GRB Polarization: A Unique Probe of GRB Physics

Abstract

Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emissions are highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field.

Hard X-Ray Polarization Catalog for a Five-year Sample of Gamma-Ray Bursts Using AstroSat CZT Imager

“…Thus, spectral analysis alone leads to ambiguity in selecting the best fit model. One way forward is to have more constraining observables such as polarization, which can help to break this degeneracy (Toma et al 2009;Covino & Gotz 2016;McConnell 2017;Gill et al 2020Gill et al , 2021. This is possible because different radiation models predict different ranges of polarization.…”

“…In the last two decades, several hard X-ray spectrographs as well as dedicated X-ray polarimeters have contributed to polarization measurements of the GRB prompt phase. A summary of the GRB polarization measurements can be found in McConnell (2017), Chattopadhyay (2021), and Gill et al (2021). However, statistically significant polarization measurements are available only for a handful of GRBs, and the obtained values exhibit a wide range.…”

Polarization in Early Optical Afterglows of Gamma-Ray Bursts Driven by Precessing Jets

“…The multiwavelength afterglow that is usually detected after the short-lived prompt γ-ray emission originates from the synchrotron radiation of electrons accelerated by forward and reverse shocks (FS and RS, respectively), produced when the jets interact with the circumburst medium (e.g., Zhang et al 2006, and references therein). In this context, the polarimetry of GRB afterglows has become an important topic, as it can provide crucial information on the properties of the magnetic fields embedded in the jets (for a review, see Gill et al 2021). In recent years, a faster response of the GRB detection technology has allowed early-time polarization measurements of several afterglows (e.g., Gorbovskoy et al 2016;Mao et al 2018;Laskar et al 2019;Buckley et al 2021).…”

Polarization in early optical afterglows of gamma-ray bursts driven by precessing jets

“…The multiwavelength afterglow that is usually detected after the short-lived prompt γ-ray emission originates from the synchrotron radiation of electrons accelerated by forward and reverse shocks (FS and RS, respectively), produced when the jets interact with the circumburst medium (e.g., Zhang et al 2006, and references therein). In this context, the polarimetry of GRB afterglows has become an important topic as it can provide crucial information on the properties of the magnetic fields embedded in the jets (for a review, see Gill et al 2021). In recent years, a faster response of the GRB detection technology has allowed early-time polarization measurements of several afterglows (e.g., Gorbovskoy et al 2016;Laskar et al 2019;Buckley et al 2021).…”