High-mass X-ray Binaries Research Articles

Impact of tides on non-coplanar orbits of progenitors of high-mass X-ray binaries

Context. An important stage in the evolution of massive binaries is the formation of a compact object in the system. It is believed that in some cases a momentum kick is imparted to the newly born object, changing the orbital parameters of the binary, such as eccentricity and orbital period, and even acquiring an asynchronous orbit between its components. In this situation, tides play a central role in the evolution of these binaries. Aims. In this work we aim to study how the orbital parameters of a massive binary change after the formation of a compact object when the stellar spin of the non-degenerate companion is not aligned with the orbital angular momentum. Methods. We used MESA, which we modified to be able to evolve binaries with different values of the inclination between the orbital planes before and just after the formation of the compact object. These modifications to the equations solved by the MESA code are extended to the case of non-solid body rotation. Results. We find that the impact of having different initial inclinations is mostly present in the evolution towards an equilibrium state that is independent of the inclination. If the binary separation is small enough such that the interaction happens when the star is burning hydrogen in its core, this state is reached before the beginning of a mass-transfer phase, while for a wider binary not all conditions characterizing the equilibrium are met. We also explore the effect of having different initial rotation rates in the stars and how the Spruit-Tayler dynamo mechanism affects the angular momentum transport for a non-coplanar binary. Conclusions. These findings show that including the inclination in the equations of tidal evolution to a binary after a kick is imparted onto a newly born compact object changes the evolution of some parameters, such as the eccentricity and the spin period of the star, depending on how large this inclination is. Moreover, these results can be used to match the properties of observed X-ray binaries to estimate the strength of the momentum kick.

Evolution of eccentric high-mass X-ray binaries

Context. The formation of neutron stars is associated with powerful astrophysical transients such as supernovae. In many cases, asymmetries in the supernova explosions are thought to be responsible for the large observed velocities of neutron stars. Aims. We aim to study the complete evolutionary history of one particular eccentric high-mass X-ray binary containing a neutron star, GX 301–2, and characterize the natal kick at the time of neutron star formation. Methods. We used the publicly available stellar-evolution code MESA to evolve binaries from their initial stages until the core-collapse scenario. We incorporated a natal kick distribution based on observations to continue the evolution during the X-ray binary phase and search for candidates matching current observations of GX 301–2. Results. We find that the range of initial masses is constrained to be less than around 30 M⊙ depending on the initial mass ratio, as higher initial masses will most likely end up producing a black hole. In the completely conservative mass-transfer scenario under study, only is an interaction between the stars when the donor is still burning Hydrogen in its core, the so-called Case A of mass transfer, able to produce progenitors for GX 301–2. The natal kick study favours kicks of variable strength, which in turn increases the tilt angle between the orbital angular momentum and the spin of the neutron star. Conclusions. We conclude that only a narrow initial progenitor parameter space is able to produce a binary such as GX 301–2 when assuming a completely conservative mass transfer. Additionally, the strength of the natal kick can span a wide range of values, but it can be constrained when considering new data concerning the systemic velocity of the binary. Finally, we derive the fraction of the expected number of binaries such as GX 301–2 in the Galaxy to be ∼6 × 10−5, implying a really low chance of finding a binary similar to GX 301–2.

Photometric Observations of Nine High Mass X-Ray Binaries and Analysis of Potential Periodicities and Variations

Beginning with American Association of Variable Star Observers Alert Notice 348 (2007 April 6) and being continually extended and updated through 2020 April 17 with Alert Notice 377, Dr. Gordon Sarty requested continuing assistance with his ongoing radial velocity observations of High Mass X-Ray Binaries. As of Alert Notice 377, nine targets were identified. These are: 1H 1936+541, RX J2030.5+4751, V2175 Cyg, V1008 Cep, V831 Cas, RX J0440.9+4431/LS V+44 17, V420 Aur, HD 249179, and SAX J0635+0533. While these Alert Notices prioritized radial velocity observations with the objective of determining orbital periods of these binary systems, optical photometry can also reveal other periods and variations due to mass transfer, pulsations, rotations, spin rates, and other effects. This Research Note will address in more detail the observation and analysis of one of these nine High Mass X-Ray Binaries, specifically 1H 1936+541.

