The Transient Nature of GRO J1655−40 and Its Evolutionary State

Abstract

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 (2.3 M☉) crossing the Hertzsprung gap. Usually in such systems the donor's radius expansion drives a near-Eddington or super-Eddington mass transfer rate that 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 in which 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.