The everlasting X-ray afterglow of GRB 060729

The X-ray afterglow of GRB 060729 had the latest detection of an X-ray afterglow ever, at 640 days after the burst. It was detected by Swift up to 125 days after the trigger and later by Chandra with follow-up observations in 2007 and 2008 with the latest observations in May 2008. These observations indicate two breaks in the X-ray light curve of the afterglow at about 1Ms and one year after the burst respectively. The first break at 1.2 Ms after the burst coincides with a hardening in the X-ray spectrum. The second break at about one year after the burst is followed by a very steep decay of F_x ∝_t^-4.59 and coincides with a spectral softening. The first break can be interpreted as a cooling break in the wind medium scenario, at which the cooling frequency of the x-ray afterglow just crosses the x-ray band, while the second break is less well-understood. Based on these interpretations, we estimated a jet half opening angle of > 14°. A comparison with other bright X-ray afterglows shows that GRB 060729 has been one of the most energetic bursts ever seen with a total beaming-corrected energy in the rest-frame 2-10 keV band of E2-10keV > 2.1 x 10 ergs.