Fermi Detection of γ-Rays from the Radio-quiet Seyfert Galaxy NGC 3281

Abstract

We report the detection of significant γ-ray emission with the Fermi Large Area Telescope from the radio-quiet Seyfert 2 galaxy NGC 3281, with a luminosity of 5.9 (±1.7) × 1041erg s−1 at a significance of 6.22σ (TS = 42.81). The power-law photon index is 2.61 (±0.24), indicative of a soft spectrum. The star formation activity in NGC 3281 is insufficient to explain its γ-ray luminosity based on the empirical relation between the infrared and γ-ray luminosities observed in other sources. The multiwavelength spectrum can be explained as due to inverse Compton scattering by relativistic electrons in the corona or the jet of seed photons from the corona, disk, and torus. The source is Compton thick, and attenuation of GeV photons due to pair production in the corona is nonnegligible (with an optical depth of about 10). The intrinsic γ-ray luminosity is inferred to be 3.4 × 1042 and 2.2 × 1041 erg s−1 for the corona and jet model, respectively. The observed γ-ray and radio luminosities are roughly consistent with the known correlation between the two quantities, among the lowest-luminosity regime. The jet origin is valid only if the radio emission is dominated by the jet.