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.