Polarization From A Radially Stratified Off-Axis GRB Outflow

A. C. Caligula do E. S. Pedreira, N. Fraija, A. Galvan-Gamez, B. Betancourt Kamenetskaia, S. Dichiara, M. G. Dainotti, R. L. Becerra, P. Veres

Submitted on 22 November 2022


While the dominant radiation mechanism gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their evolution - presenting a feasible means of probing these bursts' energetic and angular properties. Although obtaining polarization data is difficult due to the inherent complexities regarding GRB observations, advances are being made, and theoretical modeling of synchrotron polarization is now more relevant than ever. In this manuscript, we present the polarization for a fiduciary model where the synchrotron forward-shock emission evolving in the radiative-adiabatic regime is described by a radially stratified off-axis outflow. This is parameterized with a power-law velocity distribution and decelerated in a constant-density and wind-like external environment. We apply this theoretical polarization model for selected bursts presenting evidence of off-axis afterglow emission, including the nearest orphan GRB candidates observed by the Neil Gehrels Swift Observatory and a few Gravitational Wave (GWs) events that could generate electromagnetic emission. In the case of GRB 170817A, we require the available polarimetric upper limits in radio wavelengths to constrain its magnetic field geometry.


Comment: In submission. 18 pages, 7 figures, 3 tables

Subject: Astrophysics - High Energy Astrophysical Phenomena


Polarization curves for our fiducial model, considering a constant-medium. The top row shows the perpendicular magnetic field configuration, the bottom row shows the parallel one. Each column represent a different pairing $\xi$ and $\epsilon$.