Properties of Relativistic Jets in Gamma‐Ray Burst Afterglows

Abstract

We extend our calculation of physical parameters of gamma-ray burst (GRB) jets by modeling the broadband emission of the afterglows 970508, 980519, 991208, 000926, 000418, and 010222. Together with 990123, 990510, 991216, and 000301c, there are 10 well-observed afterglows for which the initial opening angle of the GRB jet can be constrained. The jet energies (after the GRB phase) obtained for this set of afterglows are within one decade around 5 Â 10 50 ergs. With the exception of 000418, which requires a jet wider than 1 2 rad, the jet initial half-angle in the other cases ranges from 2 to 20 . We find that in half of the cases, a homogeneous ambient medium accommodates the afterglow emission better than the windlike r À2 profile medium expected around massive stars. The two types of media give fits of comparable quality in four cases, with a wind medium providing a better description only for 970508. The circumburst densities we obtain are in the 0.1-100 cm À3 range, with the exception of 990123, for which it is below 10 À2 cm À3 . If in all 10 cases the observed GRB durations are a good measure of the ejecta deceleration timescale, then the parameters obtained here lead to jet Lorentz factors at the deceleration radius between 70 and 300, anticorrelated with the jet initial aperture, and jet masses around 10 À6 M . Our results on the jet energy, opening Lorentz factor, and evacuation of material until breakout provide constraints on theoretical models of GRB jets.

A VLA Study of High-redshift GRBs. I. Multiwavelength Observations and Modeling of GRB 140311A

“…These values are similar to those obtained for GRBs at z∼1 (Panaitescu & Kumar 2002;Yost et al 2003;Chandra et al 2008;Cenko et al 2010Cenko et al , 2011Laskar et al 2014), indicating no evolution in these properties with a redshift to z∼5.…”

Hubble Space Telescopeand Ground‐based Optical and Ultraviolet Observations of GRB 010222

“…Several authors have interpreted the optical afterglow in terms of the standard relativistic blast wave model (Sari, Piran, & Narayan 1998). In this case, the break results from a collimated (jetted) outflow (Sari, Piran, & Halpern 1999;Rhoads 1999); the synchrotron cooling frequency is below the optical passband, and the index p of the electron energy distribution is unusually hard, p $ 1:4 (Cowsik et al 2001;Sagar et al 2001;Stanek et al 2001;Panaitescu & Kumar 2002). Other authors prefer to interpret the break as a dynamical transition to nonrelativistic expansion (in 't Zand et al 2001;Masetti et al 2001), requiring a very energetic burst ($10 54 ergs) expanding into a dense ($10 6 cm À3 ) circumburst medium.…”

“…Following previous efforts (Cowsik et al 2001;Sagar et al 2001;Stanek et al 2001;Panaitescu & Kumar 2002), we chose to fit the data in terms of the standard afterglow model (e.g., Sari et al 1998Sari et al , 1999). We do not favor an interpretation in which the break is due to a transition of the blast wave to the nonrelativistic phase (in 't Zand et al 2001;Masetti et al 2001).…”

Multi-wavelength observations of the GRB 080319B afterglow and the modeling constraints

“…The overall afterglow behavior of long-duration GRBs, including the aforementioned examples, are explained in terms of the "collapse of very massive stars"i, i.e., so-called "collapsars" as the most favored progenitor (MacFadyen & Woosley 1999). By now, the majority of long-duration GRB afterglows have been explained in terms of constant ambient density i.e., inter stellar medium (ISM, ρ ∝ r 0 ) models (Sari, et al 1998;Wijers & Galama 1999;Panaitescu & Kumar 2002), although a Stellar Wind Medium (WM, ρ ∝ r −2 ) profile (Chevalier & Lee 2000a,b;Li & Chevalier 2001) is the natural result of massivestar environments (Zhang 2007), where ρ and r are the ambient density and the distance from the center of the progenitor star, respectively. The value of ambient density is constrained by the parameters "number density" n and the "wind parameter" A * , respectively for the ISM and WM models.…”