X‐Ray Scattering Halos from the Galactic Center: Implications for Diffuse Emission around Sagittarius A*

Abstract

We consider the absorption and scattering of X-rays observed from the Galactic center. One objective is to characterize the intrinsic X-ray emission from the central black hole, Sgr A*, in its quiescent and flaring states—crucial for our understanding of the accretion physics of supermassive black holes. We correct the fluxes observed by the Chandra and XMM-Newton telescopes for absorption and scattering, but limited knowledge about the properties of the intervening gas and dust causes large uncertainties. We use realistic models for the dust grain size distribution, consistent with many other observational constraints, as well as reasonable models for the gas and dust abundances and spatial distributions. Since much of the intervening dust is relatively close to Sgr A*, the scattered halo of X-ray photons is very concentrated; its intensity can dominate the point-spread function of Chandra inside 1'' and thus affects estimates of the point source flux. It also causes an apparent broadening of the radial intensity profiles of Galactic center sources; observations of this broadening can therefore help constrain models of the line-of-sight distribution of the dust. We estimate that the combined scattering halos from observed Galactic center sources within 10'' of Sgr A* contribute up to ~10% of the observed diffuse emission in this region. Unresolved sources may make an additional contribution. Dust-scattered photons suffer a time delay relative to the photons that arrive directly. For dust that is 100 pc toward us from the Galactic center, this delay is about 1000 s at angles of 1'' and 100 ks at 10''. We illustrate how the evolution of the scattering halo following X-ray flares from Sgr A* or other sources can also help to constrain the dust's line-of-sight distribution. We discuss the implications of X-ray scattering halos for the intensity of diffuse emission that has been reported within a few arcseconds of Sgr A*: in the most extreme, yet viable, model we consider, ~ of it is due to dust scattering of an unresolved source. The remainder results from an extended source of emission.