  A0535+26 is a slowly rotating pulsar accreting from the wind of a massive Be
star, and that 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 were observed to
date. The bright outburst of February 2011 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 ~4.9x10^37 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 ~10^37 erg/s, suggesting the 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, albeit 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.
