LATE-TIME DETECTIONS OF THE X-RAY AFTERGLOW OF GRB 060729 WITH CHANDRA-THE LATEST DETECTIONS EVER OF AN X-RAY AFTERGLOW

We report on five Chandra observations of the X-ray afterglow of the gamma-ray burst (GRB) 060729 performed between 2007 March and 2008 May. In all five observations, the afterglow is clearly detected. The last Chandra pointing was performed on 2008 May 4, 642 days after the burst-the latest detection of a GRB X-ray afterglow ever. A reanalysis of the Swift XRT light curve together with the three detections by Chandra in 2007 reveals a break at similar to 1.0 Ms after the burst with a slight steepening of the decay slope from alpha = 1.32 to 1.61. This break coincides with a significant hardening of the X-ray spectrum, consistent with a cooling break in the wind medium scenario, in which the cooling frequency of the afterglow crosses the X-ray band. The last two Chandra observations in 2007 December and 2008 May provide evidence for another break at about one year after the burst. If interpreted as a jet break, this late-time break implies a jet half-opening angle of similar to 14 degrees for a wind medium. Alternatively, this final break may have a spectral origin, in which case no jet break has been observed and the half-opening angle of the jet of GRB 060729 must be larger than similar to 15 degrees for a wind medium. We compare the X-ray afterglow of GRB 060729 in a wind environment with other bright X-ray afterglows, in particular GRBs 061121 and 080319B, and discuss why the X-ray afterglow of GRB 060729 is such an exceptionally long-lasting event.