X-ray and optical periodicities in X-ray binaries I. A0535+26

¹ Astronomical Institute of St. Petersburg University, Petrodvorets, Bibliotechnaya Pl. 2, 198504, Russia
² Isaac Newton Institute of St. Petersburg, Chile, St. Petersburg Branch, Russia
³ P. K. Shternberg Astronomical Institute, Moscow, Russia

Corresponding author: V. Larionov, VML@VL1104.spb.edu

Received: 16 January 2001
Accepted: 26 July 2001

Abstract

A homogeneous set of UBV photometry (354 data points obtained between 1983 and 1998) for the Be/X-ray binary A0535+26 = V725 Tau is analysed, aiming to look for possible periodic component(s). After subtraction of the long-term variation it was found that only a 103d periodic component remains in the power spectra in both the V and B colour bands. The probability of chance occurrence of such a peak is less than 0.1% . There are no signs of optical variability at the X-ray period (111d). We discuss possible reasons for a 103-day modulation and suggest that it corresponds to a beat frequency of the orbital period of the neutron star and the precession period (1400d) either of an accretion disc around the neutron star or a warped decretion disc around the Be star.