Abstract.We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.
Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kpc scale form, and why nearly always they are launched from the nuclei of bulge dominated elliptical galaxies and not flat spirals. Here we present the discovery of giant radio source J2345-0449 (z = 0.0755), a clear and extremely rare counter example where relativistic jets are ejected from a luminous and massive spiral galaxy on scale of ∼ 1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infra-red luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass > 2 × 10 8 M ⊙ . The very high mid-IR luminosity of the galaxy suggests that it is actively forming stars and still building a massive disk. We argue that the launch of these powerful jets is facilitated by an advection dominated, magnetized accretion flow at low Eddington rate onto this unusually massive (for a bulgeless disk galaxy) and possibly fast-spinning central black hole. Therefore, J2345-0449 is an extremely rare, unusual galactic system whose properties challenge -2the standard paradigms for black hole growth and formation of relativistic jets in disk galaxies. Thus, it provides fundamental insight into accretion diskrelativistic jet coupling processes.
We present results of a programme of multi‐epoch, intranight optical monitoring of a sample of non‐blazar‐type active galactic nuclei (AGN), which includes seven radio‐quiet quasars (RQQs) and an equal number of radio‐loud, lobe‐dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous quasi‐stellar objects (QSOs) are well matched in the redshift–optical luminosity (z–MB) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshold of ∼1 per cent variation over the night. Unambiguous detection of intranight optical variability (INOV) amplitudes in the range 1–3 per cent on day‐like or shorter time‐scales were thus made for both RQQs and LDQs. Based on these clear detections of INOV, we estimate duty cycles of 17 and 9 per cent for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15 per cent for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, first, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and, secondly, that as in LDQs, relativistic jets may also be present in RQQs. We argue that, as compared to BL Lacs, the conspicuously milder, rarer and possibly slower INOV of RQQs and LDQs can in fact be readily understood in terms of their having optical synchrotron jets which are modestly misaligned from us, but are otherwise intrinsically as relativistic and active as the jets in BL Lacs. This points toward an orientation‐based unifying scheme for the INOV of radio‐loud and radio‐quiet quasars. Variability of up to ∼0.3 mag on month‐ to year‐like time‐scales is seen for nearly all those RQQs and LDQs in our sample for which sufficient temporal coverage is available. These data have revealed an interesting event that seems most likely explained as an occultation, lasting less than six months, of much of the nuclear optical continuum source in an RQQ. The observations reported here form part of a larger ongoing project to study the intranight optical variability of four major classes of powerful AGN, including blazars.
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