Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B

TL;DR

This paper reports the detection of optical and gamma-ray emission from GRB 180720B, attributed to synchrotron and inverse-Compton processes in the reverse shock, revealing turbulence and magnetic field structures in the shock regions.

Contribution

It provides the first detailed observation of reverse shock emission in gamma rays and optical wavelengths, linking polarization changes to magnetic turbulence in GRB shocks.

Findings

Detection of optical and gamma-ray emission from the reverse shock.

Observation of polarization angle change indicating magnetic turbulence.

Evidence of low-magnetization ejecta in GRB 180720B.

Abstract

Gamma-ray bursts (GRBs) are the most electromagnetically luminous cosmic explosions. They are powered by collimated streams of plasma (jets) ejected by a newborn stellar-mass black hole or neutron star at relativistic velocities (near the speed of light). Their short-lived (typically tens of seconds) prompt $\gamma$-ray emission from within the ejecta is followed by long-lived multi-wavelength afterglow emission from the ultra-relativistic forward shock. This shock is driven into the circumburst medium by the GRB ejecta that are in turn decelerated by a mildly-relativistic reverse shock. Forward shock emission was recently detected up to teraelectronvolt-energy $\gamma$-rays, and such very-high-energy emission was also predicted from the reverse shock. Here we report the detection of optical and gigaelectronvolt-energy $\gamma$-ray emission from GRB 180720B during the first few hundred seconds, which is explained by synchrotron and inverse-Compton emission from the reverse shock propagating into the ejecta, implying a low-magnetization ejecta. Our optical measurements show a clear transition from the reverse shock to the forward shock driven into the circumburst medium, accompanied by a 90-degree change in the mean polarization angle and fluctuations in the polarization degree and angle. This indicates turbulence with large-scale toroidal and radially-stretched magnetic field structures in the reverse and forward shocks, respectively, which tightly couple to the physics of relativistic shocks and GRB jets -- launching, composition, dissipation and particle acceleration.