Jet break candidates in the X-ray and optical afterglow lightcurves

Abstract

The Swift/XRT data for 179 GRBs (050124 to 070129) and the optical afterglow data for 57 pre-and post-Swift GRBs are analyzed to explore whether the breaks in the afterglow light curves can be interpreted as jet breaks and their implications for jet energetics. We find that no textbook version of jet break is found, in which the data fully satisfy jet break criteria. By relaxing one or more requirements, candidates to various degrees are identified. The break time in the X-ray band is earlier than that in the optical bands. Among thirteen bursts having both the optical and X-ray light curves, only seven have an achromatic break, and even in these cases only one band data satisfies the closure relations. These raise concerns about interpreting the breaks as jet breaks and further inferring GRB energetics. Assuming that the candidates are jet breaks, we find that the geometrically-corrected kinetic energy (E <inf>k</inf>) distribution is much more scattered than the pre-Swift sample, but a tentative anti-correlation between jet opening angle θ<inf>j</inf> and E<inf>K,iso</inf> is found, indicating that the E<inf>K</inf> could still be quasi-universal. © 2008 American Institute of Physics.