Predicting GRB Spectro-polarimetric Signatures

The MCRaT code has been improved by implementing cyclo-synchrotron emission and absorption in addition to the prior advancements of using the full Klein-Nishina scattering cross section and the inclusion of polarization. This addition allows us to simulate the prompt emission from optical to gamma ray energies, thus allowing us to produce full spectro-polarimetric predictions to robustly test our knowledge of GRB prompt emission.

We ran the updated MCRaT code on two different GRB jets: the 16TI simulation, which has a steady jet, and the 40sp_down simulation, which has a pulsed jet. Using the results of these simulations, we used the ProcessMCRaT code to produce mock observations of the GRBs in two energy ranges:

1. Optical Wavelengths, corresponding to the Swift UVOT white filter
2. Gamma-ray Energies, coresponding to the energy range measured by the POLAR-2 polarimeter

We are able to make robust predictions that relate the spectral \(E_p\), measured \(L_\mathrm{iso}\), and time integrated (\Pi\) to one another for each energy range. This gives us a Yonetoku relation where we can also identify how the expected polarization of GRBs changes as a function of observer viewing angle.

The Yonetoku relation where the symbols represent different observer viewing angles and the line/marker outline color denotes the simulation type that was used to produce the datapoint. The fill color of the points represent either the time integrated gamma-ray polarization, in (a), or optical polarization, in (b).

We can also test how well correlated the various mock observables are with respect to one another. This test provides another way to test our model against spectro-polarimetric data.

Various spearman correlation coefficients, \(r_s\), as a function of observer viewing angle for the 16TI and 40sp_down simulations in red and green respectively. The shaded regions of red and green show the 95% confidence interval of the calculated \(r_s\).

All of the predictions that have been produced in our simulations will be able to be tested as polarimeters are becoming more precise. There are a number of polarimeters that are expected to be coming online such as POLAR-2 and LEAP which will both be able to precisely measure spectro-polarization quantities in GRBs.

This research is far from complete. There is alot of forward modeling that can be done to take the MCRaT mock observations and convolve them with detector responses in order to understand what these signals may look like in a given detector. As a result, this future work will increase the scientific output of future missions.