UV view of High Mass X-Ray Binary systems (what we know and perspectives for WSO-UV)

High Mass X-Ray Binary systems (HMXRB) are composed by a compact object orbiting around a OB massive star. The mass transfer which takes place from the massive star onto its compact companion is the responsible of the emission of high energy photons (X-rays) observed in these systems (to which they owe the name of their class). The presence of a compact object must exert a clear influence in the outer envelopes of the massive star and, thus, must modify the formation and evolution of its stellar wind. Tidal interactions, mass capture and high energy radiation heating and pressure are among the possible ways in which the compact companion and the X-ray emission will influence the stellar wind of the massive counterpart. The status of the observations of such systems from previous UV missions will be reviewed, and perspectives for the incoming WSO-UV space mission will be reported.     

The eccentric-orbit binary model for the transient X-ray sources

Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.     

On the metallicity dependence of high-mass X-ray binaries

It is commonly assumed that high-mass X-ray binary (HMXB) populations are little affected by metallicity. However, the massive stars making up their progenitor systems depend on metallicity in a number of ways, not least through their winds. We present simulations, well-matched to the observed sample of Galactic HMXBs, which demonstrate that both the number and the mean period of HMXB progenitors can vary with metallicity, with the number increasing by about a factor of 3 between solar and Small Magellanic Cloud (SMC) metallicity. However, the SMC population itself cannot be explained simply by metallicity effects; it requires both that the HMXBs observed therein primarily sample the older end of the HMXB population and that the star formation rate at the time of their formation was very large.     

Peering through the stellar wind of IGR J19140+0951 with simultaneous INTEGRAL/RXTE observations

We have used the RXTE and INTEGRAL satellites simultaneously to observe the high-mass X-ray binary (HMXB) IGR J19140+0951. The spectra obtained in the 3–80 keV range have allowed us to perform a precise spectral analysis of the system along its binary orbit. The spectral evolution confirms the supergiant nature of the companion star and the neutron star nature of the compact object. Using a simple stellar wind model to describe the evolution of the photoelectric absorption, we were able to restrict the orbital inclination angle in the range 38°–75°. This analysis leads to a wind mass-loss rate from the companion star of  ∼5 × 10⁻⁸ M_⊙ yr⁻¹  , consistent with an OB I spectral type. We have detected a soft excess in at least four observations, for the first time for this source. Such soft excesses have been reported in several HMXBs in the past. We discuss the possible origin of this excess, and suggest, based on its spectral properties and occurrences around the superior conjunction, that it may be explained as the reprocessing of the X-ray emission originating from the neutron star by the surrounding ionized gas.     

The cross-correlation between galaxies and groups: probing the galaxy distribution in and around dark matter haloes

Some massive binaries should contain energetic pulsars which inject relativistic leptons from their inner magnetospheres and/or pulsar wind regions. If the binary system is compact enough, then these leptons can initiate inverse Compton (IC) e^± pair cascades in the anisotropic radiation field of a massive star. γ-rays can be produced in the IC cascade during its development in a pulsar wind region and above a shock in a massive star wind region where the propagation of leptons is determined by the structure of a magnetic field around the massive star. For a binary system with specific parameters, we calculate phase-dependent spectra and fluxes of γ-rays escaping as a function of the inclination angle of the system and for different assumptions on injection conditions of the primary leptons (their initial spectra and location of the shock inside the binary). We conclude that the features of γ-ray emission from such massive binaries containing energetic pulsars should allow us to obtain important information on the acceleration of particles by the pulsars, and on interactions of a compact object with the massive star wind. Predicted γ-ray light curves and spectra in the GeV and TeV energy ranges from such binary systems within our Galaxy and Magellanic Clouds should be observed by future AGILE and GLAST satellites and low-threshold Cherenkov telescopes, such as MAGIC, HESS, VERITAS or CANGAROO III.     

