High-energy processes in the Galactic center

Abstract

High-energy particle interactions have been of interest to scientists ever since the discovery of cosmic rays early this century. With the realization, almost half a century later, that the prodigious radio emission seen from outer-space is due in part to these particles gyrating in cosmic magnetic fields, a new field was born which joined physics and astronomy. By studying the interaction signatures of these particles, we can gain a better understanding of the microphysics of their motion and collisions, as well as the macrophysics governing the vast and distant astrophysical objects which host them. The Galactic center is very important for this burgeoning field. Because of its proximity, years of detailed observations in particularly the radio and infrared wavebands have provided us with a good picture of what the central environment is like. Therefore, when the gamma-ray telescope EGRET detected an excess of gamma-rays over the expected background coming from within 0.2° of the center, several candidates for the emission were suggested on the basis of their characteristics known from the low-frequency observations. Two promising sources, the massive black hole candidate Sgr A* and the extended shell structure Sgr A East, are considered here. We first investigate in detail the hadronic processes contributing to the gamma-ray emission, and then compare predicted spectra to the EGRET data. We conclude that Sgr A* cannot be the source but that Sgr A East is very promising, and suggest further observational tests. Understanding the high energy processes in our Galactic center is crucial for our modeling of the same processes throughout our Galaxy as well as in distant galaxies. Because we have so much more information about the Galactic center physical environment, we have the opportunity to test our theories in a familiar surrounding before attempting to apply our ideas to places we can never hope to resolve so well. The Galactic center may hold the key to our understanding of the high energy interactions in blazars, supernova remnants and by cosmic rays.