Probing the Efficiency of Electron-Proton Coupling in Relativistic Collisionless Shocks through the Radio Polarimetry of Gamma-Ray Burst Afterglows

Understand

The late-time optical/radio afterglows of $\gamma$-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources.

• However, the fraction $f$ of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind.
• If $f<1$, the true explosion energies of GRBs are $f^{-1}$ times larger than those commonly estimated with $f=1$.
• Thus the value of $f$ gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks.