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ATEL # 1043; S. Campana (OA Brera), G.L. Israel (OA Roma)
on 4 Apr 2007; 0:02 UT
Password Certification: Sergio Campana (firstname.lastname@example.org)
Subjects: X-ray, Neutron Stars, Soft Gamma-ray Repeaters, Transients
Referred to by ATEL #: 1044
Following the discovery of a sudden spin-up and pulsed flux increase in the 6.5-s anomalous X-ray pulsar (AXP) 1E 1048.1-5937 (Dib et al. 2007, ATel \#1041), Swift observed the source on April 3 2007, UT16:01:21. The first set of data in Photon Counting mode amounts to 2409 s. The source is well detected at a count rate of 0.88+/-0.02 c/s. This is mildly piled-up so that in the following analysis we excluded the central part of the point spread function (2 pixels). We extracted photons from an annular region (2 to 20 pixels), collecting 1337 source counts. The spectrum can be well fit with the usual absorbed power law and black body typical of AXPs. With respect to previous observations with Swift the unabsorbed 1-10 keV flux increased by a factor of ~8 (now being 6.3x10^-11 erg cm^-2 s^-1). The spectral parameters are: power law photon index Gamma=3.0+/-0.3 (90% confidence level), black body temperature kT=0.65+/-0.09 keV (with the column density fixed to 1.0x10^22 cm^-2). The spectrum is also consistent with black body-only variation and not with a power law-only variation. A timing analysis clearly reveals the presence of a pulsed signal consistent within uncertainties to the known spin period. The pulse shape is complex with (at least) two peaks. The background corrected pulsed fraction, semi-amplitude of the modulation divided by the mean source count rate, is about 25% (significantly below the usual value of ~80%). We encouraged observations at other wavelengths.