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

We study the probability of close encounters between stars from a nuclear cluster and a massive black hole. 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 large 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. 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 relativistic advance of pericentre. The potential of the disc has the efffect of enlarging the loss cone and, 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_bh~10^4M_s.Comment: 10 pages, 5 figures; Astronomy & Astrophysics accepte

Orbital decay of satellites crossing an accretion disc

Motion of stellar-mass satellites is studied around a massive compact body which is surrounded by a gaseous slab of a stationary accretion disc. The satellites suffer an orbital decay due to hydrodynamical interaction with the disc medium (transitions across the disc, gap opening in the disc, density waves) and gravitational radiation. Arbitrary orbital eccentricities and inclinations are considered, and it is observed how the competing effects depend on the parameters of the model, namely, the mass and compactness of the orbiters, the osculating elements of their trajectories, and surface density of the disc. These effects have a visible impact on the satellites long-term motion, and they can produce observational consequences with respect to galactic central clusters. It is shown that the satellite-disc collisions do not impose serious restrictions on the results of gravitational wave experiments if the disc medium is diluted and the orbiter is compact but they are important in the case of environments with relatively high density. We thus concentrate on application to accretion flows in which the density is not negligible. We discuss the expected quasi-stationary structure of the cluster that is established on sub-parsec scales within the sphere of gravitational influence of the central object. Relevant to this region, we give the power-law slopes defining the radial profile of modified clusters and we show that their values are determined by satellite interaction with the accretion flow rather than their initial distribution.Comment: Astronomy & Astrophysics, in press; 11 pages and 6 figures, LaTeX2e (aa501.cls

On highly eccentric stellar trajectories interacting with a self-gravitating disc in Sgr A*

We propose that Kozai's phenomenon is responsible for the long-term evolution of stellar orbits near a supermassive black hole. We pursue the idea that this process may be driven by a fossil accretion disc in the centre of our Galaxy, causing the gradual orbital decay of stellar trajectories, while setting some stars on highly elliptic orbits. We evolve model orbits that undergo repetitive transitions across the disc over the period of ~10^7 years. We assume that the disc mass is small compared to the central black hole, and its gravitational field comparatively weak, yet non-zero, and we set the present values of orbital parameters of the model star consistent with those reported for the S2 star in Sagittarius A*. We show how a model trajectory decays and circularizes, but at some point the mean eccentricity is substantially increased by Kozai's resonance. In consequence the orbital decay of highly eccentric orbits is accelerated. A combination of an axially symmetric gravitational field and dissipative environment can provide a mechanism explaining the origin of stars on highly eccentric orbits tightly bound to the central black hole. In the context of other S-stars, we can conclude that an acceptable mass of the disc (i.e., M_d<=1 percent of the black hole mass) is compatible with their surprisingly young age and small pericentre distances, provided these stars were formed at r<=10^5 gravitational radii.Comment: Accepted for publication in A&A; 9 pages, 6 figures. Revised version with minor language corrections (no change in content

Gravitating discs around black holes

Fluid discs and tori around black holes are discussed within different approaches and with the emphasis on the role of disc gravity. First reviewed are the prospects of investigating the gravitational field of a black hole--disc system by analytical solutions of stationary, axially symmetric Einstein's equations. Then, more detailed considerations are focused to middle and outer parts of extended disc-like configurations where relativistic effects are small and the Newtonian description is adequate. Within general relativity, only a static case has been analysed in detail. Results are often very inspiring, however, simplifying assumptions must be imposed: ad hoc profiles of the disc density are commonly assumed and the effects of frame-dragging and completely lacking. Astrophysical discs (e.g. accretion discs in active galactic nuclei) typically extend far beyond the relativistic domain and are fairly diluted. However, self-gravity is still essential for their structure and evolution, as well as for their radiation emission and the impact on the environment around. For example, a nuclear star cluster in a galactic centre may bear various imprints of mutual star--disc interactions, which can be recognised in observational properties, such as the relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be available from http://www.iop.org/EJ/journal/CQ

Off-equatorial orbits in strong gravitational fields near compact objects

Near a black hole or an ultracompact star, motion of particles is governed by strong gravitational field. Electrically charged particles feel also electromagnetic force arising due to currents inside the star or plasma circling around. We study a possibility that the interplay between gravitational and electromagnetic action may allow for stable, energetically bound off-equatorial motion of charged particles. This would represent well-known generalized Stormer's 'halo' orbits, which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of Kerr-Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity.Comment: 16 pages, 7 figures, accepted in Class. Quantum Grav. (2008

Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA

Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany

Relativistic Dynamics and Extreme Mass Ratio Inspirals

It is now well-established that a dark, compact object (DCO), very likely a massive black hole (MBH) of around four million solar masses is lurking at the centre of the Milky Way. While a consensus is emerging about the origin and growth of supermassive black holes (with masses larger than a billion solar masses), MBHs with smaller masses, such as the one in our galactic centre, remain understudied and enigmatic. The key to understanding these holes - how some of them grow by orders of magnitude in mass - lies in understanding the dynamics of the stars in the galactic neighbourhood. Stars interact with the central MBH primarily through their gradual inspiral due to the emission of gravitational radiation. Also stars produce gases which will subsequently be accreted by the MBH through collisions and disruptions brought about by the strong central tidal field. Such processes can contribute significantly to the mass of the MBH and progress in understanding them requires theoretical work in preparation for future gravitational radiation millihertz missions and X-ray observatories. In particular, a unique probe of these regions is the gravitational radiation that is emitted by some compact stars very close to the black holes and which could be surveyed by a millihertz gravitational wave interferometer scrutinizing the range of masses fundamental to understanding the origin and growth of supermassive black holes. By extracting the information carried by the gravitational radiation, we can determine the mass and spin of the central MBH with unprecedented precision and we can determine how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @ Living Reviews in Relativit

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

We further study the idea that a self-gravitating accretion disc around a supermassive black hole can increase the rate of gradual orbital decay of stellar trajectories (and hence tidal disruption events) by setting some stars on eccentric trajectories. Cooperation between the gravitational field of the disc and the dissipative environment can provide a mechanism explaining the origin of stars that become bound tightly to the central black hole. We examine this process as a function of the black hole mass and conclude that it is most efficient for intermediate central masses of the order of ∼ 104Mʘ. Members of the cluster experience the stage of orbital decay via collisions with an accretion disc and by other dissipative processes, such as tidal effects, dynamical friction and the emission of gravitational waves. Our attention is concentrated on the region of gravitational dominance of the central body. Mutual interaction between stars and the surrounding environment establishes a non-spherical shape and anisotropy of the nuclear cluster. In some cases, the stellar sub-system acquires ring-type geometry. Stars of the nuclear cluster undergo a tidal disruption event as they plunge below the tidal radius of the supermassive black hole