[vsnet-grb-info 2327] GRB 051022: XRT Refined Spectral and Temporal Analysis

Wed Oct 26 22:53:29 JST 2005

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 at space.mit.edu>

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GRB 051022: XRT Refined Spectral and Temporal Analysis

N. R. Butler (UC-Berkeley), G. R. Ricker (MIT),=20
D. Q. Lamb (U.Chicago), D. N. Burrows (PSU), J.=20
Racusin (PSU), and N. Gehrels (GSFC) report:

We have performed spectral and temporal analyses=20
of the first three Swift XRT ToO follow-up=20
observations of the HETE-2--localized burst GRB=20
051022 (Olive et al., GCN 4131; Tanaka et al.=20
4137; Doty et al. 4145). Localization of the=20
X-ray afterglow of the burst was reported by=20
Racusin et al. (GCN 4141) based on the first two=20
orbits of XRT data. The PC mode data that we have=20
analyzed have an effective exposure of 14.2 ksec=20
for the time period of 12.5 to 182.3 ksec after=20
the burst. Modeling the count rate decay in the=20
0.5-10 keV band as a power-law in time, the=20
best-fit decay index is alpha =3D 1.33 =B1 0.07.=20
There is no evidence for a jet break in the XRT=20
data that we have analyzed.  The 0.2-10 keV=20
spectrum is well-fit (Chi2/nu =3D 189/172) by a=20
power-law model with absorption.  We find n_H =3D=20
(0.84 =B1 0.07) x 10^22 cm^-2, roughly consistent=20
with the value derived from the HETE-2 WXM and=20
=46REGATE data (Doty et al., GCN 4145) and greater=20
than the Galactic value in the direction of the=20
burst at the 30-sigma significance level, using=20
the Likelihood Ratio test (Delta Chi2 =3D 890 for=20
one additional degree freedom).  The photon index=20
is gamma =3D 2.0 =B1 0.1 and the mean unabsorbed flux=20
is (1.9 =B1 0.1) x 10^-11 erg cm^-2 s^-1.  The=20
spectral parameters do not appear to evolve in=20
time during the observation.

Assuming the relationship A_V =3D 0.56 N_H[10^21=20
cm^-2] + 0.23 found by Predehl and Schmitt (1995;=20
A&A, 293, 889); the expression for the extinction=20
curve given by Cardelli, Clayton, and Mathis=20
(1989; ApJ, 345, 245); R_V =3D 3.1; and a redshift=20
for the burst of z =3D 0.8 (Gal-Yam et al., GCN=20
4156), based on the positional coincidence=20
between the galaxy at this redshift and the=20
IR/optical (Castro-Tirado et al., GCN 4143; de=20
Ugarte Postigo et al., GCN 4164) and radio=20
(Cameron and Frail, GCN 4154; van der Horst, Rol,=20
and Wijers, GCN 4158) and X-ray afterglow (Patel,=20
Kouveliotou, and Rol, GCN 4163) of the burst, we=20
derive the following values for the extinction in=20
magnitudes in the observer frame: A_V =3D 8.6 =B1=20
0.7, A_R =3D 7.7 =B1 0.6, A_I =3D 6.5 =B1 0.5, A_J =3D 3.8=20
=B1 0.3, A_H =3D 2.3 =B1 0.2, and A_Ks =3D 1.5 =B1 0.1.
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<html><head><style type=3D"text/css"><!--
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 --></style><title>GRB 051022: XRT Refined Spectral and Temporal
Analysis</title></head><body>
<div><tt>GRB 051022: XRT Refined Spectral and Temporal Analysis<br>
<br>
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:<br>
<br>
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 =3D 1.33 =B1 0.07.&nbsp=
;
There is no evidence for a jet break in the XRT data that we have
analyzed.&nbsp; The 0.2-10 keV spectrum is well-fit (Chi2/nu =3D
189/172) by a power-law model with absorption.&nbsp; We find n_H =3D
(0.84 =B1 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 =3D 890 for one additional degree freedom).&nbsp; The photon
index is gamma =3D 2.0 =B1 0.1 and the mean unabsorbed flux is (1.9 =B1
0.1) x 10^-11 erg cm^-2 s^-1.&nbsp; The spectral parameters do not
appear to evolve in time during the observation.</tt><br>
<tt></tt></div>
<div><tt>Assuming the relationship A_V =3D 0.56 N_H[10^21 cm^-2] + 0.23
found by Predehl and Schmitt (1995; A&amp;A, 293, 889); the expression
for the extinction curve given by Cardelli, Clayton, and Mathis (1989;
ApJ, 345, 245); R_V =3D 3.1; and a redshift for the burst of z =3D 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 =3D 8.6 =B1 0.7, A_R =3D 7.7 =B1
0.6, A_I =3D 6.5 =B1 0.5, A_J =3D 3.8 =B1 0.3, A_H =3D 2.3 =B1 0.2, and A_Ks
=3D 1.5 =B1 0.1.</tt></div>
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