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Paper: The Galactic Center EGRET Source and the Broad-band Spectrum of SGR A East
Volume: 186, The Central Parsecs of the Galaxy
Page: 560
Authors: Fatuzzo, M.; Melia, F.; Yusef-Zadeh, F.; Markoff, S.
Abstract: High energy (30 MeV - 10 GeV) emission associated with a Galactic Center source has recently been observed by EGRET on board the Compton Gamma-ray Observatory. Continuum low-frequency radio observations of the Galactic Center reveal the presence of several viable source candidates, one of which is a prominent elliptical shell structure known as Sgr A East. Although morphologically similar to supernova remnants (SNRs), its inferred energetics and size appear to be extreme and have generated some uncertainty regarding this interpretation. Additionally, the radio spectral index determined from 6 cm and 20 cm observations of Sgr A East (~ 1) differs from the indices normally found for SNRs (~ 0.5). This difference is quite significant given that the corresponding index for the electron distribution for Sgr A East is then ~3 (versus ~2 for SNRs) and therefore unlike that thought to be produced directly from the shock acceleration processes believed to occur in such an environment. Regardless, recent observations of this region at 1720 MHz (the transition frequency of OH maser emission) have revealed the presence of several maser spots, suggesting a kinship between Sgr A East and three EGRET SNRs also observed at 1720 MHz: IC443, W28 and W44. This possible kinship suggests that Sgr A East could be the source of the EGRET emission. We show here that the pion-production and decay mechanism invoked to account for gamma-ray emission from SNRs can also naturally account for the radio through TeV spectrum of Sgr A East (including the EGRET data). The differences between the spectral features of Sgr A East and the other SNRs then results from the differences between the underlying energetics and the properties of the surrounding interstellar medium.
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