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We study the spectral and temporal behavior of the High Mass X-ray Binary A0535+26 during a `pre-outburst flare' which took place ~5 d before the peak ofa normal (type I) outburst in August/September 2005. We compare the studiedbehavior with that observed during the outburst. We analyse RXTE observationsthat monitored A 0535+26 during the outburst. We complete spectral and timinganalyses of the data. We study the evolution of the pulse period, presentenergy-dependent pulse profiles both at the initial pre-outburst flare andclose to outburst maximum, and measure how the cyclotron resonance-scatteringfeature (hereafter CRSF) evolves. We present three main results: a constantperiod P=103.3960(5)s is measured until periastron passage, followed by aspin-up with a decreasing period derivative of Pdot=(-1.69+/-0.04)x10^(-8)s/sat MJD 53618, and P remains constant again at the end of the main outburst. Thespin-up provides evidence for the existence of an accretion disk during thenormal outburst. We measure a CRSF energy of Ecyc~50kev during the pre-outburstflare, and Ecyc~46kev during the main outburst. The pulse shape, which variessignificantly during both pre-outburst flare and main outburst, evolvesstrongly with photon energy.

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