Three XMM-Newton observations of the anomalous X-ray pulsar 1E 1048.1-5937: Long term variations in spectrum and pulsed fraction

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 2-10 keV flux of 6.2 x 10-12 erg cm-2 s-1 and a pulsed fraction PF = 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=0.63 keV, photon index Gamma=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=0.2-0.3 keV and kT2=0.4-0.5 keV 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 4 sigma limits on the equivalent width in the range 10-220 eV, depending on line energy and width.