Long-term variability of the Be/X-ray binary A0535+26 -- III. Photometry

Abstract

UBV-band photometric observations of the Be/X-ray binary system HDE 245770/A0535+26 over the last decade are presented. It is shown that the emission at all wavelengths is correlated, and that no resolvable time-lag exists between variations in differing wavebands. This is suggestive of emission from only one source within the system. It is found that the optical continuum can be understood as optically thin free-free and bound-free emission from material in the circumstellar disc around the primary Oe star. We find a change in the emission measure of the disc by a factor of ∼40 over the course of the observations. If this is interpreted as a variation in the mass-loss rate from the primary, it corresponds to a change by a factor of ∼6. Period searches on the optical light curve reveal no evidence for modulation of the circumstellar envelope at the orbital period; therefore it seems unlikely that the mass-loss rate from the primary is affected by the neutron star at periastron. No positive correlation exists between the optical and X-ray light curves; such a result appears to exclude models for the X-ray behaviour in which accretion is directly fed from the dense circumstellar wind of the primary. It is likely that X-ray outbursts are instead triggered by discrete mass ejection events from the primary.