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A0535+26 is a slowly rotating pulsar accreting from the wind of a massive Bestar, and that exhibits two cyclotron absorption lines in its X-ray spectrum,at about 45 and 100 keV, respectively. Unlike similar sources, no significantvariations of the energy of its cyclotron lines with flux were observed todate. The bright outburst of February 2011 thus offers a unique occasion toprobe this peculiar behavior at flux levels not yet observed with present-dayinstruments. Here we report on the spectral and timing analysis of the datafrom the spectrometer SPI on-board INTEGRAL collected during the outburst. Atthe peak of the outburst the estimated luminosity is ~4.9x10^37 erg/s. Thefundamental cyclotron feature is detected at all flux levels, and its centroidenergy is positively-correlated with the flux of the source, confirming thatA0535+26 is accreting at a sub-critical regime. The correlation seems to falloff at ~10^37 erg/s, suggesting the transition from a Coulomb-stopping regimeto a gas-mediated shock regime. From the timing analysis we found that thepulsar was spinning up during most of the outburst, and that the spin-up ratecorrelates with the flux of the source, albeit the correlation is steeper thanthe one expected from the standard disk accretion theory. Finally, we show thatthe pulse profile of the source changes dramatically as the flux increases. Athigh luminosity the profile is highly asymmetric, implying an asymmetry in thegeometry of the accretion flow.

The INTEGRAL/SPI view of A0535+26 during the giant outburst of February 2011