Multi-wavelength Emission of Gamma-ray Burst Prompt Phase. II. Spectral Polarimetry

TL;DR

This study compares polarization spectra predictions of photosphere and magnetic reconnection models for gamma-ray burst prompt emission, highlighting key differences in polarization degree and angle behaviors across energies.

Contribution

It introduces the first analysis of polarization angle rotation spectra and identifies distinctive polarization features to differentiate the models.

Findings

Magnetic reconnection model's polarization degree increases with frequency for on-axis observations.

Polarization angles vary systematically with energy in the reconnection model, unlike in the photosphere model.

PA rotation spectra decrease with increasing energy, especially in off-axis observations.

Abstract

Polarization spectra had been predicted within the photosphere model. For the purpose of seeking more clues to distinguish between the models, both the time-resolved and time-integrated polarization spectra from optical band to MeV gamma-rays of the magnetic reconnection model are studied here. There are two newly found differences between the two models. First, the time-integrated polarization degree (PD) of the magnetic reconnection model would in general increase with frequency for on-axis observations, while it is not monotonous for the photosphere model. Second, the variations of both the time-integrated and the time-resolved polarization angles (PAs) with frequency of the magnetic reconnection model is not random, while the time-integrated PA varies randomly with frequency for the photosphere model. Therefore, future energy-resolved polarization analysis could distinguish between the two models. In addition, the PA rotation spectra are studied for the first time. The rotation value of PA within the burst duration will decrease with the increase of the observational energy band. Most significant PA rotation would happen for slightly off-axis observations in each energy band. The PA would rotate even for on-axis observations in optical band. Compared with the aligned magnetic field case, the PA rotation is quite rare in the gamma-ray band for the case with a toroidal field in the radiation region.