BROAD-LINE RADIO GALAXIES OBSERVED WITH<i>FERMI</i>-LAT: THE ORIGIN OF THE GeV γ-RAY EMISSION

Abstract

We report on a detailed investigation of the gamma-ray emission from 18 broad-line radio galaxies (BLRGs) based on two years of Fermi Large Area Telescope data. We confirm the previously reported detections of 3C 120 and 3C 111 in the GeV photon energy range; a detailed look at the temporal characteristics of the observed gamma-ray emission reveals in addition possible flux variability in both sources. No statistically significant gamma-ray detection of the other BLRGs was found, however, in the considered data set. Though the sample size studied is small, what appears to differentiate 3C 111 and 3C 120 from the BLRGs not yet detected in gamma-rays is the particularly strong nuclear radio flux. This finding, together with the indications of the gamma-ray flux variability and a number of other arguments presented, indicates that the GeV emission of BLRGs is most likely dominated by the beamed radiation of relativistic jets observed at intermediate viewing angles. In this paper we also analyzed a comparison sample of high-accretion-rate Seyfert 1 galaxies, which can be considered radio-quiet counterparts of BLRGs, and found that none were detected in gamma-rays. A simple phenomenological hybrid model applied for the broadband emission of the discussed radio-loud and radio-quiet type 1 active galaxies suggests that the relative contribution of the nuclear jets to the accreting matter is &gt;= 1% on average for BLRGs, whereas it is &lt;= 0.1% for Seyfert 1 galaxies.