Swift XRT Observations of the Magnetar 4U 0142+61

ATel #7171; R. F. Archibald (McGill), P. Scholz (McGill), V. Kaspi (McGill), N. Gehrels (GSFC), and J. Kennea (PSU).
on 3 Mar 2015; 16:44 UT
Credential Certification: Robert Archibald (rarchiba@physics.mcgill.ca)

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We report on Swift XRT observations made following an X-ray/soft gamma-ray burst detected by Swift BAT and Fermi GBM on 28 February 2015 (MJD 57081) from the direction of the magnetar 4U 0142+61 (GCN 17507, 17508). 4U 0142+61 is an 8.7 s magnetar (see eg. Dib & Kaspi 2014 and references therein) that is being monitored as part of an ongoing study of several magnetars with the Swift XRT. In an XRT WT snapshot taken at 57081.35, we detect ~7 bursts with probabilities of < 1E-3 of occurring by chance, using the mean count rate of the observation and assuming Poisson statistics. The bursts have fluences ranging from 12 to 49 counts detected in a 100 ms window (1-10 keV band). After removing these bursts, we note that the RMS pulse fraction has increased from the normal level of 6.3(1)% to 21(1)%. The average 1-10 keV flux level during this observation was 6.0(-0.1, +0.4)E-10 erg/s/cm^2, with a harder spectrum (PL index of 1.11(6) vs 3.77(6) normally). However, the 1-10 keV count rate clearly decays over the 300-s snapshot from a factor of 7.0+/-0.5 over the quiescent level to a factor of 2.0+/-0.2 over the quiescent level. By the 01 March 2015 (MJD 57082) observation, the 1-10 keV flux had returned to its normal level, 1.08(3)E-10 erg/s/cm^2 compared with a flux of 1.05(3)E-10 erg/s/cm^2 from the prior monitoring observation on 26 February 2015 (MJD 57079). In the 01 March observation, we observe one additional ~10 ms burst at 57082.198204 with 10 counts in a 100 ms bin compared to an expected count rate of 4 cps. The pulse fraction has also nearly recovered, being 8(1)%. By comparing a pulse times-of-arrival from these observations to the prediction of our pre-burst timing solution, we find that the pulse times of arrival are off by 0.35(2) and 0.50(2) in phase on MJDs 57081 and 57082 respectively, although part of this may be due to a change in pulse profile. We are currently conducting a timing campaign to characterize this tentative glitch.