This paper investigates GRB 050802 , one of the best examples of a Swift gamma-ray burst afterglow that shows a break in the X-ray lightcurve , while the optical counterpart decays as a single power-law .
This burst has an optically bright afterglow of 16.5 magnitude , detected throughout the 170 - 650 nm spectral range of the UVOT on-board Swift .
Observations began with the XRT and UVOT telescopes 286 s after the initial trigger and continued for 1.2 \times 10 ^ { 6 } s .
The X-ray lightcurve consists of three power-law segments : a rise until 420 s , followed by a slow decay with \alpha _ { 2 } = 0.63 \pm 0.03 until 5000 s , after which , the lightcurve decays faster with a slope of \alpha _ { 3 } = 1.59 \pm 0.03 .
The optical lightcurve decays as a single power-law with \alpha _ { O } = 0.82 \pm 0.03 throughout the observation .
The X-ray data on their own are consistent with the break at 5000 s being due to the end of energy injection .
Modelling the optical to X-ray spectral energy distribution , we find that the optical afterglow can not be produced by the same component as the X-ray emission at late times , ruling out a single component afterglow .
We therefore considered two-component jet models and find that the X-ray and optical emission is best reproduced by a model in which both components are energy injected for the duration of the observed afterglow and the X-ray break at 5000 s is due to a jet break in the narrow component .
This bright , well-observed burst is likely a guide for interpreting the surprising finding of Swift that bursts seldom display achromatic jet breaks .