X-ray Binaries Research Articles

X-ray Spectroscopic Study of Low-Mass X-ray Binaries: A Review of Recent Progress via the Example of GX 339-4

Low-mass X-ray binaries (LMXB) serve as natural laboratories, where the predictions of general relativity can be tested in the strong field regime. The primary object of such sources can be a neutron star (NS) or a black hole (BH), and this object captures material from the secondary object through the inner Lagrange point via a process called Roche lobe overflow. Because of the angular momentum of the infalling matter, an accretion disk is formed, in which viscous effects transport the angular momentum radially outward. In the high/soft state of these sources, the accretion disk can extend all the way to the innermost stable circular orbit (ISCO); therefore, when the primary object is a BH, its X-ray spectrum contains information about the region very close to the event horizon. This paper aims to review the theoretical and observational works related to the X-ray spectroscopy of such sources via the example of GX 339-4, which is one of the most well-known and well-studied LMXBs.

UltraCompCAT: A comprehensive catalogue of ultra-compact and short orbital period X-ray binaries

Ultracompact X-ray binaries (UCXBs) are a distinctive but elusive family of low-mass X-ray binaries (LMXBs) characterised by their tight orbits and degenerate donor stars. Here we present UltraCompCAT, the first online and comprehensive catalogue of UCXBs. The initial version of UltraCompCAT comprises 49 sources, including 20 confirmed UCXBs (those with a measured orbital period shorter than 80 min) and 25 systems that we label ‘candidates’ based on their multi-wavelength phenomenology. For completeness, we also include four LMXBs with orbital periods in the range of 80 to 120 min, since they might be related to (e.g. close progenitors) or even part of the UCXB population that evolved towards longer periods. We discuss the orbital period and Galactic distribution of the catalogue sample. We provide evidence for the presence of at least two separate groups of UCXBs, one formed by persistent systems with orbital periods shorter than 30 min and a second group of transient objects (70 %) with periods in the range of 40 to 60 min. We show that the former group is dominated by sources formed in globular clusters, while the latter accounts for the (known) UCXB population in the Galactic field. We discuss the possible evolutionary channels for the two groups.

Moving Corona and the Line Profile of the Relativistic Broad Iron Emission Line

Iron fluorescence emission lines from X-ray binaries and active galactic nuclei are important diagnostic tools for studying the physical processes near the event horizon of both the stellar-mass black holes in X-ray binaries and the supermassive black holes in active galactic nuclei. In this work, we investigate the line profile of the relativistic broad iron lines from the cool accretion disk of a black hole due to the asymmetric illumination of a moving corona, which moves away from the disk with a relativistic velocity. Both the off-axis location and the radial velocity of the moving corona are considered. Our results clearly show that the illumination and the line profile are dependent on the position and velocity of the corona, since the disk region below the corona receives more flux, which is the most important factor affecting the line profiles. As expected, if the corona is close to the receding part of the rotating disk, the red peak is enhanced, while the blue peak is weakened in the broad line profile, and the central energy of the emission line is low. Conversely, if the corona is close to the approaching part of the disk, the blue peak is strong and the central energy of the emission line is high, even higher than the intrinsic energy of the emission line. Due to the beaming effect of the moving corona, the corona with high velocity illuminates the outer region of the disk, which leads to the red peak disappearing and there being only one blue peak in the profile of the emission line.

An XMM-Newton View of the Andromeda Galaxy as Explored in a Legacy Survey (New-ANGELS). I. The X-Ray Source Catalog

We introduce the New-ANGELS program, an XMM-Newton survey of a ∼7.2 deg2 area around M31, which aims to study the X-ray populations in the M31 disk and the X-ray-emitting hot gas in the inner halo of M31 up to 30 kpc. In this first paper, we report the catalog of 4506 detected X-ray sources and attempt to cross-identify or roughly classify them. We identify 352 single stars in the foreground, 35 globular clusters, and 27 supernova remnants associated with M31, as well as 62 active galactic nuclei, 59 galaxies, and one galaxy cluster in the background. We uniquely classify 236 foreground stars and 17 supersoft sources based on their X-ray colors. X-ray binaries (83 low-mass and one high-mass) are classified based on their X-ray colors and X-ray variabilities. The remaining X-ray sources either have too low signal-to-noise ratios to calculate their X-ray colors or do not have a unique classification, so are regarded as unclassified. The X-ray source catalog is published online. Studies of the X-ray source populations and the contribution of X-ray sources in the unresolved X-ray emissions based on this catalog will be published in companion papers.

