The Be / X - ray transient 4 U 0115 + 63 / V 635 Cassiopeiae III . Quasi - cyclic variability ⋆

Context. 4U 0115+63 is one of the most active and best studied Be/X-ray transients. Previous studies of 4U 0115+63 have led to the suggestion that 4U 0115+63 undergoes relatively fast quasi-cyclic activity. However, due to the lack of good coverage of the observations, the variability time scales are uncertain. Aims. Our objective is to investigate the long-term behaviour of 4U 0115+63/V635 Cas to confirm its quasi-cyclic nature and to explain its correlated optical/IR and X-ray variability. Methods. We have performed optical/IR photometric observations and optical spectroscopic observations of 4U 0115+63/V635 Cas over the last decade with unprecedented coverage. We have focused on the Hα line variability and the long-term changes of the photometric magnitudes and colours and investigated these changes in correlation with the X-ray activity of the source. Results. The optical and infrared emission is characterised by cyclic changes with a period of ∼ 5 years. This long-term variability is attributed to the state of the circumstellar disc around the Be star companion. Each cycle involves a low state when the disc is very weak or absent and the associated low amplitude variability is orbitally modulated and a high state when a perturbed disc precesses, giving rise to fast and large amplitude photometric changes. X-ray outbursts in 4U 0115+63 come in pairs, i.e., two in every cycle. However, sometimes the second outburst is missing. Conclusions. Our results can be explained within the framework of the decretion disc model. The neutron star acts as the perturbing body, truncating and distorting the disc. The first outburst would occur before the disc is strongly perturbed. The second outburst leads to the dispersal of the disc and marks the end of the perturbed phase.