TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3659
SUBJECT: GRB 050721A: Early Swift XRT analysis results
DATE:    05/07/21 10:25:27 GMT
FROM:    Pat Romano at OAB-Swift  <romano@merate.mi.astro.it>

P. Romano (INAF-OAB), L.A. Antonelli (INAF-OAR), G. Chincarini, A. 
Moretti, G. Tagliaferri (INAF-OAB), K. Page (U. Leicester), F. Marshall 
(GSFC), P. Boyd (GSFC-UMBC) on behalf of the Swift-XRT team:

We have analyzed the Swift XRT data from the first orbit observation of 
GRB 050721A (Antonelli et al., GCN 3654). The new refined coordinates are:

RA(J2000) = 16h 53m 44.9s Dec(J2000) = -28 22 51.7

This position is 41 arcseconds from the BAT position given in GCN 3654. 
We estimate an uncertainty of 6 arcseconds radius (90% containment).

A preliminary spectral fit (simple absorbed power-law) to the combined 
PC and WT data yields a photon index of 1.7+/-0.1 in the [0.5-10] keV 
band. The derived NH is 3.16E21 cm^-2, which is higher than the Galactic 
value (1.57E21 cm^-2; Dickey & Lockman 1990). The average (194-323 
seconds from trigger for WT and 324-463 seconds from trigger for PC) 
estimated unabsorbed 0.5-10 keV flux is then 3.5E-10 ergs/s/cm^2.

The light curves in Windowed Timing (WT) and Photon Counting (PC) mode 
start 194 and 324 seconds from the BAT trigger (T0), and they show a 
fading afterglow with a preliminary decay slope -1.99+/-0.07 (first 
orbit only). The unabsorbed 0.5-10.0 keV flux at 1 hour after the burst 
is estimated to be 5E-12 ergs/s/cm^2.