The Transient Nature of Gro J1655-40 and Its Evolutionary State

We consider the evolutionary state of the black–hole X–ray source GRO J1655–40 in the context of its transient nature. Recent optical observations show that the donor in GRO J1655–40 is an intermediate–mass star (≃ 2.3M⊙) crossing the Hertzsprung gap. Usually in such systems the donor’s radius expansion drives a near-Eddington or super–Eddington mass transfer rate which would sustain a persistently bright accretion disk. We show that GRO J1655–40 is close to a narrow parameter range where disk instabilities can occur. This range corresponds to a short–lived evolutionary stage where the secondary’s radius expansion stalls (or reverses), with a correspondingly lower mass transfer rate. If GRO J1655–40 belongs to this class of transients the predicted accretion rates imply large populations of luminous persistent and transient sources, which are not seen in X–rays. The transient nature of the related system GRS 1915+105 may reflect spectral variations in a bolometrically persistent source rather than a genuine luminosity increase. Subject headings: accretion, accretion disks — binaries: close — black hole physics — stars: individual (GRO J1655–40, GRS 1915+105) Astronomy Group, University of Leicester, Leicester LE1 7RH, U.K.; [email protected]; [email protected] Max–Planck–Institut für Astrophysik, Karl–Schwarzschild–Str. 1, D 85740 Garching, Germany; [email protected] Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803-4001, USA; [email protected]