X-ray Scattering Halos from the Galactic Center: Implications for Diffuse Emission Around SgrA*

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, SgrA*, in its quiescent and flaring states. We correct the fluxes observed by the Chandra and XMM telescopes for absorption and scattering, using realistic models for the dust grain size distribution, gas and dust abundances and spatial distributions. However, there are still large uncertainties. Since much of the intervening dust is close to SgrA*, the scattered halo of X-ray photons is very concentrated: its intensity can dominate the Chandra PSF inside 1", and so affects estimates of the point source flux. It also broadens the radial intensity profiles of Galactic center sources - observations of this effect can help constrain models of the spatial distribution of the dust. We estimate that the combined scattering halos from observed Galactic center sources within 10" of SgrA* contribute up to ~10% of the observed diffuse emission in this region. Dust-scattered photons suffer a time delay relative to the photons that arrive directly. For dust that is 100 pc towards 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 SgrA* 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 SgrA*: in the most extreme, yet viable, model we consider, ~1/3 of it is due to dust scattering of an unresolved source. The remainder results from an extended source of emission.