The Environments of the Most Energetic Gamma-Ray Bursts

Abstract

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/nIR wavelengths. The evolution of LGRB afterglows depends on the density profile of the external medium, enabling us to separate wind or 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 ( B ). We show that 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 which 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.

The Late-time Afterglow Evolution of Long Gamma-Ray Bursts GRB 160625B and GRB 160509A

“…Thus lateral expansion at the speed of sound can still account for the observed late-time decline. Gompertz et al (2018) found a best fit of p = 2.06 ± 0.13 for GRB 160625B, which is consistent with our results in both bands within σ p 13 . In any case, for this burst some form of lateral expansion is required, and the edge effect alone is insufficient.…”

“…A complication was noted by Gompertz et al (2018), who find that using different synchrotron closure relations to estimate p (such as using the spectral index or the pre-or post-break decline) typically results in different estimates, with an intrinsic scatter on the value of p of 0.25 ± 0.04 (we will denote this as σ p ). They argue this is probably caused by emission from GRB afterglows not behaving exactly as the rather simplified analytical models predict 12 .…”

Closure Relations of Gamma-Ray Bursts in High Energy Emission

“…for this approach (e.g., Maxham et al 2011;Zhang et al 2011), in many studies, the temporal and spectral properties of the GeV extended emission are consistent with the external forward shock model (Kumar & Barniol Duran 2009Ghisellini et al 2010;Beniamini et al 2015;Panaitescu 2017;Gompertz et al 2018;Tak et al 2019).…”

Plasmas in Gamma-Ray Bursts: Particle Acceleration, Magnetic Fields, Radiative Processes and Environments

“…In later studies, [e.g., 332,333] ISM-like environments continued to be found in long GRB afterglows. Further measurements of the spectral and temporal indices for optical [334] and X-ray [71,[335][336][337] afterglows of long GRBs all point to a split in environment types between wind and ISM.…”