A Study of the Afterglows of Four GRBs: Constraining the Explosion and Fireball Model

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 ǫB. Good fits can be produced with magnetic energy accumulating or decaying with the shock strength over the afterglow as ǫB ∝ γ ; −2 ≤ x ≤ +1 . The data are not very sensitive to increasing density profiles, allowing good fits even with density ∝ r. 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 profiles may produce reasonable fits, but steeper profiles, even r, will not fit the data. Subject headings: gamma rays: bursts Division of Physics, Mathematics and Astronomy, 105-24, California Institute of Technology, Pasadena, CA 91125 Theoretical Astrophysics 130-33, California Institute of Technology, Pasadena, CA 91125 National Radio Astronomy Observatory, P.O. BOX ‘O’, Socorro, NM 87801