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The 2009 outburst of accreting millisecond pulsar IGR J17511-3057 as observed by SWIFT and RXTE

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.