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Register a new userDuty cycle of the radio galaxy B2 0258+35
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Radio loud Active Galactic Nuclei are episodic in nature, cycling through periods of activity and quiescence. In this work we investigate the duty cycle of the radio galaxy B2~0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio source consists of a pair of kpc-scale jets embedded in two large-scale lobes (~240 kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity. We have combined new LOFAR data at 145 MHz and new SRT data at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral properties of the outer lobes and derive their age. Interestingly, the spectrum of both the outer Northern and Southern lobes is not ultra-steep as expected for an old ageing plasma with spectral index values equal to $rm alpha_{1400}^{145}=0.48pm0.11$ and $rm alpha_{6600}^{1400}=0.69pm0.20$ in the outer Northern lobe, and $rm alpha_{1400}^{145}=0.73pm0.07$ in the outer Southern lobe. Moreover, despite the wide frequency coverage available for the outer Northern lobe (145-6600~MHz), we do not identify a significant spectral curvature (SPC$simeq$0.2$pm0.2$). While mechanisms such as in-situ particle reacceleration, mixing or compression can temporarily play a role in preventing the spectrum from steepening, in no case seem the outer lobes to be compatible with being very old remnants of past activity as previously suggested (with age $gtrsim$ 80 Myr). We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.
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Restarted radio galaxies represent a unique tool to investigate the duty cycle of the jet activity in Active Galactic Nuclei. Due to a sharp discontinuity of the GHz spectral index distribution within its lobes, the radio galaxy 3C388 has for long being claimed to be a peculiar example of AGN with multi-epoch activity. In this work we have performed, for the first time, a spatially resolved study of the radio spectrum of this source down to MHz-frequencies, aimed at investigating its radiative age and duty cycle. We have used new low frequency observations at 144 MHz performed with LOFAR and at 350 MHz performed with the VLA combined with archival data at higher frequencies. We find that the spectral indices in the lower frequency range 144-614 MHz have flatter values ($sim$0.55-1.14) than those observed in the higher frequency range 1400-4850 MHz ($sim$0.75-1.57) but follow the same distribution across the lobes, with a systematic steepening towards the edges. However, the spectral shape throughout the source is not uniform and often deviates from standard models. This suggests that mixing of different particle populations is occurring, although it remains difficult to understand whether this is caused by observational limitations (insufficient spatial resolution and/or projection effects) or by the intrinsic presence of multiple particle populations, possibly related to the two different outbursts. By using single-injection radiative models we compute that the total source age is $lesssim$80 Myr and that the duty cycle is about $rm t_{on}/t_{tot}sim$ 60%, which is enough to prevent the intracluster medium from cooling according to X-ray estimates. While to date the radio spectral distribution of 3C388 remains a rare case among radio galaxies, upcoming multi-frequency surveys will soon allow us to investigate whether more sources with the same characteristics do actually exist.
Steep spectrum radio sources associated with active galactic nuclei (AGN) may contain remnants of past AGN activity episodes. Novel instruments like the LOw Frequency ARray (LOFAR) are enabling studies of these fascinating structures to be made at tens to hundreds of MHz with sufficient resolution to analyse their complex morphology. Our goal is to characterize the integrated and resolved spectral properties of VLSS J1431+1331 and estimate source ages based on synchrotron radio emission models, thus putting constraints on the AGN duty cycle. Using a broad spectral coverage, we have derived spectral and curvature maps, and used synchrotron ageing models to determine the time elapsed from the last time the source plasma was energized. We used LOFAR, Giant Metrewave Radio Telescope (GMRT) and Jansky Very Large Array (VLA) data. Based on our ageing analysis, we infer that the AGN that created this source currently has very low levels of activity or that it is switched off. The derived ages for the larger source component range from around 60 to 130 Myr, hinting that the AGN activity decreased or stopped around 60 Myr ago. Our analysis suggests that VLSS J1431.8+1331 is an intriguing, two-component source. The larger component seems to host a faint radio core, suggesting that the source may be an AGN radio relic. The spectral index we observe from the smaller component is distinctly flatter at lower frequencies than the spectral index of the larger component, suggesting the possibility that the smaller component may be a shocked plasma bubble. From the integrated source spectrum, we deduce that its shape and slope can be used as tracers of the activity history of this type of steep spectrum radio source.
We present an analysis of deep Chandra X-ray observations of the galaxy cluster MS 0735.6+7421, which hosts the most energetic radio AGN known. Our analysis has revealed two cavities in its hot atmosphere with diameters of 200-240 kpc. The total cavity enthalpy, mean age, and mean jet power are $9times 10^{61}$ erg, $1.6times 10^{8}$ yr, and $1.7times 10^{46}$ erg/s, respectively. The cavities are surrounded by nearly continuous temperature and surface brightness discontinuities associated with an elliptical shock front of Mach number 1.26 (1.17-1.30) and age of $1.1times 10^{8}$ yr. The shock has injected at least $4times 10^{61}$ erg into the hot atmosphere at a rate of $1.1times 10^{46}$ erg/s. A second pair of cavities and possibly a second shock front are located along the radio jets, indicating that the AGN power has declined by a factor of 30 over the past 100 Myr. The multiphase atmosphere surrounding the central galaxy is cooling at a rate of 36 Msun/yr, but does not fuel star formation at an appreciable rate. In addition to heating, entrainment in the radio jet may be depleting the nucleus of fuel and preventing gas from condensing out of the intracluster medium. Finally, we examine the mean time intervals between AGN outbursts in systems with multiple generations of X-ray cavities. We find that, like MS0735, their AGN rejuvenate on a timescale that is approximately 1/3 of their mean central cooling timescales, indicating that jet heating is outpacing cooling in these systems.
The evolutionary stage of a powerful radio source originated by an AGN is related to its linear size. In this context, compact symmetric objects (CSOs), which are powerful and intrinsically small objects, should represent the young stage in the individual radio source life. However, the fraction of young radio sources in flux density-limited samples is much higher than what expected from the number counts of large radio sources.This indicates that a significant fraction of young radio sources does not develop to the classical Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the radio jets are expanding within the dense and inhomogeneous interstellar medium,the ambient may play a role in the jet growth, for example slowing down or even disrupting its expansion when a jet-cloud interaction takes place. Moreover, this environment may provide the thermal seed photons that scattered by the lobes electrons may be responsible for high energy emission, detectable by Fermi-LAT.
We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper we use HI observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the Active Galactic Nucleus and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [OIII] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely HI-rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio-AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.
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