111-day periodicity of X-ray transient A0535 + 26

Abstract

A0535 + 26 is the prototype object of the hard spectrum, pulsating X-ray transients associated with Be companions. Our analysis of its 1969–79 X-ray (3–12 keV) time history, measured by the Vela 5B satellite, shows a period of 111.0 ± 0.4 days, confirming the suggestion of Nagase et al.¹. A likely explanation for the period is binary motion in a wide orbit. ‘Giant’ outbursts (flux ≥ 1 Crab ≃ 40 Vela counts s⁻¹) observed in 1975 and 1980 are not in phase with the 111-day maxima. The source became significantly more active after the 1975 event, averaging ≃3 Vela counts s⁻¹, compared with ≃0.5 counts s⁻¹ before. The latter count rate corresponds to a time averaged X-ray luminosity of 1×10³⁵ erg s⁻¹ at 1.3 kpc. The mass loss rate from the companion, HDE245770, required to power this luminosity by wind accretion is 10⁻⁵ M_⊙ yr⁻¹, which is excessive for a B0V star. It is more likely that the transferred matter is lost from the equatorial region of the rapidly rotating B star.