Gompertz, Benjamin P., Fruchter, A. S. and Pe’er, A. (2018) The environments of the most energetic Gamma-Ray bursts. The Astrophysical Journal, 866 (2). 162. doi:10.3847/1538-4357/aadba8

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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/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 (epsilon B ). We show that epsilon B must be $\ll {10}^{-2}$ 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.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science, Engineering and Medicine > Science > Physics
Library of Congress Subject Headings (LCSH):	Gamma ray bursts, Red shift, Afterglow (Physics)
Journal or Publication Title:	The Astrophysical Journal
Publisher:	Institute of Physics Publishing, Inc.
ISSN:	0004-637X
Official Date:	24 October 2018
Dates:	Date Event 24 October 2018 Published 16 August 2018 Accepted
Volume:	866
Number:	2
Article Number:	162
DOI:	10.3847/1538-4357/aadba8
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
RIOXX Funder/Project Grant:	Project/Grant ID RIOXX Funder Name Funder ID 725246 H2020 European Research Council http://dx.doi.org/10.13039/100010663 618499 Seventh Framework Programme http://dx.doi.org/10.13039/100011102 UNSPECIFIED University of Leicester http://dx.doi.org/10.13039/501100000738

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