Dual Nature of Hard X-Ray Outbursts from the Superluminal X-Ray Transient Source GRO J1655-40

We report the results of multiwavelength observations of the superluminal X-ray transient GRO J1655-40 during and following the prominent hard X-ray outburst of 1995 March-April. GRO J1655-40 was monitored continuously by BATSE on board the Compton Gamma Ray Observatory and observed repeatedly in the radio and optical bands from the ground. About a month after the onset of the hard X-ray outburst, GRO J1655-40 was observed twice by the Hubble Space Telescope in 1995 April 25 and 27, with the aim of detecting faint optical emission from ejected plasmoids. Despite the similarity of the hard X-ray emission in 1995 April with previous events in 1994, no radio or optical emission from GRO J1655 -40 related to plasmoids was detected. We conclude that GRO J1655-40 is subject to complex behavior showing radio-loud hard X-ray outbursts with strong radio emission (of flux f(r) greater than or similar to 100 mJy), both from a "core" source and from propagating plasmoids (as those in 1994), and radio-quiet hard X-ray outbursts with no detectable radio emission and plasmoid activity (f(r) less than or similar to 0.5 mJy) (as those in 1995). Our results can constrain models of particle acceleration and radiation of relativistic plasmoids.