Broad Line Radio Galaxies Observed with Fermi-LAT: The Origin of the GeV Gamma-Ray Emission

TL;DR

This study analyzes gamma-ray emissions from 18 broad line radio galaxies using Fermi-LAT data, confirming detections in two sources and suggesting jet beaming as the primary emission mechanism, with differences from Seyfert galaxies.

Contribution

First detailed gamma-ray analysis of a small BLRG sample, linking gamma-ray emission to jet orientation and nuclear radio flux, and contrasting with Seyfert galaxies.

Findings

3C 120 and 3C 111 detected in gamma-rays with possible flux variability.

Gamma-ray emission likely dominated by relativistic jets at intermediate viewing angles.

No gamma-ray detections in Seyfert 1 galaxies, indicating different jet contributions.

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 (LAT) 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 however found in the considered dataset. 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, indicate 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 none were detected in gamma-rays. A simple phenomenological hybrid model applied for the broad-band 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 > 1 percent on average for BLRGs, whilst <0.1 percent for Seyfert 1 galaxies.