2008A&A...480L..17C

We study the spectral and temporal behavior of the High Mass X-ray Binary A0535+26 during a ``pre-outburst flare'' which took place ∼5d 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 A0535+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 {dot}(P)=(-1.69±0.04)x10^–8s/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 E_cyc∼50keV during the pre-outburst flare, and E_cyc∼46keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.