Abstract: Abstract Low-frequency quasiperiodic oscillations (LFQPOs) are commonly observed in X-ray light curves of black hole X-ray binaries (BHXRBs); however, their origin remains a topic of debate. In order to thoroughly investigate variations in spectral properties on the quasiperiodic oscillation (QPO) timescale, we utilized the Hilbert–Huang transform technique to conduct phase-resolved spectroscopy across a broad energy band for LFQPOs in the newly discovered BHXRB Swift J1727.8–1613. This is achieved through quasi-simultaneous observations from Neutron Star Interior Composition Explorer, Nuclear Spectroscopic Telescope Array, and Hard X-ray Modulation Telescope. Our analysis reveals that both the nonthermal and disk–blackbody components exhibit variations on the quasiperiodic oscillation (QPO) timescale, with the former dominating the QPO variability. For the spectral parameters, we observe modulation of the disk temperature, spectral indices, and reflection fraction with the QPO phase with high statistical significance (≳5 σ ). Notably, the variation in the disk temperature is found to precede the variations in the nonthermal and disk fluxes by ∼0.4–0.5 QPO cycles. We suggest that these findings offer further evidence that the type-C QPO variability is a result of geometric effects of the accretion flow.

Abstract: We present the spectro-polarimetric results obtained from simultaneous X-ray observations with and of the dipping neutron star X-ray binary This source is the most polarized Atoll source so far observed with with a polarization degree of $2.7<!PCT!> 0.9<!PCT!>$ in the 2--8 keV band during the nondip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system ($i The spectra are well described by the combination of soft thermal emission, a Comptonized component, and reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons that contribute significantly even though their fraction in the total flux is not high.

Abstract: ABSTRACT The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness–intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340 + 0 covering the complete ‘Z’-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/Soft X-ray Telescope and NICER (the soft colour is defined as a ratio of 3–6 to 0.5–3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using large area X-ray proportional counter and the first NICER detection of 33.1 ± 1.1 Hz HB oscillation (HBO) in 3–6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41–52 Hz) than the HBOs (16–31 Hz) and NB oscillations (6.2–8 Hz) but significantly lower rms (∼1.6 per cent). The HBO is also limited to the energy range 3–20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarization during HB.

Abstract: Abstract LS I+61°303 is a high-mass X-ray binary system comprising a massive Be star and a rapidly rotating neutron star. Its spectral energy distribution across multiwavelengths categorizes it as a γ -ray binary system. In our analysis of LS I+61°303 using Fermi Large Area Telescope observations, we not only confirmed the three previously discussed periodicities of orbital, superorbital, and orbital–superorbital beat periods observed in multiwavelength observations, but also identified an additional periodic signal. This newly discovered signal exhibits a period of ∼26.3 days at a ∼7 σ confidence level. Moreover, the power spectrum peak of the new signal gradually decreases as the energy increases across the energy ranges of 0.1–0.3, 0.3–1.0, and 1.0–500.0 GeV. Interestingly, a potential signal with a similar period was found in data obtained from the Owens Valley Radio Observatory 40 m telescope. We suggest that the newly discovered periodic signal may originate from a coupling between the orbital period and the retrograde stellar precession period.

Abstract: Abstract The electromagnetic emission and the afterglow observations of the binary neutron star merger event GW170817A confirmed the association of the merger with a short gamma-ray burst (GRB) harboring a narrow (5°–10°) and powerful (10 49 –10 50 erg) jet. Using the 1 s long neutrino-radiation general relativistic MHD simulation of coalescing neutron stars of K. Kiuchi et al., and following the semi-analytical estimates of M. Pais et al., we inject a narrow, powerful, unmagnetized jet into the post-merger phase. We explore different opening angles, luminosities, central engine durations, and times after the merger. We explore early (0.1 s following the merger) and late (1 s) jet launches; the latter is consistent with the time delay of ≈1.74 s observed between GW170817 and GRB 170817A. We demonstrate that the semi-analytical estimates correctly predict the jets’ breakout and collimation conditions. When comparing our synthetic afterglow light curves to the observed radio data of GW170807, we find a good agreement for a 3 × 10 49 erg jet launched late with an opening angle in the range ≃5°–7°.

