DISCOVERY OF GeV EMISSION FROM THE CIRCINUS GALAXY WITH THE FERMI LARGE AREA TELESCOPE

We report the discovery of gamma-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. Circinus is a nearby (similar to 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 (similar or equal to 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) x 10(-9) photon 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.