Broadband study and the discovery of pulsations from the Be/X-ray binary eRASSU J052914.9−662446 in the Large Magellanic Cloud

Context.The Magellanic Clouds are our nearest star-forming galaxies. While the population of high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud is relatively well studied, our knowledge about the Large Magellanic Cloud (LMC) is far from complete given its large angular extent and the insufficient coverage with X-ray observations.Aims.We conducted a search for new HMXBs in the LMC using data from eROSITA, the soft X-ray instrument on board the Spektrum-Roentgen-Gamma satellite.Methods.After confirming the nature of eRASSU J052914.9−662446 as a hard X-ray source that is positionally coincident with an early-type star, we followed it up with optical spectroscopic observations from the South African Large Telescope (SALT) and a dedicatedNuSTARobservation.Results.We study the broadband timing and spectral behaviour of the newly discovered HMXB eRASSU J052914.9−662446 through eROSITA,Swift, andNuSTARdata in X-rays and the Optical Gravitational Lensing Experiment (OGLE) and SALT RSS data at the optical wavelength. We report the detection of a spin period at 1412 s and suggest that the orbital period of the system is ∼151 days. We thereby establish that eRASSU J052914.9−662446 is an accreting pulsar. Furthermore, through optical spectroscopic observations and the detection of Hαemission, the source is identified as a Be X-ray binary pulsar in the LMC. We also investigated the variability of the source in the optical and X-ray regime over the past decades and provide estimates of the possible magnetic field strength of the neutron star.

Decomposition of galactic X-ray emission with PHOX

Context.X-ray observations of galaxies with high spatial resolution instruments such asChandrahave revealed that major contributions to their diffuse emission originate from X-ray-bright point sources in the galactic stellar field. It has been established that these point sources, called X-ray binaries, are accreting compact objects with stellar donors in a binary configuration. They are classified according to the predominant accretion process: wind-fed in the case of high-mass donors and Roche-lobe mass transfer in the case of low-mass donors. Observationally, it is challenging to reliably disentangle these two populations from each other because of their similar spectra.Aims.We provide a numerical framework with which spatially and spectrally accurate representations of X-ray binary populations can be studied from hydrodynamical cosmological simulations. We construct average spectra, accounting for a hot gas component, and verify the emergence of observed scaling relations between galaxy-wide X-ray luminosity (LX) and stellar mass (M*) and betweenLXand the star-formation rate (SFR).Methods.Using simulated galaxy halos extracted from the (48 h−1 cMpc)3volume of the Magneticum Pathfinder cosmological simulations atz = 0.07, we generate mock spectra with the X-ray photon-simulator PHOX. We extend the PHOXcode to account for the stellar component in the simulation and study the resulting contribution in composite galactic spectra.Results.Well-known X-ray binary scaling relations with galactic SFR andM*emerge self-consistently, verifying our numerical approach. Average X-ray luminosity functions are perfectly reproduced up to the one-photon luminosity limit. Comparing our resultingLX − SFR − M*relation for X-ray binaries with recent observations of field galaxies in the Virgo galaxy cluster, we find significant overlap. Invoking a metallicity-dependent model for high-mass X-ray binaries yields an anticorrelation between mass-weighted stellar metallicity and SFR-normalized luminosity. The spatial distribution of high-mass X-ray binaries coincides with star-formation regions of simulated galaxies, while low-mass X-ray binaries follow the stellar mass surface density. X-ray binary emission is the dominant contribution in the hard X-ray band (2–10 keV) in the absence of an actively accreting central super-massive black hole, and it provides a ∼50% contribution in the soft X-ray band (0.5–2 keV), rivaling the hot gas component.Conclusions.We conclude that our modeling remains consistent with observations despite the uncertainties connected to our approach. The predictive power and easily extendable framework hold great value for future investigations of galactic X-ray spectra.

