X-ray study of a sample of FR0 radio galaxies: unveiling the nature of the central engine

TL;DR

This study presents the first X-ray analysis of FR0 radio galaxies, revealing their similarities to FRIs and suggesting intrinsic jet properties or intermittent activity as reasons for their compactness.

Contribution

It provides the first X-ray spectral analysis of a sample of FR0 radio galaxies, comparing their properties with FRIs, BL Lacs, and young radio sources.

Findings

FR0s have X-ray properties similar to FRIs.

X-ray emission likely originates from jets, correlated with radio luminosity.

FR0s show low accretion rates and lack extended radio structures.

Abstract

FR0s are compact radio sources that represent the bulk of the Radio-Loud (RL) AGN population, but they are still poorly understood. Pilot studies on these sources have been already performed at radio and optical wavelengths: here we present the first X-ray study of a sample of 19 FR0 radio galaxies selected from the SDSS/NVSS/FIRST sample of Best & Heckman (2012), with redshift $\leq$ 0.15, radio size $\leq$ 10 kpc and optically classified as low-excitation galaxies (LEG). The X-ray spectra are modeled with a power-law component absorbed by Galactic column density with, in some cases, a contribution from thermal extended gas. The X-ray photons are likely produced by the jet as attested by the observed correlation between X-ray (2-10 keV) and radio (5 GHz) luminosities, similar to FRIs. The estimated Eddington-scaled luminosities indicate a low accretion rate. Overall, we find that the X-ray properties of FR0s are indistinguishable from those of FRIs, thus adding another similarity between AGN associated with compact and extended radio sources. A comparison between FR0s and low luminosity BL Lacs, rules out important beaming effects in the X-ray emission of the compact radio galaxies. FR0s have different X-ray properties with respect to young radio sources (e.g. GPS/CSS sources), generally characterized by higher X-ray luminosities and more complex spectra. In conclusion, the paucity of extended radio emission in FR0s is probably related to the intrinsic properties of their jets that prevent the formation of extended structures, and/or to intermittent activity of their engines.