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GCN Circular 6181

Subject
GRB 070125: X-ray light curve analysis
Date
2007-03-08T21:34:34Z (17 years ago)
From
David Burrows at PSU/Swift <dnburrows@gmail.com>
D. N. Burrows and J. Racusin report on behalf of the Swift XRT team:

Garnavich et al. (GCN Circ. 6165) have suggested that the X-ray light 
curve of GRB 070125 has a late break to a steep slope,  in agreement 
with the optical break reported by Mirabel, Halpern & Thorstensen 
(GCN 6096) and confirmed by their observations.

We have re-examined the XRT light curve, which extends from ~44 ks to 
~1.5 Ms post-burst, and reconfirm our original conclusions.  Full 
details, including a plot of the X-ray light curve showing several 
possible fits of single and broken power laws, are given in GCN 
Report 28.3 (http://gcn.gsfc.nasa.gov/reports/report_28_3.pdf).  We find:

1) the X-ray light curve is best fit by a broken power law, but with 
a break time at 1.35 +/- 0.35 days (90% confidence), not > 4 days as 
required by the optical data (Mirabel, Halpern & Thorstensen, GCN 
6096).  However, this fit, with reduced chi**2=1.6 for 15 degrees of 
freedom, is rather poor (P=0.065), due primarily to large residuals 
between 100 and 200 ks.

2) a better fit can be obtained under the assumption that there is a 
small X-ray flare at about 110 ks.  If these data points are 
excluded, the remaining X-ray data can be fit by a single power law 
of slope 1.57 +/- 0.07 (90% confidence) with reduced chi**2 of 0.82 
for 14 degrees of freedom.  A broken power law fit to this data set 
is slightly worse and has very poor constraints on the fit parameters.

We conclude that we cannot distinguish between a single power law fit 
with a small flare at 110 ks, and a broken power law fit with 
additional flaring (to account for the poor residuals).  Therefore 
the X-ray data do not show evidence for a jet break: they are 
consistent with a jet break coincident with the optical break, but 
are equally consistent with no break at all.
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