We report on 5 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-04, 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 about 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 about 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 about 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.
We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.
