The transition from quasar radio-loud to radio-quiet state in the framework of the black hole scalability hypothesis

Toruń Centre for Astronomy, N. Copernicus University, 87-100 Toruń, Poland
e-mail: amr@astro.uni.torun.pl

Received: 23 July 2010
Accepted: 23 September 2010

Abstract

Aims. There are several lines of evidence that active galactic nuclei (AGN) can be regarded as scaled-up X-ray binaries (XRB). The timescales of the evolutionary phenomena in these two classes are proportional to the black hole (BH) masses. Consequently, unlike in the case of XRBs, the evolution of AGNs is too slow to be followed directly. What could be done, however, is to assign particular types of active galaxies to different evolutionary stages observable in XRBs. We studied such an assignment for three quasars with clear signatures of a recent transition from the radio-loud to the radio-quiet state.

Methods. The quasars we investigated have large-scale radio lobes that are clearly asymmetric – one lobe is of Fanaroff-Riley II type, while the other one is a diffuse relic devoid of a hotspot. We suggest that the prime cause of the asymmetry of these radio sources is that the nuclei of their host galaxies currently produce no jets. To prove that, we observed them with milliarcsecond resolution to check if they are similar to those in radio-quiet quasars.

Results. The observations carried out with the EVN revealed that the nuclei of the quasars under investigation are not of a core-jet type that is characteristic for radio-loud, lobe-dominated quasars. It follows that the lobes are no longer fuelled and that the apparent asymmetry results from the orientation, which causes a time lag of the order of 10⁶ years between their images: the lobe perceived as a relic is nearer than the lobe with a hotspot and so it is observed in a later stage of the decay.

Conclusions. The three AGNs under investigation were radio-loud earlier, but now they have switched to the radio-quiet state. In the framework of the XRB/AGN unification, the above means that they have left the very high state and have moved now to the high/soft state. If this scenario is correct it poses a challenge to the so-called spin paradigm. While a radio-loud AGN must have a spinning BH in its centre, the BH of a radio-quiet AGN does not necessarily have low spin; AGNs with high-spin BHs, like those we deal with here, may become radio-quiet.