Relativistic tidal compressions of a star by a massive black hole

Laboratoire Univers et Théories, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France e-mail: [matthieu.brassart;jean-pierre.luminet]@obspm.fr

Received: 9 October 2009
Accepted: 28 November 2009

Abstract

Aims. We investigate the stellar pancake mechanism during which a solar-type star is tidally flattened within its orbital plane as it passes close to a black hole.

Methods. We simulated the relativistic orthogonal compression process and follow the associated shock waves formation. We considered a one-dimensional hydrodynamical stellar model moving in the relativistic gravitational field of a non-rotating black hole. The model is numerically solved using a Godunov-type shock-capturing source-splitting method to correctly reproduce the shock wave profiles.

Results. Simulations confirm that the space-time curvature can induce several successive orthogonal compressions of the star, which give rise to several strong shock waves. The shock waves finally escape from the star and repeatedly heat up the stellar surface to high-energy values. Such a shock-heating could interestingly provide a direct observational signature of strongly disruptive star – black hole encounters through the emission of hard X or soft γ-ray bursts. Timescales and energies of such a process are consistent with some observed events such as GRB 970815.