The pre-outburst flare of the A 0535+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 RXTEobservations 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) sis measured until periastron passage, followed by a spin-up with a decreasing period derivative of $\dot{P}$ = (-1.69 ±0.04) $\times$10-8 ss-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 $E_{{\rm cyc}}$~ 50 keV during the pre-outburst flare, and $E_{{\rm cyc}}$~ 46 keVduring the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.