Continued X-ray Monitoring of Magnetar Candidate SWIFT J1822.3-1606

ATel #3553; P. Scholz, M. A. Livingstone, V. M. Kaspi (McGill U.)
on 13 Aug 2011; 02:09 UT
Credential Certification: Victoria Kaspi (vkaspi@physics.mcgill.ca)

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We report on Swift/XRT and RXTE/PCA observations of the new 8.4-s Galactic magnetar candidate SWIFT J1822.3-1606, also referred to as SGR J1822.3-1606 (ATELs #3488, #3489, #3490, #3493, #3495, #3496, #3501, #3503, #3543).

The persistent X-ray flux from the source continues to fade in ongoing XRT monitoring observations. For data in the MJD range 55757 to 55781, the best-fit power-law index, alpha, for the decay of the absorbed 1-10 keV flux is -0.47 +/- 0.02, assuming a decay of functional form F(t) = F0 + F0*(t-T)^alpha, where T is the epoch of the Swift/BAT trigger (ATEL #3488). This power-law index is shallower than those in the decays of SGR 1627-41 following its 1998 outburst (Esposito et al. 2008, MNRAS, 390, L34) and 1E 2259+586 following its 2002 outburst (Zhu et al. 2008, ApJ, 686, 520) but steeper than those of SGR 1627-41 following its 2008 activation (Esposito et al. 2008), and of 1E 1547-5408 following its 2009 outburst (Scholz & Kaspi, 2011, ApJ, in press). An exponential decay also provides a reasonable fit to the flux decay of SWIFT J1822.3-1606. We further note in our preliminary analysis of XRT data for SWIFT J1822.3-1606 no significant spectral changes as the flux decreases.

Also, we note the discrepancy in the reported spin-down rate (and hence inferred dipole B field) of Serim et al. (ATEL #3543) and Gogus et al (ATEL #3503). Our independent analysis of timing data during in the MJD range 55758.5 to 55780.5 (13 XTE/PCA and 15 Swift/XRT observations) shows no significant spin-down, with best-fit frequency derivative (5.94+/-8.15)E-15 Hz^2 where the reported uncertainty is 3 times the formal uncertainty (99% confidence upper limit). This implies an upper limit on the inferred dipolar magnetic field strength of B<7.1E13 G, assuming the standard dipole equation. This is consistent with the results of Serim et al. but not with those of Gogus et al.