Recent X-ray observations of active galactic nuclei with ASCA suggest that we may be observing Fe-K line emission from very close to the central black-black hole. We provide here the relevant formalism to interpret those observations. We show how to model the appearance of an accretion disk around a rotating black hole and line profiles from disk emission. We try to present in a coherent way results that are scattered in the litterature and/or that are limited to particular cases. We extend these results to the more general case of a rotating black hole (Kerr metric). We finally discuss the relation of the full width at zero intensity of the line to the angular momentum of the black hole.

Context. The compact radio source Sagittarius A * (Sgr A * ) at the centre of our Galaxy harbours a supermassive black hole, whose mass (≈3.7 × 10 6 M ) has been measured from stellar orbital motions. Sgr A * is therefore the nearest laboratory where super-massive black hole astrophysics can be tested, and the environment of black holes can be investigated. Since it is not an active galactic nucleus, it also offers the possibility of observing the capture of small objects that may orbit the central black hole. Aims. We study the effects of the strong gravitational field of the black hole on small objects, such as a comet or an asteroid. We also explore the idea that the flares detected in Sgr A * might be produced by the final accretion of single, dense objects with mass of the order of 10 20 g, and that their timing is not a characteristic of the sources, but rather of the space-time of the central galactic black hole in which they are moving. Methods. The problem of tidal disruption of small objects by a black hole is studied numerically, using ray-tracing techniques, in a Schwarzschild background. Results. We find that tidal effects are strong enough to melt sufficiently massive, solid objects, and present calculations of the temporal evolution of the light curve of infalling objects as a function of various parameters. Our modelling of tidal disruption suggests that during tidal squeezing, the conditions for synchrotron radiation can be met. We show that the light curve of a flare can be deduced from dynamical properties of geodesic orbits around black holes and that it depends only weakly on the physical properties of the source.

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Context. Low-mass satellites, like asteroids and comets, are expected to be present around the black hole at the Galactic center. We consider small bodies orbiting a black hole, and we study the evolution of their orbits due to tidal interaction with the black hole. Aims. In this paper we investigate the consequences of the existence of plunging orbits when a black hole is present. We are interested in finding the conditions that exist when capture occurs. Methods. Earlier analysis of the evolution of classical Keplerian orbits was extended to relativistic orbits around a Schwarzschild black hole. Results. The main difference between the Keplerian and black hole cases is in the existence of plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes through a "final" unstable circular orbit. On this orbit, tidal energy is released on a characteristic black hole timescale. Conclusions. This process may be relevant for explaining how small, compact clumps of material can be brought onto plunging orbits, where they may produce individual short duration accretion events. The available energy and the characteristic timescale are consistent with energy released and the timescale typical of Galactic flares.