Multiwavelength observations of an outburst from the Be/X-ray transient 4U0115+63 in 1994

We present OSSE observations of the recurrent X-ray transient 4U0115+63 during the 1994 May-June outburst, spanning the brightest period of the outburst and covering more than a full binary orbit. We also present long-term observations of the optical counterpart V635 Cassiopeiae and the long-term hard X-ray light curve determined by BATSE pulsed observations. Optical and infrared photometry as well as H alpha spectroscopy reveals that the correlation between the optical and X-ray behaviour of the source, although evident, is not simple. We discuss the recent history of the system, paying particular attention to the extremely unusual behaviour of the Be star. We investigate the possible mechanisms for this behaviour and find that the centrifugal inhibition of accretion theory provides an adequate explanation for the X-ray activity. Our data, however, point strongly against the previous assumption that the accretion disc around the neutron star is responsible for the very large variations in optical luminosity. These variations, as well as those in the shape and strength of the H alpha line, must originate in the Be circumstellar envelope.