Swift observation of the AXP 4U 0142+61 following a bright X-ray burst

ATel #998; S. Campana (OA Brera), S. Patel (NASA/MSFC), G. L. Israel (OA Roma), N. Rea (Sron), C. Kouveliotou (NASA/MSFC)
on 9 Feb 2007; 21:32 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)

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Following the bright X-ray burst reported by Gavriil et al. (2007, ATel #993), Swift observed the anomalous X-ray pulsar 4U 0142+61 on Feb. 8th 2007 (36.7 hr after the burst). The XRT instrument collected 4844 s in Photon Counting (PC) mode and 138 s in Window Timing (WT) mode. The source is well detected at a piled-up rate of 2.7 c/s (corresponding to about 5 c/s of true rate). We extracted spectra for the two modes (in PC in an annulus 5-30 pixels wide to avoid pile-up) and fit them with the usual absorbed black body plus power law model. The best fit parameters are N_H=(1.0+/-0.2)x1E22 cm-2, T_BB=0.43+/-0.03 keV (R_BB=3.9+/-1.8 km assuming a source distance of 3 kpc) and photon index 3.7+/-0.2 (all errors are at 90% c.l.). The fit is good with a reduced chi2 of 1.0 with 171 d.o.f., the black body component is required (F-test probability of 1E-6). The 0.5-10 keV unabsorbed flux is (8.2+/-1.0)x1E-10 erg/cm2/s. These parameters are in line with previous XMM-Newton (Gohler et al. 2005, A&A 433 1079) and Chandra (Patel et al. 2003, ApJ 587 367; Juett et al. 2002, ApJ 568 L31) and recent (July 2006) Swift observations. The PC light curve shows a significant flux decrease consistent with an exponential decay (starting at the time of the X-ray burst) with an e-folding time tau=39+/-10 ks (even if the XRT light curve can be fit equally well with a linear decay). We analysed separately the PC spectra of the two consecutive orbits, keeping fixed the black body temperature and the photon index at their best overall values. We find a small (30%) decrease in the black body normalization, although it is not statistically significant. No bursts were clearly detected in either PC or WT data. However, one short burst could have triggered the automatic mode change (from PC to WT) that occurred during the middle of the first orbit data. A weak pulsation is detected at the expected period of 8.688 s with a semi-amplitude of 5+/-2%.