TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4165
SUBJECT: GRB 051022: XRT Refined Spectral and Temporal Analysis
DATE:    05/10/26 13:51:17 GMT
FROM:    George Ricker at MIT  <grr@space.mit.edu>

N. R. Butler (UC-Berkeley), G. R. Ricker (MIT), D. Q. Lamb 
(U.Chicago), D. N. Burrows (PSU), J. Racusin (PSU), and N. Gehrels 
(GSFC) report:

We have performed spectral and temporal analyses of the first three 
Swift XRT ToO follow-up observations of the HETE-2--localized burst 
GRB 051022 (Olive et al., GCN 4131; Tanaka et al. 4137; Doty et al. 
4145). Localization of the X-ray afterglow of the burst was reported 
by Racusin et al. (GCN 4141) based on the first two orbits of XRT 
data. The PC mode data that we have analyzed have an effective 
exposure of 14.2 ksec for the time period of 12.5 to 182.3 ksec after 
the burst. Modeling the count rate decay in the 0.5-10 keV band as a 
power-law in time, the best-fit decay index is alpha = 1.33 +/- 0.07. 
There is no evidence for a jet break in the XRT data that we have 
analyzed.  The 0.2-10 keV spectrum is well-fit (Chi2/nu = 189/172) by 
a power-law model with absorption.  We find n_H = (0.84 +/- 0.07) x 
10^22 cm^-2, roughly consistent with the value derived from the 
HETE-2 WXM and FREGATE data (Doty et al., GCN 4145) and greater than 
the Galactic value in the direction of the burst at the 30-sigma 
significance level, using the Likelihood Ratio test (Delta Chi2 = 890 
for one additional degree freedom).  The photon index is gamma = 2.0 
+/- 0.1 and the mean unabsorbed flux is (1.9 +/- 0.1) x 10^-11 erg 
cm^-2 s^-1.  The spectral parameters do not appear to evolve in time 
during the observation.

Assuming the relationship A_V = 0.56 N_H[10^21 cm^-2] + 0.23 found by 
Predehl and Schmitt (1995; A&A, 293, 889); the expression for the 
extinction curve given by Cardelli, Clayton, and Mathis (1989; ApJ, 
345, 245); R_V = 3.1; and a redshift for the burst of z = 0.8 
(Gal-Yam et al., GCN 4156), based on the positional coincidence 
between the galaxy at this redshift and the IR/optical (Castro-Tirado 
et al., GCN 4143; de Ugarte Postigo et al., GCN 4164) and radio 
(Cameron and Frail, GCN 4154; van der Horst, Rol, and Wijers, GCN 
4158) and X-ray afterglow (Patel, Kouveliotou, and Rol, GCN 4163) of 
the burst, we derive the following values for the extinction in 
magnitudes in the observer frame: A_V = 8.6 +/- 0.7, A_R = 7.7 +/- 
0.6, A_I = 6.5 +/- 0.5, A_J = 3.8 +/- 0.3, A_H = 2.3 +/- 0.2, and 
A_Ks = 1.5 +/- 0.1.

[GCN OPS NOTE(26oct05):  This is a reissue of GCN Circ 4165 with the 
binary-conversion characters removed and the line-break formating fixed.]