Jet-breaks in the X-ray Light-Curves of Swift GRB Afterglows

In the set of 236 GRB afterglows observed by Swift between January 2005 and March 2007, we identify 30 X-ray light-curves whose power-law fall-off exhibit a steepening (”break”) at 0.1–10 day after trigger, to a decay steeper than t−1.5. For most of these afterglows, the X-ray spectral slope and the decay indices before and after the break can be accommodated by the standard jet model although a different origin of the breaks cannot be ruled out. In addition, there are 27 other afterglows whose X-ray light-curves may also exhibit a late break to a steep decay, but the evidence is not that compelling. The X-ray emissions of 38 afterglows decay slower than t−1.5 until after 3 day, half of them exhibiting such a slow decay until after 10 day. Therefore, the fraction of well-monitored Swift afterglows with potential jet-breaks is around 60 percent, whether we count only the strongest cases for each type or all of them. This fraction is comparable to the 75 percent of pre-Swift afterglows whose optical light-curves displayed similar breaks at ∼ 1 day. The properties of the prompt emission of Swift afterglows with light-curve breaks show the same correlations (peak energy of GRB spectrum with the burst isotropic output and with burst collimated output) as previously found for pre-Swift optical afterglows with light-curve breaks (the Amati and Ghirlanda relations, respectively). However, we find that Ghirlanda relation is largely a consequence of Amati’s and that the addition of a new observable (the jet-break time) leads to a stronger correlation only when the few outliers to the Amati relation are taken into account.