GRB Polarization

Variation of the GRB polarization degree and the polarization angle has been observed in very few sources (Götz et al. 2009; Yonetoku et al. 2011; Zhang et al. 2019; Sharma et al. 2019; Chattopadhyay et al. 2022). Time-resolved polarization studies are challenging due to insufficient photon counts during a GRB outburst. However, such observations can be a valuable tool to understand the internal structure of the GRB jet and the nature of emission. Different theoretical models have put forth possible predictions of variation of the polarization angle, for example, the evolution of viewing angle cone resulting in observing different magnetic field geometries (Ghisellini & Lazzati 1999) or inherently patchy, non-axisymmetric emission due to internal fluid inhomogeneities (Lazzati & Begelman 2009). The strength of the observed polarization fraction and nature of the variation will help distinguish between such models (e.g. as reported in  Yonetoku et al. 2011; Sharma et al. 2019), which will provide valuable constraints on the nature of GRB outflow.

Daksha, with its pixelated CZT detectors with large collecting area will be able to measure polarization of hard X-rays in the prompt phase for GRBs having sufficient brightness. Figure 5 shows simulation results showing hard X-ray polarization measurement capabilities of Daksha. Our preliminary study finds that the MDP for Daksha will be 0.31 for a fluence of 10−4 erg cm−2, and we expect to measure polarization for nearly 5–8 GRBs per year with a fluence of more than 10−4 erg cm−2. In the lifetime of Daksha, it will obtain a homogeneous GRB polarization sample to constrain physical models of GRB prompt emission. The details of the polarization sensitivity of Daksha (including the measurement method) are published in Bala & Mate et al., 2023.