Polarization Signatures in GRBs

The MCRaT code had been improved by implementing the full Klein-Nishina scattering cross section and polarization through the Stokes parameters. These additions to the code allow us to simulate the time integrated and time resolved polarization expected from relativistic outflows.

We ran the MCRaT code on two different GRB jets, the 16TI simulation, which had a steady jet, and the 40sp_down simulation, which had a pulsed jet. We find that in both cases, the time integrated polarization increases with the observer viewing angle. At large viewing angles, however, the polarization decreases due to the fact that the photons are not fully decoupled from the flow.

Additionally, with MCRaT, we are able to produce time resolved polarization predictions expected from LGRBs which can help distinguish various models for GRB radiation mechanisms. This is especially important as future detectors are expected to have better polarization measurement capabilities compared to current GRB polarimeters.

The 16TI light curve and polarization at an oberver viewing angle of 7\(^\circ\). The polarization percentage of this GRB is very low due to the fact that it has a steady jet and very little angular structure.

We find that the time resolved polarization in the 16TI simulation is very small due to the lack of structure in the jet. On the other hand, the 40sp_down simulation has alot of spatial and temporal structure and we find that there are time periods in the light curve where the polarization gets as high as ~8%. Additionally, the polarization angle is observed to change as a function of time as different shells of material move into the observer’s line of sight.

To show the evolution of the polarization angle \(\chi\) we rebin the 40sp_down light curve into uniform 0.5 s bins. At the beginning of the light curve, there is a clear evolution in \(\chi\) from 90\(^\circ\) to 0\(^\circ\) to -90\(^\circ\) at the beginning of the light curve.

While these results are interesting, we still need to conduct large domain GRB jet simulations in order to ensure that photons at large viewing angles are decoupled from the flow. This will allow us to make better predictions of the expected polarization signatures seen from the photospheric model.