A thin diffuse component of the Galactic ridge X-ray emission and heating of the interstellar medium contributed by the radiation of Galactic X-ray binaries

Abstract

We predict a thin diffuse component of the Galactic Ridge X-ray emission (GRXE) arising from the scattering of the radiation of bright X-ray binaries (XBs) by the interstellar medium. This scattered component has the same scale height as that of the gaseous disk (∼ 80 pc) and is therefore thinner than the GRXE of stellar origin (scale height ∼ 130 pc). The morphology of the scattered component is furthermore expected to trace the clumpy molecular and HI clouds. We calculate this contribution to the GRXE from known Galactic XBs assuming that they are all persistent. The known XBs sample is incomplete, however, because it is flux limited and spans the lifetime of X-ray astronomy (∼ 50 years), which is very short compared with the characteristic time of 1000-10000 years that would have contributed to the diffuse emission observed today due to time delays. We therefore also use a simulated sample of sources, to estimate the diffuse emission we should expect in an optimistic case assuming that the X-ray luminosity of our Galaxy is on average similar to that of other galaxies. In the calculations we also take into account the enhancement of the total scattering cross-section due to coherence effects in the elastic scattering from multi-electron atoms and molecules. This scattered emission can be distinguished from the contribution of low X-ray luminosity stars by the presence of narrow fluorescent K-α lines of Fe, Si, and other abundant elements present in the interstellar medium and by directly resolving the contribution of low X-ray luminosity stars. We find that within 1• latitude of the Galactic plane the scattered emission contributes on average 10 − 30% of the GRXE flux in the case of known sources and over 50% in the case of simulated sources. In the latter case, the scattered component is found to even dominate the stellar emission in certain parts of the Galactic plane. X-rays with energies 1 keV from XBs should also penetrate deep inside the HI and molecular clouds, where they are absorbed and heat the interstellar medium. We find that this heating rate dominates the heating by cosmic rays (assuming a solar neighborhood energy density) in a considerable part of the Galaxy.

Probing Large-scale Coherence between Spitzer IR and Chandra X-Ray Source-subtracted Cosmic Backgrounds

“…This Diffuse Galactic Light is very faint at 3.6 and 4.5 µm, and is generally estimated through extrapolation from, or cross correlation with, much brighter interstellar emission at other wavelengths (e.g., Kashlinsky et al 2005;Arendt et al 2010;Zemcov et al 2014;Matsumoto et al 2011;Seo et al 2015). Galactic X-rays also scatter in the diffuse ISM (Molaro et al 2014) and thus might correlate with the IR. However, X-ray scattering is predominantly a small angle phenomenon, dropping sharply with increasing angular scale (Smith & Dwek 1998;Valencic & Smith 2015), so the X-ray sources would have to be within ∼1000 ′′ of the survey fields.…”

Scale heights and equivalent widths of the iron K-shell lines in the Galactic diffuse X-ray emission

“…Therefore the Fe I Kα line would mainly originate from MCs. Molaro et al (2014) claimed that ∼10-30% of the total luminosity of the GRXE would be the scattered flux of LMXBs. Using the best-fit parameters listed in table 2, the 5-8 keV band luminosity in the (|l * | = 10…”

Low-luminosity X-ray sources and the Galactic ridge X-ray emission

“…The 6.4-keV emission may in fact originate in an entirely different way, namely in the scattering of the radiation of bright XRBs by the intersteller medium. In a recent paper Molaro et al (2014) argue that between 10-30 per cent of the broad-band GRXE flux might originate in this way. The results presented in this paper do not exclude this possibility, particularly if the contribution from scattering in our fiducial direction (l = 28.5 • ) is at the lower-end of the quoted range.…”

“…Superimposed on this, at the level of 10 percent or more, is the diffuse scattered X-ray emission from bright XRBs, which carries the imprint of the 6.4 keV fluorescent line. As a tracer of dense molecular gas and HI clouds, this component is predicted to have a clumpy distribution (Molaro et al 2014). Finally, a further modest contribution (perhaps ∼ 10 percent) may be contributed by other relatively young Galactic source populations, which show a narrow concentration on the plane in specific Galactic complexes and features.…”