The Be/X-ray binary A0535+26 during its recent 2009/2010 outbursts

TL;DR

This paper reports on the 2009/2010 outbursts of the Be/X-ray binary A0535+26, analyzing spectral and timing data to understand its behavior and cyclotron line variations during these events.

Contribution

It provides new observational insights into the spectral and timing properties of A0535+26 during its recent outbursts, focusing on cyclotron line behavior.

Findings

Detection of a giant outburst reaching ~5.14 Crab flux.

Observation of a peculiar double-peaked outburst in August 2009.

Identification of cyclotron line variations with mass accretion rate.

Abstract

The Be/X-ray binary A0535+26 showed a giant outburst in December 2009 that reached ~5.14 Crab in the 15-50 keV range. Unfortunately, due to Sun constraints it could not be observed by most X-ray satellites. The outburst was preceded by four weaker outbursts associated with the periastron passage of the neutron star. The fourth of them, in August 2009, presented a peculiar double-peaked light curve, with a first peak lasting about 9 days that reached a (15-50 keV) flux of 440 mCrab. The flux then decreased to less than 220 mCrab, and increased again reaching 440 Crab around the periastron. The outburst was monitored with INTEGRAL, RXTE, and Suzaku TOO observations. One orbital period (~111 days) after the 2009 giant outburst, a new and unexpectedly bright outburst took place (~1.4Crab in the 15-50 keV range). It was monitored with TOO obs ervations with INTEGRAL, RXTE, Suzaku, and Swift. First results of the spectral and timing analysis of these observations are presented, with a specific focus on the cyclotron lines present in the system and its variation with the mass accretion rate.