Photospheric Prompt Emission From Long Gamma Ray Burst Simulations -- II. Spectropolarimetry

Abstract

Although Gamma Ray Bursts (GRBs) have been detected for many decades, the lack of knowledge regarding the radiation mechanism that produces the energetic flash of radiation, or prompt emission, from these events has prevented the full use of GRBs as probes of high energy astrophysical processes. While there are multiple models that attempt to describe the prompt emission, each model can be tuned to account for observed GRB characteristics in the gamma and X-ray energy bands. One energy range that has not been fully explored for the purpose of prompt emission model comparison is that of the optical band, especially with regards to polarization. Here, we use an improved MCRaT code to calculate the expected photospheric optical and gamma-ray polarization signatures ($Π_\mathrm{opt}$ and $Π_γ$, respectively) from a set of two relativistic hydrodynamic long GRB simulations, which emulate a constant and variable jet. We find that time resolved $Π_\mathrm{opt}$ can be large ($\sim 75\%$) while time-integrated $Π_\mathrm{opt}$ can be smaller due to integration over the asymmetries in the GRB jet where optical photons originate; $Π_γ$ follows a similar evolution as $Π_\mathrm{opt}$ with smaller polarization degrees. We also show that $Π_\mathrm{opt}$ and $Π_γ$ agree well with observations in each energy range. Additionally, we make predictions for the expected polarization of GRBs based on their location within the Yonetoku relationship. While improvements can be made to our analyses and predictions, they exhibit the insight that global radiative transfer simulations of GRB jets can provide with respect to current and future observations.