Discovery of GeV Emission from the Circinus Galaxy with the Fermi Large Area Telescope

We report the discovery of γ -ray emission from the Circinus galaxy using the Large Area Telescope (LAT) on board 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 yr of LAT observations reveals a significant (7.3σ) 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 γ -ray spectrum yields a flux of (18.8±5.8)×10−9 photon cm−2 s−1 and photon index 2.19±0.12, corresponding to an isotropic γ -ray luminosity of 3×1040 erg s−1. This observed γ -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.