Abstract: In this work, we investigate an alternative channel for the formation of fast-spinning black hole-neutron star (BHNS) binaries, in which super-Eddington accretion is expected to occur in accreting BHs during the stable mass transfer phase within BH-stripped helium (BH--He-rich) star binary systems. We evolve intensive \texttt{MESA} grids of close-orbit BH--He-rich star systems to systematically explore the projected aligned spins of BHs in BHNS binaries, as well as the impact of different accretion limits on the tidal disruption probability and electromagnetic (EM) signature of BHNS mergers. Most of the BHs in BHNS mergers cannot be effectively spun up through accretion, if the accretion rate is limited to $\lesssim10\,\dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}$ is the standard Eddington accretion limit. In order to reach high spins (e.g., $\chi_{\rm BH} \gtrsim 0.5$), the BHs are required to be born less massive (e.g., $\lesssim3.0\,M_\odot$) in binary systems with initial periods of $\lesssim0.2-0.3\,{\rm days}$ and accrete material at $\sim100\,\dot{M}_{\rm Edd}$. However, even under this high accretion limit, $\gtrsim6\,M_\odot$ BHs are typically challenging to significantly spin up and generate detectable associated EM signals. Our population simulations suggest that different accretion limits have a slight impact on the ratio of tidal disruption events. However, as the accretion limit increases, the EM counterparts from the cosmological BHNS population can become bright overall.

Abstract: Abstract Recent gravitational wave observations include possible detections of black hole - neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the gravitational wave signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the gravitational waves emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.113 to 0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the cases with the least compact neutron star, the gravitational waves emitted, in terms of mismatches, are the most distinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star.

Abstract: Recent gravitational wave observations include possible detections of black hole - neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the gravitational wave signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the gravitational waves emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.123 to 0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the case with the highest mass ratio and most compact neutron star, the gravitational waves emitted, in terms of mismatches, are almost indistinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star.

Abstract: Abstract XB 1254−690 is a neutron star low-mass X-ray binary with an orbital period of 3.88 hrs, and it exhibits energy-dependent intensity dips, thermonuclear bursts, and flares. We present the results of an analysis of a long observation of this source using the AstroSat satellite. The X-ray light curve gradually changed from a high-intensity flaring state to a low-intensity one with a few dips. The hardness intensity diagram showed that the source is in a high-intensity banana state with a gradually changing flux. Based on this, we divide the observation into four flux levels for a flux-resolved spectral study. The X-ray spectra can be explained by a model consisting of absorption, thermal emission from the disc and non-thermal emission from the corona. From our studies, we detect a correlation between the temperature of the thermal component and the flux and we examine the implications of our results for the accretion disc geometry of this source.

Abstract: We consider the final evolutionary stages of a neutron star-black hole pair. According to the current paradigm, such systems eventually coalesce, which in some cases is accompanied by neutron-star tidal disruption. Using analytical methods, we show that the scenario of slow (of the order of several seconds) neutron-star stripping by the black hole is also possible, depending on the system parameters (the initial masses and intrinsic angular momenta of the components, the equation of state for the neutron star). Reaching the lower mass limit (about one tenth of the solar mass), the neutron star explodes to produce a comparatively powerful electromagnetic transient. Our population calculations show that the stripping mechanism is possible in 50-90% of the cases among all coalescing neutron star-black hole pairs, depending on the model assumptions about the evolution of close binary systems (the common-envelope efficiency parameter, the supernova explosion mechanism) and the initial metallicity of the stellar population. Because of the large mass of the ejected material, the kilonova emission in this scenario has good prospects of detection.

