2005A&A...437..997T

We report the results of a recent (July 2004) XMM-Newton Target of Opportunity observation of the Anomalous X-ray Pulsar 1E 1048.1-5937 together with a detailed re-analysis of previous observations carried out in 2000 and 2003. In July 2004 the source had a 2-10keV flux of 6.2x10^–12erg/cm²/s and a pulsed fraction P_F=0.68. Comparison of the three data sets shows the presence of an anti-correlation between flux and pulsed fraction, implying that previous estimates of the source energetics based on the assumption of a large and constant pulsed fraction might be significantly underestimated. The source spectrum is well described by a power law plus blackbody model (kT∼0.63keV, photon index Γ∼2.7-3.5) or, alternatively, by the sum of two blackbodies of which the hotter is Comptonized by relativistic electrons. In this case the temperatures are kT₁∼0.2-0.3keV and kT₂∼0.4-0.5keV and the emitting area of the cooler component is consistent with the whole neutron star surface. The long-term luminosity variation of a factor >2 is accompanied by relatively small variations in the spectral shape. Phase-resolved spectroscopy indicates a harder spectrum together with the pulse maximum. No spectral features have been detected with 4σ limits on the equivalent width in the range ∼10-220eV, depending on line energy and width.