Population Of High-mass X-ray Binaries Research Articles

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.

XMM-Newton and INTEGRAL observations of new absorbed supergiant high-mass X-ray binaries

Context. During the first year in operation, INTEGRAL, the European Space Agency’s γ-ray observatory, has detected more than 28 new bright sources in the galactic plane which emit the bulk of their emission above 10 keV. Aims. To understand the nature of those sources we have obtained follow-up observations in the X-ray band with XMM-Newton. Methods. We derive accurate X-ray positions, propose infrared counterparts and study the source high energy long and short term variability and spectra. Results. 70% of the sources are strongly absorbed (NH ≥ 10 23 atom cm −2 ). More than half of these absorbed sources show pulsations with periods ranging from 139 to 1300 s, i.e., they are slow X-ray pulsars. The candidate infrared counterparts are not as strongly absorbed demonstrating that part of the absorbing matter is local to the sources. Conclusions. Many of these new sources are supergiant high-mass X-ray binaries (HMXB) in which the stellar wind of the companion star is accreted onto the compact object. The large local absorption in these new sources can be understood if the compact objects are buried deep in their stellar winds. These new objects represent half of the population of supergiant HMXB.

X-ray Luminosity Functions and Star Formation Rate

I will discuss the connection between the luminosity function of a population of high-mass X-ray binaries in a galaxy and the star formation rate in the this galaxy. The understanding of this connection provides on the one hand an independent measure of an important galaxy property, and on the other hand new insights into populations of high-mass X-ray binaries. In particular, observations with the Chandra X-ray telescope are uniquely suited to investigate the X-ray part of this connection and I will present examples of this.

High-Mass X-ray Binaries population in the Galaxy

We study high mass X-ray binaries (HMXBs) in the Galaxy using data of the INTEGRAL observatory. High sensitivity survey of the whole Galaxy with a possibility to detect absorbed sources significantly enlarged our sample of HMXBs in a comparison with the previous studies. Large fraction of detected high mass X-ray binaries is highly photoabsorbed. We investigated the HMXBs distribution along the Galactic plane and found their strong concentrations toward Galactic spiral arms, confirming previous results of Grimm et al. (2002) obtained using smaller sample of sources. We conclude that the mapping of Galactic HMXBs should be important tool to trace the star formation regions at the opposite side of the Galaxy.

INTEGRAL discovers a population of persistent intrinsically absorbed super-giant High-Mass X-Ray Binaries

During the first year in operation, INTEGRAL has detected more than 28 new bright sources which emit the bulk of their emission above 10 keV. Follow-up observations of a subset of these sources in the X-ray band with XMM-Newton indicate that 80% of them are very strongly absorbed. More than half of these absorbed sources show strong pulsations with long periods ranging from 139 to 1300s, i.e., they are slow X-ray pulsars. Many of these new sources are super-giant high-mass X-ray binaries (HMXB) in which the stellar wind of the companion star is accreted onto the compact object. The large local absorption in these new sources can be understood if the compact objects are buried deep in their stellar winds. These new objects represent half of the population of active super-giant HMXB.

Protostellar mergers in protoclusters and the origin of ultra-luminous X-ray sources

I suggest that stellar coalescence in mid-size protoclusters ( are likely to disperse quickly into loose OB associations, where most ULXs are found. Sufficiently high protostellar density may be achieved when clustered star formation is triggered by galaxy collisions and mergers. Low metallicity may then be necessary to ensure that a large fraction of the stellar mass ends up in a black hole. In this scenario, most ULXs are naturally explained as the extreme end of the high-mass X-ray binary population.

HMXB, ULX and star formation

Based on recent X-ray observations of the Milky Way, Magellanic Clouds and nearby starburst galaxies we study the population of high-mass X-ray binaries (HMXBs), their connection with ultra-luminous X-ray sources (ULXs) and the relation to the star formation rate (SFR). Although more subtle SFR-dependent effects are likely to exist, the data in the log( L X )∼ 36–40.5 luminosity range are broadly consistent with the existence of a “universal” luminosity function of HMXBs, which can be roughly described as a power law with a differential slope of ∼ 1.6 and a cutoff at log( L X )∼ 40.5. The ULX sources found in many starburst galaxies occupy the high-luminosity end of this single-slope power-law distribution, whereas its low-luminosity part is composed of “ordinary” high-mass X-ray binaries, observed, e.g. in the Milky Way and Magellanic Clouds. As the normalization of the “universal” luminosity function is proportional to the star formation rate, the number and/or the collective X-ray luminosity of HMXBs can be used to measure the current value of the SFR in the host galaxy. Distant (unresolved) starburst galaxies observed by Chandra at redshifts of z ∼ 0.2–1.3 obey the same L X –SFR relation as local galaxies, indicating that the ULXs at these redshifts were not significantly more luminous than those found in nearby galaxies.

X-ray Binaries in the Milky Way and Other Galaxies

We performed a study of the X-ray binary population in the Milky Way. The results of this study, spatial distribution and in particular luminosity function, can be used for comparison with the X-ray binary populations of other galaxies. In the second part we give an example by investigating the connection between the star formation rate and the high mass X-ray binary population in galaxies observed by CHANDRA.

