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 nearApril 25, 1996. Optical photometry obtained April 20-24, 1996 shows a steadybrightening of the source in B, V, R, and I beginning about six days before thestart of the X-ray outburst. The onset of the optical brightening was earliestin I and latest in B. However, the rate of the optical brightening was fastestin B and slowest in I. The order of the increases in the different opticalfilters suggests that the event was an "outside-in" disturbance of theaccretion disk. The substantial delay between the optical rise and the rise ofthe X-rays may provide indirect support for the advection-dominated accretionflow model of the inner regions of the accretion disk.Comment: 8 pages, 2 figures, Latex (uses the standard AAS style file aas2pp4.sty), accepted for publication in the ApJ Letter