Abstract: Abstract We present the results obtained from X-ray and optical analysis of the Be/X-ray binary IGR J06074+2205, focusing on before, during, and after the X-ray outbursts in October and December 2023. The properties of the neutron star in the binary are investigated using NICER and NuSTAR observations during the X-ray outbursts. The pulse profiles across a broad energy range, are found to be strongly dependent on luminosity and energy, revealing the complex nature of the emitting region. An absorbed power-law can describe each NICER spectrum in the 1-7 keV band. The 3–79 keV NuSTAR spectrum can be well-described by a negative and positive power-law with an exponential cut-off model. Utilizing the MAXI/GSC long-term light curve, we estimate the probable orbital period to be 80 or 80/n (n=2,3,4) days. We investigate the evolution of the circumstellar disc around the Be star by using optical spectroscopic observations of the system between 2022 and 2024. We observe variable Hα and FeII emission lines with an increase in equivalent width, indicating the presence of a dynamic circumstellar disc. A distinct variation in the V/R value for Hα and FeII lines is also observed. The appearance of additional emission lines, such as HeI (5875.72 Å), HeI (6678 Å), and HeI (7065 Å), during the post-outburst observation in February 2024 suggests the growing of a larger or denser circumstellar disc. The disc continues to grow without any noticeable mass loss, even during the 2023 X-ray outbursts, which may lead to a future giant X-ray outburst.

Abstract: Gamma-ray binary LS I+61$^{\circ}$ 303 consists of a neutron star orbiting around a Be star with a period of $P_{\rm orb}\simeq26.5\ {\rm d}$. Apart from orbital modulations, the binary shows long-term flux variations with a superorbital period of $P_{\rm sup}\simeq4.6\ {\rm yrs}$ as seen in nearly all wavelengths. The origin of this superorbital modulation is still not well understood. Under the pulsar wind-stellar outflow interaction scenario, we propose that the superorbital modulations of LS I+61$^{\circ}$ 303 could be caused by the precession of the Be disk. Assuming X-rays arise from synchrotron radiation of the intrabinary shock, we develop an analytical model to calculate expected flux modulations over the orbital and superorbital phases. The asymmetric two-peak profiles in orbital light curves and sinusoidal-like long-term modulations are reproduced under the precessing disk scenario. The observed orbital phase drifting of the X-ray peak and our fitting of long-term X-ray data indicate that the neutron star is likely orbiting around the star with a small eccentricity and periastron phase around $\Phi_{\rm p}\sim0.6$. We compare the Corbet diagrams of LS I+61$^{\circ}$ 303 with other Be/X-ray binaries and the linear correlation in the $P_{\rm sup}-P_{\rm orb}$ diagram suggests that the precession of the Be disk in LS I+61$^{\circ}$ 303 is induced by the tidal torque of its neutron star companion.

Abstract: GW170817 and GRB 170817A provided direct evidence that binary neutron star (NSNS) mergers can produce short gamma-ray bursts (sGRBs). However, questions remain about the nature of the central engine. Depending on the mass, the remnant from a NSNS merger may promptly collapse to a black hole (BH), form a hypermassive neutron star (HMNS) which undergoes a delayed collapse to a BH, a supramassive neutron star (SMNS) with a much longer lifetime, or an indefinitely stable NS. There is strong evidence that a BH with an accretion disk can launch a sGRB-compatible jet via the Blandford-Znajek mechanism, but whether a supramassive star can do the same is less clear. We have performed general relativistic magnetohydrodynamics (GRMHD) simulations of the merger of both irrotational and spinning, equal-mass NSNSs constructed from a piecewise polytropic representation of the SLy equation of state, with a range of gravitational (ADM) masses that yield remnants with mass above and below the supramassive limit. Each NS is endowed with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. We examine cases with different initial binary masses, including a case which produces a HMNS which collapses to a BH, and lower mass binaries that produce SMNS remnants. We find similar jet-like structures for both the SMNS and HMNS remnants that meet our basic critera for an incipient jet. The outflow for the HMNS case is consistent with a Blandford-Znajek (BZ) jet. There is sufficient evidence that such BZ-powered outflows can break out and produce ulrarelativistic jets so that we can describe the HMNS system as a sGRB progenitor. However, the incipient jets from the SMNS remnants have much more baryon pollution and we see indications of inefficient acceleration and mixing with the surrounding debris. Therefore, we cannot conclude that SMNS outflows are the progenitors of sGRBs.

