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arXiv · 1310.1913

Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT

Masaaki Hayashida·Lukasz Stawarz·Chi C. Cheung·Keith Bechtol·Greg M. Madejski·Marco Ajello·Francesco Massaro·Igor V. Moskalenko·Andrew Strong·Luigi Tibaldo
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Abstract

We report the discovery of gamma-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Circinus is a nearby (~4 Mpc) starburst with a heavily obscured Seyfert-type active nucleus, bipolar radio lobes perpendicular to the spiral disk, and kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected during 4 years of LAT observations reveals a significant (~ 7.3 sigma) excess above the background. We find no indications of variability or spatial extension beyond the LAT point-spread function. A power-law model used to describe the 0.1-100 GeV gamma-ray spectrum yields a flux of (18.8+/-5.8)x10^{-9} ph cm^{-2} s^{-1} and photon index 2.19+/-0.12, corresponding to an isotropic gamma-ray luminosity of 3 x 10^{40} erg s^{-1}. This observed gamma-ray luminosity exceeds the luminosity expected from cosmic-ray interactions in the interstellar medium and inverse Compton radiation from the radio lobes. Thus the origin of the GeV excess requires further investigation.

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Masaaki Hayashida, Lukasz Stawarz, Chi C. Cheung, Keith Bechtol, Greg M. Madejski, Marco Ajello, Francesco Massaro, Igor V. Moskalenko, Andrew Strong, Luigi Tibaldo. 2013-10-07. Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT. https://doi.org/10.1088/0004-637x/779/2/131

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