Soft γ-ray selected radio galaxies: favouring giant size discovery

Abstract

Using the recent INTEGRAL/IBIS and Swift/BAT surveys we have extracted a sample of 64 confirmed plus 3 candidate radio galaxies selected in the soft gamma-ray band. The sample covers all optical classes and is dominated by objects showing a FR II radio morphology; a large fraction (70%) of the sample is made of "radiative mode" or High Excitation Radio Galaxies (HERG). We have measured the source size on NVSS, FIRST and SUMSS images and have compared our findings with data in the literature obtaining a good match. We surprisingly found that the soft gamma-ray selection favours the detection of large size radio galaxies: 60% of objects in the sample have size greater than 0.4 Mpc while around 22% reach dimension above 0.7 Mpc at which point they are classified as Giant Radio Galaxies or GRGs, the largest and most energetic single entities in the Universe. Their fraction among soft gamma ray selected radio galaxies is significantly larger than typically found in radio surveys, where only a few percent of objects (1-6%) are GRGs. This may partly be due to observational biases affecting radio surveys more than soft gamma ray surveys, thus disfavouring the detection of GRGs at lower frequencies. The main reasons and/or conditions leading to the formation of these large radio structures are still unclear with many parameters such as high jet power, long activity time and surrounding environment all playing a role; the first two may be linked to the type of AGN discussed in this work and partly explain the high fraction of GRGs found in the present sample. Our result suggests that high energy surveys may be a more efficient way than radio surveys to find these peculiar objects.

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

“…The 15 GRG subject of this work have been extracted from a parent sample of 64 radio galaxies selected via cross-correlation of hard X-ray catalogues and radio surveys. Indeed, Bassani et al (2016) produced a catalogue of radio galaxies detected by the IN-TEGRAL/IBIS (Imager on Board the INTEGRAL Satellite) and/or Swift/BAT (Burst Alert Telescope), at energies >20 keV, identified using the two complementary radio surveys covering the whole sky: the NRAO Very Large Array Sky Survey (NVSS, 1.4 GHz, Condon et al 1998), and the Sydney University Molonglo Sky Survey (SUMSS, 843 MHz, Bock et al 1999). The same authors found that ∼25% of the objects in that sample have a projected linear size >0.7 Mpc, and can then be considered GRG (see Tab.…”

“…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).…”

“…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). 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).…”

An Updated Catalog of Giant Radio Sources

“…Furthermore, the recent investigations of Bassani et al (2016) show that a large fraction of soft gamma-ray selected radio sources become GRSs. The all-sky observations of the INTErnational Gamma-Ray Astrophysics Laboratory (IN-TEGRAL;Winkler 1994) and Swift (Gehrels et al 2004) satellites reveal a large population of AGNs.…”

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

“…We measured a linear size of 1.2 Mpc (1125 arcsec from lobe to lobe, at a spatial scale of 1.087 kpc/arcsec) so it falls in the GRG category. Indeed, it is included in the recently-published sample of GRG selected in the soft gamma-ray band by Bassani et al (2016), and classified as an FRII. The symmetric structure has a position angle of about 45 degrees, with the southern jet less extended than the Northern one, which could be due to a projection effect.…”