Abstract: We present the first X-ray observation of the energetic millisecond pulsar binary PSR\,J1431$-$4715, performed with and complemented with fast optical multi-band photometry acquired with the instrument at ESO-NTT. It is found as a faint X-ray source without a significant orbital modulation. This contrasts with the majority of systems that instead display substantial X-ray orbital variability. The X-ray spectrum is dominated by non-thermal emission and, due to the lack of orbital modulation, does not favour an origin in an intrabinary shock between the pulsar and companion star wind. While thermal emission from the neutron star polar cap cannot be excluded in the soft X-rays, the dominance of synchrotron emission favours an origin in the pulsar magnetosphere that we describe at both X-ray and gamma-ray energies with a synchro-curvature model. The optical multi-colour light curve folded at the 10.8\,h orbital period is double-humped and dominated by ellipsoidal effects, but also affected by irradiation. The light curves are fit with several models encompassing direct heating and a cold spot, or heat redistribution after irradiation either through convection or convection plus diffusion. Despite the inability to constrain the best irradiation models, the fits provide consistent system parameters, giving an orbital inclination of 59$ circ $ and a distance of 3.1pm 0.3\,kpc. The companion is found to be an F-type star, underfilling its Roche lobe RL with a mass of 0.20$ odot $, confirming the redback status, but hotter than the majority of redbacks. The stellar dayside and nightside temperatures of 7500\,K and 7400\,K, respectively, indicate a weak irradiation effect on the companion, likely due to its high intrinsic luminosity. Although the pulsar mass cannot be precisely derived, a heavy (1.8-2.2\ odot $) neutron star is favoured.

Abstract: Abstract Some fast radio bursts (FRBs) exhibit repetitive behaviors, and their origins remain enigmatic. It has been argued that repeating FRBs could be produced by the interaction between a neutron star and an asteroid belt. Here, we consider the systems in which an asteroid belt dwells around a massive star, while a neutron star, as a companion of the massive star, interacts with the belt through gravitational force. Various orbital configurations are assumed for the system. Direct N -body simulations are performed to investigate the dynamical evolution of the asteroids' belt. It is found that a larger orbital eccentricity of the neutron star will destroy the belt more quickly, with a large number of asteroids being scattered out of the system. A low inclination not only suppresses the collisions but also inhibits the ejection rate at early stages. However, highly inclined systems may undergo strong oscillations, resulting in the Kozai–Lidov instabilities. Among the various configurations, a clear periodicity is observed in the collision events for the case with an orbital eccentricity of 0.7 and mutual inclination of 0 ◦ . It is found that such a periodicity can be sustained for at least eight neutron star orbital periods, supporting this mechanism as a possible explanation for periodically repeating FRBs. Our studies also suggest that the active stage of these kinds of FRB sources should be limited, since the asteroid belt would finally be destroyed by the neutron star after multiple passages.

