1996A&AS..120C.201T

Two hard X-ray transients (GRS 1915+10 and GRO J1655-40) recently discovered by SIGMA and BATSE show occasional ejections of radio plasmoids moving at apparent superluminal speeds. The plasmoid propagation geometry resembles that one of SS 433 with counter-streaming jets. We discuss the possible nature of the compact stars producing superluminal ejection episodes driven by an accretion disk instability. Recent data indicate that hard X-ray emission from superluminal transient sources is a necessary but not sufficient condition for plasmoid formation and acceleration. GRO J1655-40 (and probably also GRS 1915+10) shows two types of hard X-ray outbursts with and without radio jet emission. We briefly discuss the nature of the disk instability producing the radio flaring behavior. Multiwavelength observations and monitoring of superluminal X-ray transients will greatly improve our current understanding of jet formation from compact objects.