Effects of a Black Hole's Gravitational Field on the Luminosity of a Star during a Close Encounter

To complement hydrodynamic studies of the tidal disruption of the star by a massive black hole, we present the study of stellar luminosity and its variations produced by the strong gravitational field of the black hole during a close encounter. By simulating the relativistically moving star and its emitted light and taking into account general relativistic effects on particle and light trajectories, our results show that the black hole's gravity alone induces apparent stellar luminosity variations on typical timescales of a few r sub(g)/c [=(5 s)m sub(bh)/(10 super(6) M sub( ))] to a few 100 r sub(g)/c [6(10 minutes)m sub(bh)/(10 super(6) M sub( ))], where r sub(g) = Gm sub(bh)/c super(2). We discern different cases with respect to the strength of tidal interaction and focus on two: (1) a star encountering a giant black hole traces spacetime almost as a point particle, so the apparent luminosity variations are dominated by clearly recognizable general relativistic effects, and (2) in a close encounter of a star with a black hole of similar size, the stellar debris is spread around the black hole by processes in which hydrodynamics plays an important role. We discuss limitations and results of our approach.