HeI doubled emission lines from A0535+26 ≡ HDE 245770. A possible interpretation

Abstract

We report results of the spectroscopy for V725 Tau (HDE 245770, BD +26°883), commonly known as Flavia's star, which is the optical counterpart of the X-ray pulsar A0535+26, carried out at the 1.5 m Loiano telescope during one run of observations of this star spread over a period of years. The HeI emission lines clearly show doubling, which is good evidence for the presence of a disc. In this paper we critically discuss the possibility that this disc is a temporary accretion disc around the neutron star, a view that contrasts to the usual interpretation, which considers that this sort of doubling in the HeI emission lines is due to a disc formed by gas expelled from the Be star. In the former case the outer radius of the accretion disc could range from 5.9 × 10¹⁰ cm to 2.1 × 10¹¹ cm, taking the most probable range into account for the orbital inclination of the binary system, from 35° to 39°. The presence of such a temporary accretion disc around the neutron star was predicted by Giovannelli & Ziółkowski (1990).