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We report the results of a recent (July 2004) XMM-Newton Target ofOpportunity observation of the Anomalous X-ray pulsar 1E 1048.1-5937, togetherwith a detailed re-analysis of previous observations carried out in 2000 and2003. In July 2004 the source had a 2-10 keV flux of 6.2$\times10^{-12}$ ergcm$^{-2}$ s$^{-1}$ and a pulsed fraction P$_F$=0.68. The comparison of thethree data sets shows the presence of an anti-correlation between flux andpulsed fraction, implying that previous estimates of the source energeticsbased on the assumption of a large and constant pulsed fraction might besignificantly underestimated. The source spectrum is well described by a powerlaw plus blackbody model (kT~0.63 keV, photon index $\Gamma$~2.7-3.5) or,alternatively, by the sum of two blackbodies of which the hotter is Comptonizedby relativistic electrons. In this case the temperatures are kT${_1}$~0.2-0.3keV and kT${_2}$~0.4-0.5 keV and the emitting area of the cooler component isconsistent with the whole neutron star surface. The long term luminosityvariation of a factor >~2 is accompanied by relatively small variations in thespectral shape. Phase resolved spectroscopy indicates a harder spectrum incorrespondence of the pulse maximum. No spectral features have been detectedwith 4$\sigma$ limits on the equivalent width in the range ~10-220 eV,depending on line energy and width.Comment: 10 pages, 6 figures; accepted for publication in A&

ThreeXMM-Newtonobservations of the anomalous X-ray pulsar 1E 1048.1–5937: Long term variations in spectrum and pulsed fraction