The universal luminosity function of binary X-ray sources in galaxies

The empirically found universal power-law form of the X-ray luminosity function for a population of high-mass X-ray binaries in galaxies can be explained by the fundamental mass-luminosity and mass-radius relations for massive stars.

X‐Rays from NGC 3256: High‐Energy Emission in Starburst Galaxies and Their Contribution to the Cosmic X‐Ray Background

The infrared-luminous galaxy NGC 3256 is a classic example of a merger-induced nuclear starburst system. We find here that it is the most X-ray-luminous star-forming galaxy yet detected (L0.5-10 keV = 1.6 × 1042 ergs s-1). Long-slit optical spectroscopy and a deep, high-resolution ROSAT X-ray image show that the starburst is driving a superwind which accounts for ~20% of the observed soft X-ray emission. Analysis of X-ray spectral data from ASCA indicates this gas has a characteristic temperature of kT ≈ 0.3 keV. Our model for the broadband X-ray emission of NGC 3256 contains two additional components: a warm thermal plasma (kT ≈ 0.8 keV) associated with the central starburst, and a hard power-law component with an energy index of αX ≈ 0.7. We discuss the energy budget for the two thermal plasmas and find that the input of mechanical energy from the starburst is more than sufficient to sustain the observed level of emission. We also examine possible origins for the power-law component, concluding that neither a buried AGN nor the expected population of high-mass X-ray binaries can account for this emission. Inverse Compton scattering, involving the galaxy's copious flux of infrared photons and the relativistic electrons produced by supernovae, is likely to make a substantial contribution to the hard X-ray flux. Such a model is consistent with the observed radio and IR fluxes and the radio and X-ray spectral indices. We explore the role of X-ray-luminous starbursts in the production of the cosmic X-ray background radiation. The number counts and spectral index distribution of the faint radio source population, thought to be dominated by star-forming galaxies, suggest that a significant fraction of the hard X-ray background could arise from starbursts at moderate redshift.

High-mass X-ray binary populations. 1: Galactic modeling

Modern stellar evolutionary tracks are used to calculate the evolution of a very large number of massive binary star systems (M<SUB>tot</SUB> greater than or = 15 solar mass) which cover a wide range of total masses, mass ratios, and starting separations. Each binary is evolved accounting for mass and angular momentum loss through the supernova of the primary to the X-ray binary phase. Using the observed rate of star formation in our Galaxy and the properties of massive binaries, we calculate the expected high-mass X-ray binary (HMXRB) population in the Galaxy. We test various massive binary evolutionary scenarios by comparing the resulting HMXRB predictions with the X-ray observations. A major goal of this study is the determination of the fraction of matter lost from the system during the Roche lobe overflow phase. Curiously, we find that the total numbers of observable HMXRBs are nearly independent of this assumed mass-loss fraction, with any of the values tested here giving acceptable agreement between predicted and observed numbers. However, comparison of the period distribution of our HMXRB models with the observed period distribution does reveal a distinction among the various models. As a result of this comparison, we conclude that approximately 70% of the overflow matter is lost from a massive binary system during mass transfer in the Roche lobe overflow phase. We compare models constructed assuming that all X-ray emission is due to accretion onto the compact object from the donor star's wind with models that incorporate a simplified disk accretion scheme. By comparing the results of these models with observations, we conclude that the formation of disks in HMXRBs must be relatively common. We also calculate the rate of formation of double degenerate binaries, high velocity detached compact objects, and Thorne-Zytkow objects.

Soft X-ray observations of the interacting galaxies NGC 1808 and NGC 1792

The soft X-ray emission from both galaxies NGC 1808 and NGC 1792, which we investigated using the ROSAT HRI and PSPC, comes most probably from X-ray binaries and/or from hot ionized gas in powerful supernovae and supernova remnants. The distribution of the soft X-ray emission in NGC 1808, which is very well correlated with the distribution of "radio knots" in the central starburst, suggests that hot gas dominates the emission in the ROSAT band. This is consistent with the results of PSPC observations by Junkes et al. (1994). The total soft X-ray luminosity in the ROSAT band of NGC 1808 of 1.2 x 10^41^ ergs s^-1^ is relatively high compared with other nearby starburst galaxies. Soft X-ray emission of diffuse hot ionized gas that is associated with the outflow traced by the conspicuous dust filaments protruding from the plane has been detected. Its luminosity in the ROSAT band is &gt;= 3 x 10^39^ ergs s^-1^, i.e., several percent of the total soft X-ray luminosity. Thus, NGC 1808 is another example for a "superwind" galaxy. The soft X-ray radiation from NGC 1792 is more likely to be dominated by a population of high-mass X-ray binaries or young powerful supernovae which are associated with the high-level star formation going on in the very prominent H II regions along its spiral arms, with possibly an additional contribution of diffuse hot ionized gas. The soft X-ray luminosities of individual sources lie in the range of 5 X 10^38^ to 2.7 x 10^39^ ergs s^-1^, thus exceeding by far the Eddington luminosity of an accreting neutron star. The peaks of some of these soft X-ray luminous sources are offset with respect to the H II regions by a few hundred parsecs. Accordingly, if the soft X-ray sources should originate from the H II regions, their relative velocities with respect to the ambient medium have to be as high as ~100 km s^-1^.