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

摘要

We predict a thin (scale height $\sim$ 80 pc) diffuse component of theGalactic Ridge X-ray emission (GRXE) arising from the scattering of theradiation of bright X-ray binaries (XBs) by the interstellar medium (ISM). Themorphology of this scattered component is expected to trace the clumpymolecular and HI clouds. We calculate this contribution to the GRXE from knownGalactic XBs assuming that they are all persistent. The known XBs sample isincomplete, however, because it is flux limited and spans the lifetime of X-rayastronomy ($\sim 50$ years). We therefore also use a simulated sample ofsources, to estimate the diffuse emission we should expect in an optimisticcase assuming that the X-ray luminosity of our Galaxy is on average similar tothat of other galaxies. In the calculations we also take into account theenhancement of the total scattering cross-section due to coherence effects inthe elastic scattering from multi-electron atoms and molecules. This scatteredemission can be distinguished from the contribution of low X-ray luminositystars by the presence of narrow fluorescent K-$\alpha$ lines of Fe, Si, andother abundant elements present in the ISM and by directly resolving thecontribution of low X-ray luminosity stars. We find that within $1^\circ$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 thecase of simulated sources. In the latter case, the scattered component is foundto even dominate the stellar emission in certain parts of the Galactic plane.X-rays with energies $\gtrsim 1$ keV from XBs should also penetrate deep insidethe HI and molecular clouds, where they are absorbed and heat the ISM. We findthat this heating rate dominates the heating by cosmic rays (assuming a solarneighborhood energy density) in a considerable part of the Galaxy. [abridged]