Hard X-ray-selected giant radio galaxies – I. The X-ray properties and radio connection

Abstract

We present the first broad-band X-ray study of the nuclei of 14 hard X-ray selected giant radio galaxies, based both on the literature and on the analysis of archival X-ray data from NuSTAR, XMM-Newton, Swift and INTEGRAL. The X-ray properties of the sources are consistent with an accretion-related X-ray emission, likely originating from an X-ray corona coupled to a radiatively efficient accretion flow. We find a correlation between the X-ray luminosity and the radio core luminosity, consistent with that expected for AGNs powered by efficient accretion. In most sources, the luminosity of the radio lobes and the estimated jet power are relatively low compared with the nuclear X-ray emission. This indicates that either the nucleus is more powerful than in the past, consistent with a restarting of the central engine, or that the giant lobes are dimmer due to expansion losses.

A young and obscured AGN embedded in the giant radio galaxy Mrk 1498

“…The X-ray spectra are indeed modelled by a power-law emission in the 2-195 keV energy band, and this emission is highly absorbed by a column density of 1.7±0.1×10 23 cm −2 , in good agreement with Eguchi et al (2009) and Ursini et al (2018). If we compare the column density with those of other optically classified Seyfert 1.8/1.9 sources (Hernández-García et al 2017b), only two out of the fifteen studied sources have N H >10 23 cm −2 , thus Mrk 1498 is among the most obscured Seyfert 1.8/1.9, in the range of Seyfert 2s (Hernández-García et al 2015).…”

Hard X-ray selected giant radio galaxies – II. Morphological evidence of restarted radio activity

“…From lower to higher frequencies: TGSS (Intema et al 2017), GMRT (this work), SUMSS (Bock et al 1999), NVSS (Condon et al 1998), ATCA (Saripalli et al 2008(Saripalli et al , 2013 X-ray emission: this could be the case if the core was undergoing a restarted activity. Bruni et al (2019) built the radio spectra of the cores of these GRG, showing that more than half show a spectrum peaking in the GHz domain, consistent with being young radio sources with an age of the order of tens of kyr, in contrast with the Myr timescale needed to produce the extended lobes: this came out in favor of the restarted scenario proposed by Ursini et al (2018).…”

“…Bassani et al (2016), who presented the parent sample of these GRG, found that this way of selecting radio galaxies favours large-sized objects, since ∼60% of their sample of radio galaxies emitting at hard X-rays has a size larger than 0.4 Mpc, and the fraction of GRG (22%) is about four times larger than what seen in radio surveys to date (∼6%, Ishwara-Chandra & Saikia 1999). From our previous follow-up study in the X-ray domain, the nuclei of these GRG already showed an excess of nuclear X-ray emission with respect to the lobes radio luminosities and estimated jets power, indicating a possible restarted nuclear activity (Ursini et al 2018). Furthermore, a first single-dish radio follow-up conducted with the Effelsberg 100-m telescope, and aiming at building the radio spectra of the cores of these GRG, revealed the presence of young radio sources in 8 over 15 targets.…”

“…This fraction is almost four times larger than what previously found in radio galaxies samples selected from the radio (∼6% in the 3CR catalog, see Ishwara-Chandra & Saikia 1999 and 8% in the 3CRR sample (Laing et al 1983). Ursini et al (2018) (hereinafter referred as paper I, preceding this work) presented a X-ray follow-up of the sample, finding that in most sources the jet power and the radio luminosity of the lobes are lower than what is expected from the nuclear 1 https://sites.google.com/inaf.it/grace/ Table 1. Details about data collected for this work.…”

“…In particular, LOFAR could provide at the same time high-sensitivity and high-resolution images of these GRG, making it possible to identify diffuse, low surface brightness emission. This work, presenting the morphology of the GRG sample from Bassani et al (2016), is the third in a series after Ursini et al (2018) and Bruni et al (2019) aiming at characterizing the combined X-ray and radio properties of the first GRG sample selected from INTE-GRAL/IBIS and Swift/BAT hard X-ray catalogues (>20 keV). Bassani et al (2016), who presented the parent sample of these GRG, found that this way of selecting radio galaxies favours large-sized objects, since ∼60% of their sample of radio galaxies emitting at hard X-rays has a size larger than 0.4 Mpc, and the fraction of GRG (22%) is about four times larger than what seen in radio surveys to date (∼6%, Ishwara-Chandra & Saikia 1999).…”