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 regard to polarization. Here, we use an improved Monte Carlo radiation transfer code to calculate the expected photospheric optical and gamma-ray polarization signatures (Πoptand Π
A complete understanding of gamma-ray bursts (GRBs) has been difficult to achieve, due to our incomplete knowledge of the radiation mechanism that is responsible for producing the prompt emission. This emission, which is detected in the first tens of seconds of the GRB, is typically dominated by hard X-ray and gamma-ray photons, although there have also been a few dozen prompt optical detections. These optical detections have the potential to discriminate between plausible prompt emission models, such as the photospheric and synchrotron shock models. In this work, we use an improved MCRaT code, which includes cyclo-synchrotron emission and absorption, to conduct radiative transfer calculations from optical to gamma-ray energies under the photospheric model. The calculations are conducted using a set of two-dimensional relativistic hydrodynamic long GRB jet simulations, consisting of a constant and a variable jet. We predict the correlations between the optical and gamma-ray light curves as functions of observer angle and jet variability, and find that there should be extremely dim optical prompt precursors for large viewing angles. Additionally, the detected optical emission originates from dense regions of the outflow, such as shock interfaces and the jet-cocoon interface. Our results also show that the photospheric model is unable to account for the current set of optical prompt detections that have been made and therefore additional radiative mechanisms are needed to explain these prompt optical observations. These findings show the importance of conducting global radiative transfer simulations using hydrodynamically calculated jet structures.
more » « less- Award ID(s):
- 1907955
- NSF-PAR ID:
- 10485950
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 922
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 257
- Size(s):
- ["Article No. 257"]
- Sponsoring Org:
- National Science Foundation
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Abstract γ , respectively) from a set of two relativistic hydrodynamic long GRB simulations, which emulate a constant and variable jet. We find that time-resolved Πoptcan be large (∼75%) while time-integrated Πoptcan be smaller due to integration over the asymmetries in the GRB jet where optical photons originate; Πγ follows a similar evolution as Πoptwith smaller polarization degrees. We also show that Πoptand Πγ 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. -
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