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 April 25, 1996. Optical photometry obtained April 20-24, 1996 shows a steady brightening of the source in B, V , R, and I beginning about six 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. Subject headings: binaries: spectroscopic — black hole physics — X-rays: stars — stars: individual (GRO J1655-40) 1Visiting Astronomer at Cerro Tololo Inter-American Observatory (CTIO), which is operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 2National Young Investigator