Correlations between X-ray outbursts and relativistic ejections in the X-ray transient GRO J1655 – 40

ALTHOUGH objects that emit radio jets have been known for many years, the physical mechanism responsible for the jets has been unclear. Accretion of mass onto a compact object (such as a black hole) has often been invoked in models of their formation1. X-ray emission from such sources is a potentially powerful probe of the processes taking place, because it seems to arise much closer to the central object. Here we report the detection of X-ray and radio emission from the recently discovered2,3 transient source X-ray Nova Scorpii 1994 (GRO J1655 –- 40). The radio outbursts, presumably reflecting the feeding of material into relativistic jets, generally follow bursts of hard X-ray emission (20 – 400 keV) with a delay that varies from a few days to about two weeks. This suggests that the mechanism behind the X-ray emission is not related to the ejection process in a simple way. Nevertheless, GRO J1655 – 40 may be the best example of a compact object/accretion-disk system in which models of jet formation and X-ray production can be tested directly.