A high-mass X-ray binary pulsar 4U 1907+09 with multiple absorption-line features in the spectrum

ABSTRACT We report X-ray observations of the high mass X-ray binary (HMXB) pulsar 4U 1907+09. Spectral and timing analysis of the source has been performed using Nuclear Spectroscopic Telescope Array (NuSTAR) observation. Timing analysis of the photon events revealed the coherent X-ray pulsation of the source with a pulse period of 442.92 ± 0.03 s. It is observed that the source is spinning down at a rate of 0.1971(4) s yr−1. The pulse profile is characterized by a decaying amplitude of the secondary peak and relative growth in the amplitude of the primary peak with an increase in energy. The broad-band spectral coverage of NuSTAR has been used to observe multiple absorption features in the X-ray continuum of the source. We confirm the presence of two prominent cyclotron absorption features at ∼17 and ∼38 keV, respectively. In addition, we have detected an absorption-line feature at ∼8 keV, with an equivalent width of ∼1.3 keV. The variation of the spectral parameters with pulse phase has been observed using phase-resolved spectroscopy and the relevant variabilities of the parameters have been discussed with the underlying physical implications. The continuum evolution and variations in spectral parameters have also been studied by time-resolved spectroscopy.

Revisiting the Magnetic Field Distribution of Normal Pulsars: Implications for the Multiple Origins for Neutron Stars

In this study, we revisit the magnetic field (B-field) distribution of normal pulsars, motivated by the fact that the number of known pulsars has exceeded 3300. Here, we divided the normal pulsar samples into three subgroups by constant lines of characteristic age τ ch, i.e., young, middle-aged, and old pulsars. We note that τ ch is not used as the time indicator in this study; instead, it just served as cutting lines to divide the pulsar samples. Then, we applied several statistical tests, i.e., the Anderson–Darling, Shapiro–Wilk, Kolmogorov–Smirnov, and Mann–Whitney–Wilcoxon tests, to the selected normal pulsar samples (N = 1970) and to a data set of 32 neutron stars (NSs) in high-mass X-ray binaries (HMXBs) for comparison purposes. We obtained that (i) the conclusion on the characteristic B field (B ch) log-normal distribution for the normal pulsars by the previous studies is no longer appropriate, while only young pulsars (N = 24, τ ch < 15 kyr) follow a log-normal distribution, indicating that only the B ch of young pulsars is close to real B fields. (ii) In the directly measured B-field range of NS-HMXBs (∼1012–1013 G), the B ch of young pulsars (N = 15) and the real B fields of NS-HMXBs (N = 32) are inferred to be log-normal, and they are further verified to come from the same distribution, implying that there is no significant decay for real B fields, at least within the timescale of ∼10 Myr for normal pulsars. (iii) Statistically, young pulsars (N = 24) are inferred to be self-contained, suggesting that the initial spin period of pulsars is less than 515 ms. (iv) The B ch distributions of three normal pulsar subsamples are different, hinting at the existence of multiple origins for NSs.

Classification of 3 accreting binaries with VLT/X-Shooter spectra

ABSTRACT Since its launch, the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has discovered hundreds of X-ray sources, many of which lack proper classification. This mission also led to the discovery of new categories of high mass X-ray binaries (HMXB). We use the spectra of the X-Shooter instrument at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) to better understand the nature of 3 accreting binaries (IGR J10101−5654, IGR J11435−6109, and IGR J12489−6243) discovered by INTEGRAL. We mainly focused on the lines and continuum from the X-Shooter spectra. We used atlases to constrain the nature of the sources and also complemented the spectra with measurements taken by Spitzer and the Wide-field Infrared Survey Explorer (WISE) in infrared, and parallaxes from Gaia for the distances. We determined the nature of each binary system: a BeHMXB system with a companion star of spectral type B0.5 Ve with peculiar carbon emission for IGR J10101−5654 and IGR J11435−6109, and a CV system with an evolved K star (K0IV–K2IV) for IGR J12489−6243. We also estimated some geometrical parameters of the decretion disc and neutron star’s orbit in the case of IGR J11435−6109.

Do High-spin High-mass X-Ray Binaries Contribute to the Population of Merging Binary Black Holes?

