On the accretion flow geometry in A0535+26

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Central Astronomical Observatory of the Russian Academy of Science at Pulkovo, Pulkovo 65-1, 196140 Saint-Petersburg, Russia
³ Astronomical institute of St. Petersburg University, St. Petersburg, Russia
⁴ Isaac Newton Institute of Chile, St. Petersburg Branch, Russia

Corresponding author: N. R. Ikhsanov, ikhsanov@mpifr-bonn.mpg.de

Received: 28 November 2000
Accepted: 20 March 2001

Abstract

The geometry of accretion flow in the Be/X-ray transient A0535+26 is explored. It is shown that neither moderate nor giant X-ray flaring events observed in the system can be interpreted within the spherically symmetrical accretion model and hence the formation of an accretion disk around the neutron star magnetosphere during the both types of flares is required. The accretion disk can be formed at the periastron if (i) the expansion velocity of the Be star envelope in the equatorial plane is and (ii) the parameter accounting for the accretion flow inhomogeneities, ξ, satisfies the following condition: , where is the rate of mass capture by the neutron star expressed in units of . We suggest that the "missing" outburst phenomenon can be associated with the spherically symmetrical accretion onto the interchange-stable magnetosphere of the neutron star. The average spin up rate of the neutron star during moderate flares 3.5 10^-12 Hz s^-1 is predicted.