The pre-outburst flare of the A 0535+26 August/September 2005 outburst

TL;DR

This study analyzes the spectral and timing behavior of the A 0535+26 binary system during a pre-outburst flare, revealing insights into its pulse period evolution, cyclotron line energy, and pulse profile changes, indicating accretion disk presence.

Contribution

It provides the first detailed comparison of pre-outburst flare behavior with main outburst characteristics in A 0535+26, highlighting spectral and timing evolution.

Findings

Constant pulse period before periastron passage

Detection of spin-up indicating an accretion disk

Variation in cyclotron line energy between flare and outburst

Abstract

We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a `pre-outburst flare' which took place ~5 d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst. We analyse RXTE observations that monitored A 0535+26 during the outburst. We complete spectral and timing analyses of the data. We study the evolution of the pulse period, present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves. We present three main results: a constant period P=103.3960(5)s is measured until periastron passage, followed by a spin-up with a decreasing period derivative of Pdot=(-1.69+/-0.04)x10^(-8)s/s at MJD 53618, and P remains constant again at the end of the main outburst. The spin-up provides evidence for the existence of an accretion disk during the normal outburst. We measure a CRSF energy of Ecyc~50kev during the pre-outburst flare, and Ecyc~46kev during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.