The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-Ray Properties of SGR 1806-20 from 1993 to 2005
Abstract
We report on the evolution of key spectral and temporal parameters of SGR 1806-20 prior to and following the highly energetic giant flare of 2004 December 27. Using RXTE, we track the pulse frequency of the SGR and find that the spin-down rate varied erratically in the months before and after the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for a discrete jump in spin frequency at the time of the December 27th flare (|Δν/ν|<5×10-6). In the months surrounding the flare, we find a strong correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson et al. As with the flare in SGR 1900+14, the pulse morphology of SGR 1806-20 changes drastically following the flare. Using Chandra and other publicly available imaging X-ray detector observations, we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual magnetar persistent emission spectral model of a power law plus a blackbody provides an excellent fit to the data. We confirm the earlier finding by Mereghetti et al. of increasing spectral hardness of SGR 1806-20 between 1993 and 2004. However, our results indicate significant differences in the temporal evolution of the spectral hardening. Rather than a direct correlation between torque and spectral hardness, we find evidence for a sudden torque change that preceded a gradual hardening of the energy spectrum on a timescale of years. Interestingly, the spectral hardness, spin-down rate, phase-averaged flux, and pulsed flux of SGR 1806-20 all peak months before the flare epoch.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2007
- DOI:
- 10.1086/507459
- arXiv:
- arXiv:astro-ph/0602402
- Bibcode:
- 2007ApJ...654..470W
- Keywords:
-
- Stars: Pulsars: Individual: Alphanumeric: SGR 1806-20;
- X-Rays: Bursts;
- Astrophysics
- E-Print:
- 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To appear in the Oct 20 2006 edition