Environmental Effects in Galaxies: Molecular Gas and Nuclear Activity

Galactic X-ray emitters have been described by Giovannelli andSabau-Graziati (1993). We would address the bulk of this paper on HighMass X-Ray Binaries (HMXBs) and particularly in demonstrating theimportance of these systems as powerful laboratories to test the propertiesof high-energy physical processes through the multi-frequency studies oftheir behavior. Interactions between the two components of those systemsoriginate processes that manifest themselves along a large part of theelectromagnetic spectrum. Therefore it is possible to study indirectlyhigh-energy phenomena through the analysis oflow-energy phenomena with enormous advantages since the latter can bemainly studied via ground-based experiments, meanwhile the former can bestudied only by mean of space-based experiments, at least up to energiesof order 100 GeV.Moreover, since HMXBs are galactic, their emissions are the highestmeasurable; this renders such systems the most powerful laboratories to testtheories on collapsed objects, which can be scaled to extra-galactic distancesand dimensions. This fact suggests that HMXBs can be in general useful targetseven for small satellites, and in particular, in the X-ray energy range,very suitable targets for experiments like SIXE (Spanish Italian X-rayExperiment) to be launched in small satellites such as the Spanish MINISAT-02.     

High-mass X-ray binaries and recent star formation history of the Small Magellanic Cloud

We study the relation between high-mass X-ray binary (HMXB) population and recent star formation history (SFH) for the Small Magellanic Cloud (SMC). Using archival optical SMC observations, we have approximated the color-magnitude diagrams of the stellar population by model stellar populations and, in this way, reconstructed the spatially resolved SFH of the galaxy over the past 100 Myr. We analyze the errors and stability of this method for determining the recent SFH and show that uncertainties in the models of massive stars at late evolutionary stages are the main factor that limits its accuracy. By combining the SFH with the spatial distribution of HMXBs obtained from XMM-Newton observations, we have derived the dependence of the HMXB number on the time elapsed since the star formation event. The number of young systems with ages ? 10 Myr is shown to be smaller than the prediction based on the type-II supernova rate. The HMXB number reaches its maximum ～20–50 Myr after the star formation event. This may be attributable, at least partly, to a low luminosity threshold in the population of X-ray sources studied, L _min ～ 10³⁴ erg s^?1. Be/X systems make a dominant contribution to this population, while the contribution from HMXBs with black holes is relatively small.     

The Evolution of Massive Close Binaries

We present results of evolutionary computations for massive close binaries with the Brussels simultaneous evolution code for conservative and non-conservative Roche lobe overflow (RLOF). We discuss mass transfer in massive close binaries during phases of RLOF, common envelope, spiral-in and merging. We examine the effects of stellar wind during successive stellar evolution phases and the final fate of primaries. We show how our library can be used to explain well-known binaries such as the WR + OB system V444 Cyg, HMXBs Vela X-1 and Wray 977, LMXBs like Her X-1, and binary pulsars. More details on the evolution of massive close binaries can be found in “The Brightest Binaries” (Vanbeveren et al., 1998).     

High-mass X-ray binaries and the spiral structure of the host galaxy

We investigate the manifestation of the spiral structure in the distribution of high-mass X-ray binaries (HMXBs) over the host galaxy. We construct the simplest kinematic model. It shows that the HMXBs should be displaced relative to the spiral structure observed in such traditional star formation rate indicators as the Hα and far-infrared emissions because of their finite lifetimes. Using Chandra observations of M51, we have studied the distribution of X-ray sources relative to the spiral arms of this galaxy observed in Hα. Based on K-band data and background source number counts, we have separated the contributions from high-mass and low-mass X-ray binaries and active galactic nuclei. In agreement with model predictions, the distribution of HMXBs is wider than that of bright H II regions concentrated in the region of ongoing star formation. However, the statistical significance of this result is low, as is the significance of the concentration of the total population of X-ray sources to the spiral arms. We also predict the distribution of HMXBs in our Galaxy in Galactic longitude. The distribution depends on the mean HMXB age and can differ significantly from the distributions of such young objects as ultracompact H II regions.     

An estimate of the supernova kick velocities for high-mass X-ray binaries in the Small Magellanic Cloud

This work investigates the possible supernova kick velocities imposed on high-mass X-ray binary (HMXB) systems in the Small Magellanic Cloud. Comparisons are made between the location of such systems and the locations of young stellar clusters on the premise that these may represent the birthplace of many of these systems. Measurements of the separation of clusters and HMXBs, and an estimate of the typical lifetimes of these systems, lead to a minimum average space velocity of 30 km s⁻¹. This value is compared to theoretical estimates.     

