Advection‐dominated Flows around Black Holes and the X‐Ray Delay in the Outburst of GRO J1655−40

We show that the time delay between the optical and X-ray outbursts of theblack-hole soft X-ray transient source GRO J1655-40, observed in April 1996,requires that the accretion flow in this object must consist of two components:a cold outer accretion disk and an extremely hot inner advection-dominatedaccretion flow (ADAF). In quiescence, the model predicts a spectrum which is ingood agreement with observations, with most of the observed radiation comingfrom the ADAF. By fitting the observed spectrum, we estimate the mass accretionrate of the quiescent system and the transition radius between the disk and theADAF. We present a detailed numerical simulation of a dwarf-nova typeinstability in the outer disk. The resulting heat front reaches the ADAF cavitypromptly; however, it must then propagate inward slowly on a viscous timescale, thereby delaying the onset of the X-ray flux. The model reproduces theobserved optical and X-ray light curves of the April 1996 outburst, as well asthe 6-day X-ray delay. Further, the model gives an independent estimate of thequiescent mass accretion rate which is in very good agreement with the rateestimated from fitting the quiescent spectrum. We show that a pure thin diskmodel without an ADAF zone requires significant tuning to explain the X-raydelay; moreover, such a model does not explain the quiescent X-ray emission ofGRO J1655-40.Comment: 13 pages, 4 figures Submitted to The Astrophysical Journa