Abstract: On April 25, 2019, the LIGO-Virgo Collaboration discovered a gravitational-wave (GW) signal from a binary neutron star (BNS) merger, that is, GW190425. Due to the inferred large total mass, the origin of GW190425 remains unclear. Assuming GW190425 originated from the standard isolated binary evolution channel, its immediate progenitor is considered to be a close binary system, consisting of a He-rich star and a NS just after the common envelope phase. We aim to study the formation of GW190425 in a solar-like environment by using the detailed binary evolution code MESA We perform detailed stellar structure and binary evolution calculations that take into account mass loss, internal differential rotation, and tidal interactions between a He-rich star and a NS companion. We explore the parameter space of the initial binary properties, including initial NS and He-rich masses and initial orbital period. We find that the immediate post-common-envelope progenitor system, consisting of a primary $ NS and a secondary He-rich star with an initial mass of $ in a close binary with an initial period of $ days days $), that experiences stable Case BB/BC mass transfer (MT) during binary evolution, can reproduce the formation of GW190425-like BNS events. Our studies reveal that the secondary He-rich star of the GW190425's progenitor before its core collapse can be efficiently spun up through tidal interaction, finally remaining as a NS with rotational energy even reaching $ erg $, which is always much higher than the neutrino-driven energy of the supernova (SN) explosion. If the newborn secondary NS is a magnetar, we expect that GW190425 can be the remnant of a magnetar-driven SN, namely a magnetar-driven ultra-stripped SN, a superluminous SN, or a broad-line Type Ic SN. Our results show that GW190425 could be formed through the isolated binary evolution, which involves a stable Case BB/BC MT just after the common envelope phase. On top of that, we show the He-rich star can be tidally spun up, potentially forming a spinning magnetized NS (magnetar) during the second SN explosion.

Abstract: We consider the observational implications of the binary neutron star (BNS) merger GW170817 leaving behind a rapidly rotating massive neutron star that launches a relativistic, equatorial outflow as well as a jet. We show that if the equatorial outflow (ring) is highly beamed in the equatorial plane, its luminosity can be "hidden" from view until late times, even if carrying a significant fraction of the spin-down energy of the merger remnant. This hidden ring reveals itself as a re-brightening in the light curve once it slows down enough for Earth to be within the ring's relativistic beaming solid angle. We compute semi-analytic light curves using this model and find they are in agreement with the observations thus far, and we provide predictions for the ensuing afterglow.

Abstract: In May 2023, the LIGO Livingston observatory detected the likely black hole-neutron star (BHNS) merger GW230529_181500. That event is expected to be the merger of a 2.5-4.5 $M_{\odot}$ primary with a secondary compact object of mass between 1.2-2.0 $M_{\odot}$. This makes it the first BHNS merger with a significant potential for the production of electromagnetic (EM) counterparts, and provides further evidence for compact objects existing within the suspected lower mass gap. To produce post-merger EM transients, the component of the black hole spin aligned with the orbital angular momentum must be sufficiently high, allowing the neutron star to be tidally disrupted. The disrupting BHNS binary may then eject a few percent of a solar mass of matter, leading to an observable kilonova driven by radioactive decays in ejecta, and/or a compact-binary GRB (cbGRB) resulting from the formation of an accretion disk and relativistic jet. Determining which mergers lead to disruption of the neutron star is necessary to predict the prevalence of EM signals from BHNS mergers, yet most BHNS simulations so far have been performed far from the minimum spin required for tidal disruption. Here, we use the Spectral Einstein Code (SpEC) to explore the behavior of BHNS mergers in a mass range consistent with GW230529_181500 close to that critical spin, and compare our results against the mass remnant model currently used by the LVK collaboration to predict the probability of tidal disruption. Our numerical results reveal the emergence of non-zero accretion disks even below the predicted NS disruption limit, of low mass but capable of powering cbGRBs. Our results also demonstrate that the remnant mass model underpredicts the disk mass for the DD2 EOS, while they are within expected modeling errors for SFHo. In all of our simulations, any kilonova signal would be dim and dominated by post-merger disk outflows.

Abstract: Accretion disks formed in binary neutron star mergers play a central role in many astrophysical processes of interest, including the launching of relativistic jets or the ejection of neutron-rich matter hosting heavy element nucleosynthesis. In this work we analyze in detail the properties of accretion disks from 44 ab initio binary neutron star merger simulations for a large set of nuclear equations of state, binary mass ratios and remnant fates, with the aim of furnishing reliable initial conditions for disk simulations and a comprehensive characterization of their properties. We find that the disks have a significant thermal support, with an aspect ratio decreasing with the mass ratio of the binary from $\sim 0.7$ to 0.3. Even if the disk sample spans a broad range in mass and angular momentum, their ratio is independent from the equation of state and from the mass ratio. This can be traced back to the rotational profile of the disc, characterized by a constant specific angular momentum (as opposed to a Keplerian one) of $3-5 \times 10^{16} \rm ~ cm^2~s^{-1}$. The profiles of the entropy per baryon and of the electron fraction depend on the mass ratio of the binary. For more symmetric binaries, they follow a sigmoidal distribution as a function of the rest mass density, for which we provide a detailed description and a fit. The disk properties discussed in this work can be used as a robust set of initial conditions for future long-term simulations of accretion disks from binary neutron star mergers, posing the basis for a progress in the quantitative study of the outflow properties.

