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

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 Xray study of a sample of 19 FR0 radio galaxies selected from the SDSS/NVSS/FIRST sample of Best & Heckman (2012), with redshift ≤ 0.15, radio size ≤ 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.

Building a cluster: shocks, cavities, and cooling filaments in the group–group merger NGC 6338

“…We therefore agree with Torresi et al (2018), who concluded that the AGN of NGC 6338 is X-ray faint, and place stronger limits on its luminosity. It should be noted that the high central density found in NGC 6338 (see Section 3.2) is therefore very unlikely to be biased by AGN emission.…”

The High Energy View of FR0 Radio Galaxies

“…We use two FR 0s as reference ( Figure 4): while one is Tol 1326 − 379, detected by Fermi, the other source is a nearby (z=0.04) FR 0 radio galaxy, J160426.51+17. It shows a flat X-ray spectral index (Γ X = 1.1 ± 0.3) and X-ray luminosity similar to Tol 1326 − 379 (see [30]) that make it a potential candidate for a γ-ray detection. As an exercise, we adjusted the model adopted for Tol 1326 − 379 to reproduce the SED of J160426.51+17 (Figure 4b): the output in the GeV window could be just at the Fermi detection limit.…”

“…Another proof of the misaligned nature of FR 0s comes from the distribution of the ratio between [O III] and X-ray luminosities (R [OI I I]/X ). While FR 0s and FR Is show similar values of R [OI I I]/X ∼ −1.7 (e.g., [38,44]), low-luminosity blazars have R [OI I I]/X ∼ −3.3 (e.g., [30,45]) as expected if the X-ray emission is beamed. Are they small fading FR Is?…”

“…The black solid line is the linear regression for FR 0s + FR Is: log L X = (7.8 ± 0.6) + (0.8 ± 0.1)log L 5 GHz . Black dashed lines are uncertainties on the slope, taken from[30].…”

Infrared Diagnostics of the ISM in the Circumnuclear Environments of the Youngest Radio Galaxies

“…We also do not discuss here the more evolved radio galaxies, classified morphologically as Medium Symmetric Objects (MSOs), and/or spectrally as Compact Steep Spectrum (CSS) sources (see Kunert-Bajraszewska et al 2014). Finally, we do not include the Fanaroff-Riley type 0 (FR0) radio galaxies (Glowacki et al 2017;Torresi et al 2018 WISE is an all sky survey mission covering the entirety of the northern and southern sky. The telescope obtains measurements in four bands (W1-W4) centered at 3.4, 4.6, 12, and 22 µm respectively.…”