Abstract
We show that the time delay between the optical and X-ray outbursts of the black hole soft X-ray transient source GRO J1655-40, observed in 1996 April, suggests that the accretion flow in this object must consist of two components: a cold outer accretion disk and an extremely hot inner advection-dominated accretion flow (ADAF). In quiescence, the model predicts a spectrum that is in good agreement with observations, with most of the observed radiation coming from the ADAF. By fitting the observed spectrum, we estimate the mass accretion rate of the quiescent system and the transition radius between the disk and the ADAF. We present a detailed numerical simulation of a dwarf nova-type instability in the outer disk. The resulting heat front reaches the ADAF cavity promptly; however, it must then propagate inward slowly on a viscous timescale, thereby delaying the onset of the X-ray flux. The model reproduces the observed optical and X-ray light curves of the 1996 April outburst, as well as the 6 day X-ray delay. Further, the model gives an independent estimate of the quiescent mass accretion rate that is in very good agreement with the rate estimated from fitting the quiescent spectrum. We show that a pure thin disk model without an ADAF zone requires significant tuning to explain the X-ray delay; moreover, such a model does not explain the quiescent X-ray emission of GRO J1655-40.
Highlights
The binary X-ray source GRO J1655–40 is a member of the class of so-called “Soft X-ray Transients” (SXTs) or “X-ray Novae”
We examine only one aspect of the complex behavior of GRO J1655–40 in outburst, namely, the properties of the optical precursor and the X-ray outburst, and what they imply for models of quiescent black-hole transient (BHT) and for the outburst mechanism
We show that the observations of the April 1996 outburst of GRO J1655–40 imply the presence of a two-component accretion flow in this system, and that the parameters deduced from observations agree very well with a model comprised of an outer cold disk and an inner hot advection-dominated accretion flow (ADAF), as proposed by NBM
Summary
The binary X-ray source GRO J1655–40 ( called X-ray Nova Scorpii 1994) is a member of the class of so-called “Soft X-ray Transients” (SXTs) or “X-ray Novae” In these systems, a low mass Roche-lobefilling secondary star transfers mass through an accretion disk onto a compact object: a neutron star or a black hole. A natural candidate for such an instability is the thermal (and viscous) instability resulting from abrupt changes in opacities when hydrogen becomes partially ionized Such a mechanism explains successfully dwarf nova outbursts in the framework of the disk instability model (DIM) (see Cannizzo 1993 and references therein). NBM have shown that self-consistent models for the spectra of V404 Cyg and A0620–00 can be obtained by an ADAF that extends outward to Rtr ∼ 104 Schwarzschild radii For these models, the accretion disk is cool and the optical/UV flux is mostly supplied by synchrotron emission from the ADAF.
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