Abstract: Context. The coincident detection of GW170817 and GRB170817A marked a milestone for the connection between binary neutron star (BNS) mergers and short gamma-ray bursts (sGRBs). These mergers can lead to the formation of a black hole surrounded by a disk and the generation of a powerful jet. It spends energy to break free from the merger ejecta, and then a portion of it, is dissipated to produce observable emissions. Aims. Our primary goal is to enhance our comprehension of BNS mergers by constraining the disk mass for a selection of sGRBs, utilizing isotropic gamma-ray luminosity and corresponding emission times as key indicators. Methods. In this study, we leverage data from GW170817 to estimate the disk mass surrounding the BNS merger remnant and subsequently infer the accretion-to-jet efficiency. Then statistically examine other sGRBs observations to estimate the possibility of being induced by BNS mergers Results. Our findings suggest that, when employing similar physical parameters as in the sole observed BNS-powered GRB event, GRB170817A, a substantial fraction of sGRBs necessitate an unrealistically massive disk remnant. Conclusions. This observation raises the possibility that either a different mechanis

Abstract: We present optical follow-up of IGR J16194-2810, a hard X-ray source discovered by the INTEGRAL mission. The optical counterpart is a $\sim500\,L_\odot$ red giant at a distance of $2.1$ kpc. We measured 16 radial velocities (RVs) of the giant over a period of $\sim 300$ days. Fitting these RVs with a Keplerian model, we find an orbital period of $P_{\rm orb} = 192.73 \pm 0.01$ days and a companion mass function $f(M_2) = 0.361 \pm 0.005 \,M_{\odot}$. We detect ellipsoidal variability with the same period in optical light curves from the ASAS-SN survey. Joint fitting of the RVs, light curves, and the broadband SED allows us to robustly constrain the masses of both components. We find a giant mass of $M_\star = 1.02\pm 0.01\,M_{\odot}$ and a companion mass of $M_{2} = 1.35^{+0.09}_{-0.07}\,M_{\odot}$, implying that the companion is a neutron star (NS). We recover a $4.06$-hour period in the system's TESS light curve, which we tentatively associate with the NS spin period. The giant does not yet fill its Roche lobe, suggesting that current mass transfer is primarily via winds. MESA evolutionary models predict that the giant will overflow its Roche lobe in $5$-$10$ Myr, eventually forming a recycled pulsar + white dwarf binary with a $\sim 900$ day period. IGR J16194-2810 provides a window on the future evolution of wide NS + main sequence binaries recently discovered via Gaia astrometry. As with those systems, the binary's formation history is uncertain. Before the formation of the NS, it likely survived a common envelope episode with a donor-to-accretor mass ratio $\gtrsim 10$ and emerged in a wide orbit. The NS likely formed with a weak kick ($v_{\rm kick}\lesssim 100\,\rm km\,s^{-1}$), as stronger kicks would have disrupted the orbit.

