Gamma-Ray Burst Jet Breaks Revisited

Abstract

Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow light curve allows a measurement of the jet opening angle and true energetics of GRBs. In this paper, we re-investigate this problem using a large sample of GRBs that have an optical jet break that is consistent with being achromatic in the X-ray band. Our sample includes 99 GRBs from 1997 February to 2015 March that have optical and, for Swift GRBs, X-ray light curves that are consistent with the jet break interpretation. Out of the 99 GRBs we have studied, 55 GRBs are found to have temporal and spectral behaviors both before and after the break, consistent with the theoretical predictions of the jet break models, respectively. These include 53 long/soft (Type II) and 2 short/hard (Type I) GRBs. Only 1 GRB is classified as the candidate of a jet break with energy injection. Another 41 and 3 GRBs are classified as the candidates with the lower and upper limits of the jet break time, respectively. Most jet breaks occur at 90 ks, with a typical opening angle θ j =(2.5 ± 1.0)°. This gives a typical beaming correction factor-f 1000for Type II GRBs, suggesting an even higher total GRB event rate density in the universe. Both isotropic and jetcorrected energies have a wide span in their distributions: log(E γ,iso /erg)=53.11 with σ=0.84; log(E K,iso /erg)=54.82 with σ=0.56; log(E γ /erg)=49.54 with σ=1.29; and log(E K /erg)=51.33 with σ=0.58. We also investigate several empirical correlations (Amati, Frail, Ghirlanda, and Liang-Zhang) previously discussed in the literature. We find that in general most of these relations are less tight than before. The existence of early jet breaks and hence small opening angle jets, which were detected in the Swfit era, is most likely the source of scatter. If one limits the sample to jet breaks later than 10 4 s, the Liang-Zhang relation remains tight and the Ghirlanda relation still exists. These relations are derived from Type II GRBs, and Type I GRBs usually deviate from them.

A statistical method to detect non-stationarities of gamma-ray burst jets

“…It is possible that currently available datasets (see e.g. Wang et al 2018) do not provide a sufficient number of such GRBs for a p < 0.03 detection of angular jet motions, even if the consequential V − θ c connection exists in nature. However also note, that if in the future, a solid connection between the gamma light curve variabilities (measured from prompt gamma light curves) and the characteristic jet angles (measured from optical afterglows) is found, this could directly provide a method for giving estimates (at least upper limits) on θ c values of GRBs solely from their prompt gamma light curves.…”

“…There is a well known standard method for deriving the characteristic angles of GRB jets, θ c , from jet break times of GRB afterglows (see e.g. Wang et al 2018 (Li & Paczyński 2006). Note that both groups of authors assumed that GRB jets are stationary, and that variabilities of GRB light curves are the result of the intrinsic time dependence of the gamma emission process.…”

A Comparison between Radio Loud and Quiet Gamma-Ray Bursts, and Evidence for a Potential Correlation between Intrinsic Duration and Redshift in the Radio Loud Population

“…From their sample, 21 overlapped our radio bright sample and 8 overlapped our radio quiet sample. We found the average jet opening angle in the radio loud and quiet sample using the Wang et al (2018) data is < θ j >= 2.6 deg, and < θ j >= 2.04 deg, respectively.…”

“…• Finally, we used data from Wang et al (2018), who examined a sample of GRBs that indicated jet breaks in their optical light curves, and that were consistent with an achromatic break in the X-ray light curve at the same time (the achromatic nature of the break is indicative of a physical jet, as opposed to spectral-energy-dependent evolution). From their sample, 21 overlapped our radio bright sample and 8 overlapped our radio quiet sample.…”

Exceptionally bright optical emission from a rare and distant gamma-ray burst

“…Furthermore, its true kinetic energy E kin,θ = 3.8 +10.2 −2.7 × 10 52 erg is consistent with the energy of the population of supernovae associated with long GRBs 64 . An opening angle of ∼ 1 deg is rarely observed in GRBs 65 and is found in less than 10% of studied populations 66 , which are, however, usually embedded in environments with densities orders of magnitude larger. Extremely narrow opening angles < 1.5 o and jets with steep structure k > 4 are expected in long GRBs due to the initial propagation of the jet into the progenitor material 67,68 .…”