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

TL;DR

This paper proposes that magnetospheric instability causes X-ray flaring in A0535+26 at the start of outbursts, explaining spectral and pulse profile variations observed during these events.

Contribution

It introduces a magnetospheric instability model to explain flaring activity and spectral changes at outburst onset in the X-ray pulsar A0535+26, linking instability to observed phenomena.

Findings

Explains shift in cyclotron line energy during pre-outburst spike

Accounts for differences in pulse profiles and energy evolution

Links magnetospheric instability to flaring activity

Abstract

We argue that X-ray flaring variability observed in the transient X-ray pulsar A 0535+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.