Swift monitoring of the new accreting millisecond X-ray pulsar IGR J17511-3057 in outburst

Context. A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring programme. Aims. To study the evolution of the source X-ray flux and spectral properties during the outburst, we requested a Swift monitoring of IGR J17511-3057. Methods. In this paper we report on the results of the first two weeks of monitoring the source. Results. The persistent emission of IGR J17511-3057 during the outburst is modelled well with an absorbed blackbody (kT similar to 0.9 keV) and a power-law component (Gamma similar to 1-2), similar to what has been observed from other previously known millisecond pulsars. Swift also detected three type-I Xray bursts from this source. By assuming that the peak luminosity of these bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we derived an upper limit to the source distance of similar to 10 kpc. The theoretically expected recurrence time of the bursts according to the helium burst hypothesis is 0.2-0.9 days, in agreement with the observations.