Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHard X-ray selected giant radio galaxies -- II. Morphological evidence of restarted radio activity
301
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
About 6% of Radio Galaxies (RG) can reach linear sizes larger than 0.7 Mpc, and are then classified as Giant Radio Galaxies (GRG). The conditions that make possible the formation of such big structures is still not clear - either core accretion properties or environmental factors. Recent studies have shown that GRG can be up to four times more abundant in hard X-ray selected (i.e. from INTEGRAL/IBIS and Swift/BAT at >20 keV) RG samples. Moreover, a high fraction of young radio sources found in their cores suggests a recently restarted activity, as suggested from the discrepancy between the measured jet and lobes power, with respect to the one expected from core X-ray luminosity. Here we present a radio morphological study of a sample of 15 hard X-ray selected GRG, discussing low-frequency images from our GMRT campaign complemented with others from the literature: among them, 7/15 show evidence of restarted radio activity either in the form of double-double/X-shaped morphology, or as a cocoon emission embedding more recent jets. This, together with the objects from this sample already found hosting a young radio source in their core, suggests that at least 13 over 15 of these hard X-ray selected GRG show features which are consistent with the possibility of restarted radio activity.
rate research
Read More
Giant radio galaxies (GRGs), with extended structures reaching hundreds of kpc, are among the most spectacular examples of ejection of relativistic plasma from super-massive black holes. In this work, third of a series, we present LOw Frequency ARray (LOFAR) images at 144 MHz, collected in the framework of the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources extracted from our sample of hard X-ray selected GRGs (HXGRG, i.e. from INTEGRAL/IBIS and Swift/BAT catalogues at >20 keV). Thanks to the resolution and sensitivity of LoTSS, we could probe the complex morphology of these GRGs, unveiling cases with diffuse (Mpc-scale) remnant emission, presence of faint off-axis wings, or a misaligned inner jet. In particular, for one source (B21144+35B), we could clearly detect a $sim$300 kpc wide off-axis emission, in addition to an inner jet which orientation is not aligned with the lobes axis. For another source (J1153.9+5848) a structure consistent with jet precession was revealed, appearing as an X-shaped morphology with relic lobes having an extension larger than the present ones, and with a different axis orientation. From an environment analysis, we found 2 sources showing an overdensity of cosmological neighbours, and a correspondent association with a galaxy cluster from catalogues. Finally, a comparison with radio-selected GRGs from LoTSS DR1 suggested that, on average, HXGRG can grow to larger extents. These results highlight the importance of deep low-frequency observations to probe the evolution of radio galaxies, and ultimately estimate the duty cycle of their jets.
Giant Radio Galaxies (GRG) are the largest single entities in the Universe, having a projected linear size exceeding 0.7 Mpc, which implies that they are also quite old objects. They are not common, representing a fraction of only about 6% in samples of bright radio galaxies. While a census of about 300 of these objects has been built in the past years, still no light has been shed on the conditions necessary to allow such an exceptional growth, whether of environmental nature or linked to the inner accretion properties. Recent studies found that samples of radio galaxies selected from hard X-ray AGN catalogs selected from INTEGRAL/IBIS and Swift/BAT (thus at energies >20 keV) present a fraction of GRG four times larger than what found in radio-selected samples. We present radio observations of 15 nuclei of hard X-ray selected GRG, finding for the first time a large fraction (61%) of young radio sources at the center of Mpc-scale structures. Being at the center of GRG, these young nuclei may be undergoing a restarting activity episode, suggesting a link between the detected hard X-ray emission - due to the ongoing accretion - and the reactivation of the jets.
As a rule, both lobes of Fanaroff-Riley (FR) type-II radio sources are terminated with hotspots, but the 3C328 radio galaxy is a specimen of an FR II-like object with a hotspot in only one lobe. A conceivable reason for such asymmetry is that the nucleus of 3C328 was temporarily inactive. There was no energy transfer from it to the lobes during the period of quiescence, and so they began to fade out. However, under the assumption that the axis connecting the two lobes makes an appreciable angle with the sky plane, and hence one is considerably farther from the observer than the other, the lobes are observed at two distinct stages of evolution due to the light-travel lag. While the far-side lobe is still perceived as being of the FR II type with a hotspot, decay of the near-side lobe is already apparent. No jets are visible in the VLA images, but the VLBA observations of the inverted-spectrum core component of 3C328 have revealed that it has a jet of a sub-arcsecond length pointing towards the lobe that shows evidence of decay. Since the jet always points to the near side, its observed orientation is in line with the scenario proposed here. The presence of the jet supports the inference that the nucleus of 3C328 is currently active; however, given the fact that the jet is short (approx. 200 pc in projection), the activity must have restarted very recently. The lower and upper limits of the quiescent period length have been calculated.
Using the latest 70 month Swift-BAT catalog we examined hard X-ray selected Seyfert I galaxies which are relatively little known and little studied, and yet potentially promising to test the ionized relativistic reflection model. From this list we chose 13 sources which have been observed by XMM-Newton for less than 20 ks, in order to explore the broad band soft to hard X-ray properties with the analysis of combined XMM-Newton and Swift data. Out of these we found seven sources which exhibit potentially promising features of the relativistic disc reflection, such as a strong soft excess, a large Compton hump and/or a broadened Fe line. Longer observations of four of these sources with the currently operating satellite missions, such as Suzaku, XMM-Newton and NuStar and two others by such future missions as ASTRO-H, will be invaluable, in order to better understand the relativistic disc reflection closest to the central black hole and constrain such important effects of strong gravity as the black hole spin.
Because the disc--jet coupling likely depends on various properties of sources probed, the sample control is always an important but challenging task. In this work, we re-analyzed the INTEGRAL hard X-ray-selected sample of Seyfert galaxies. We only consider sources that have measurements in black hole mass, and luminosities in radio and X-rays. Our sample includes 64 sources, consists of both bright AGNs and low-luminosity ones. We first find that, because of the similarity in the $L_{HX}/L_X$ distribution, the X-ray origin of radio-loud Seyferts may be the same to that of radio-quiet ones, where we attribute to the hot accretion flow (or similarly, the corona). We then investigate the connections between luminosities in radio and X-rays. Since our sample suffers a selection bias of a black hole mass $M_{BH}$ dependence on $L_X/L_{Edd}$, we focus on the correlation slope $xi_X$ between the radio (at 1.4 GHz) and X-ray luminosities in Eddington unit, i.e. $(L_R/L_{Edd})propto(L_X/L_{Edd})^{xi_X}$. We classify the sources according to various properties, i.e. 1) Seyfert classification, 2) radio loudness, and 3) radio morphology. We find that, despite these differences in classification, all the sources in our sample are consistent with a universal correlation slope $xi_X$, with $xi_X=0.77pm0.10$. This is unexpected, considering various possible radio emitters in radio-quiet systems. For the jet interpretation, our result may suggest a common/universal but to be identified jet launching mechanism among all the Seyfert galaxies, while properties like black hole spin and magnetic field strength only play secondary roles. We further estimate the jet production efficiency $eta_{jet}$ of Seyfert galaxies, which is $eta_{jet}approx1.9^{+0.9}_{-1.5}times10^{-4}$ on average. We also find that $eta_{jet}$ increases as the system goes fainter.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
