical shell around the neutron star. The simulations are performed with the fully relativistic Monte Carlo code monk capable of computing the X-ray polarization degree and angle for various physical input parameters of…
Neutron stars offer the opportunity to study the behaviour of matter at densities and temperatures inaccessible to terrestrial experiments. Gravitational-wave observations of binary neutron star coalescences can constrain the neutron-star equation of state before and after merger. After the neutron star binary merges, hyperons can form in the remnant, changing the behaviour of the neutron-star equation of state. In this study, we use finite-entropy equations of state to show that a post-merger remnant can spin up due to cooling. The magnitude of the spin-up depends on the neutron-star equation…
We study the current sample of rapidly rotating neutron stars in both accreting and non-accreting binaries in order to determine whether the spin distribution of accreting neutron stars in low-mass X-ray binaries can be reconciled with current accretion torque models. We perform a statistical analysis of the spin distributions and show that there is evidence for two sub-populations among low-mass X-ray binaries, one at relatively low spin frequency, with an average of ~300 Hz and a broad spread, and a peaked population at higher frequency with average spin frequency of ~575 Hz. We show that th…
We investigate the kilohertz quasi-periodic oscillations of low-mass X-ray binaries within the Hartle-Thorne spacetime. On the basis the relativistic precession model we extract the total mass $M$, angular momentum $J$, and quadrupole moment $Q$ of a compact object in a low-mass X-ray binary by analyzing the data of the Z -source GX 5-1. In view of the recent neutron star model we compute the radius, angular velocity and other parameters of this source by imposing the observational and theoretical constraints on the mass-radius relation.
Neutron star X-ray binaries emit a compact, optically thick, relativistic radio jet during low-luminosity, usually hard states, as Galactic black-hole X-ray binaries do. When radio emission is bright, a hard power-law tail without evidence for an exponential cutoff is observed in most systems. We have developed a jet model that explains many spectral and timing properties of black-hole binaries in the states where a jet is present. Our goal is to investigate whether our jet model can reproduce the hard tail, with the correct range of photon index and the absence of a high-energy cutoff, in neu…
Merging neutron star binaries are prime candidate sources for heavy r-process nucleosynthesis. The amount of heavy r-process material is consistent with the mass ejection and rates of mergers, and abundances of relic radioactive materials suggest that heavy r-process material is produced in rare events. Observations of possible macronovae provide further support for this model. Still, some concerns remain. One is the observation of heavy r-process elements in Ultra Faint Dwarf (UFD) galaxies. The escape velocities from UFDs are so small that the natal kicks, taking place at neutron stars birth…
The dust properties of the line-of-sight materials in neutron star low-mass X-ray binaries (LMXBs) can be probed by X-ray observations and laboratory experiments. We use a Markov chain Monte Carlo (MCMC) method to conduct a spectral analysis of Chandra ACIS-S/HETG archival data of a sample of LMXBs, including GX 5-1 and GX 13+1. Our MCMC-based analysis puts constraints on the Si K-edge dust properties of the outflowing disk winds in this sample. Further X-ray observations of other LMXBs will help us better understand the grain features of dense outflows and accretion flows in neutron star bina…
We search the literature for reports on the spectral properties of neutron-star low-mass X-ray binaries when they have accretion luminosities between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal rela…
Physical collisions and close approaches between stars play an important role in the formation of exotic stellar systems. Standard theories suggest that collisions are rare, occurring only via random encounters between stars in dense clusters. We present a different formation pathway, the triple evolution dynamical instability (TEDI), in which mass loss in an evolving triple star system causes orbital instability. The subsequent chaotic orbital evolution of the stars triggers close encounters, collisions, exchanges between the stellar components, and the dynamical formation of eccentric compac…
We discuss mass transfer in eccentric binaries containing a white dwarf and a neutron star (WD--NS binaries). We show that such binaries are produced from field binaries following a series of mass transfer episodes that allow the white dwarf to form before the neutron star. We predict the orbital properties of binaries similar to the observed WD--NS binary J1141+6545, and show that they will undergo episodic mass transfer from the white dwarf to the neutron star. Furthermore, we describe oil-on-water, a two-phase SPH formalism that we have developed in order to model mass transfer in such bina…
This article gives a very brief introduction about measuring the mass and radius of neutron star from X-ray observations. The masses and radii of neutron stars can be determined from photospheric radius expansion bursts in low-mass X-ray binaries, X-ray pulse profile modeling in accreting X-ray pulsars, gravitational redshift measurement in low-mass X-ray binaries and thermal X-ray spectral fitting in quiescent low-mass X-ray binaries.
