Based on Chandra and ASCA observations of nearby starburst galaxies and RXTE/ASM, ASCA and MIR‐KVANT/TTM studies of high‐mass X‐ray binary (HMXB) populations in the Milky Way and Magellanic Clouds, we propose that the number and/or the collective X‐ray luminosity of HMXBs can be used to measure the star formation rate (SFR) of a galaxy. We show that, within the accuracy of the presently available data, a linear relation between HMXB number and star formation rate exists. The relation between SFR and collective luminosity of HMXBs is non‐linear in the low‐SFR regime, LX∝ SFR∼ 1.7, and becomes linear only for a sufficiently high star formation rate, SFR ≳ 4.5 M⊙ yr−1 (for M* > 8 M⊙). The non‐linear LX–SFR dependence in the low‐SFR limit is not related to non‐linear SFR‐dependent effects in the population of HMXB sources. It is rather caused by the fact that we measure the collective luminosity of a population of discrete sources, which might be dominated by the few brightest sources. Although more subtle SFR‐dependent effects are likely to exist, over the entire range of SFRs the data are broadly consistent with the existence of a universal luminosity function of HMXBs that can be roughly described as a power law with a differential slope of ∼1.6, a cut‐off at LX∼ few × 1040 erg s−1 and a normalization proportional to the star formation rate. We apply our results to (spatially unresolved) starburst galaxies observed by Chandra at redshifts up to z∼ 1.2 in the Hubble Deep Field North and show that the calibration of the collective luminosity of HMXBs as an SFR indicator based on the local sample agrees well with the SFR estimates obtained for these distant galaxies with conventional methods.

Based on a homogeneous set of X-ray, infrared and ultraviolet observations from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies and their relation with the star formation rate (SFR). In agreement with previous results, we find that HMXBs are a good tracer of the recent star formation activity in the host galaxy and their collective luminosity and number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the scaling relations still bear a rather large dispersion of ~0.4 dex, which we believe is of a physical origin. We present the catalog of 1057 X-ray sources detected within the $D25$ ellipse for galaxies of our sample and construct the average X-ray luminosity function (XLF) of HMXBs with substantially improved statistical accuracy and better control of systematic effects than achieved in previous studies. The XLF follows a power law with slope of 1.6 in the logLx~35-40 luminosity range with a moderately significant evidence for a break or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the Eddington limit for a neutron star or a stellar mass black hole. We discuss implications of our results for the theory of binary evolution. In particular we estimate the fraction of compact objects that once upon their lifetime experienced an X-ray active phase powered by accretion from a high mass companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is the life time of the X-ray active phase). This is ~4 orders of magnitude more frequent than in LMXBs. We also derive constrains on the mass distribution of the secondary star in HMXBs.Comment: 23 pages, 14 figures, 5 tables, MNRAS - Accepted 2011 September 2

Using results of Chandra observations of old stellar systems in 11 nearby galaxies of various morphological types and the census of low‐mass X‐ray binaries (LMXBs) in the Milky Way, we study the population of LMXBs and their relation to the mass of the host galaxy. We show that the azimuthally averaged spatial distributions of the number of LMXBs and, in the majority of cases, of their collective luminosity closely follow that of the near‐infrared light. Considering galaxies as a whole, we find that, in a broad stellar mass range, log(M*) ∼ 9–11.5, the total number of LMXBs and their combined luminosity are proportional to the stellar mass of the host galaxy. Within the accuracy of the light‐to‐mass conversion, we cannot rule out the possibility of a weak dependence of the X/M* ratio on morphological type. However, the effect of such a dependence, if any, does not exceed a factor of ∼1.5–2. The luminosity distributions of LMXBs observed in different galaxies are similar to each other and, with the possible exception of NGC 1553, are consistent with the average luminosity function derived from all data. The average X‐ray luminosity function of LMXBs in nearby galaxies has a complex shape and is significantly different from that of high‐mass X‐ray binaries (HMXBs). It follows a power law with a differential slope of ≈1 at low luminosities, gradually steepens at log(LX) ≳ 37.0–37.5 and has a rather abrupt cut‐off at log(LX) ∼ 39.0–39.5. This value of the cut‐off luminosity is significantly, by an order of magnitude, lower than found for HMXBs.

