Time delay between the optical and X-ray outbursts in the high-mass X-ray transient A0535+26/HDE245770

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: franco.giovannelli@iaps.inaf.it
² Space Research Institute, Profsoyuznaya 84/32, 117810 Moscow, Russia
³ National Research Nuclear University MEPhI, Kashirskoe Shosse 31, 115409 Moscow, Russia

Received: 28 November 2012
Accepted: 3 September 2013

Abstract

The optical behavior of the Be star in the high-mass X-ray transient A0535+26/HDE245770 shows that at periastron the luminosity is typically enhanced by 0.02 to a few tenths mag, and the X-ray outburst occurs eight days after the periastron. We constructed a quantitative model for this event based on a nonstationary accretion disk behavior, connected with a high ellipticity of the orbital motion. We explain the observed time delay between the peaks of the optical and X-ray outbursts in this system by the time of radial motion of the matter in the accretion disk, after an increase of the mass flux in the vicinity of a periastral point in the binary. This time is determined by the turbulent viscosity with a parameter of α = 0.1−0.3. The increase of the mass flux is a sort of flush that reaches the external part of the accretion disk around the neutron star, which enhances the optical luminosity. The subsequent X-ray flare occurs when the matter reaches the hot central parts of the accretion disk and the neutron star’s surface.