THE ROLE OF BINARY STARS IN UNDERSTANDING THE PHYSICS AND EVOLUTION OF STARS

The diversity of close binary stars (CBS) and the rich manifestation of their activity as a result of the interaction of stellar components have turned their observed family into a very developed and effective tool for studying the evolution of stars. This review presents the main features of modern ideas about the evolution of CBS from their origin to the formation of finite compact remnants of components: degenerate dwarfs, neutron stars and stellar black holes. The main phenomena related to their interaction with each other and accompanying the process of fusion of compact components of the CBS are also listed, taking into account the effect of common envelopes, radiation of gravitational waves in cataclysmic and X-ray binaries, supernovae (SN Ia, SN Ib), gamma-bursters and other systems. The paper is based on a talk presented at the astrophysical memorial seminar “Novelties in Understanding the Evolution of Binary Stars”, dedicated to the 90th anniversary of Professor M.A. Svechnikov.

Chance Coincidences between Black Hole Low-mass X-Ray Binaries and Supernova Remnants

I argue that black hole low-mass X-ray binaries (BH LMXBs) are very unlikely to be physically associated with supernova remnants (SNRs). The timescales of BH LMXBs are so much longer than those of SNRs, that there is only a 0.2% chance of any BH LMXB being identified within its natal SNR. However, the probability of a BH LMXB being projected within an SNR is significant; I estimate that two BH LMXBs should be projected within SNRs from our perspective. I look more closely at the suggestion by Balakrishnan and collaborators of an association between the BH X-ray binary Swift J1728.9-3613 and the SNR G351.9-0.9, and show that this is most likely a chance coincidence.

Investigating the Lower Mass Gap with Low-mass X-Ray Binary Population Synthesis

Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio pulsars have been used to identify a gap between the most massive neutron stars (NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs are typically only possible for transient systems: outburst periods enable detection via all-sky X-ray monitors, while quiescent periods enable radial velocity measurements of the low-mass donor. We quantitatively study selection biases due to the requirement of transient behavior for BH mass measurements. Using rapid population synthesis simulations (COSMIC), detailed binary stellar-evolution models (MESA), and the disk instability model of transient behavior, we demonstrate that transient LMXB selection effects introduce observational biases, and can suppress mass-gap BHs in the observed sample. However, we find a population of transient LMXBs with mass-gap BHs form through accretion-induced collapse of an NS during the LMXB phase, which is inconsistent with observations. These results are robust against variations of binary evolution prescriptions. The significance of this accretion-induced collapse population depends upon the maximum NS birth mass . To reflect the observed dearth of low-mass BHs, COSMIC and MESA models favor . In the absence of further observational biases against LMXBs with mass-gap BHs, our results indicate the need for additional physics connected to the modeling of LMXB formation and evolution.

Spectral Energy Distributions of Southern Binary X-Ray Sources

The rapid variability of X-ray binaries (XRBs) produces a wide range of X-ray states that are linked to activity across the electromagnetic spectrum. It is particularly challenging to study a sample of sources large enough to include all types in their various states, and to cover the full range of frequencies that show flux density variations. Simultaneous observations with many telescopes are necessary. In this project, we monitor 48 XRBs with seven telescopes across the electromagnetic spectrum from 5 × 109 to 1019 Hz, including ground-based radio, IR, and optical observatories, and five instruments on two spacecraft over a 1 week period. We construct spectral energy distributions and matching X-ray color–intensity diagrams for 20 sources that have the most extensive detections. Our observations are consistent with several models of expected behavior proposed for the different classes: we detect no significant radio emission from pulsars or atoll sources, but we do detect radio emission from Z sources in the normal or horizontal branch, and from black holes in the high/soft, low/hard, and quiescent states. The survey data provide useful constraints for more detailed models predicting behavior from the different classes of sources.

