Correlated V/R and infrared photometric variations in the Be/X-ray binary LS I +61° 235/RX J0146.9+6121

Abstract

We report on the long-term variability of the Be/X-ray binary LS I+61° 235/RX J0146.9+6121. New optical spectroscopic and infrared photometric observations confirm the presence of global one-armed oscillations in the circumstellar disc of the Be star, and allow us to derive a V/R band quasi-period of 1240 ± 30d. Pronounced shell events, reminiscent of the spectacular variations in Be stars, are also seen. We have found that the J, H and K infrared photometric bands vary in correlation with the spectroscopic V/R variations, implying that the one-armed disc oscillations are prograde. The effect of the oscillations is not only seen in the Hα line but is also seen in the He I λ6678 and Paschen lines. As these lines are formed at different radii in the equatorial disc of the Be star, such effects confirm the global nature of the perturbation. The Keplerian disc has been found to be denser than the average for a sample of isolated Be stars, which may be indicative of some kind of interaction with the compact companion. Finally, from a Rossi X-ray Timing Explorer observation we derive a spin period of the neutron star of 1404.5 ± 0.5 s.