Pulse Profile Changes and a Possible Timing Anomaly in the Magnetar SGR J1745-29

ATel #6064; Ryan S. Lynch (McGill University), Victoria M. Kaspi (McGill University), and Paul Scholz (McGill University)
on 12 Apr 2014; 22:35 UT
Credential Certification: Ryan Lynch (rlynch@physics.mcgill.ca)

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We have been observing the magnetar SGR J1745-29 using the Green Bank Telescope (GBT) at a center frequency of 8.7 GHz. The magnetar was monitored with approximately weekly cadence from 11 August 2013 to 25 January 2014, and with approximately monthly cadence thereafter. Yusef-Zadeh et al. (ATel #6041) reported a factor of ~20 increase in flux at 44 GHz in VLA observations on 21 February 2014, compared to an upper limit obtained on 31 August 2011. Our GBT observations from 10 March 2014 and 4 April 2014 do not show a dramatic increase in the 8.7 GHz flux compared to its mean level over the course of our observations, given the uncertainties in our flux calibration, which we estimate to be about 15%. The flux on 10 March was slightly below the mean level, while the flux on 4 April was < 2 times greater than the mean. This level of epoch-to-epoch flux variation is consistent with what we have observed previously for SGR J1745-29. We do, however, observe the emergence of a new pulse profile component beginning on 10 March. The centroid of the new peak is separated from the main profile component by ~0.1 turns. Its amplitude relative to the profile peak varied between our observing epochs, decreasing from about 60% to about 20%.

These changes contrast with the slow and steady profile changes that were observed prior to 10 March. Between 11 August 2013 and 25 January 2014, the period and period derivative evolved smoothly (P=3.763733 s , P-dot=1.75828 10^-11, referenced to MJD 56587.0). Incoherent measurements of the spin period do not show evidence for a glitch in our most recent observations, but we no longer have unambiguous phase connection. The residual phase difference is larger than can be accounted for due to the profile changes. This indicates a possible abrupt change in the spin-down rate of the magnetar, which has been seen before (Kaspi et al. 2014, accepted to ApJ, arxiv:1403.5344), though this may also be due to a high degree of timing noise. Further timing observations are required to differentiate between these possibilities.

Example integrated pulse profiles from before and after the 44 GHz brightening can be found at
http://www.physics.mcgill.ca/~rlynch/Files/Public/SGRJ1745-29_profile_change.png