SwiftandXMM‐NewtonObservations of the Extraordinary Gamma‐Ray Burst 060729: More than 125 Days of X‐Ray Afterglow

We report the results of the Swift and XMM observations of theSwift-discovered long Gamma-Ray Burst GRB 060729 ($T_{90}$=115s). The afterglowof this burst was exceptionally bright in X-rays as well as at UV/Opticalwavelengths showing an unusually long slow decay phase ($\alpha$=0.14\plm0.02)suggesting a larger energy injection phase at early times than in other bursts.The X-ray light curve displays a break at about 60 ks after the burst. TheX-ray decay slope after the break is $\alpha$=1.29\plm0.03. Up to 125 daysafter the burst we do not detect a jet break, suggesting that the jet openingangle is larger than 28 degrees. In the first 2 minutes after the burst (restframe) the X-ray spectrum of the burst changed dramatically from a hard X-rayspectrum to a very soft one. We find that the X-ray spectra at this early phasecan all be fitted by an absorbed single power law model or alternatively by ablackbody plus power law model. The power law fits show that the X-ray spectrumbecomes steeper while the absorption column density decreases. In Swift'sUV/Optical telescope the afterglow was clearly detected up to 9 days after theburst in all 6 filters and even longer in some of the UV filters with thelatest detection in the UVW1 31 days after the burst. A break at about 50 ks isclearly detected in all 6 UVOT filters from a shallow decay slope of about 0.3and a steeper decay slope of 1.3. In addition to the \swift observations wealso present and discuss the results from a 61 ks ToO observation by XMM.(Abriviated)Comment: Accepted to be published in the Astrophysical Journal, 28 pages, 10 figure