On one mechanism for the generation of a reverse solar wind shock in the magnetosheath in front of the Earth’s magnetosphere

A possible mechanism for the generation of a reverse fast shock in the magnetosheath in the solar wind flow around the Earth’s magnetosphere is considered. It is shown that such a shock can emerge through the breaking of a nonlinear fast magnetosonic compression wave reflected from the magnetopause toward the bow shock rear. In this case, the magnetopause is represented as a tangential discontinuity with a zero normal magnetic field component at it and the mechanism under consideration is assumed to be secondary with respect to the sudden disturbance of the bow shock-Earth’s magnetosphere system by a nonstationary solar wind shock. A possible confirmation of the process under study by in-situ SC3 experimental observations of the bow shock front motion on the Cluster spacecraft is pointed out.     

Gamma-Ray Emission from Be/X-ray Binaries

Be/X-ray binaries are systems formed by a massive Be star and a magnetized neutron star, usually in an eccentric orbit. The Be star has strong equatorial winds occasionally forming a circumstellar disk. When the neutron star intersects the disk the accretion rate dramatically increases and a transient accretion disk can be formed around the compact object. This disk can last longer than a single orbit in the case of major outbursts. If the disk rotates faster than the neutron star, the Cheng-Ruderman mechanism can produce a current of relativistic protons that would impact onto the disk surface, producing gamma-rays from neutral pion decays and initiating electromagnetic cascades inside the disk. In this paper we present calculations of the evolution of the disk parameters during both major and minor X-ray events, and we discuss the generation of gamma-ray emission at different energies within a variety of models that include both screened and unscreened disks.     

Case Study on the γ-algorithm:Possible Tests from the Luminosity versus Displacement Correlation of High Mass X-Ray Binaries

Using the apparent correlation of luminosity(Lx)versus displacement(R)of high mass X-ray binaries(HMXBs),we aim to constrain the common envelope(CE)mechanism,wh...     

Late stages of the evolution of close binaries

Close binaries can evolve through various ways of interaction into compact objects (white dwarfs, neutron stars, black holes). Massive binary systems (mass of the primaryM ₁ larger than 14 to 15M ₀) are expected to leave, after the first stage of mass transfer a compact component orbiting a massive star. These systems evolve during subsequent stages into massive X-ray binaries. Systems with initial large periode evolve into Be X-ray binaries.Low mass X-ray sources are probably descendants of lower mass stars, and various channels for their production are indicated. The evolution of massive close binaries is examined in detail and different X-ray stages are discussed. It is argued that a first X-ray stage is followed by a reverse extensive mass transfer, leading to systems like SS 433, Cir X1. During further evolution these systems would become Wolf-Rayet runaways. Due to spiral in these system would then further evolve into ultra short X-ray binaries like Cyg X-3.Finally the explosion of the secondary will in most cases disrupt the system. In an exceptional case the system remains bound, leading to binary pulsars like PSR 1913+16. In such systems the orbit will shrink due to gravitational radiation and finally the two neutron stars will coalesce. It is argued that the millisecond pulsar PSR 1937+214 could be formed in this way.A complete scheme starting from two massive ZAMS stars, ending with a millisecond pulsar is presented.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia 3–7 June, 1983.     

Neutron Stars in X-ray Binaries and their Environments

Neutron stars in X-ray binary systems are fascinating objects that display a wide range of timing and spectral phenomena in the X-rays. Not only parameters of the neutron stars, like magnetic field strength and spin period evolve in their active binary phase, the neutron stars also affect the binary systems and their immediate surroundings in many ways. Here we discuss some aspects of the interactions of the neutron stars with their environments that are revelaed from their X-ray emission. We discuss some recent developments involving the process of accretion onto high magnetic field neutron stars: accretion stream structure and formation, shape of pulse profile and its changes with accretion torque. Various recent studies of reprocessing of X-rays in the accretion disk surface, vertical structures of the accretion disk and wind of companion star are also discussed here. The X-ray pulsars among the binary neutron stars provide excellent handle to make accurate measurement of the orbital parameters and thus also evolution of the binray orbits that take place over time scale of a fraction of a million years to tens of millions of years. The orbital period evolution of X-ray binaries have shown them to be rather complex systems. Orbital evolution of X-ray binaries can also be carried out from timing of the X-ray eclipses and there have been some surprising results in that direction, including orbital period glitches in two X-ray binaries and possible detection of the most massive circum-binary planet around a Low Mass X-ray Binary.     

