The pre-outburst flare of the A0535+26 August/September 2005 outburst

Aims. 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. Methods. 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. Results. 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 P ̇ = (−1.69 ± 0.04) × 10−8 s s−1 at MJD 53 618, 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 ∼ 50 keV during the pre-outburst flare, and Ecyc ∼ 46 keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.