GRB Polarization

Polarization from GRB emission can be an important tool to probe the physics of emission mechanisms in GRBs, the geometry of the emission region, and the origin and nature of the magnetic field at the emission region (Covino & Gotz 2016; Lazzati et al. 2004; Waxman 2003). Till date polarization has been detected in only a handful of sources (Gill et al. 2020; Chattopadhyay et al. 2022). There is significant debate in the literature regarding the source of the polarization e.g. synchrotron with ordered magnetic fields, synchrotron with random magnetic fields at shocks, and inverse Compton interactions (Beloborodov 2011; Toma et al. 2009; Covino & Gotz 2016; Gill et al. 2020). Different theoretical models predict varying degrees of maximum polarization, based on the magnetic field, geometry and the viewing angle (Ghisellini & Lazzati 1999; Lazzati et al. 2004; Gill et al. 2020). If the polarization is due to synchrotron processes, as is often conjectured (Burgess et al. 2020), then a high polarization fraction would imply ordered magnetic fields within the jet structure. A disordered magnetic field structure that many theories propose may arise at the forward shock (Medvedev & Loeb 1999) would lower the polarization. Contemporary work on GRB spectra indicate the prompt emission resulting from synchrotron radiation(non-thermal) (Burgess et al. 2019; Troja et al. 2017). However, spectral modeling of photospheric emission(thermal) has also provided adequate fits to a subset of GRBs (Vianello et al. 2018). A combined study of the GRB spectrum and polarization will break the degeneracy between various such theoretical models.

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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.

An artist's impression of a Gamma Ray Burst jet over time, and the small patches of magnetic fields present, as revealed by new research. Credit: Dr Kitty Yeung