Gravitational-wave observations of binary black hole (BBH) systems point to black hole spin magnitudes being relatively low. These measurements appear in tension with high spin measurements for high-mass X-ray binaries (HMXBs). We use grids of MESA simulations combined with the rapid population-synthesis code COSMIC to examine the origin of these two binary populations. It has been suggested that Case-A mass transfer while both stars are on the main sequence can form high-spin BHs in HMXBs. Assuming this formation channel, we show that depending on the critical mass ratios for the stability of mass transfer, 48%–100% of these Case-A HMXBs merge during the common-envelope phase and up to 42% result in binaries too wide to merge within a Hubble time. Both MESA and COSMIC show that high-spin HMXBs formed through Case-A mass transfer can only form merging BBHs within a small parameter space where mass transfer can lead to enough orbital shrinkage to merge within a Hubble time. We find that only up to 11% of these Case-A HMXBs result in BBH mergers, and at most 20% of BBH mergers came from Case-A HMXBs. Therefore, it is not surprising that these two spin distributions are observed to be different.

First insights into the ISM at z &amp;gt; 8 with JWST: possible physical implications of a high [O iii] λ4363/[O iii] λ5007

ABSTRACT We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at z &amp;gt; 7.5. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3 − 7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift ‘analogue’ populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at z &amp;gt; 7.5. We find that S06355 at z = 7.67 and S10612 at z = 7.66 are similar to local Green Peas and Blueberries. In contrast, S04590 at z = 8.50 appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O iii] λ4363/[O iii] λ5007 ratio (RO3) of 0.048 (or 0.055 when dust-corrected), implying either extremely high electron temperatures, &amp;gt;3 × 104 K, or gas densities &amp;gt;104 cm−3. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterizing the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.

Finding the birthplace of HMXBs in the Galaxy using Gaia EDR3: Kinematical age determination through orbit integration

Context. High-mass X-ray binaries (HMXBs) are produced after the first supernova event in a massive binary. These objects are intrinsically young and can suffer from a significant natal kick. As such, the progenitors of HMXBs are likely to have formed away from the current location of the X-ray emitting systems. Aims. We aim to find the birthplace of the known HMXBs of our Milky Way. Specifically, we want to answer the question of whether the formation of HMXBs can be associated with open stellar clusters and/or Galactic spiral structures and, from that, infer the time elapsed since the first supernova event. Methods. We used astrometric data from Gaia Early Data Release 3 to initialise the position and velocity of each known HMXB from the Galaxy, and to integrate their motion back in time. In parallel, we performed the same calculations on a sample of 1381 open clusters detected by Gaia as well as for four Galactic spiral arms for which shape and motion have also been recently modelled using Gaia data. We report on all the encounter candidates between HMXBs and clusters or spiral arms in the past 100 Myr. Results. In our sample of 26 HMXBs, we infer that seven were born in clusters and eight were born near a Galactic spiral arm, and we conclude that seven others could have formed isolated from these Galactic structures. The birthplaces of the remaining four HMXBs are still inconclusive due to a combination of great distance, poor astrometric data, and a lack of a known open cluster in the vicinity. We provide the kinematical age since the supernova of 15 HMXBs. Conclusions. The astrometry from Gaia and the orbit integration we employed are effective at finding the birthplaces of HMXBs in the Milky Way. By considering the biases in our data and method, we find it is likely that the progenitors of HMXBs preferentially formed alongside other massive stars in open clusters.

Constraints to neutron-star kicks in high-mass X-ray binaries withGaiaEDR3

Context.All neutron star progenitors in neutron-star high-mass X-ray binaries (NS HMXBs) undergo a supernova event that may lead to a significant natal kick impacting the motion of the whole binary system. The space observatoryGaiaperforms a deep optical survey with exquisite astrometric accuracy, for both position and proper motions, that can be used to study natal kicks in NS HMXBs.Aims.Our aim is to survey the observed Galactic NS HMXB population and to quantify the magnitude of the kick imparted onto their NSs, and to highlight any possible differences arising between the various HMXB types.Methods.We performed a census of Galactic NS HMXBs and cross-matched existing detections in X-rays, optical, and infrared with theGaiaEarly Data Release 3 database. We retrieved their parallaxes, proper motions, and radial velocities (when available), and performed a selection based on the quality of the parallax measurement. We then computed their peculiar velocities with respect to the rotating reference frame of the Milky Way, and including their respective masses and periods, we estimated their kick velocities through Markov chain Monte Carlo simulations of the orbit undergoing a supernova event.Results.We infer the posterior kick distributions of 35 NS HMXBs. After an inconclusive attempt at characterising the kick distributions with Maxwellian statistics, we find that the observed NS kicks are best reproduced by a gamma distribution of mean 116−15+18km s−1. We note that supergiant systems tend to have higher kick velocities than Be HMXBs. The peculiar velocity versus non-degenerate companion mass plane hints at a similar trend, supergiant systems having a higher peculiar velocity independently of their companion mass.

