Article

LATE-TIME DETECTIONS OF THE X-RAY AFTERGLOW OF GRB 060729 WITH<i>CHANDRA</i>—THE LATEST DETECTIONS EVER OF AN X-RAY AFTERGLOW

D. GrupeDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]D. N. BurrowsDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]Xue-Feng WuDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]Xiang-Yu WangDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]Bing ZhangDepartment of Physics, University of Nevada, Las Vegas, NV 89154, USAEn-Wei LiangDepartment of Physics, Guangxi University, Nanning 530004, ChinaG. P. GarmireDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]J. A. NousekDepartment of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]N. GehrelsAstrophysics Science Division, Astroparticle Physics Laboratory, Code 661, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAG. RickerMassachusetts Institute of Technology, 77 Massachusetts Av., Cambridge, MA 02139-4307, USAMarshall W. BautzMassachusetts Institute of Technology, 77 Massachusetts Av., Cambridge, MA 02139-4307, USA

2010en

ABI

Abstract

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.

Identifiers

DOI: 10.1088/0004-637x/711/2/1008

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