Phase changes of the Be/X-ray binary X Persei

¹ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
² Astronomy Centre, CPES, Univ. of Sussex, Falmer, Brighton BN1 9QH, UK
³ Crimean Astrophysical Observatory and Isaac Newton Institute of Chile, Crimean Branch, Naucny, 98409 Crimea, Ukraine
⁴ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan
⁵ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁶ Department of Physics & Astronomy, University of Leicester Leicester LE1 9RH, UK
⁷ Sternberg Astronomical Institute, Universiteskii pr. 13, 119899 Moscow, Russia

Corresponding author: J. S. Clark, jsc@star.ucl.ac.uk

Received: 8 August 2001
Accepted: 18 October 2001

Abstract

We present high resolution optical spectroscopy and V band photometry obtained during the period 1987-2001 for the Be star X Persei/HD 24534, the counterpart to the X-ray pulsar 4U 0352+30. We find that throughout this interval X Per is highly active, with significant photometric and spectroscopic variability. We identify one episode of complete disc loss during this period (1988 May-1989 June), characterised by significant mag optical fading and the presence of purely photospheric Hα and He i 6678 Å lines. Two further episodes of pronounced optical fading which did not result in the complete dispersal of the circumstellar disc were also identified (1994 October-1995 October and 1999 November-present). The emission line profiles of both Hα and He i 6678 Å also show significant variability. Cyclic changes in the strength of the peaks in both emission lines are observed, with periods ranging from 0.6-2 yrs -we attribute these to the presence of a one armed density wave in the inner circumstellar disc. Additional structure at large projected velocities is also present in the He i line -suggesting the presence of a significant density enhancement in the disc near the stellar surface (the "double disc" of Tarasov & Roche). The evolution of the outer edge of the Hα emitting region of the circumstellar disc is followed during disc formation, and is found to increase rather slowly. This observation, combined with the presence of the one armed density wave and the rate of disc formation and loss all provide strong evidence for the hypothesis that the circumstellar disc of X Per is a viscous decretion disc, with angular momentum being supplied by an as yet unknown physical mechanism near the stellar surface.