TITLE: GCN CIRCULAR NUMBER: 6186 SUBJECT: GRB 070125: Chandra X-ray Confirmation of Jet Break DATE: 07/03/09 21:18:41 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, A. M. Soderberg (Caltech), D. A. Frail (NRAO), and D. B. Fox (Penn State) report on behalf of a larger collaboration: The Chandra X-ray Observatory + ACIS observed the field of GRB070125 (Hurley et al, GCN 6024) beginning 5 March 2007 21:28 UT for a 30 ks exposure (mean epoch ~ 39.76 days after the burst). No source is detected at the location of the optical afterglow (Cenko & Fox, GCN 6028). Formally, using a circular aperture with 1" diameter, we detect 0.9 +/- 5.0 photons from 0.3-10 keV. Using spectral properties derived from early XRT observations (Gamma=2.0, nH=8e20; Racusin and Vetere, GCN 6030), we estimate an upper limit on the afterglow flux of < 2e-15 erg cm^-2 s^-1. Had the X-ray flux seen by the Swift XRT (Burrows & Racusin, GCN 6181) continued to decay as a single power-law with index ~ -1.5, we would expect a flux ~ 5e-15 erg cm^-2 s^-1 at the Chandra epoch (~ 20 Chandra ACIS-S photons), more than a factor of 2 above this upper limit. We therefore conclude the X-ray decay has steepened since the last XRT detection, confirming the jet break seen in the optical light curve (Mirabal, Halpern, & Thorstensen, GCN 6096; Garnavich et al., GCN 6165). A plot of the X-ray light curve can be found at: http://www.srl.caltech.edu/~cenko/public/grb070125_xray.jpg We would like to thank the entire Chandra X-ray Center staff for their execution of this observation and the rapid processing of the data.