Gamma rays from a reverse shock with turbulent magnetic fields 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.

5 pages, 4 figures (main) plus Methods and Supplementary Methods, accepted for publication

References in corpus (18)

• Teraelectronvolt emission from the $\gamma$-ray burst GRB 190114C
• A very-high-energy component deep in the Gamma-ray Burst afterglow
• A decade of Gamma-Ray Bursts observed by $Fermi$-LAT: The 2${}^{nd}$ GRB catalog
• Observation of inverse Compton emission from a long $\gamma$-ray burst
• Klein-Nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum
• A comprehensive analysis of Swift/XRT data: I. Apparent spectral evolution of GRB X-ray tails
• Magnetic field evolution in relativistic unmagnetized collisionless shocks
• GRB 060418 and 060607A: the medium surrounding the progenitor and the weak reverse shock emission
• Production of magnetic energy by macroscopic turbulence in GRB afterglows
• Constraining the magnetic field structure in collisionless relativistic shocks with a radio afterglow polarization upper limit in GW170817
• Analysis and modelling of the multi-wavelength observations of the luminous GRB 190114C
• External Inverse-Compton and Proton Synchrotron Emission from the Reverse Shock as the Origin of VHE Gamma-Rays from the Hyper-Bright GRB 221009A
• Shock Corrugation by Rayleigh-Taylor Instability in GRB Afterglow Jets
• Temporal Evolution of the Gamma-ray Burst Afterglow Spectrum for an Observer: GeV--TeV Synchrotron Self-Compton Light Curve
• Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A
• Analytic Modeling of Synchrotron-Self-Compton Spectra: Application to GRB 190114C
• A Cloud-based architecture for the Cherenkov Telescope Array observation simulations. Optimisation, design, and results
• Particle In Cell Simulations of Mildly Relativistic Outflows in Kilonova Emissions

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• Afterglow Linear Polarization Signatures from Steep GRB Jets: Implications for Orphan Afterglows
• New Insights on Gamma-Ray Burst Radiation Mechanisms from Multiwavelength Observations
• The most distant optically polarised GRB afterglow: GRB 240419A at z = 5.178
• Synchrotron self-Compton Reverse Shock in Energy-Injection and Radiative Scenarios
• The Double-Burst Nature and Early Afterglow Evolution of Long GRB 110801A