Gamma-ray bursts (GRBs) are  the most powerful explosions in the Universe. They are powered by ultra-relativistic jets (highly collimated outflows moving at ~99.995% of the speed of light). The huge isotropic-equivalent gamma-ray luminosities of Lɣ,iso~1051 – 1054 erg/s of their brightest prompt emission phase places GRBs as the most (electromagnetically) luminous transient events in nature. This makes them detectable out to the far reaches of the Universe, from down to barely a billion years after the Big Bang. The exact radiation mechanism that produces this prompt emission is still a mystery. The two most popular candidates are synchrotron emission from relativistic electrons with a power-law energy distribution and Compton scattering of thermal radiation by warm electrons. Both mechanisms can explain the majority of the observed prompt GRB spectra equally well, making it hard to distinguish between them. However, they can lead to very different linear polarization, depending on the magnetic field geometry (for synchrotron emission) and the jet’s angular structure (for both). Therefore, high-sensitivity linear polarization measurements can help distinguish between them and reveal the dominant radiation mechanism. Moreover, for synchrotron emission they can also teach us about the magnetic field structure in the emission region.

GRB Polarization: In this work, Dr. Ramandeep Gill (ARCO), Dr. Merlin Kole (University of Geneva) and Prof. Jonathan Granot (ARCO) provide a comprehensive review of the predictions for prompt GRB linear polarization from theoretical models and the current status of polarization measurements. The work gives a concise overview of the fundamental questions in GRB physics, namely what are the jet composition and dynamics, how and where is the energy dissipated, and what are the different candidate radiation mechanisms capable of producing the non-thermal prompt GRB spectrum. It presents relevant formulae and polarization predictions (both time-integrated and time-resolved) for different radiation mechanisms, magnetic field configurations (relevant for synchrotron emission), jet angular structures and dynamics. On the observational front, this work presents the basics of gamma-ray polarimetry and chronicles the GRB polarization measurements made by different detectors over the years. The figure shows example lightcurve (black) and time-resolved polarization (Π) curves for synchrotron emission from different magnetic field configurations in one realization of a uniform (lacking angular structure) jet seen at an angle from the symmetry axis of the jet (for a single pulse in the GRB prompt emission lightcurve).

The detailed polarization predictions presented in this work will enable observers to compare high-sensitivity measurements from upcoming gamma-ray polarimeters with theoretical models and constrain fundamental properties of the GRB relativistic jet and prompt emission mechanism.