We investigate the characteristics of young ( 1 ? 1036?erg?s?1) high-mass X-ray binaries (HMXBs) and find the population to be strongly metallicity dependent. We separate the model populations among two distinct formation pathways: (1) systems undergoing active Roche lobe overflow (RLO) and (2) wind accretion systems with donors in the (super)giant stage, which we find to dominate the HMXB population. We find metallicity to primarily affect the number of systems which move through each formation pathway, rather than the observable parameters of systems which move through each individual pathway. We discuss the most important model parameters affecting the HMXB population at both low and high metallicities. Using these results, we show that (1) the population of ultra-luminous X-ray sources can be consistently described by very bright HMXBs which undergo stable RLO with mild super-Eddington accretion and (2) the HMXB population of the bright starburst galaxy NGC?1569 is likely dominated by one extremely metal-poor starburst cluster.

The effect of starburst metallicity on bright x-ray binary formation pathways

Accretion Power in Astrophysics: ACCRETION FLOWS

We investigate the characteristics of young ( 1e36 erg/s) High-Mass X-ray Binaries (HMXBs) and find the population to be strongly metallicity-dependent. We separate the model populations among two distinct formation pathways: (1) systems undergoing active Roche Lobe Overflow (RLO), and (2) wind accretion systems with donors in the (super)giant (SG) stage, which we find to dominate the HMXB population. We find metallicity to primarily affect the number of systems which move through each formation pathway, rather than the observable parameters of systems which move through each individual pathway. We discuss the most important model parameters affecting the HMXB population at both low and high metallicities. Using these results, we show that (1) the population of ultra-luminous X-Ray sources can be consistently described by very bright HMXBs which undergo stable Roche Lobe overflow with mild super-Eddington accretion and (2) the HMXB population of the bright starburst galaxy NGC~1569 is likely dominated by one extremely metal-poor starburst cluster.

The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways

We introduce a method for obtaining the X-ray luminosity function (XLF) and the binary-period distribution of populations of high-mass X-ray binaries (HMXBs) in the stellar fields (i.e.,outside globular clusters) of normal galaxies. We start from standard distributions of the parameters of those primordial binaries which are the progenitors of HMXBs and follow the transformation of these distributions with the aid of a Jacobian formalism as the former evolve into the latter through the processes of the first mass transfer and the supernova (SN) that follows. We discuss the distributions of the post-SN binaries and the HMXBs. We show that our calculated model XLF has a differential slope ? 1.6 with a flattening at low luminosities, in excellent agreement with observations. The calculated binary-period distribution, which basically has a slightly sloping plateau-like character at intermediate periods, with a rise to this plateau at shorter periods and fall-off from it at longer periods, is in agreement with the observed distribution within observational uncertainties. We discuss the physical origin of these distributions. We demonstrate that, while the effects of both (1) the distribution of the properties of the massive companion in the HMXBs and (2) the primordial orbital distribution and the SN dynamics are important, the former appear to be dominant in determining the XLF, and the latter in determining the HMXB binary-period distribution. We discuss the possible roles of stellar-mass black holes and ultraluminous X-ray sources in the observed universal XLF of HMXBs.

Collective properties of x-ray binary populations of galaxies. i. luminosity and orbital period distributions of high-mass x-ray binaries

The aim of this PhD Thesis is to characterize a representative sample of Supergiant X-ray Binaries (SGXBs) formed by 4 sources: XTE J1855-026, a classical SGXB with long-term stable X-ray flux; AX J1841.0-0535 and AX J1845.0-0433, two supergiant fast X-ray transients (SFXTs) with the X-ray emission mostly dominated by flaring; and IGR J00370+6122, something in between these 2 sub-groups. The physical processes that produce these observable differences are still a matter of debate. In this PhD Thesis I performed a study of these 4 different systems to provide new data to constrain the models. This study consists of:(i) the determination of the orbital solution,(ii) a systematic study of the wind behavior along the orbit by the measure of Halpha variations,(iii) a model of stellar atmospheres of the donor star,(iv) establish whether there are X-ray flux variations modulated by the orbital period. The study of the wind shows that Halpha variations are dominated by intrinsic wind processes. The stellar atmospheres study shows that the supergiant stars that harbor these binaries have a higher projected rotational velocity, higher He abundance and higher N/C ratio than that of isolated supergiant stars. The results of this study show that the eccentricity of the binary does not have a simple correlation with the differences in the X-ray flux of the different sub-groups. Therefore, the idea of a more complex scenario is consolidated. The discovery of a new type of system, IGR J00370+6122, which properties do not fit in any of the established sub-groups, reinforces the idea of a continuum in the observed properties more than a strict classification of systems. Furthermore, I have developed a pipeline to reduce spectra of the FRODOSpec spectrograph at the Liverpool Telescope optimized for the reduction of the red spectra of the obscured supergiants that we find in these SGXBs.

Fundamental properties of High-Mass X-ray Binaries

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.

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X-ray binaries in the Milky Way and other galaxies

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.

X-ray luminosity functions and star formation rate

We study the statistical properties of the combined emission of a population of discrete sources. In particular, we consider the dependence of their total luminosity ( L tot = ∑ k L k ) and fractional rms tot variability on the number of sources n or on the normalization of the luminosity function. We show that due to small number statistics a regime exists, in which L tot grows non-linearly with n . This is in apparent contradiction with the seemingly obvious prediction L tot = ∫ LdN / dLdL ~ n . In this non-linear regime, the rms tot decreases with n significantly more slowly than expected from the rms ~ 1/ n 1/2 averaging law. Only in the limit of n >> 1 do these quantities behave as intuitively expected, L tot ~ n and rms tot ~ 1/ n 1/2 . Using the total X-ray luminosity of a galaxy due to its X-ray binary population as an example, we show that the L X –SFR and L X –M * relations predicted from the respective ``universal'' luminosity functions of high and low mass X-ray binaries are in a good agreement with observations.

Hans-Jakob Grimm

Papers

X-ray binaries in the Milky Way and other galaxies

X-ray luminosity functions and star formation rate

Statistical properties of the combined emission of a discrete source population: astrophysical implications