Abstract: Compact and double compact binary systems are excellent natural laboratories for the study of a variety of physical phenomena. In this Thesis I study black hole binary systems and neutron star low-mass X-ray binaries in order to investigate binary stellar evolution pathways and accretion mechanisms. Estimating the binary black hole merger rate and comparing it with predictions from gravitational-wave observatories is an important method to constrain binary stellar evolution channels. I investigate the chemically homogeneous evolution of close binary systems consisting of rapidly rotating, massive stars at low metallicities, by combining, for the first time, realistic binary models with detailed cosmological calculations of the chemical and star-formation history of the Universe. By constraining the population properties and determining the cosmological and detection rates of binary black hole mergers more precisely than before, I find that the chemically homogeneous evolution pathway can be an important source of aLIGO events. Probing accretion processes in neutron star low-mass X-ray binaries offers unique tests of general relativity and helps to constrain the elusive equation of state of neutron stars. In the next part of the Thesis, I explore one of the most promising models to explain the accreting quasi-periodic oscillations observable in the power density spectra of neutron stars and black holes in binary systems: the relativistic precession model. I analyse the RXTE data of the neutron star X-ray binary 4U1608-52, presenting the first case study where more than one usable set of the necessary three quasi-periodic oscillations (a so-called triplet) are found for one single source, and for which multiple tests of the relativistic precession model can be carried out. I find that the mass and spin values resulting from such triplets cluster around physically realistic values, which can be considered reliable estimates of the neutron star fundamental parameters. If confirmed, my results indicate that 4U1608-52 would be one of the heaviest neutron stars known to date. Finally, I analyse the RXTE data of the 11 Hz accreting pulsar IGR J17480-2446. Previous studies failed to find in this source the very low-frequency quasi-periodic oscillation associated with frame-dragging in the relativistic precession model, thus casting doubt on the validity of the model itself. In this study I find previously undetected very low-frequency quasi-periodic oscillations and show that these are consistent with being produced via frame-dragging, thus re-establishing the validity of the relativistic precession model.

Abstract: Abstract A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary and companion parameters for NS LMXBs using the stellar evolution code MESA. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency (ν) evolution, which depends on various parameters, including accretion rate, fractional mass loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as ν, orbital period (Porb), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the ν, Porb and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.

Abstract: Abstract The Double Neutron Star binary PSR J1846-0513 is discovered by the Five hundred-meter Aperture Spherical radio Telescope (FAST) in Commensal Radio Astronomy FAST Survey. The pulsar is revealed to be harbored in an eccentric orbit with e = 0.208 and orbital period of 0.613 day. The total mass of the binary is constrained to be 2.6287(35)M⊙, with the upper mass limit of the pulsar at 1.3455 M⊙ and the lower mass limit of the com panion at 1.2845 M⊙. To reproduce its evolution history, we simulated the 1D model of its progenitor from neutron star - helium (He) star binary system via MESA code. Since the large eccentricity is widely believed to originate from an asymmetric supernova explosion, we examined the dynamical effects of the supernova. Our results show that the binary may&#xD;stem from a helium star with initial mass of 3.3 − 4.0 M⊙ accompanied by a neutron star in a circle orbit with initial period of ∼ 0.5 day.

Abstract: Abstract Ser X–1 is a low-mass neutron star X-ray binary and has been persistently accreting since its discovery in the 1960s. It has always been observed to be in a soft spectral state and has never showed substantial long-term X-ray variability. Ser X–1 has one previous radio observation in the literature in which radio emission was detected during this soft state, which is contrary to the behavior of black hole X-ray binaries. We have recently obtained 10 randomly sampled radio epochs of Ser X–1 in order to further investigate its anomalous soft-state radio emission. Out of 10 epochs, we find 8 nondetections and 2 detections at 10 GHz flux densities of 19.9 ± 4.2 μ Jy and 32.2 ± 3.6 μ Jy, respectively. We do not detect polarization in either epoch, ruling out very high polarization levels (≲63% and 34%). We compare these Ser X–1 results to other X-ray binaries and consider explanations for its long-term variable radio behavior.

