Discovery of GeV Emission tfrom the Circinus Galaxy with the Fermi-Lat

We report the discovery of γ-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 σ) 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− 100GeV γ-ray spectrum yields a flux of (18.8± 5.8)× 10 ph cm s and photon index 2.19±0.12, corresponding to an isotropic γ-ray luminosity of 3×10 erg s. 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. Subject headings: radiation mechanisms: non-thermal — galaxies: active — galaxies: individual (Circinus) — galaxies: jets — galaxies: Seyfert — gamma rays: galaxies