A Study of the Afterglows of Four Gamma‐Ray Bursts: Constraining the Explosion and Fireball Model

Abstract

We employ a fireball model of the gamma-ray burst explosion to constrain intrinsic and environmental parameters of four events with good broadband afterglow data; GRB 970508, GRB 980329, GRB 980703, and GRB 000926. Using the standard assumptions of constant circumburst density and no evolution of the fraction of the explosion energy in the post-shock magnetic field, we investigate the uniformity of the derived explosion and shock physics parameters among these events. We find a variety of parameters: densities that range from those of the ISM to diffuse clouds, energies comparable to the total GRB γ-ray energy, collimations from near-isotropy to 0.04 radians, substantial electron energy fractions of 10-30% with energy distribution indices of 2.1-2.9 and magnetic energy fractions from 0.2-25%. We also investigate the level to which the data constrain the standard model assumptions, such as the magnetic field evolution, and the allowed density profiles of the medium. Fits generally improve slightly with an increasing magnetic energy fraction ǫ B . Good fits can be produced with magnetic energy accumulating or decaying with the shock strength over the afterglow as ǫ B ∝ γ x ; −2 ≤ x ≤ +1 . The data are not very sensitive to increasing density profiles, allowing good fits even with density ∝ r 10 . Some parameter values change by up to an order of magnitude under such altered assumptions; the parameters of even good fits cannot be taken at face value. The data are sensitive to decreasing densities; r −2 profiles may produce reasonable fits, but steeper profiles, even r −2.5 , will not fit the data.

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.…”

GRB 091127: The cooling break race on magnetic fuel

“…There is a growing number of studies which have modelled broad-band GRB light curves, and these have yielded results for B which span several orders of magnitude between different GRBs, with values from ∼10 −5 to ∼10 −1 (Panaitescu & Kumar 2001, 2002Yost et al 2003), raising questions whether the assumption of B being constant in the simplest fireball model is consistent with the observations. Lately, the idea of B increasing in time as a power-law has been discussed and is receiving increasing support from observational data (e.g., Panaitescu et al 2006;Kong et al 2010).…”

On the nature of X-ray flashes

“…The possibility of non-constant equipartition parameters has been considered by Chevalier (2003) and used in afterglow modelling by Yost et al (2003) who assumed that α B varies with the shock Lorentz factor. For the prompt phase, have shown that the condition 2x + y < 1 (which therefore excludes the standard values x = 1/2 and y = 5/2) is often required to obtain good fits of the temporal and spectral evolution of GRB pulses.…”