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

TL;DR

This paper models the 8-day delay between optical and X-ray outbursts in A0535+26/HDE245770 using a nonstationary accretion disk with high orbital ellipticity, explaining the observed timing through turbulent viscosity effects.

Contribution

It provides a quantitative model linking optical and X-ray outbursts via accretion disk dynamics and turbulence parameters in a high ellipticity binary system.

Findings

The optical outburst precedes the X-ray by about 8 days.

The delay is explained by radial matter motion in the accretion disk.

Turbulent viscosity parameter α is estimated between 0.1 and 0.3.

Abstract

The optical behaviour of the Be star in the high mass X-ray transient A0535+26/HDE245770 shows that at the periastron typically there is an enhancement in the luminosity of order 0.02 to few tenths mag, and the X-ray outburst happens about 8 days after the periastron. We construct a quantitative model of this event, basing on the a nonstationary accretion disk behavior, connected with a high ellipticity of the orbital motion. The ephemeris used in this paper -- JD$_{\rm opt-outb}$ = JD$_0$(2,444,944) $\pm$ n(111.0 $\pm$ 0.4) days are derived from the orbital period of the system P$_{\rm orb} = 111.0 \pm 0.4$ days, determined by Priedhorsky & Terrell (1983), and from the optical flare of December 5, 1981 (Giovannelli et al., 1985) (here after 811205-E; E stands for the Event occurred at that date) that triggered the subsequent X-ray outburst of December 13, 1981 (Nagase et al., 1982) (here after 811213-E). 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 the parameter $\alpha=0.1-0.3$. The increase of the mass flux is a sort of flush that reaches the external part of the accretion disk around the neutron star, producing an enhancement in the optical luminosity. The consequent X-ray flare happens when the matter reaches the hot central parts of the accretion disk, and the neutron star surface.