Classical OBe Stars as Post-supernova Runaways: Confirming Binary Origins

Massive binaries play an important role in fields ranging from gravitational-wave astronomy to stellar evolution. We provide several lines of evidence that classical OBe stars in the Small Magellanic Cloud (SMC) obtain their rapid rotation from mass and angular momentum transfer in massive binaries, which predicts that the subsequent supernovae should often eject OBe stars into the field. We find that (1) OBe stars have a higher field frequency than OB stars; (2) our cumulative distribution function (CDF) of stellar distances from O stars shows that OBe stars are indeed much more isolated than ordinary OB stars of corresponding spectral types; (3) the CDFs of OBe stars approach that of high-mass X-ray binaries (HMXBs), which are confirmed post-supernova objects; and (4) Oe stars are as isolated from clusters as Be stars, implying that their final masses are relatively independent of their initial masses, consistent with major mass transfer. Lastly, we also find that the spatial distribution of supergiant OBe stars differs from that of classical OBe stars, consistent with the different mechanisms responsible for their emission-line spectra.

A TESS search for donor-star pulsations in high-mass X-ray binaries

ABSTRACT Ground-based optical photometry of the counterparts of high-mass X-ray binaries (HMXBs) has revealed the presence of periodic modulations on time-scales of ∼0.3–0.5 d. More recent space-based observations (CoRoT and TESS) of OB and Be stars have shown that pulsations caused by p and g modes are common in early-type stars. We have therefore undertaken a systematic search for variability in the optical counterparts of 23 HMXBs (mostly neutron star systems, but including one black hole, Cyg X-1) using TESS data primarily in 2 min cadence mode. After removing the orbital period modulation in four systems, we find that all 23 sources show evidence for quasi-periodic variability on periods shorter than ∼1 d. We compare their power spectra with those from observations of other OB- and Be-type stars. In two systems, V725 Tau and HD 249179 (which may not be an HMXB), we find evidence for an outburst, the former being simultaneous with an X-ray flare. We search for changes in the power spectra over the outburst duration and compare them with outbursts seen in other Be systems.

Are Binary Black Hole Mergers and Long Gamma-Ray Bursts Drawn from the Same Black Hole Population?

This paper compares the population of binary black hole (BBH) mergers detected by LIGO/Virgo with selected long gamma-ray burst (GRB) world models convolved with a delay function (LGRBs are used as a tracer of stellar-mass BH formation). The comparison involves the redshift distribution and the fraction of LGRBs required to produce the local rate of BBH mergers. We find that BBH mergers and LGRBs cannot have the same formation history, unless BBH mergers have a long coalescence time of several Gyr. This would imply that BHs born during the peak of long GRB formation at redshift z ≈ 2−3 merge within the horizon of current GW interferometers. We also show that LGRBs are more numerous than BBH mergers, meaning that most of them do not end their lives in BBH mergers. We interpret these results as an indication that BBH mergers and LGRBs constitute two distinct populations of stellar-mass BHs, with LGRBs being more frequent than BBH mergers. We speculate that the descendants of LGRBs may resemble galactic high-mass X-ray binaries more than BBH mergers. Finally, we discuss the possible existence of a subpopulation of fast-spinning LGRB descendants among BBH mergers, showing that this population, if it exists, is expected to become dominant beyond redshift z ≈ 1, leading to a change in the observed properties of BBH mergers.

