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Relativistic simulations of long-lived reverse shocks in stratified ejecta: the origin of flares in GRB afterglows

Lamberts, A. and Daigne, F. (2018) Relativistic simulations of long-lived reverse shocks in stratified ejecta: the origin of flares in GRB afterglows. Monthly Notices of the Royal Astronomical Society, 474 (2). pp. 2813-2827. ISSN 0035-8711.

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The X-ray light curves of the early afterglow phase from gamma-ray bursts (GRBs) present a puzzling variability, including flares. The origin of these flares is still debated, and often associated with a late activity of the central engine. We discuss an alternative scenario where the central engine remains short-lived and flares are produced by the propagation of a long-lived reverse shock in a stratified ejecta. Here we focus on the hydrodynamics of the shock interactions. We perform one-dimensional ultrarelativistic hydrodynamic simulations with different initial internal structure in the GRB ejecta. We use them to extract bolometric light curves and compare with a previous study based on a simplified ballistic model. We find a good agreement between both approaches, with similar slopes and variability in the light curves, but identify several weaknesses in the ballistic model: the density is underestimated in the shocked regions, and more importantly, late shock reflections are not captured. With accurate dynamics provided by our hydrodynamic simulations, we confirm that internal shocks in the ejecta lead to the formation of dense shells. The interaction of the long-lived reverse shock with a dense shell then produces a fast and intense increase of the dissipated power. Assuming that the emission is due to the synchrotron radiation from shock-accelerated electrons, and that the external forward shock is radiatively inefficient, we find that this results in a bright flare in the X-ray light curve, with arrival times, shapes, and duration in agreement with the observed properties of X-ray flares in GRB afterglows.

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Lamberts, A.0000-0001-8740-0127
Additional Information:© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 November 14. Received 2017 November 14; in original form 2017 February 14. Published: 05 December 2017. The authors thank Zakaria Meliani for helpful advice on the set-up of the simulations and Robert Mochkovitch for his insight on this paper. The authors thank the anonymous referee for comments that greatly improved the discussion of the X-ray properties of the ejecta. The authors thank the French Programme National Hautes Energies (PNHE) and the Centre National d'Etudes Spatiales (CNES) for financial support. Support for AL was provided by an Alfred P. Sloan Research Fellowship, NASA ATP Grant NNX14AH35G, and NSF Collaborative Research Grant 1411920 and CAREER grant 1455342. Numerical simulations were run on supercomputer Pleiades from the NASA Supercomputing Division and supercomputer Stampede from the Texas Advanced Supercomputing Center.
Funding AgencyGrant Number
Programme National Hautes Energies (PNHE)UNSPECIFIED
Centre National d'Etudes Spatiales (CNES)UNSPECIFIED
Alfred P. Sloan FoundationUNSPECIFIED
Subject Keywords:hydrodynamics – radiation mechanisms: non-thermal – relativistic processes – shock waves – methods: numerical – gamma-ray burst: general
Issue or Number:2
Record Number:CaltechAUTHORS:20180301-103617147
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Official Citation:A Lamberts, F Daigne; Relativistic simulations of long-lived reverse shocks in stratified ejecta: the origin of flares in GRB afterglows, Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 2, 21 February 2018, Pages 2813–2827,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85038
Deposited By: Tony Diaz
Deposited On:01 Mar 2018 20:25
Last Modified:03 Oct 2019 19:25

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