Inner disc obscuration in GRS 1915+105 based on relativistic slim disc model

Abstract

We study the observational signatures of the relativistic slim disc of 10 M_sun black hole, in a wide range of mass accretion rate, mdot, dimensionless spin parameter, a_ast, and viewing angle, i. In general, the innermost temperature, T_in increases with the increase of i for a fixed value of mdot and a_ast, due to the Doppler effect. However, for i > 50 and mdot > mdot_turn, T_in starts to decrease with the increase of mdot. This is a result of self-obscuration -- the radiation from the innermost hot part of the disc is blocked by the surrounding cooler part. The value of mdot_turn and the corresponding luminosities depend on a_ast and i. Such obscuration effects cause an interesting behavior on the disc luminosity (L_disc) -- T_in plane for high inclinations. In addition to the standard-disc branch which appears below mdot_turn and which obeys L_disc propto T_in^4 -relation, another branch above mdot_turn, which is nearly horizontal, may be observed at luminosities close to the Eddington luminosity. We show that these features are likely observed in a Galactic X-ray source, GRS 1915+105. We support a high spin parameter (a_ast > 0.9) for GRS 1915+105 since otherwise the high value of T_in and small size of the emitting region (r_in < 1r_S) cannot be explained.