We show that the 1996 outburst of the X-ray binary transient system GRO
J1655-40 can be explained by the standard dwarf-nova type disc instability,
followed by an episode of enhanced mass transfer from the secondary if the mass
transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit.
We argue that irradiation of the secondary during the onset of the outburst
driven by the thermal instability in the outer disc can increase the mass
transfer rate above the minimum value required for stable accretion. This will
then produce the period of near-constant X-ray emission seen in this system.
This scenario can also explain the observed anti-correlation between the
optical and X-ray fluxes. It is generally accepted that optical emission in
low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays.
There is also strong circumstantial evidence that in order for the outer disc
to see the irradiating flux, it must be warped. Depending on the warp
propagation mechanism, either a burst of mass from the secondary or viscous
decay are likely to decrease the degree of warping, thereby causing the
decrease in the observed optical flux while the X-ray flux remains constant or
even increases, exactly as observed in GRO J1655-40. Finally, the decrease of
the disc warping and, therefore, irradiation will cause the disc to become
unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres
