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 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.
