  The twelfth accretion-powered millisecond pulsar, IGR J17511-3057, was
discovered in September 2009. In this work we study its spectral and timing
properties during the 2009 outburst based on Swift and RXTE data. Our spectral
analysis of the source indicates only slight spectral shape evolution during
the entire outburst. The equivalent width of the iron line and the apparent
area of the blackbody emission associated with the hotspot at the stellar
surface both decrease significantly during the outburst. This is consistent
with a gradual receding of the accretion disc as the accretion rate drops. The
pulse profile analysis shows absence of dramatic shape evolution with a
moderate decrease in pulse amplitude. This behaviour might result from a
movement of the accretion column footprint towards the magnetic pole as the
disc retreats. The time lag between the soft and the hard energy pulses
increase by a factor of two during the outburst. A physical displacement of the
centroid of the accretion shock relative to the blackbody spot or changes in
the emissivity pattern of the Comptonization component related to the
variations of the accretion column structure could cause this evolution. We
have found that IGR J17511-3057 demonstrates outburst stages similar to those
seen in SAX J1808.4-3658. A transition from the "slow decay" into the "rapid
drop" stage, associated with the dramatic flux decrease, is also accompanied by
a pulse phase shift which could result from an appearance of the secondary spot
due to the increasing inner disc radius.
