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

F. Yusef-Zadeh(1), M. Muno(2), M. Wardle(3), D.C. Lis(4)


(1) Department of Physics and Astronomy, Northwestern University, Evanston, Il. 60208
(2) Dept Physics and Astronomy, University of California, Los Angeles, Box 951547, Los Angeles, CA 90095
(3) Department of Physics, Macquarie University, Sydney NSW 2109, Australia
(4) California Institute of Technology, MC 320-47, Pasadena, CA 91125

Paper: ApJ, Feb 2007, in press


Abstract:

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. Many of the best studied sources in this region exhibit a mixture of 6.4 keV Fe K alpha emission, molecular line emission and nonthermal radio continuum radiation. 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. This correlation, when combined with the distribution of molecular gas suggests against the irradiation of Sgr C by Sgr A*. Instead, 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 K alpha 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 103 eV cm-3 for Sgr C, Sgr B1, Sgr B2, and ``the 45 and -30 km s-1'' clouds. We also generalize this idea to explain the cosmic-ray heating of molecular gas, the interstellar cosmic ray ionization, the pervasive production of diffuse K alpha line and TeV emission from the Galactic center molecular clouds. In particular, we suggest that Inverse Compton 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.


Preprints available from the authors at zadeh@northwestern.edu , or the raw TeX (no figures) if you click here.

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