Abstract: Understanding the nature of galactic populations of double compact binaries (where both stars are a neutron star or black hole) has been a topic of interest for many years, particularly the coalescence rate of these binaries. The only observed systems thus far are double neutron star systems containing one or more radio pulsars. However, theorists have postulated that short-duration gamma-ray bursts may be evidence of coalescing double neutron star or neutron star-black hole binaries, while long-duration gamma-ray bursts are possibly formed by tidally enhanced rapidly rotating massive stars that collapse to form black holes (collapsars). The work presented here examines populations of double compact binary systems and tidally enhanced collapsars. We make use of binpop and binkin, two components of a recently developed population synthesis package. Results focus on correlations of both binary and spatial evolutionary population characteristics. Pulsar and long-duration gamma-ray burst observations are used in concert with our models to draw the conclusions that (i) double neutron star binaries can merge rapidly on time-scales of a few million years (much less than that found for the observed double neutron star population), (ii) common-envelope evolution within these models is a very important phase in double neutron star formation and (iii) observations of long gamma-ray burst projected distances are more centrally concentrated than our simulated coalescing double neutron star and collapsar Galactic populations. Better agreement is found with dwarf galaxy models although the outcome is strongly linked to the assumed birth radial distribution. The birth rate of the double neutron star population in our models ranges from 4 to 160 Myr-1 and the merger rate ranges from 3 to 150 Myr-1. The upper and lower limits of the rates result from including electron-capture supernova kicks to neutron stars and decreasing the common-envelope efficiency, respectively. Our double black hole merger rates suggest that black holes should receive an asymmetric kick at birth.

Abstract: In eclipsing X-ray binary systems, the direct X-ray emission is blocked by the companion star during the eclipse. We observe only reprocessed emission that contains clues about the environment of the compact object and its chemical composition, ionization levels, etc. We have found flares in some X-ray binaries during their eclipses. The study of eclipse flares provides additional clues regarding the size of the reprocessing region and helps distinguish between different components of the X-ray spectrum observed during the eclipse. In the archival data, we searched for flares during eclipses of high-mass X-ray binaries and found flares in three sources: Vela X-1, LMC X-4, and 4U 1700-37. Comparing spectral properties of the eclipse flare and non-flare data, we found changes in the power-law photon index in all three sources and multiple emission lines in Vela X-1 and 4U 1700-37. The fluxes of prominent emission lines showed a similar increase as the overall X-ray flux during the eclipse flare, suggesting the lines originate in the binary environment and not in the interstellar medium. We also observed a soft excess in 4U 1700-37 that remains unchanged during both eclipse flare and non-flare states. Our analysis suggests that this emission originates from the extremely thin shell of the stellar wind surrounding the photosphere of its companion star. The detection of short (100-200 seconds) count-rate doubling timescale in 4U 1700-37 and LMC X-4 indicates that the eclipse reprocessing occurs in a region larger than, but comparable to the size of the companion star.

Abstract: CXOU J005245.0-722844 is an X-ray source in the Small Magellanic Cloud (SMC) that has long been known as a Be/X-ray binary (BeXRB) star, containing an OBe main sequence star and a compact object. In this paper, we report on a new very fast X-ray outburst from CXOU J005245.0-722844. X-ray observations taken by Swift constrain the duration of the outburst to less than 16 days and find that the source reached super-Eddington X-ray luminosities during the initial phases of the eruption. The XRT spectrum of CXOU J005245.0-722844 during this outburst reveals a super-soft X-ray source, best fit by an absorbed thermal blackbody model. Optical and Ultraviolet follow-up observations from the Optical Gravitational Lensing Experiment (OGLE), Asteroid Terrestrial-impact Last Alert System (ATLAS), and Swift identify a brief ~0.5 magnitude optical burst coincident with the X-ray outburst that lasted for less than 7 days. Optical photometry additionally identifies the orbital period of the system to be 17.55 days and identifies a shortening of the period to 17.14 days in the years leading up to the outburst. Optical spectroscopy from the Southern African Large Telescope (SALT) confirms that the optical companion is an early-type OBe star. We conclude from our observations that the compact object in this system is a white dwarf (WD), making this the seventh candidate Be/WD X-ray binary. The X-ray outburst is found to be the result of a very-fast, ultra-luminous nova similar to the outburst of MAXI J0158-744.