Abstract. We study the Log(N)-Log(S ) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, L X ≥ 10 34 −10 35 erg s −1 , X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant differences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge whereas HMXBs tend to avoid the inner ∼3−4 kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of ≈150 and ≈410 pc for HMXB and LMXB correspondingly) and show clear signatures of the spiral structure in their spatial distribution. The Log(N)-Log(S ) distributions and the X-ray luminosity functions are also noticeably different. LMXB sources have a flatter Log(N)-Log(S ) distribution and luminosity function. The integrated 2-10 keV luminosities of all X-ray binaries in the Galaxy, averaged over 1996-2000, are ∼2−3 × 10 39 (LMXB) and ∼2−3 × 10 38 (HMXB) erg s −1 . Normalised to the stellar mass and the star formation rate, respectively, these correspond to ∼5 × 10 28 erg s −1 M −1 for LMXBs and ∼5 × 10 37 erg s −1 /(M yr −1 ) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the ∼5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of ∼2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4 M neutron star, at least 12 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.

It is shown that the energy dependence of the time-lags in Cygnus X-1 excludes any significant contribution of the standard reflected component to the observed lags. The conclusion is valid in the 0:1-10 Hz frequency range where time-lags have been detected with sufficient significance. In fact, the data hint that the reflected component is working in the opposite direction, reducing the lags at energies where the contribution of the reflected component is significant.We argue that the observed logarithmic dependence of time-lags on energy could be due to the small variations of the spectral index in the frame of a very simple phenomenological model. We assume that an optically thin flow/corona, emitting a power law like spectrum, is present at a range of distances from the compact object. The slope of the locally emitted spectrum is a function of distance, with the hardest spectrum emitted in the innermost region. If perturbations with different time-scales are introduced to the accretion flow at different radii, then X-ray lags naturally appear, caused by the inward propagation of perturbations on the diffusion time-scales.

No abstract

Abstract. X-ray transients provide unique opportunity to probe accretion regimes of at a vastly different accretion rates. We analyze a collection of the RXTE observations (Galactic Center scans, ASM monitoring and a pointed observation) of enigmatic transient source high mass X-ray binary V4641 Sgr and argue that they broadly support the hypothesis that giant September 1999 outburst was associated with an episode of super-Eddington accretion onto the black hole. During the outburst an extended optically thick envelope/outflow has been formed around the source making the observational appearance of V4641 Sgr in many aspects very similar to that of SS433. These results suggest that objects like V4641 Sgr and SS433 indeed represent the class of objects accreting matter at a rate comparable or above Eddington value and the formation of an envelope /outflow is a generic characteristic of supercritical accretion. When the accretion rate decreased the envelope vanished and the source short term variability and spectral properties started to resemble those of other galactic black hole candidates accreting at a rate well below the Eddington value. Interestingly that during this phase the source spectrum was very similar to the Cygnus X-1 spectrum in the low state inspite of more than order of magnitude larger X-ray luminosity.

Two‐component X‐ray spectra (soft multicolour black‐body and harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick and optically thin respectively) in the vicinity of the compact source. Most of the aperiodic variability of the X‐ray flux on the short time‐scales is associated with the harder component. We suggest that drastically different amplitudes of variability of these two components are simply related to the very different viscous time‐scales in the geometrically thin and geometrically thick parts of the accretion flow. In the geometrically thin discs, variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical or thermal time‐scales do not cause any significant variations of the mass accretion rate at smaller radii because of a very long diffusion time. Any variations on the time‐scales shorter than the diffusion time‐scale are effectively dampened. On the contrary such variations can easily survive in the geometrically thick flows and as a result the mass accretion rate in the innermost region of the flow will reflect modulations of the mass accretion rate added to the flow at any distance from the black hole. Therefore if primary instabilities operate on the short time‐scales then the stability of the soft component (originating from the geometrically thin and optically thick flow) and variability of the hard component (coming from the geometrically thick and optically thin flow) are naturally explained. For Cygnus X‐1, the overall shape of the power density spectra (PDS) in the soft and hard spectral states can be qualitatively explained if the geometrically thin disc is sandwiched by the geometrically thick corona extending in a radial direction up to a large distance from the compact object. In the hard state the thin disc is truncated at some distance from the black hole followed by the geometrically thick flow. The break in the PDS is then associated with the characteristic frequencies in the accretion flow at the thin disc truncation radius.