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

TL;DR

This study analyzes Swift GRB X-ray afterglow light-curves, identifying jet-breaks in about 60% of cases, and compares these findings with pre-Swift optical afterglow data, revealing consistent correlations and some new insights into jet models.

Contribution

It provides the first comprehensive analysis of jet-breaks in Swift X-ray afterglows, confirming their prevalence and relation to prompt emission properties, and compares these with pre-Swift optical observations.

Findings

Approximately 60% of Swift afterglows show potential jet-breaks.

Swift and pre-Swift afterglows exhibit similar break fractions and correlations.

The Ghirlanda relation is largely derived from the Amati relation, with stronger correlations when outliers are included.

Abstract

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 use of the jet-break time leads to a stronger Ghirlanda correlation only when the few outliers to the Amati relation are included.