Energy-resolved pulse profiles of accreting pulsars: Diagnostic tools for spectral features

Aims. We introduce a method for extracting spectral information from energy-resolved light curves folded at the neutron star spin period (known as pulse profiles) in accreting X-ray binaries. Spectra of these sources are sometimes characterized by features superimposed on a smooth continuum, such as iron emission lines and cyclotron resonant scattering features. We address here the question on how to derive quantitative constraints on such features from energy-dependent changes in the pulse profiles. Methods. We developed a robust method for determining in each energy-selected bin the value of the pulsed fraction using the fast Fourier transform opportunely truncated at the number of harmonics needed to satisfactorily describe the actual profile. We determined the uncertainty on this value by sampling through Monte Carlo simulations a total of 1000 faked profiles. We rebinned the energy-resolved pulse profiles to have a constant minimum signal-to-noise ratio throughout the whole energy band. Finally we characterize the dependence of the energy-resolved pulsed fraction using a phenomenological polynomial model and search for features corresponding to spectral signatures of iron emission or cyclotron lines using Gaussian line profiles. Results. We apply our method to a representative sample of NuSTAR observations of well-known accreting X-ray pulsars. We show that, with this method, it is possible to characterize the pulsed fraction spectra, and to constrain the position and widths of such features with a precision comparable with the spectral results. We also explore how harmonic decomposition, correlation, and lag spectra might be used as additional probes for detection and characterization of such features.

Catalog of the Galactic Population of X-Ray Pulsars in High-mass X-Ray Binary Systems

A catalog of the Galactic population of X-ray pulsars in high-mass X-ray binary (HMXB) systems is presented. It contains information about 82 confirmed sources: 18 persistent and 64 transient pulsars. Their basic parameters include spin period, spin evolution with global and local spin-up/spin-down and duration, orbital period, X-ray luminosity, magnetic field strength measured by cyclotron line analysis, distance, spectral and luminosity class, and observable parameters of massive companions, which are shown in the tables provided, with corresponding references. Candidates of HMXB pulsars are also listed for further careful consideration.

Observations of X-ray Binaries at the Caucasus Mountain Observatory of SAI MSU

Observations of X-ray Binaries at the Caucasus Mountain Observatory of SAI MSU

Formation and Evolution of Transient Jets and Their Cavities in Black Hole X-Ray Binaries

We propose a model explaining the origin of transient/episodic jets in black hole X-ray binaries, in which they are caused by transitions from a collimated, strongly magnetized jet to a wide, uncollimated outflow. The change occurs when the accretion flow leaves the magnetically choked state due to an increase of the accretion rate for a weakly varying magnetic flux. The formed powerful jet then detaches from its base, and propagates as a discrete ejection. The uncollimated outflow then produces a relativistic plasma that fills the surroundings of the black hole, contributing to the formation of a low-density cavity. While the pressure in the cavity is in equilibrium with the surrounding interstellar medium (ISM), its inertia is orders of magnitude lower than that of the ISM. This implies that the plasma cannot efficiently decelerate the ejecta, explaining most of the observations. The modest deceleration within the cavities observed in some cases can then be due to the presence of clouds and/or filaments, forming a wide transition zone between the cavity and the ISM.

Probing the jet size of two black hole X-ray binaries in the hard state

ABSTRACT Using multifrequency very long baseline interferometry (VLBI) observations, we probe the jet size in the optically thick hard state jets of two black hole X-ray binary (BHXRB) systems, MAXI J1820+070 and V404 Cygni. Because of optical depth effects, the phase-referenced VLBI core positions move along the jet axis of the BHXRB in a frequency-dependent manner. We use this ‘core shift’ to constrain the physical size of the hard state jet. We place an upper limit of 0.3 au on the jet size measured between the 15 and 5 GHz emission regions of the jet in MAXI J1820+070, and an upper limit of 1.0 au between the 8.4 and 4.8 GHz emission regions of V404 Cygni. Our limit on the jet size in MAXI J1820+070 observed in the low-hard state is a factor of 5 smaller than the values previously observed in the high-luminosity hard state (using time lags between multifrequency light curves), thus showing evidence of the BHXRB jet scaling in size with jet luminosity. We also investigate whether motion of the radio-emitting region along the jet axis could affect the measured VLBI parallaxes for the two systems, leading to a mild tension with the parallax measurements of Gaia. Having mitigated the impact of any motion along the jet axis in the measured astrometry, we find the previous VLBI parallax measurements of MAXI J1820+070 and V404 Cygni to be unaffected by jet motion. With a total time baseline of 8 yr, due to having incorporated 14 new epochs in addition to the previously published ones, our updated parallax measurement of V404 Cygni is 0.450 ± 0.018 mas (2.226 ± 0.091 kpc).

