Leptonic origin of the 100 MeV gamma-ray emission from the Galactic Centre

D. Malyshev, M. Chernyakova, A. Neronov, R. Walter

Published 2015-03-17Version 1

The Galactic Centre is a bright gamma-ray source with the GeV-TeV band spectrum composed of two distinct components in the 1-10 GeV and 1-10 TeV energy ranges. The nature of these two components is not clearly understood. We investigate the gamma-ray properties of the GC in order to clarify the origin of the observed emission. We report imaging, spectral and timing analysis of the data of 74 months of observations of the Galactic Centre by FERMI/LAT gamma-ray telescope complemented by the sub-MeV data from ~10 years of INTEGRAL/PICsIT observations. We find that in the GeV band the Galactic Centre is spatially consistent with a point source. The 3 sigma upper limit on its radius is 0.13 degree. The spectrum of the source in the 100 MeV energy range does not have a characteristic turnover which would point to the pion decay origin of the signal. Instead, the source spectrum is consistent with a model of inverse Compton scattering by high-energy electrons. In such a model, the GeV bump in the spectrum originates from an episode of injection of high-energy particles which happened ~300 years ago. This injection episode coincides with the known activity episode of the Galactic Centre region, previously identified using X-ray observations. The hadronic model of source activity could be still compatible with the data, if bremsstrahlung emission from high-energy electrons is present in addition to the pion decay emission.