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We report on a 63-ks long XMM-Newton observation of the accreting millisecondpulsar SAX J1808.4-3658 during the latest X-ray outburst which started onSeptember 21st 2008. The pn spectrum shows a highly significant emission linein the energy band where the iron K-alpha line is expected, and which weidentify as emission from neutral (or mildly ionized) iron. The line profileappears to be quite broad (more than 1 keV FWHM) and asymmetric; the mostprobable explanation for this profile is Doppler and relativistic broadeningfrom the inner accretion disc. From a fit with a diskline profile we find aninner radius of the disc of 8.7^(+3.7)_(-2.7) R_g, corresponding to18.0^(+7.6)_(-5.6) km for a 1.4 Msun neutron star. The disc therefore appearstruncated inside the corotation radius (31 km for SAX J1808.4-3658) inagreement with the fact that the source was still showing pulsations during theXMM-Newton observation.

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