Large scale X-ray jets that extend to >100 kpc distances from the host galaxy indicate the importance of jets interactions with the environment on many different physical scales. Morphology of X-ray clusters indicate that the radio-jet activity of a cD galaxy is intermittent. This intermittency might be a result of a feedback and/or interactions between galaxies within the cluster. Here we consider the radiation pressure instability operating on short timescales (<10^5 years) as the origin of the intermittent behaviour. We test whether this instability can be responsible for short ages (< 10^4 years) of Compact Symmetric Objects measured by hot spots propagation velocities in VLBI observations. We model the accretion disk evolution and constrain model parameters that may explain the observed compact radio structures and over-abundance of GPS sources. We also describe effects of consequent outbursts.
The idea of intermittency is not new. Some observational evidence was given already in early 60-ties. For example Burbidge & Burbidge (1965) suggested intermittent outbursts of NGC1275 the cD galaxy in the center of Perseus A cluster. Kellermann (1966) derived the intermittency timescales of ∼ 10 4 -10 6 years required for the production of relativistic particles responsible for the observed synchrotron spectra in radio sources and quasars. Signatures of the past recurrent activity in nuclei of normal galaxies were presented by Bailey & Clube (1978), but their paper was not really noticed and has only 24 citations to date. Shields & Wheeler (1978) examined quasar models and suggested that their accumulate the mass during quiescent periods and then through the instability they transfer the mass onto a central black hole during a short period of an outburst of the activity. These are just a few examples of the AGN intermittency that has been considered since the early days of studies of the nuclear emission in galaxies. It is now that we are looking closely at this behaviour as 2 Siemiginowska et al.
it becomes evident that it is an important component to our understanding of the evolution of structures in the universe. However, there are still many open questions about the origin of the intermittent behaviour. Is it related to the unsteady fuel supply, or accretion flow? Are there many quiescent and outburst phases? What is the mechanism regulating the intermittent behaviour?
Observations show a range of timescales for the AGN outbursts. Quasar lifetimes estimated based on large samples of SDSS quasars are of order of 10 7 years (Martini & Weinberg 2001). Signatures of outbursts in recent observations of X-ray clusters indicated similar timescales (see McNamara & Nulsen 2007, for the review). However, episodes of activity on timescales shorter than < 10 5 years have been also observed for example in compact radio sources (Owsianik et al. 1998;Reynolds & Begelman 1997) or as light echos in nearby galaxies (Lintott et al. 2009). In this review we will focus on the short timescales of the intermittent jet activity. We discuss the radio source evolution, observational evidence for the intermittent activity and present the model for the origin and nature of the short term activity based on the accretion disk physics.
Jets provide the evidence for energetic AGN outflows. They highlight the fact that the energy released in the nuclear region in the close vicinity of a black hole can influence the environment at large distances. They also trace the source age and activity timescales. In many recently discovered X-ray jets associated with powerful quasars the continuous X-ray jet emission extends to hundreds of kpc distances from the core (Siemiginowska et al. 2002;Sambruna et al. 2002;Harris & Krawczynski 2006). These jets are straight, sometimes curved or bent, and have many knots. For example in PKS 1127-145 jet associated with z=1.18 quasar the separation and size of the knots may indicate separate outbursts of jet activity with timescales ∼ 10 5 years (Siemiginowska et al. 2007).
Host galaxy scale jets, smaller than < 10 kpc provide a direct information about the jet interactions with the ISM and feedback. Compact radio sources that are entirely contained within the host galaxy represent the initial phase of radio source growth and they are young. The most compact Gigahertz Peaked Spectrum (GPS) sources have linear radio sizes below ∼ 1 kpc. Their age can be probed directly by studying the expansion velocity of symmetric radio structures and it is typically less than 10 3 years (Polatidis & Conway 2003;Gugliucci et al. 2005). Compact Steep Spectrum (CSS) radio sources are slightly larger, but still contained within the host galaxy. Their age is given by synchrotron ageing measurements and is smaller than 10 5 years (Murgia et al. 1999).
Tengstrand et al. ( 2009) discuss the XMM-Newton and Chandra X-ray Observatory observations of a complete sample of compact GPS galaxies. All these sources are unresolved in X-rays, but they are very powerful in both the radio and X-rays bands. In Figure 1 we mark the location of GPS galaxies in radio vs. X-ray luminosity plane. We also mark the locations of the large scale FRI and FRII sources. The GPS sources cover an upper right corner of that diagram and the plotted regression lines indicate possible evolutionary path for the GPS sources to grow into large scale radio sources. However, it is unclear which way and how the evolution of the GPS radio sources proceeds. Are they fade in radio and X-rays simultaneously or maybe only the radio fades while the X-ray emission remains unchanged indicating that the X-ray emission process is independent of the radio one? On the other hand maybe the GPS sources remain in this part of the diagram due to repetitive outbursts. O’ Dea & Baum (1997) show that there is an overabundance of compact radio sources, so they need to be short-liv
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