XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658

We report on a 63-ks long XMM-Newton observation of the accreting millisecond pulsar SAX J1808.4-3658 during the latest X-ray outburst which started on September 21st 2008. The pn spectrum shows a highly significant emission line in the energy band where the iron K-alpha line is expected, and which we identify as emission from neutral (or mildly ionized) iron. The line profile appears to be quite broad (more than 1 keV FWHM) and asymmetric; the most probable explanation for this profile is Doppler and relativistic broadening from the inner accretion disc. From a fit with a diskline profile we find an inner radius of the disc of 8.7(-2.7)(+3.7) R-g, corresponding to 18.0(-5.6)(+7.6) km for a 1.4 M circle dot neutron star. The disc therefore appears truncated inside the corotation radius (31 km for SAX J1808.43658) in agreement with the fact that the source was still showing pulsations during the XMM-Newton observation.