X-ray outburst of 4U 0115+634 and ROTSE observations of its optical counterpart V635 Cas

ROTSE IIId (The Robotic Optical Transient Experiment) observations of the X-ray binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January make it possible, for the first time, to study the correlation between optical and type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron passage where the optical brightness was reduced by 0.3 mag for a few days. We interpret the sharp reduction of optical brightness as a sign of mass ejection from the outer parts of the disc of the Be star. After this sharp decrease, the optical brightness rose again and reached the pre-X-ray outburst level. Afterwards, a gradual decrease of the optical brightness followed until a minimum then a gradual increase started again. Qualitatively, the change of the optical lightcurve suggests a precession of the Be star disc of around a few hundred days. We also investigate the periodic signatures from the archival RXTE-ASM (Rossi X-ray Timing Explorer – All Sky Monitor) light curve covering a time span of ∼9 years. We find significant orbital modulation in the ASM light curve during the type I X-ray outburst.