2018ApJ...866..162G

We analyze the properties of a sample of long gamma-ray bursts (LGRBs) detected by the Fermi satellite that have a spectroscopic redshift and good follow-up coverage at both X-ray and optical/near infrared wavelengths. The evolution of LGRB afterglows depends on the density profile of the external medium, enabling us to separate wind or interstellar medium (ISM)-like environments based on the observations. We do this by identifying the environment that provides the best agreement between estimates of p, the index of the underlying power-law distribution of electron energies, as determined by the behavior of the afterglow in different spectral/temporal regimes. At 11 rest-frame hours after trigger, we find a roughly even split between ISM-like and wind-like environments. We further find a 2σ separation in the prompt emission energy distributions of wind-like and ISM-like bursts. We investigate the underlying physical parameters of the shock, and calculate the (degenerate) product of density and magnetic field energy (ε_B). We show that ε_B must be ≪10^–2 to avoid implied densities comparable to the intergalactic medium. Finally, we find that the most precisely constrained observations disagree on p by more than would be expected based on observational errors alone. This suggests additional sources of error that are not incorporated in the standard afterglow theory. For the first time, we provide a measurement of this intrinsic error that can be represented as an error in the estimate of p of magnitude 0.25 ± 0.04. When this error is included in the fits, the number of LGRBs with an identified environment drops substantially, but the equal division between the two types remains.