The Be/X-ray transient 4U 0115+63/V635 Cassiopeiae II. Outburst mechanisms

We present multi-wavelength long-term monitoring observations of V635 Gas, the optical counterpart to the transient X-ray pulsar 4U 0115+63. The evolu tion of emission lines and photometric magnitudes indicates that the Be sta r undergoes relatively fast (similar to3 - 5 yr) quasi-cyclic activity, los ing and reforming its circumstellar disc. We show that the general optical, infrared and X-ray behaviour can be explained by the dynamical evolution o f the viscous circumstellar disc around the Be star. After each disc-loss e pisode, the disc starts reforming and grows until it reaches the radius at which the resonant interaction of the neutron star truncates it. At some po int, the disc becomes unstable to (presumably radiative) warping and then t ilts and starts precessing. The tilting is very large and disc precession l eads to a succession of single-peaked and shell profiles in the emission li nes. Type II X-ray outbursts take place after the disc has been strongly di sturbed and we speculate that the distortion of the disc leads to interacti on with the orbiting neutron star. T;Ve discuss the implications of these c orrelated optical/X-ray variations for the different models proposed to exp lain the occurrence of X-ray outbursts in Be/X-ray binaries. We show that t he hypothesis of mass ejection events as the cause of the spectacular varia bility and the X-ray outbursts is unlikely to be meaningful for any Be/X-ra y binary.