TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4374 SUBJECT: GRB 051221: Swift XRT refined analysis DATE: 05/12/21 18:46:53 GMT FROM: Milvia Capalbi at ISAC/ASDC M. Capalbi, M. Perri (ASDC), D.N. Burrows, D. Grupe (PSU), P. Boyd (GSFC-UMBC), W. Voges (MPE) report on behalf of the Swift XRT team: We have analyzed the first six orbits of the Swift XRT data of GRB 051221 (Parsons et al., GCN 4363). Data in Windowed Timing (WT) mode started at 01:52:48 UT, 92 seconds after the BAT trigger, then the XRT switched to Photon Counting mode. The light curve shows an initial fading behavior up to 2.5 ks with a decay index of -1.3 ± 0.7. At later times, an indication of a re-brightening is present. A preliminary spectral fit (simple absorbed power-law) to the WT data in the 0.5-10. 0 keV band yields a photon index of 2.0+/-0.4 and a column density value consistent with the Galactic one (6.7E20 cm-2; Dickey & Lockman 1990). Extrapolating the initial decay, the unabsorbed 0.5-10.0 keV flux at 24 hours after the burst is estimated to be ~5E-14 erg cm^-2 s^-1.