Three new high-mass X-ray binaries in the Large Magellanic Cloud

Context. The Magellanic Clouds host a large population of high-mass X-ray binary (HMXB) systems, and although the Large Magellanic Cloud (LMC) is an order of magnitude more massive than the Small Magellanic Cloud, there are significantly fewer known HMXBs in the former. Aims. We conducted a search for new HMXBs in XMM-Newton observations that were performed with the aim of investigating supernova remnant candidates in the supergiant shells LMC5 and LMC7. The three observed fields are located in regions that have not been widely explored in the X-ray band. Methods. We analysed the XMM-Newton data to look for sources with hard X-ray spectrum and their counterparts with optical colours and brightness values that are typical of HMXBs. Results. We report the discovery of three new Be/X-ray binaries, two of them showing pulsations in their X-ray flux. With a luminosity of 6.5 × 1034 erg s−1, we see that XMMU J045315.1−693242 in LMC7 was relatively X-ray faint. The long-term OGLE I-band light curve of the V = 15.5 mag counterpart suggests a 49.6 day or 24.8 day orbital period for the binary system. Then, XMMU J045736.9−692727, which is also located in LMC7, was brighter, with a luminosity of 5.6 × 1035 erg s−1 and hard spectrum with a power-law photon index of 0.63. The X-ray flux revealed clear pulsations with a period of 317.7 s. We obtained optical high resolution spectra from the V = 14.2 mag counterpart using the SALT-HRS spectrograph. Hα and Hβ were observed in emission with complex line profiles and equivalent widths of −8.0 Å and −1.3 Å, respectively. The I-band light curve obtained from OGLE shows a series of four strong outbursts followed by a sudden drop in brightness by more than 1 mag within 73–165 days and a recovery to the level from before the outbursts. RX J0524.2−6620, previously classified as X-ray binary candidate, is located at the eastern part of LMC5. We report the discovery of 360.7 s pulsations. During the XMM-Newton observation the luminosity was at ∼4 × 1035 erg s−1 and the source showed a hard spectrum with a power-law photon index of 0.78. The Hα emission line profile obtained from SALT-HRS is characterised by two broad peaks with a separation corresponding to ∼178 km s−1, along with an equivalent width of −4.2 Å. The long-term OGLE I-band light curve of the V = 14.9 mag counterpart reveals a quasi-periodic flaring activity while the colour evolution during the flares follows a hysteresis loop with redder colour during the rise. Based on the modelling the Hα line profiles measured from XMMU J045736.9−692727 and RX J0524.2−6620, we derived constraints on the size of the Be disks. Conclusions. Our discovery of two pulsars among three new Be/X-ray binaries increases the number of known HMXB pulsars in the LMC to 25.

Common patterns in pulse profiles of high-mass X-ray binaries

Context. X-ray pulsars are binary systems which consist of a neutron star in orbit with a mass donor (companion). In these systems the neutron accretes matter from the companion star, which creates accretion columns or hot spots on the neutron star surface and gives rise to pulsations in the X-ray light curve. The pulse profiles carry information about the accretion and magnetic field geometry. Here we present a study and classification of energy-resolved pulse profiles of a sample of X-ray pulsars, focusing on high-mass X-ray binaries. Aims. Our goal is to perform a classification of X-ray pulsars based on their observed pulse profiles and look for correlations between this classification and their principle physical observables. The analysis pipeline is available online. Methods. We analysed the pulse profiles of a sample of X-ray pulsars using data obtained with the X-ray Multi-Mirror Mission (XMM-Newton) and the Nuclear Spectroscopic Telescope Array (NuSTAR). We fitted the energy-resolved pulse profiles with a Fourier series of up to five harmonics. We then used the energy evolution of the different Fourier components to classify the pulse profiles into groups. We investigated relationships between the pulse profile properties and other observables of the systems (e.g. orbital period, magnetic field strength, and luminosity) to study the extreme physics of these systems. Results. The sources were divided into three groups using a classification based on the shape, the dominance of the fitted Fourier harmonics, and their respective evolution with energy. We do not find a conclusive correlation between the pulse profile shapes or groups and other parameters of the systems. However, a weak trend was found when comparing our classification to the sources’ locations in the spin period-orbital period diagram. Further studies are required to confirm this trend. Conclusions. Despite the large variety of pulse profiles of the X-ray pulsars, we found that with our approach clear categories emerge which we use to classify their behaviour as a function of energy. As we do not find a clear relationship between our classification scheme and other parameters, like the luminosity, the magnetic field strength, or the orbital and spin periods, we conclude that X-ray pulse profiles are influenced by other hidden variables.

