Enhanced activity of massive black holes by stellar capture assisted by a self-gravitating accretion disc

Abstract

Aims. We study the probability of close encounters between stars from a nuclear cluster and a massive black hole (10 4 M M • 10 8 M ). The gravitational field of the system is dominated by the black hole in its sphere of influence. It is further modified by the cluster mean field (a spherical term) and a gaseous disc/torus (an axially symmetric term) causing a secular evolution of stellar orbits via Kozai oscillations. Intermittent phases of high eccentricity increase the chance that stars become damaged inside the tidal radius of the central hole. Such events can produce debris and lead to recurring episodes of enhanced accretion activity. Methods. We introduce an effective loss cone and associate it with tidal disruptions during the high-eccentricity phases of the Kozai cycle. By numerical integration of the trajectories forming the boundary of the loss cone, we determine its shape and volume. We also include the effect of a relativistic advance of the pericentre. Results. The potential of the disc has the efffect of enlarging the loss cone, therefore, the predicted number of tidally disrupted stars should grow by factor of 10 2 . On the other hand, the effect of the cluster mean potential, together with the relativistic pericentre advance, act against the eccentricity oscillations. In the end we expect the tidal disruption events to be approximately ten times more frequent in comparison with the model in which the three effects -the cluster mean field, the relativistic pericentre advance, and the Kozai mechanism -are all ignored. The competition of different influences suppresses the predicted star-disruption rate as the black hole mass increases. Hence, the process under consideration is more important for intermediate-mass black holes, M • 10 4 M .

Tidal disruption of stars by supermassive black holes: The X-ray view

“…Also, because in AGN with a permanent accretion disk, and with a large reservoir of obscuring material, there is a number of alternate mechanisms which can produce or mimik largeamplitude variability (e.g., Grupe et al 2012a,b). Yet, TDEs are expected to occur in AGN, and disruption rates may be even higher than in non-active galaxies (e.g., Karas & Subr 2007). Especially, the AGN IC 3599 is remarkable for its high-amplitude X-ray outburst associated with a strong change in optical emission lines, and several mechanisms were considered to explain the data: highamplitude Narrow-line Seyfert 1 variability, variants of accretion-disk instabilities, or a TDE (Brandt et al 1995, Grupe et al 1995, Komossa & Bade 1999).…”

An Enhanced Rate of Tidal Disruptions in the Centrally Overdense E+a Galaxy NGC 3156

“…Observational estimates for the TDE rate are typically~---10 yr galaxy 5 1 1 (Donley et al 2002;Khabibullin & Sazonov 2014;van Velzen & Farrar 2014), a number discrepant with theoretical estimates by an order of magnitude or more (Stone & Metzger 2016). Other processes can in principle enhance the TDE rate above the floor set by two-body relaxation, such as non-conservation of angular momentum in axisymmetric or triaxial potentials (Magorrian & Tremaine 1999;Merritt & Poon 2004), interactions with massive perturbers such as molecular clouds (Perets et al 2007) and large-scale accretion disks (Karas & Šubr 2007), or gravitational wave recoil of the central SMBH (Stone & Loeb 2011). The impact of these more exotic mechanisms is more difficult to quantify observationally.…”

Enhancing the rate of tidal disruptions of stars by a self-gravitating disc around a massive central black hole

“…Therefore we wanted to learn whether the Kozai mechanism is still relevant for the long-term dynamics of nuclear stars, even if relativistic pericentre advance and the effect of the nuclear cluster are taken into account. Figure 3 shows the fraction F 1 (R t ; a) [7]. We find that the damping effects are weak if the mass of the axisymmetric 01003-p.3 EPJ Web of Conferences component (i.e.…”

“…[7] we characterised the influence of highly eccentric orbits on the star-capture rate by introducing fractional rates, given by the number of stars that exhibit pericentre distances below a certain threshold radius R min . We denote this fraction F 2 (R min ; a, C 1 ) and write it in the Tidal Disruption Events and AGN Outbursts Structure of the nuclear cluster interacting with an embedded accretion disc.…”

“…In this context it is of a particular interest to study processes that can influence the orbital motion of stars in the close vicinity of the central black hole. Here we will summarize our prior works [5,7,16,17] on various perturbation effects that can cause the orbital decay, so that stars of the nuclear cluster migrate towards the centre. Near the center the stars eventually plunge below the horizon, or they become disrupted by tidal forces of the supermassive black hole.…”