Astronomy and Astrophysics Seminars

Constraining the GRB prompt emission mechanism, magnetic field geometry and jet structure with linear polarization

by Dr. Ramandeep Gill (George Washington University, USA)

Friday, February 28, 2020 from to (Asia/Kolkata)
at Lecture Theater ( AG-66 )
Description
The prompt emission mechanism in gamma-ray bursts (GRBs) remains unclear after many decades of work. The non-thermal band-like spectrum has been shown to be explained by a few emission models, including synchrotron emission, inverse-Compton scattering of quasi-thermal radiation released at the photosphere, and Compton drag. Measurements of linear polarization can break the degeneracy between these models and also provide key insights into the structure of relativistic outflows in GRBs. To date, high levels of prompt emission linear polarization above 50 percent with a 3-sigma detection significance have been measured by various instruments (e.g., POLAR, AstroSat). In regards, the afterglow emission, linear polarization of only a few percent have been measured. This provides stringent constraints on the post-shock magnetic field structure in relativistic shocks. The first part of the talk will present the theoretical predictions of linear polarization for the different emission mechanisms and for different jet angular structures. I will argue that a single robust measurement of linear polarization above 50 percent would strongly point towards the outflow having a large scale or globally ordered magnetic field and synchrotron emission as the underlying mechanism. On the basis of Monte Carlo simulations, I will further argue that such an emission mechanism will also be favored if most GRBs are linearly polarized at levels higher than 20 percent. The second part of the talk will look at the robust constraints on the structure of the post-shock magnetic field in relativistic shocks offered by the polarization upper limit of the radio afterglow of GW170817/GRB170817A. I will present new results that show that current theoretical models and particle-in-cell simulations don't yet capture the true structure of the post-shock field.