Neutron stars are the densest objects known in the Universe. Being the final product of stellar evolution, their internal composition and structure is rather poorly constrained by measurements. It is the purpose of this paper to put some constrains on the mass and moment of inertia of neutron stars based on the interpretation of kHz quasi-periodic oscillations observed in low mass X-ray binaries. We use observations of high-frequency quasi-periodic observations (HF-QPOs) in low mass X-ray binaries (LMXBs) to look for the average mass and moment of inertia of neutron stars. This is done by appl…
We calculate moment of inertia of neutron star with different exotic constituents such as hyperons and (anti)kaon condensates and study its variation with mass and spin frequency. The sets of equation of state, generated within the framework of relativistic mean field model with density-dependent couplings are adopted for the purpose. We follow the quasi-stationary evolution of rotating stars along the constant rest mass sequences, that varies considerably with different constituents in the equation of state. We also explore the universal relations associated with some of the normalised proper…
There are over 100 Be stars that are known to have neutron star companions but only one such system with a black hole. Previous theoretical work suggests this is not due to their formation but due to differences in X-ray luminosity. It has also been proposed that the truncation of the Be star's circumstellar disc is dependent on the mass of the compact object. Hence, Be star discs in black hole binaries are smaller. Since accretion onto the compact object from the Be star's disc is what powers the X-ray luminosity, a smaller disc in black hole systems leads to a lower luminosity. In this paper…
Axion stars could form binaries with neutron stars. Given the extremely strong external magnetic field exhibited by individual neutron stars, there can be a substantial conversion of axions to photons in these binaries. The photon emission is doubly modulated due to the neutron star spinning and the axion star orbiting, yielding a unique discovery signal. Similar features are also generated in binaries between a neutron star and an axion-clouded black hole. Encouragingly, such binaries are found to be within the reach of ongoing and upcoming experiments (e.g., the Five hundred meter Aperture S…
The magnetic inclination angle $χ$, namely the angle between the spin and magnetic axes of a neutron star (NS), plays a vital role in its observational characteristics. However, there are few systematic investigations on its long-term evolution, especially for accreting NSs in binary systems. Applying the model of \citet{2021MNRAS.505.1775B} and the binary evolution code \mesa{}, we simultaneously simulate the evolution of the accretion rate, spin period, magnetic field, and magnetic inclination angle of accreting NSs in intermediate/low X-ray binaries (I/LMXBs). We show that the evolution of …
We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs with masses less than a critical mass of $M_{\rm WD,crit}=0.2\,M_\odot$ undergo stable mass transfer and evolve into ultra-compact $X$-ray binaries. Systems with higher-mass white dwarfs e…
Neutron stars in X-ray binary systems are fascinating objects that display a wide range of timing and spectral phenomena in the X-rays. Not only parameters of the neutron stars, like magnetic field strength and spin period evolve in their active binary phase, the neutron stars also affect the binary systems and their immediate surroundings in many ways. Here we discuss some aspects of the interactions of the neutron stars with their environments that are revelaed from their X-ray emission. We discuss some recent developments involving the process of accretion onto high magnetic field neutron s…
Ultra-luminous X-ray sources (ULXs) are off-nuclear point sources in nearby galaxies with luminosities well exceeding the Eddington limit for stellar-mass objects. It has been recognized after the discovery of pulsating ULXs (PULXs) that a fraction of these sources could be accreting neutron stars in high-mass X-ray binaries (HMXBs) though the majority of ULXs are lacking in coherent pulsations. The earliest stage of some HMXBs may harbor rapidly rotating neutron stars propelling out the matter transferred by the massive companion. The spin-down power transferred by the neutron-star magnetosph…
We review the current status of general relativistic studies for coalescences of black hole--neutron star binaries. First, high-precision computations of black hole--neutron star binaries in quasiequilibrium circular orbits are summarized, focusing on the quasiequilibrium sequences and the mass-shedding limit. Next, the current status of numerical-relativity simulations for the merger of black hole--neutron star binaries is described. We summarize our understanding for the merger process, tidal disruption and its criterion, properties of the merger remnant and ejected material, gravitational w…
In a recent paper, we determined the measurement accuracy of physical parameters for eccentric, precessing, non-spinning, inspiraling, stellar-mass black hole - black hole (BH-BH) binaries for the upcoming second-generation LIGO/VIRGO/KAGRA detector network at design sensitivity using the Fisher matrix method. Here we extend that study to a wide range of binary masses including neutron star - neutron star (NS-NS), NS-BH, and BH-BH binaries with BH masses up to $110 \, M_{\odot}$. The measurement error of eccentricity $e_{10 \,\rm Hz}$ at a gravitational-wave (GW) frequency of $10 \, {\rm Hz}$ …
The author presented the results on the evolution of NS LMXBs and the formation of UCXBs(Ma & Li 2009 for details), and proposed a scenario for the formation of UCXBs from L/IMXBs with the aid of a CB disk in this work.
