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 March-April 1995. GRO J1655-40 was continuously monitored by BATSE on board CGRO, and repeatedly observed 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 HST on April 25 and 27 1995, with the aim of detecting faint optical emission from ejected plasmoids. Despite the similarity of the hard X-ray emission in April 1995 with previous events in 1994, no radio or optical emission related to plasmoids was detected. We conclude that GRO J1655-40 is subject to a complex behavior showing: radio-loud hard X-ray outbursts with strong radio emission (of flux $f_r \goe 100$~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 ($f_r \loe 0.5$~mJy) (as those in 1995). Our results can constrain models of particle acceleration and radiation of relativistic plasmoids.