On 2004 December 27 , a highly-energetic giant flare was recorded from the magnetar candidate SGR 1806 - 20 .
In the months preceding this flare , the persistent X-ray emission from this object began to undergo significant changes .
Here , we report on the evolution of key spectral and temporal parameters prior to and following this giant flare .
Using the Rossi X-ray Timing Explorer , we track the pulse frequency of SGR 1806 - 20 and find that the spin-down rate of this SGR 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 27 { th } flare ( | \frac { \Delta \nu } { \nu } | < 5 \times 10 ^ { -6 } ) .
In the months surrounding the flare , we find a correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson , Lyutikov & Kulkarni ( 2002 ) .
As with the flare in SGR 1900 + 14 , the pulse morphology of SGR 1806 - 20 changes drastically following the flare .
Using the Chandra X-ray Observatory and other publicly available imaging X-ray detector observations , we construct a spectral history of SGR 1806 - 20 from 1993 to 2005 .
As we reported earlier , the X-ray spectrum of SGR 1806 - 20 can not be fitted to a simple power-law model .
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 .
( 2005a ) of increasing spectral hardness of SGR 1806 - 20 between 1993 and 2004 .
We find both an increase in blackbody temperature and a flattening of the power-law photon index .
However , our results indicate significant differences in the temporal evolution of the spectral hardening .
Rather than a direct correlation as proposed by Mereghetti et al. , 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 , and pulsed flux of SGR 1806 - 20 all peak months before the flare epoch .