[AAVSO-HEN] GRB 051221: Swift XRT refined analysis

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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  <capalbi at asdc.asi.it>

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.