Statistical Properties of X-Ray Flares from the Supergiant Fast X-Ray Transients

Supergiant fast X-ray transients are a sub-class of high mass X-ray binaries, in which the compact object is a neutron star and it is accreting materials from its non-degenerate companion star. The sample of supergiant fast X-ray transients from XMM-Newton public observations have been intensively studied, and hence the corresponding parameters for identifying X-ray flares have been obtained by Sidoli et al., such as the rise time, the decay time, the duration time, the waiting time, the peak luminosity, the isotropic energy and the mean luminosity. In order to investigate the origin of the X-ray flares from the supergiant fast X-ray transients, we apply the cumulative distribution method with the power-law index to the target sample. We then find that the relevant parameters can be well described by the cumulative distribution with indices 1.0–1.9 for IGR J16418-4532, IGR J16328-4726 and IGR J18450-0435, respectively. This finding indicates that the X-ray flares are likely to be produced by the self-organizing critical process, one possible scenario may be due to reconnection in magnetized stellar wind blobs approaching into the magnetosphere of magnetized neutron star. Therefore, we suggest that the high mass X-ray binaries with similar X-ray flares likely belong to the self-organized criticality systems.

Characteristics and evolution of Be-type high-mass X-ray binaries as potential ultraluminous X-ray sources

ABSTRACT Some ultraluminous X-ray sources (ULXs) exhibit X-ray pulses, and their central sources are thought to be neutron stars. It has also been suggested that some are transient sources with Be-type donors. In this study, we use the mass accretion model of a Be-type high-mass X-ray binary (BeHMXB) to estimate the conditions under which a giant X-ray burst caused by a BeHMXB exceeds the Eddington luminosity. Moreover, we investigate the duration for which BeHMXBs can be observed as transient ULXs with bursts above the Eddington luminosity during binary evolutions. The results indicate that BeHMXBs could be ULXs for a typical duration of approximately 1 Myr. Comparisons with nearby observed BeHMXBs indicate that many binary systems have the potential to become ULXs during their evolution. Particularly, a BeHMXB system tends to become a ULX when the Be donor has a dense deccretion disc aligned with the orbital plane. Because BeHMXBs are very common objects and a significant number of them can become ULXs, we conclude that a reasonable fraction of the observed ULXs could consist of evolved BeHMXBs.

The 21-cm signal from the cosmic dawn: metallicity dependence of high-mass X-ray binaries

ABSTRACT X-rays from high-mass X-ray binaries (HMXBs) are likely the main source of heating of the intergalactic medium (IGM) during Cosmic Dawn (CD), before the completion of reionization. This Epoch of Heating (EoH; z ∼10–15) should soon be detected via the redshifted 21-cm line from neutral hydrogen, allowing us to indirectly study the properties of HMXBs in the unseen, first galaxies. Low-redshift observations, as well as theoretical models, imply that the integrated X-ray luminosity to star formation rate of HMXBs (LX/SFR) should increase in metal-poor environments, typical of early galaxies. Here, we study the impact of the metallicity (Z) dependence of LX/SFR during the EoH. For our fiducial models, galaxies with star formation rates of order 10−3–$10^{-1}\, M_\odot$ yr−1 and metallicities of order 10−3–$10^{-2}\, Z_\odot$ are the dominant contributors to the X-ray background (XRB) during this period. Different LX/SFR–Z relations result in factors of ∼ 3 differences in these ranges, as well as in the mean IGM temperature and the large-scale 21-cm power, at a given redshift. We compute mock 21-cm observations adopting as a baseline a 1000 h integration with the upcoming Square Kilometer Array (SKA) for two different LX/SFR–Z relations. We perform inference on these mock observations using the common simplification of a constant LX/SFR, finding that constant LX/SFR models can recover the IGM evolution of the more complicated LX/SFR–Z simulations only during the EoH. At z &amp;lt; 10, where the typical galaxies are more polluted, constant LX/SFR models overpredict the XRB and its relative contribution to the early stages of the reionization.