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Spin and Pulsed X-ray Flux Properties of SGR 1806-20 after the Giant Flare

ATel #407; Peter M. Woods (USRA/NSSTC), Chryssa Kouveliotou (MSFC/NSSTC), Ersin Gogus (Sabanci Univ), Mark Finger, Sandy Patel (USRA/NSSTC), Jean Swank (NASA/GSFC), and Kevin Hurley (UCB)
on 7 Feb 2005; 23:04 UT
Credential Certification: Peter Woods (Peter.Woods@nsstc.nasa.gov)

Subjects: X-ray, Neutron Star, Soft Gamma-ray Repeater

The Soft Gamma Repeater SGR 1806-20 emitted a giant flare that was recorded on 2004 December 27 (GCN 2920). We triggered an ongoing sequence of ToO observations of the SGR with RXTE that began on 2005 January 22, the earliest RXTE could observe due to Sun-angle constraints. We find no evidence for a sudden change in spin frequency of the SGR following this flare and place a limit on such a frequency jump of |delta nu| < 2 x 10^-5 Hz. On the other hand, the spin-down rate has decreased relative to the pre-flare value. The current frequency derivative is (-3.15 +/- 0.09) x 10^-12 Hz/s, some ~2.7 times smaller than the average value of -8.6 x 10^-12 Hz/s observed over the ~4 years leading up to the flare. Finally, the pulsed flux of SGR 1806-20 is significantly less than it was pre-flare (ATEL 313). Assuming that the pulsed fraction has not changed since before the flare, we infer the current, unabsorbed X-ray flux of SGR 1806-20 to be ~2 x 10^-11 ergs cm^-2 s^-1 (2-10 keV).