As neutron star - black hole binaries are expected to be discovered through future pulsar surveys using upcoming facilities, it is necessary to understand various observable properties of such systems. In the present work, we study the advance of the periastron of such binaries under the post-Newtonian formalism over a wide range of parameters. We find that the first and second order post-Newtonian effects and the leading order spin-orbit coupling effects are significant for such binaries but higher order effects can be neglected.
The merger of black hole-neutron star binaries can eject substantial material with the mass ~0.01-0.1M_sun when the neutron star is disrupted prior to the merger. The ejecta shows significant anisotropy, and travels in a particular direction with the bulk velocity ~0.2c. This is drastically different from the binary neutron star merger, for which ejecta is nearly isotropic. Anisotropic ejecta brings electromagnetic-counterpart diversity which is unique to black hole-neutron star binaries, such as viewing-angle dependence, polarization, and proper motion. The kick velocity of the black hole, gr…
We study the feasibility of detecting exotic cores in merging neutron stars with ground-based gravitational-wave detectors. We focus on models with a sharp nuclear/exotic matter interface, and assume a uniform distribution of neutron stars in the mass range $[1,2] M_\odot$. We find that the existence of exotic cores can be confirmed at the 70% confidence level with as few as several tens of detections. Likewise, with such a sample, we find that some models of exotic cores can be excluded {with high confidence}.
We propose a novel method to examine whether Galactic double neutron star binaries are formed in the LISA band. In our method, we assign an effective time fraction $τ$ to each double neutron star binary detected by LISA. This fraction is given as a function of the observed orbital period and eccentricity and should be uniformly distributed in the absence of in-band binary formation. Applying statistical techniques such as the Kolmogorov-Smirnov test to the actual list of $τ$, we can inspect the signature of the in-band binary formation. We discuss the prospects of this method, paying close att…
The discovery of two neutron star-black hole coalescences by LIGO and Virgo brings the total number of likely neutron stars observed in gravitational waves to six. We perform the first inference of the mass distribution of this extragalactic population of neutron stars. In contrast to the bimodal Galactic population detected primarily as radio pulsars, the masses of neutron stars in gravitational-wave binaries are thus far consistent with a uniform distribution, with a greater prevalence of high-mass neutron stars. The maximum mass in the gravitational-wave population agrees with that inferred…
The Galactic Center (GC) is dominated by the gravity of a super-massive black hole (SMBH), Sagittarius A$^*$, and is suspected to contain a sizable population of binary stars. Such binaries form hierarchical triples with the SMBH, undergoing Eccentric Kozai-Lidov (EKL) evolution, which can lead to high eccentricity excitations for the binary companions' mutual orbit. This effect can lead to stellar collisions or Roche-lobe crossings, as well as orbital shrinking due to tidal dissipation. In this work we investigate the dynamical and stellar evolution of such binary systems, especially with reg…
Mergers of binary neutron stars and black hole-neutron star binaries are one of the most promising sources for the ground-based gravitational-wave (GW) detectors and also a high-energy astrophysical phenomenon as illustrated by the observations of gravitational waves and electromagnetic (EM) waves in the event of GW170817. Mergers of these neutron-star binaries are also the most promising site for r-process nucleosynthesis. Numerical simulation in full general relativity (numerical relativity) is a unique approach to the theoretical prediction of the merger process, GWs emitted, mass ejection …
We discuss the prospects of LISA for detecting neutron star binaries (NSBs) in the local group galaxies such as LMC and M31. Using the recently estimated merger rate $ {\rm 1540 \, Gpc^{-3}yr^{-1}}$ and inversely applying the conventional arguments based on the $B$-band galaxy luminosities, we estimate the frequency distributions of NSBs in the local galaxies. We find that, after 10 year observation with its current design sensitivity, LISA might detect $\sim 5$ NSBs both in LMC and M31 with signal-to-noise ratios larger than 10. Some of the NSBs might be three-dimensionally localized well wit…
We develop a method to compute low-eccentricity initial data of black hole--neutron star binaries in the puncture framework extending previous work on other types of compact binaries. In addition to adjusting the orbital angular velocity of the binary, the approaching velocity of a neutron star is incorporated by modifying the helical Killing vector used to derive equations of the hydrostationary equilibrium. The approaching velocity of the black hole is then induced by requiring the vanishing of the total linear momentum of the system, differently from the case of binary black holes in the pu…
