Pair-dominated GeV-optical flash in GRB 130427A

Abstract

We show that the light curve of the double GeV+optical flash in GRB 130427A is consistent with radiation from the blast wave in a wind-type medium with density parameter $A=\rho r^2\sim 5\times 10^{10}$ g cm${}^{-1}$. The peak of the flash is emitted by copious $e^{\pm }$ pairs created and heated in the blast wave; our first-principle calculation determines the pair-loading factor and temperature of the shocked plasma. Using detailed radiative transfer simulations we reconstruct the observed double flash. The optical flash is dominated by synchrotron emission from the thermal plasma behind the forward shock, and the GeV flash is produced via inverse Compton (IC) scattering by the same plasma. The seed photons for IC scattering are dominated by the prompt MeV radiation during the first tens of seconds, and by the optical to X-ray afterglow thereafter. IC cooling of the thermal plasma behind the forward shock reproduces all GeV data from a few seconds to $\sim 1$ day. We find that the blast wave Lorentz factor at the peak of the flash is $\Gamma \approx 200$, and the forward shock magnetization is ${\epsilon }_B\sim 2\times 10^{-4}$. An additional source is required by the data in the optical and X-ray bands at times $>10^2$ s; we speculate that this additional source may be a long-lived reverse shock in the explosion ejecta.

5 pages, 3 figures, accepted to ApJL

References in corpus (6)

• Fermi-LAT Observations of the Gamma-ray Burst GRB 130427A
• On the Mechanism of Gamma-Ray Burst Afterglows
• Can the early X-ray afterglow of GRBs be explained by a contribution from the reverse shock?
• The Bright Optical flash and Afterglow from the Gamma-Ray Burst GRB 130427A
• NuSTAR Observations of GRB130427A establish a single component synchrotron afterglow origin for the late optical to multi-GeV emission
• An external-shock model for GRB afterglow 130427A

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