XRBcats: Galactic High Mass X-ray Binary Catalogue

Context. We present a new catalogue of high mass X-ray binaries (HMXBs) in the Galaxy that we call the Galactic High Mass X-ray Binary Catalogue (XRBcats), which improves upon the most recent of such catalogues. We include new HMXBs discovered since previous publications and revise the classification for several objects previously considered HMXBs or HMXB candidates. The catalogue includes basic information (e.g. source names, coordinates, types), other data (e.g. distance and X-ray flux estimates, binary system parameters), and other characteristic properties of the 169 HMXBs catalogued. We also present finding charts in several bands from the infrared to hard X-rays for each object. Aims. The aim of this catalogue is to provide a list of all currently known Galactic HMXBs, including basic information on both compact objects and non-degenerate counterpart properties (where available). We also include objects tentatively classified as HXMBs in the literature and give a brief motivation for the classification in each case. Methods. The catalogue is compiled based on a search of known HMXBs and candidates in all publicly available databases and literature published before May 2023. The relevant properties in various wavelength bands were collected for all objects, either from the literature or using data provided by large-scale surveys. For the latter case, the counterparts in each individual survey were found by cross-correlating positions of identified HMXBs with relevant databases. Results. An up-to-date catalogue of Galactic HMXBs is presented to facilitate research in this field. Our goal was to collect a larger set of relevant HMXB properties in a more uniform way compared to previously published works.

Simple Convective Accretion Flows (SCAFs): Explaining the ≈ −1 Density Scaling of Hot Accretion Flows around Compact Accretors

Recent simulations find that hot gas accretion onto compact accretors is often highly turbulent and diskless, and shows a power-law density profile with slope α ρ ≈ −1. These results are consistent with observational constraints, but do not match existing self-similar solutions of radiatively inefficient accretion flows. We develop a theory for this new class of accretion flows, which we dub simple convective accretion flows (SCAFs). We use a set of hydrodynamic simulations to provide a minimalistic example of SCAFs, and develop an analytic theory to explain and predict key flow properties. We demonstrate that the turbulence in the flow is driven locally by convection, and argue that radial momentum balance, together with an approximate up–down symmetry of convective turbulence, yields α ρ = −1 ± few 0.1. Empirically, for an adiabatic hydrodynamic flow with γ ≈ 5/3, we get α ρ ≈ −0.8; the resulting accretion rate (relative to the Bondi accretion rate), , agrees very well with the observed accretion rates in Sgr A*, M87*, and a number of wind-fed supergiant X-ray binaries. We also argue that the properties of SCAFs are relatively insensitive to additional physical ingredients such as cooling and magnetic field; this explains their common appearance across simulations of different astrophysical systems.

Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038

Transitional millisecond pulsars are an emerging class of sources that link low-mass X-ray binaries to millisecond radio pulsars in binary systems. These pulsars alternate between a radio pulsar state and an active low-luminosity X-ray disc state. During the active state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray to optical pulsations are observed only during the high mode. The root cause of this puzzling behaviour remains elusive. This paper presents the results of the most extensive multi-wavelength campaign ever conducted on the transitional pulsar prototype, PSR J1023+0038, covering from the radio to X-rays. The campaign was carried out over two nights in June 2021 and involved 12 different telescopes and instruments, including XMM-Newton, HST, VLT/FORS2 (in polarimetric mode), ALMA, VLA, and FAST. By modelling the broadband spectral energy distributions in both emission modes, we show that the mode switches are caused by changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on top of a compact jet, as testified by the detection of at least one short-duration millimetre flare with ALMA at the high-to-low mode switch. The pulsar is subsequently re-enshrouded, completing our picture of the mode switches.

Bayesian analysis of a relativistic hadronic model constrained by recent astrophysical observations

ABSTRACTWe use Bayesian analysis in order to constrain the equation of state for nuclear matter from astrophysical data related to the recent measurements from the NICER mission, LIGO/Virgo collaboration, and probability distributions of mass and radius from other 12 sources, including thermonuclear busters, and quiescent low-mass X-ray binaries. For this purpose, we base our study on a relativistic hadronic mean field model including an ω − ρ interaction. Our results indicate optimal ranges for some bulk parameters at the saturation density, namely, effective mass, incompressibility, and symmetry energy slope (L0). For instance, we find $L_0 = 50.79^{+15.16}_{-9.24}$ MeV (Case 1) and $L_0 = 75.06^{+8.43}_{-4.43}$ MeV (Case 2) in a 68 per cent confidence interval for the two cases analysed (different input ranges for L0 related to the PREX-II data). The respective parametrizations are in agreement with important nuclear matter constraints, as well as observational neutron star data, such as the dimensionless tidal deformability of the GW170817 event. From the mass–radius curves obtained from these best parametrizations, we also find the ranges of 11.97 km ≤ R1.4 ≤ 12.73 km (Case 1) and 12.34 km ≤ R1.4 ≤ 13.06 km (Case 2) for the radius of the $1.4\, \mathrm{M}_\odot$ neutron star.

