A Self-Consistent Model for the Broad-band Spectrum of Sgr A East at the Galactic Center

Abstract

Sgr A East is a very prominent elongated shell structure surrounding (though off-centered from) the Galactic nucleus. Its energetics (~4x10^52 ergs), based on the power required to carve out the radio synchrotron remnant within the surrounding dense molecular cloud, appear to be extreme compared to the total energy (~10^51 ergs) released in a typical supernova (SN) explosion. Yet it shares several characteristics in common with SN remnants (SNRs), the most significant of which is the ~0.1-10 GeV gamma-ray spectrum measured by EGRET, if we associate the Galactic center source 2EGJ1746-2852 with this nonthermal shell. We here show that the highest-energy component in Sgr A East's spectrum, like that of SNRs, can be fitted with the gamma-rays produced in neutral pion decays. Further, we demonstrate in a self-consistent manner that the leptons released in the associated charged pion decays produce an e+e- distribution that can mimic a power-law with index ~3, like that inferred from the VLA data for this source. These relativistic electrons and positrons also radiate by bremsstrahlung, and inverse Compton scattering with the intense IR and UV fields from the nucleus. We show that the overall emissivity calculated in this way may account for Sgr A East's broadband spectrum ranging from GHz frequencies all the way to TeV energies, where Whipple has thus far set an upper limit to the flux corresponding to a 2.5-sigma signal above the noise.