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Phase Drifts of Sub-pulses during the 2004 Giant Flare of SGR 1806-20 and Settling of the Magnetic Fields

We analyzed the observations of SGR 1806-20 performed with the \textit{Rossi X-ray Timing Explorer} (RXTE) during its 2004 giant flare. We studied the phase evolution of the sub-pulses identified in the X-ray waveform and found that the sub-pulses varied in phase with time and then gradually settled, which might indicate drifts of the emission regions in relative to the neutron star surface, or changes in the local emission geometry before the magnetic field became stable. The characteristic e-folding timescale of the phase drifts measured starting about 15 s following the initial flux spike are in the range between 37 s and 84 s. This leads to the first measurements of the characteristic timescale for the magnetic field of the neutron star to settle after a field reconfiguration during the giant flare.