The recent inference of a 70M_sun black hole (BH) in the Galactic, detached binary LB-1 has sparked cross-disciplinary debate since a stellar remnant of such large mass is well above what can be expected from stellar-evolutionary theory, especially in an enriched environment like that of the Milky Way. This study focusses on the possibilities of formation of extraordinarily massive BHs at solar and globular cluster (GC)-like metallicities via evolution of massive stellar binaries. A population-synthesis approach is followed utilizing the recently updated BSE program. BHs in the mass range of 5…
Gravitational waveforms from numerical simulations are a critical tool to test and analytically calibrate the waveform models used to study the properties of merging compact objects. In this paper, we present a series of high-accuracy waveforms produced with the SpEC code for systems involving at least one neutron star. We provide for the first time waveforms with sub-radian accuracy over more than twenty cycles for low-mass black hole-neutron star binaries, including binaries with non-spinning objects, and binaries with rapidly spinning neutron stars that maximize the impact on the gravitatio…
Binary systems with neutron stars and double degenerate systems are crucial objects to test current stellar evolution models and Einstein's general relativity. In this study, we present angular momentum loss mechanism via gravitational radiation and magnetized stellar winds for some selected systems with a neutron star. We calculated and plotted their time scales for angular momentum loss. Gravitational wave amplitudes of binary systems with a neutron star components are also estimated and their detectability with a gravitational wave detector (LISA) has been plotted.
Gravitational wave detectors in the LIGO/Virgo frequency band are able to measure the individual masses and the composite tidal deformabilities of neutron-star binary systems. This paper demonstrates that high accuracy measurements of these quantities from an ensemble of binary systems can in principle be used to determine the high density neutron-star equation of state exactly. This analysis assumes that all neutron stars have the same thermodynamically stable equation of state, but does not use simplifying approximations for the composite tidal deformability or make additional assumptions ab…
Using 250 neutron star merger simulations with microphysics, we explore for the first time the role of nuclear incompressibility in the prompt collapse threshold for binaries with different mass ratios. We demonstrate that observations of prompt collapse thresholds, either from binaries with two different mass ratios or with one mass ratio but combined with the knowledge of the maximum neutron star mass or compactness, will constrain the incompressibility at the maximum neutron star density, $K_{\rm max}$, to within tens of percent. This, otherwise inaccessible, measure of $K_{\rm max}$ can po…
We investigate evolutionary pathways leading to neutron star formation through the collapse of oxygen-neon white dwarf (ONe WD) stars in interacting binaries. We consider (1) non-dynamical mass transfer where an ONe WD approaches the Chandrasekhar mass leading to accretion-induced collapse (AIC) and (2) dynamical timescale merger-induced collapse (MIC) between an ONe WD and another WD. We present rates, delay times, and progenitor properties for two different treatments of common envelope evolution. We show that AIC neutron stars are formed via many different channels and the most dominant cha…
We investigate the form of the radiation emitted by an axion star-neutron star binary using a $f(r)=\text{sech(r/R)}$ profile for the axion star. Our analysis takes into account the co-rotating plasma of the neutron star. We find that that there is significant enhancement to the radiated power if the neutron star's spin is tilted towards the plane of the axion star-neutron star orbit, compared to the case where it is perpendicular. We also examine whether the neutron star's co-rotating plasma can play a role in the emitted power and we find that even though dilute axion stars can in principle …
Neutron star-helium white dwarf (NS+He WD) binaries are important evolutionary products of close-orbit binary star systems. They are often observed as millisecond pulsars and may continue evolving into ultra-compact X-ray binaries (UCXBs) and continuous gravitational wave (GW) sources that will be detected by space-borne GW observatories, such as LISA, TianQin and Taiji. Nevertheless, the stability of NS+He WD binaries undergoing mass transfer is not well studied and still under debate. In this paper, we model the evolution of NS+He WD binaries with WD masses ranging from 0.17-0.45 $M_{\odot}$…
