An optical precursor to the recent X-ray outburst of the black hole binary GRO J1655-40

The All-Sky Monitor on the Rossi X-Ray Timing Explorer detected an X-ray (2-12 keV) outburst from the black hole binary GRO J1655-40 beginning near 1996 April 25. Optical photometry obtained on 1996 April 20-24 shows a steady brightening of the source in B, V, R, and I beginning about 6 days before the start of the X-ray outburst. The onset of the optical brightening was earliest in I and latest in B. However, the rate of the optical brightening was fastest in B and slowest in I. The order of the increases in the different optical filters suggests that the event was an ''outside-in'' disturbance of the accretion disk. The substantial delay between the optical rise and the rise of the X-rays may provide indirect support for the advection-dominated accretion flow model of the inner regions of the accretion disk.