Letter to the Editor

The appearance of magnetospheric instability in flaring activity at the onset of X-ray outbursts in A0535+26

¹ Institut für Astronomie und Astrophysik, University of Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: Kpostnov@gmail.com
² Sternberg Astronomical Institute, 119999, Moscow, Russia
³ European Space Agency, European Space Astronomy Center, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 19 December 2007
Accepted: 14 January 2008

Abstract

We argue that X-ray flaring variability observed in the transient X-ray pulsar A0535+26 is due to low-mode magnetospheric instability. This instability develops at the onset of accretion, in the thin boundary layer between the accretion disk and neutron star magnetosphere. As a result, the matter collected in the boundary layer can rapidly fall onto the NS surface close to the magnetic poles, but not exactly along the field lines by which the stationary accretion proceeds. This explains the shift in cyclotron line energy measured using RXTE data in a pre-outburst spike, with respect to the line energy observed during the main outburst. Furthermore, the instability can account for the difference in pulse profiles, and their energy evolution that is different in the pre-outburst flare and main outburst.