A 0535+26 in outburst: Magnetospheric instabilities and accretion geometry

Abstract

The Be/X-ray binary A 0535+26 showed a normal (type I) outburst in August/September 2005, which reached a maximum X-ray flux of 400mCrab in the 5-100keV range. The outburst was observed by INTEGRAL and RXTE. The energy of the fundamental cyclotron line has been measured with INTEGRAL and RXTE at ∼45keV. Flaring activity was observed during the rise to the peak of the outburst. RXTE observations during one of these flares found the energy of the fundamental cyclotron line shifted to a significantly higher position than during the rest of the outburst, where it remains constant. Also, the energy-dependent pulse profiles during the flare differ significantly from the rest of the outburst. These differences have been interpreted with the presence of magnetospheric instabilities at the onset of the accretion. A decomposition method is applied to A 0535+26 pulse profiles. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles, and that the emission regions have axisymmetric beam patterns. Using pulse profiles obtained from RXTE observations, the contribution of the two emission regions has been disentangled. Constraints on geometry of the pulsar and a possible solution of the beam pattern are given. First results of the comparison of the reconstructed beam pattern with a geometrical model that includes relativistic light deflection are presented.