Preheating of the early universe by radiation from high-mass X-ray binaries

Using a reliablymeasured intrinsic (i.e., corrected for absorption effects) present-day luminosity function of high-mass X-ray binaries (HMXBs) in the 0.25–2 keV energy band per unit star formation rate, we estimate the preheating of the early Universe by soft X-rays from such systems. We find that X-ray irradiation, mainly executed by ultraluminous and supersoft ultraluminous X-ray sources with luminosity L _X > 10³⁹ erg s^?1, could significantly heat (T >T _CMB, where T _CMB is the temperature of the cosmic microwave background) the intergalactic medium by z ~ 10 if the specific X-ray emissivity of the young stellar population in the early Universe was an order of magnitude higher than at the present epoch (which is possible due to the low metallicity of the first galaxies) and the soft X-ray emission from HMXBs did not suffer strong absorption within their galaxies. This makes it possible to observe the 21 cm line of neutral hydrogen in emission from redshifts z < 10.     

Ultracompact HII regions: the impact of newly formed massive stars on their environment

Summary Ultracompact (UC)HII regions are manifestations of newly formed massive stars that are still embedded in their natal molecular cloud. They are among the brightest and most luminous single objects in the Galaxy at far infrared and radio wavelengths. Recent high spatial resolution studies, particularly at radio wavelengths, have greatly contributed to our understanding of these dynamic objects and the impact they have on their environment. A summary is given of our current understanding of the physical properties, morphologies, dynamics, number and distribution in the Galaxy, and molecular environments of UCHII regions. Recent models of the circumnebular dust imply that the graphite/silicate abundance ratio is about half that of dust in the diffuse interstellar medium. The dust cocoons are large, cool, and optically thick shortward of a few microns. There are apparently between 1700 and 3000 UCHII regions in the Galaxy. This represents 10–20% of the total O star population. There are too many UCHII regions (just counting those studied with the VLA) to be consistent with the short dynamical lifetimes of this very compact stage of evolution. Both the morphologies and the large number can be understood if UC HII regions are bow shocks. Models of stellar wind supported bow shocks are discussed and consequences for the dynamics and morphologies of the ionized and molecular gas are explored.     

A Statistical Study of the Mass Distribution of Neutron Stars

By reviewing the methods of mass measurements of neutron stars in four different kinds of systems, i.e., the high-mass X-ray binaries (HMXBs), low-mass X-ray binaries (LMXBs), double neutron star systems (DNSs) and neutron star-white dwarf (NS-WD) binary systems, we have collected the orbital parameters of 40 systems. By using the boot-strap method and the Monte-Carlo method, we have rebuilt the likelihood probability curves of the measured masses of 46 neutron stars. The statistical analysis of the simulation results shows that the masses of neutron stars in the X-ray neutron star systems and those in the radio pulsar systems exhibit different distributions. Besides, the Bayes statistics of these four different kind systems yields the most-probable probability density distributions of these four kind systems to be (1.340 ± 0.230)M8, (1, 505 ± 0.125)M8,(1.335 ± 0.055)M8 and (1.495 ± 0.225)M8, respectively. It is noteworthy that the masses of neutron stars in the HMXB and DNS systems are smaller than those in the other two kind systems by approximately 0.16M8. This result is consistent with the theoretical model of the pulsar to be accelerated to the millisecond order of magnitude via accretion of approximately 0.2M8. If the HMXBs and LMXBs are respectively taken to be the precursors of the BNS and NS-WD systems, then the influence of the accretion effect on the masses of neutron stars in the HMXB systems should be exceedingly small. Their mass distributions should be very close to the initial one during the formation of neutron stars. As for the LMXB and NS-WD systems, they should have already under- gone the process of suffcient accretion, hence there arises rather large deviation from the initial mass distribution.