A comprehensive analysis of Swift XRT data. III. Jet break candidates in X-ray and optical afterglow light curves

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 observed breaks in the afterglow light curves can be interpreted as jet breaks, as well as their implications for jet energetics. We find that no burst is included in our "Platinum" sample, in which the data fully satisfy the jet break criteria. By relaxing one or more of the requirements for a jet break, candidates to various degrees are identified. In the X-ray band, 42 of 103 well-sampled X-ray light curves have a decay slope ≲ 1.5 in the postbreak segment (the "Bronze" sample), and 27 of these also satisfy the closure relations of the forward-shock models ("Silver" sample). The numbers of "Bronze" and "Silver" candidates in the optical light curves are 27 and 23, respectively. The X-ray break time is earlier than that in the optical bands. Among 13 bursts having both optical and X-ray light curves, only seven have an achromatic break, and even in these cases, only in one band do the data satisfy the closure relations ("Gold" sample). These results raise concerns about interpreting the breaks as jet breaks and further inferring GRB energetics. Assuming that the "Silver" and "Gold" breaks are jet breaks, we derive jet opening angles (θ<inf>j</inf>) and kinetic energies (E<inf>K</inf>) or lower limits on them and find that the E<inf>K</inf> distribution is much more scattered than the pre-Swift sample, but a tentative anticorrelation between 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. The American Astronomical Society. All rights reserved.