Bright end of the luminosity function of high-mass X-ray binaries: contributions of hard, soft and supersoft sources

Abstract

Using a spectral analysis of bright Chandra X-ray sources located in 27 nearby galaxies and maps of star-formation rate (SFR) and ISM surface densities for these galaxies, we constructed the intrinsic X-ray luminosity function (XLF) of luminous high-mass X-ray binaries (HMXBs), taking into account absorption effects and the diversity of HMXB spectra. The XLF per unit SFR can be described by a power law dN/d log L X,unabs ≈ 2.0(L X,unabs /10 39 erg s −1 ) −0.6 (M yr −1 ) −1 from L X,unabs = 10 38 to 10 40.5 erg s −1 , where L X,unabs is the unabsorbed luminosity at 0.25-8 keV. The intrinsic number of luminous HMXBs per unit SFR is a factor of ∼ 2.3 larger than the observed number reported before. The intrinsic XLF is composed of hard, soft and supersoft sources (defined here as those with the 0.25-2 keV to 0.25-8 keV flux ratio of < 0.6, 0.6-0.95 and > 0.95, respectively) in ∼ 2:1:1 proportion. We also constructed the intrinsic HMXB XLF in the soft X-ray band (0.25-2 keV). Here, the numbers of hard, soft and supersoft sources prove to be nearly equal. The cumulative present-day 0.25-2 keV emissivity of HMXBs with luminosities between 10 38 and 10 40.5 erg s −1 is ∼ 5×10 39 erg s −1 (M yr −1 ) −1 , which may be relevant for studying the X-ray preheating of the early Universe.

The intrinsic collective X-ray spectrum of luminous high-mass X-ray binaries

“…Using high-quality maps of atomic (HI) and molecular (H 2 ) gas in the sampled galaxies, we demonstrated in Sazonov & Khabibullin (2017a) that the line-of-sight absorption column densities, N H , inferred from the spectra of the studied X-ray sources (typically a few 10 21 cm −2 ) can be attributed to the ISM of their host galaxies. We further took advantage of the HI, H 2 and SFR maps of the sampled galaxies to evaluate observational biases associated with the detection of X-ray sources by Chandra, which arise due to intrinsic diversity of HMXB spectra and X-ray absorption in the ISM.…”

Impact of ultraluminous X-ray sources on photoabsorption in the first galaxies

“…In this connection, it is also worth noting that a typical galaxy in the early Universe would exhibit only relatively low-luminosity HMXBs at a given time. Indeed, given the HMXB LF measured in the local Universe (Mineo, Gilfanov, & Sunyaev 2012;Sazonov & Khabibullin 2017b) and assuming (perhaps optimistically) that the specific occurence rate of HMXBs was a factor of ∼ 10 higher at z ∼ 10, a typical galaxy at that epoch, with a SFR 0.01M yr−1, is not expected to have HMXBs with L X > a few × 10 37 erg s −1 at a given instant. The combined luminosity of such sources is therefore less than 10 38 erg s −1 .…”

“…Such a modification is supposed to render the numerical calculations better-behaved because of the absence of discontinuities in the spectrum of incident radiation. For the spectrum given by equation (23), the fraction of the bolometric luminosity contained in the 0.1-10 keV and 0.25-8 keV bands (the latter was used in Sazonov & Khabibullin 2017b,c for construction of the X-ray luminosity function and collective X-ray spectrum of HMXBs in the local Universe) is 0.87 and 0.69, respectively.…”

New outburst from the luminous supersoft source SSS1 in NGC 300 with periodic modulation

“…Many of these ultraluminous SSSs have been detected in nearby galaxies: two in M 101 Kong et al 2004), one in M 51 , one in M 81 (Swartz et al 2002;Liu 2008), one in the Antennae (Fabbiano et al 2003), one in NGC 4631 (Carpano et al 2007), two in NGC 300 Carpano et al 2006) and one in NGC 247 (Jin et al 2011). Eleven more candidates have been spotted by Sazonov & E-mail: scarpano@mpe.mpg.de Khabibullin (2017), based on their hardness-ratios (0.25-2 keV to 0.25-8 keV flux ratio >0.95).…”