Mass transfer from the donor of GRS 1915+105

Observatory, Box 14, 00014 University of Helsinki, Finland

Corresponding author: osmi.vilhu@helsinki.fi

Accepted: 21 February 2002

Abstract

A scenario for a periodic filling and emptying of the accretion disc of the microquasar GRS1915+105 is proposed, by estimating the mass transfer rate from the donor and comparing it with the observed accretion rate onto the primary black hole. The mass of the Roche-lobe-filling donor (), the primary black hole mass () and the binary orbital period of 33.5 d (Greiner et al. 2001b) predict for the donor spectral type and K-magnitude around K6 III and -2.6, respectively. The He-core of 0.28  of such a giant leads to evolutionary expansion along the giant branch with a conservative mass transfer rate of /year. On the other hand, the average observed accretion rate onto the primary is ten times larger: ( kpc)/y, where η is the efficiency of converting accretion into radiation. We propose a duty cycle with (5–10)(η/0.1) per cent active ON-state. The timescale of the (recurrent) OFF-state is identified as the viscosity time scale at the circularization radius (14 ) and equals /0.001)^-4/5 years, where α is the viscosity parameter in the α-prescription of a classical disc. If the viscosity at the outer edge of the disc is small and η is close to the maximum available potential energy (per rest mass energy) at the innermost stable orbit, the present activity phase may still last another 10–20 years. We also discuss other solutions allowing a broader range of donor masses (0.6–2.4 ).