A double-peaked outburst of A 0535+26 observed with INTEGRAL, RXTE, and Suzaku

TL;DR

This paper analyzes a unique double-peaked outburst of the Be/X-ray binary A 0535+26 using data from INTEGRAL, RXTE, and Suzaku, focusing on timing, spectral properties, and possible causes for the unusual light curve.

Contribution

It provides the first detailed timing and spectral analysis of the double-peaked outburst, revealing no significant differences from typical outbursts and discussing potential explanations for the double peaks.

Findings

No significant variation in cyclotron line energy during the outburst.

Spectral hardening observed with increasing luminosity.

The source was accreting in the sub-critical regime.

Abstract

The Be/X-ray binary A 0535+26 showed a normal (type I) outburst in August 2009. It is the fourth in a series of normal outbursts associated with the periastron, but is unusual by presenting a double-peaked light curve. The two peaks reached a flux of ~450 mCrab in the 15-50 keV range. We present results of the timing and spectral analysis of INTEGRAL, RXTE, and Suzaku observations of the outburst. The energy dependent pulse profiles and their evolution during the outburst are studied. No significant differences with respect to other normal outbursts are observed. The centroid energy of the fundamental cyclotron line shows no significant variation during the outburst. A spectral hardening with increasing luminosity is observed. We conclude that the source is accreting in the sub-critical regime. We discuss possible explanations for the double-peaked outburst.