Cyclical behaviour and disc truncation in the Be/X-ray binary A0535+26

A0535+26 is shown to display quantized infrared excess flux states, which are interpreted as the first observational verification of the proposed resonant truncation scheme for Be X-ray binaries (BeXRBs). The simultaneity of X-ray activity with transitions between these states strongly suggests a broad mechanism for outbursts, in which material lost from the disc during the reduction of the truncation radius is accreted by the neutron star (NS). Furthermore, changes between states are shown to be governed by a ∼1500-d period, probably due to precession of the Be disc, which profoundly dictates the global behaviour of the system. Such a framework appears to be applicable to BeXRBs in general. Additionally, the subsonic radial expansion of the Be disc is measured, and photometric evidence for a NS accretion disc during a type II outburst discussed. Values for the stellar inclination, mass and rotational velocity are derived, and photometric modulation at Porb detected.