Probing Particle Acceleration through Broadband Early Afterglow Emission of MAGIC Gamma-Ray Burst GRB 190114C

TL;DR

This paper models the early afterglow emission of GRB 190114C to constrain particle acceleration timescales, electron fractions, and plasma parameters, providing insights into shock physics and non-thermal emission mechanisms.

Contribution

It introduces a numerical simulation approach to analyze early afterglow emission, constraining acceleration efficiency and electron fractions in relativistic shocks.

Findings

Electron acceleration timescale likely shorter than 20 gyroperiods.

Thermal synchrotron emission explains early optical emission with low non-thermal electron fraction.

X-ray data suggest high energy transfer efficiency from protons to electrons.

Abstract

Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index reported by {\it Fermi}. This gives an interesting constraint on the acceleration efficiency for Weibel-mediated shocks. We also constrain the number fraction of non-thermal electrons $f_{\rm e}$, and the temperature of the thermal electrons. The early optical emission can be explained by the thermal synchrotron emission with $f_{\rm e} \lesssim 0.01$. On the other hand, the X-ray light curves restrict efficient energy transfer from protons to the thermal electrons, and $f_{\rm e}\sim1$ is required if the energy fraction of the thermal electrons is larger than $\sim10$\%. The parameter constraints obtained in this work give important clues to probing plasma physics with relativistic shocks.