Seyfert?2 galaxies in the GeV band: jets and starburst

摘要

Context. The Fermi/LAT collaboration recently reported the detection of starburt galaxies in the high energy γ-ray domain, as well as radio-loud narrow-line Seyfert?1 objects. Aims. Motivated by the presence of sources close to the location of composite starburst/Seyfert?2 galaxies in the first year Fermi/LAT catalogue, we aim at studying high energy γ-ray emission from such objects, and at disentangling the emission of starburst and Seyfert activity. Methods. We analysed 1.6 years of Fermi/LAT data from NGC?1068 and NGC?4945, which count among the brightest Seyfert?2 galaxies. We search for potential variability of the high energy signal, and derive a spectrum of these sources. We also analyse public INTEGRAL IBIS/ISGRI data over the last seven years to derive their hard X-ray spectrum. Results. We find an excess of high energy γ-rays of 8.3σ and 9.2σ for 1FGL?J0242.7+0007 and 1FGL?J1305.4?4928, which are found to be consistent with the position of the Seyfert?2 galaxies NGC?1068 and NGC?4945, respectively. The energy spectrum of the sources can be described by a power?law with a photon index of Γ?=?2.31??±? 0.13 and a flux of F100???MeV?100???GeV??=? (8.60?±?2.27)??×??10-12?erg?cm-2?s-1 for NGC?1068, while for NGC?4945, we obtain a photon index of Γ?=?2.31??±? 0.10 and a flux of F100???MeV?100???GeV??=? (1.58?±?0.32)??×??10-11?erg?cm-2?s-1. For both sources, we detect no significant variability nor any indication of a curvature of the spectrum. While the high energy emission of NGC?4945 is consistent with starburst activity, that of NGC?1068 is an order of magnitude above expectations, suggesting dominant emission from the active nucleus. We show that a leptonic scenario can account for the multi-wavelength spectral energy distribution of NGC?1068. Conclusions. High energy γ-ray emission is revealed for the first time in a Seyfert?2 galaxy. If?this result is confirmed in other objects, new perspectives would be opened up into the GeV?band, with the discovery of a new class of high energy γ-ray emitters.