2015ApJ...806..193S

A0535+26 is a slowly rotating pulsar accreting from the wind of a massive Be star and exhibits two cyclotron absorption lines in its X-ray spectrum, at about 45 and 100 keV, respectively. Unlike similar sources, no significant variations of the energy of its cyclotron lines with flux have been observed to date. The bright outburst of 2011 February thus offers a unique occasion to probe this peculiar behavior at flux levels not yet observed with present-day instruments. Here we report on the spectral and timing analysis of the data from the spectrometer SPI on board INTEGRAL collected during the outburst. At the peak of the outburst the estimated luminosity is ∼4.9x10³⁷ erg/s. The fundamental cyclotron feature is detected at all flux levels, and its centroid energy is positively correlated with the flux of the source, confirming that A0535+26 is accreting at a sub-critical regime. The correlation seems to fall off at ∼10³⁷ erg/s, suggesting a transition from a Coulomb-stopping regime to a gas-mediated shock regime. From the timing analysis we found that the pulsar was spinning up during most of the outburst and that the spin-up rate correlates with the flux of the source, although the correlation is steeper than the one expected from the standard disk accretion theory. Finally, we show that the pulse profile of the source changes dramatically as the flux increases. At high luminosity the profile is highly asymmetric, implying an asymmetry in the geometry of the accretion flow.