Probing the inner disk origin of the iron line in SAX J1808.4-3658

We propose a 100-ks triggered XMM observation of the accreting millisecond pulsar J1808 in its next outburst to gather strong evidence of the inner disk origin of its broad iron emission line. The accretion disk during short-outburst transients is expected to be truncated both at the inner radius (by the neutron star magnetosphere) and at an outer radius of a few hundreds Gravitational radii because the disk is rapidly emptying. The ring-like shape of the disk will produce a clear double-peaked line profile, unique signature of the disk origin of the line. The clear detection of this profile by XMM will give the ultimate proof of the disk origin of the Fe line.