The Origin of Diffuse X-ray and $γ$-ray Emission from the Galactic Center Region: Cosmic Ray Particles

Abstract

(Abridged) The inner couple hundred pcs of our Galaxy is characterized by significant amount of synchrotron-emitting gas, which appears to co-exist with a large reservoir of molecular gas. The spatial correlation between fluorescent Fe K-alpha line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present multi-wavelength study of a representative Galactic center cloud Sgr C using Chandra, VLA and FCRAO. We note a correlation between the nonthermal radio filaments in Sgr C and the X-ray features, suggesting that the two are related. We account for this distribution in terms of the impact of the relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both a nonthermal bremsstrahlung X-ray continuum emission, as well as diffuse 6.4 keV line emission. The production rate of Fe Kalpha photons associated with the injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with the synchrotron emitting radio filaments or extended features is estimated to be in the range between 20 and $\sim10^3$ eV cm^-3 for Sgr C, Sgr B1, Sgr B2, and ``the 45 and -30 \kms'' clouds. We also generalize this idea to explain the pervasive production of diffuse Kalpha line and TeV emission from the Galactic center molecular clouds. In particular, we suggest that IC scattering of the sub-millimeter radiation from dust by relativistic electrons may contribute substantially to the large-scale diffuse TeV emission observed towards the central regions of the Galaxy.