IGR J18249-3243: a new GeV-emitting FR II and the emerging population of high energy radio galaxies

TL;DR

This paper reports the discovery and classification of a new FR II radio galaxy emitting at GeV energies, highlighting the importance of radio surveys in identifying high-energy extragalactic sources and suggesting lobes as the emission origin.

Contribution

It presents the reclassification of a Fermi-LAT source as a new FR II radio galaxy and identifies nine new radio galaxies with GeV emission, expanding the population of high-energy radio galaxies.

Findings

The new FR II galaxy emits GeV gamma rays likely from lobes.

Nine new radio galaxies with GeV emission are identified.

Lobe emission via inverse Compton scattering may dominate gamma-ray production.

Abstract

The advent of new all-sky radio surveys such as the VLA Sky Survey (VLASS) and the Rapid ASKAP Continuum Survey (RACS), performed with the latest generation radio telescopes, is opening new possibilities on the classification and study of extragalactic $\gamma$-ray sources, specially the underrepresented ones like radio galaxies. In particular, the enhanced sensitivity (sub-mJy level) and resolution (a few arcsec) provides a better morphological and spectral classification. In this work, we present the reclassification of a Fermi-LAT source as a new FRII radio galaxy from the INTEGRAL sample found to emit at GeV energies. Through a broad-band spectral fitting from radio to $\gamma$-ray, we find that the commonly invoked jet contribution is not sufficient to account for the observed $\gamma$-ray flux. Our modeling suggests that the observed emission could mainly originate in the lobes (rather than in the radio core) by inverse Compton scattering of radio-emitting electrons off the ambient photon fields. In addition, we cross-correlated the latest generation radio surveys with a list of Fermi-LAT objects from the literature considered to be candidate misaligned AGN, finding four new radio galaxies with a double-lobed morphology. Additional four objects could be classified as such thanks to previous studies in the literature, for a total of nine new radio galaxies with GeV emission presented in this work. We foresee that further objects of this class might be found in the near future with the advent of the Square Kilometer Array (SKA), populating the GeV sky.