The application of standard accretion theory to observations of X-ray binaries provides valuable insights into neutron star properties, such as their spin period and magnetic field. However, most studies concentrate on relatively old systems, where the neutron star is in its late propeller, accretor, or nearly spin equilibrium phase. Here we use an analytic model from standard accretion theory to illustrate the evolution of high-mass X-ray binaries early in their life. We show that a young neutron star is unlikely to be an accretor because of the long duration of ejector and propeller phases. …
The fundamental constituent of matter at high temperature and density has intrigued physicists for quite some time. Recent results from heavy-ion colliders have enriched the Quantum Chromodynamics phase diagram at high temperatures and low baryon density. However, the phase at low temperatures and finite (mostly intermediate) baryon density remain unexplored. Theoretical Quantum Chromodynamics calculation predicts phase transition from hadronic matter to quark matter at such densities. Presently, the best laboratories available to probe such densities lie at the core of neutron stars. Recent r…
Merging neutron star (NS) binaries may be detected by ground-based gravitational wave (GW) interferometers (e.g. LIGO/VIRGO) within this decade and may also generate electromagnetic radiation detectable by wide-field, fast imaging telescopes that are coming online. The GWs can provide new constraint on the NS equation of state (including mass-radius relation and the related nuclear symmetry energy through resonant g-modes). This paper reviews various hydrodynamical (including equilibrium and dynamical/resonant tides) and electrodynamical processes in coalescing NS binaries, with focus on the p…
The gravitational-wave signal from the merger of two neutron stars cannot be easily differentiated from the signal produced by a comparable-mass mixed binary of a neutron star and a black hole. Indeed, both binary types can account for the gravitational-wave signal GW170817 even if its electromagnetic counterpart emission is taken into account. We propose a method that {requires neither information from the post-inspiral phase of the binary nor an electromagnetic counterpart} to identify mixed binaries of neutron stars merging with low-mass black holes using gravitational-waves alone. This met…
Neutron star binaries offer a rich phenomenology in terms of gravitational waves and merger remnants. However, most general relativistic studies have been performed for nearly circular binaries, with the exception of head-on collisions. We present the first numerical relativity investigation of mergers of eccentric equal-mass neutron-star binaries that probes the regime between head-on and circular. In addition to gravitational waves generated by the orbital motion, we find that the signal also contains a strong component due to stellar oscillations (f-modes) induced by tidal forces, extending…
Coalescing neutron star (NS)-black hole (BH) binaries are promising sources of gravitational-waves (GWs) to be detected within the next few years by current GW observatories. If the NS is tidally disrupted outside the BH innermost stable circular orbit, an accretion torus may form, and this could eventually power a short gamma-ray burst (SGRB). The observation of an SGRB in coincidence with gravitational radiation from an NS-BH coalescence would confirm the association between the two phenomena and also give us new insights on NS physics. We present here a new method to measure NS radii and th…
Over the past half century, X-ray and radio observations of accreting neutron stars and stellar mass black holes have yielded a rich observational picture with common features including state transitions and jet formation, but also sharp differences. While black hole X-ray binaries overwhelmingly suppress jets in so-called soft states, accreting neutron stars are less restrictive, with a soft state wind observed in some sources to co-exist with a jet. We propose an explanation for these differences that leads to a generalization of a foundational element, the hardness-intensity diagram of Fend…
We show that gravitational wave emission from neutron star binaries can be used to discover any generic long-ranged muonic force due to the large inevitable abundance of muons inside neutron stars. As a minimal consistent example, we focus on a gauged U(1)$_{L_μ- L_τ}$ symmetry. In pulsar binaries, such U(1)$_{L_μ- L_τ}$ vectors induce an anomalously fast decay of the orbital period through the emission of dipole radiation. We study a range of different pulsar binaries, finding the most powerful constraints for vector masses below ${\cal O}(10^{-18} {\rm eV})$. For merging binaries the presenc…
We discuss the role of dynamical tidal effects for inspiralling neutron star binaries, focussing on features that may be considered "unmodelled" in gravitational-wave searches. In order to cover the range of possibilities, we consider i) individual oscillation modes becoming resonant with the tide, ii) the elliptical instability, where a pair of inertial modes exhibit a nonlinear resonance with the tide, and iii) the non-resonant p-g instability which may arise as high order p- and g-modes in the star couple nonlinearly to the tide. In each case, we estimate the amount of additional energy los…
