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

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano “G.Occhialini”, via Bassini 15, 20133 Milano, Italy e-mail: tiengo@mi.iasf.cnr.it
² Università degli Studi di Milano, Dipartimento di Fisica, via Celoria 16, 20133 Milano, Italy
³ Università di Padova, Dipartimento di Fisica, via Marzolo 8, 35131 Padova, Italy
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK
⁵ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
⁶ SRON – National Institute for Space Research, Sorbonnelaan, 2, 3584 CA, Utrecht, The Netherlands

Received: 3 January 2005
Accepted: 12 March 2005

Abstract

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  10^-12 erg cm^-2 s^-1 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 ~ 0.63 keV, 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  ~ 0.2–0.3 keV and  ~ 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 4σ limits on the equivalent width in the range ~10–220 eV, depending on line energy and width.