TITLE: GCN CIRCULAR NUMBER: 6181 SUBJECT: GRB 070125: X-ray light curve analysis DATE: 07/03/08 21:34:34 GMT FROM: David Burrows at PSU/Swift 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.