Spiral-wave-driven accretion in quiescent dwarf novæ

Context. In dwarf novæ (DNe) and low-mass X-ray binaries, the tidal potential excites spiral waves in the accretion disc. Spiral-wave-driven accretion may be important in quiescent discs, where the angular momentum transport mechanism has yet to be identified. Previous studies were limited to unrealistically high temperatures for numerical studies or to specific regimes for analytical studies. Aims. We perform the first numerical simulation of spiral-wave-driven accretion in the cold temperature regime appropriate to quiescent discs, which have Mach numbers ≳100. Methods. We used the new GPU-accelerated finite-volume code IDEFIX to produce global hydrodynamics 2D simulations of the accretion discs of DN systems with a sufficiently fine spatial resolution to capture the short scale-height of cold, quiescent discs with Mach numbers ranging from 80 to 370. Results. Running the simulations on timescales of tens of binary orbits shows transient angular momentum transport that decays as the disc relaxes from its initial conditions. We find the angular momentum parameter α drops to values of ≪10−2, too weak to drive accretion in quiescence.

Modelling the expected very high energy γ-ray emission from accreting neutron stars in X-ray binaries

ABSTRACT The detection of γ-ray emission from accreting pulsars in X-ray binaries (XRBs) has long been sought after. For some high-mass X-ray binaries (HMXBs), marginal detections have recently been reported. Regardless of whether these will be confirmed or not, future telescopes operating in the γ-ray band could offer the sensitivity needed to achieve solid detections and possibly spectra. In view of future observational advances, we explored the expected emission above 10 GeV from XRBs, based on the Cheng & Ruderman model, where γ-ray photons are produced by the decay of π0 originated by protons accelerated in the magnetosphere of an accreting pulsar fed by an accretion disc. We improved this model by considering, through Monte Carlo simulations, the development of cascades inside of and outside the accretion disc, taking into account pair and photon production processes that involve interaction with nuclei, X-ray photons from the accretion disc, and the magnetic field. We produced grids of solutions for different input parameter values of the X-ray luminosity (Lx), magnetic field strength (B), and for different properties of the region where acceleration occurs. We found that the γ-ray luminosity spans more than five orders of magnitude, with a maximum of ∼1035 erg s−1. The γ-ray spectra show a large variety of shapes: some have most of the emission below ∼100 GeV, others are harder (emission up to 10–100 TeV). We compared our results with Fermi/LAT and VERITAS detections and upper limits of two HMXBs: A0535+26 and GRO J1008−57. More consequential comparisons will be possible when more sensitive instruments will be operational in the coming years.

Investigating the energy-dependent temporal nature of black hole binary system H 1743-322

ABSTRACT Black hole X-ray binaries routinely exhibit quasi-periodic oscillations (QPOs) in their power density spectrum. Studies of QPOs have demonstrated immense ability to understand these dynamical systems although their unambiguous origin still remains a challenge. We investigate the energy-dependent properties of the Type-C QPOs detected for H 1743-322 as observed with AstroSat in its two X-ray outbursts of 2016 and 2017. The combined broad-band LAXPC and SXT spectrum is well modelled with a soft thermal and a hard Comptonization component. The QPO exhibits soft/negative lags i.e. variation in soft-band lags the variation in hard band, although the upper harmonic shows opposite behaviour i.e. hard/positive lags. Here, we model energy-dependent properties (fractional root mean square and time-lag variation with energy) of the QPO and its upper harmonic individually with a general scheme that fits these properties by utilizing the spectral information and consequently allows to identify the radiative component responsible for producing the variability. Considering the truncated disc picture of accretion flow, a simple model with variation in inner disc temperature, heating rate, and fractional scattering with time delays is able to describe the fractional rms and time-lag spectra. In this work, we show that this technique can successfully describe the energy-dependent features and identify the spectral parameters generating the variability.