¹ Observatoire de Strasbourg, 11 rue de l'Université, F67000 Strasbourg, France
² SAX SDC, ASI, c/o Nuova Telespazio, via Corcolle 19, I00131 Rome, Italy
³ Astrophysics Research Institute, Liverpool John Moores University, Byrom St., Liverpool, L3 3AF, UK
⁴ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan
⁵ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, U.K.
⁶ Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100, Burjassot, Valencia, Spain
⁷ Physics and Astronomy Dept., University of Southampton, Southampton, SO17 BJ1, U.K.
⁸ Osservatori Astronomici di Padova e Asiago, via dell'Osservatorio 8, 36012 Asiago (Vicenza), Italy
⁹ Centro Interdipartimentale di Studi ed Attività Spaziali (C.I.S.A.S.) "G. Colombo", Università di Padova, Italy

We present multi-wavelength long-term monitoring observations of V635 Cas, the optical counterpart to the transient X-ray pulsar 4U 0115+63. The evolution of emission lines and photometric magnitudes indicates that the Be star undergoes relatively fast (3-5 yr) quasi-cyclic activity, losing and reforming its circumstellar disc. We show that the general optical, infrared and X-ray behaviour can be explained by the dynamical evolution of the viscous circumstellar disc around the Be star. After each disc-loss episode, the disc starts reforming and grows until it reaches the radius at which the resonant interaction of the neutron star truncates it. At some point, the disc becomes unstable to (presumably radiative) warping and then tilts and starts precessing. The tilting is very large and disc precession leads to a succession of single-peaked and shell profiles in the emission lines. Type II X-ray outbursts take place after the disc has been strongly disturbed and we speculate that the distortion of the disc leads to interaction with the orbiting neutron star. We discuss the implications of these correlated optical/X-ray variations for the different models proposed to explain the occurrence of X-ray outbursts in Be/X-ray binaries. We show that the hypothesis of mass ejection events as the cause of the spectacular variability and X-ray outbursts is unlikely to be meaningful for any Be/X-ray binary.

Accepted by Astronomy & Astrophysics
Preprints from ignacio@astro.u-strasbg.fr
or on the web at http://xxx.lanl.gov/abs/astro-ph/0101208