**1. Introduction**

Gamma-ray bursts (GRBs) are one of the most energetic, and electromagnetically the brightest, transient phenomena in the Universe. They are the ideal test beds for understanding nature at its extreme that involves an explosive release of energy over a short timescale, producing a burst of *γ*-rays with an isotropic-equivalent luminosity of *<sup>L</sup>γ*,iso <sup>∼</sup> <sup>10</sup><sup>51</sup> <sup>−</sup> <sup>10</sup><sup>54</sup> erg s−<sup>1</sup> . It is now well established that most GRBs are cosmological sources and that they are powered by ultrarelativistic (with bulk Lorentz factors Γ & 100) bipolar beamed outflows driven by a central engine–a compact object. The identity of the central engine, which could be either a black hole (BH) or a millisecond magnetar, is not entirely clear as the highly variable emission is produced far away from it at a radial distance of *<sup>R</sup>* <sup>∼</sup> <sup>10</sup><sup>12</sup> <sup>−</sup> <sup>10</sup>16cm. The most luminous phase of the burst, referred to as the "prompt" phase is short lived with a bimodal duration distribution, where the short GRBs have typical durations of *<sup>t</sup>*GRB <sup>∼</sup> <sup>10</sup>−<sup>1</sup> s and the long GRBs typically last for *t*GRB ∼ 30 s while the dividing line sits at *t* ∼ 2 s [1]. These two classes of GRBs are also distinct spectrally, with the short GRBs being spectrally harder as compared to the long GRBs that produce softer *γ*-rays. Other clues, e.g., the association of long-soft GRBs with star-forming regions [2] and type-Ib/c supernovae [3–5] and that of the short-hard GRBs with early type galaxies [6,7] lead to the identification of two distinct progenitors. The

**Citation:** Gill, R.; Kole, M.; Granot, J. GRB Polarization: A Unique Probe of GRB Physics. *Galaxies* **2021**, *9*, 82. https://doi.org/10.3390/ galaxies9040082

Academic Editors: Elena Moretti and Francesco Longo

Received: 28 August 2021 Accepted: 19 October 2021 Published: 27 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

long-soft GRBs are associated with the core-collapse of massive (& (20 − 30)*M*) Wolf– Rayet stars [8], whereas the short-hard GRBs were theorized to originate in compact object mergers, namely, that of two neutron stars (NSs) or a NS-BH pair [9,10]. The unequivocal proof of the latter association had to wait until the gravitational wave (GW) detectors, LIGO and Virgo, became operational, which led to the coincident detection of GWs from the merger of two NSs and a short-hard GRB by *Fermi*-GBM and the INTEGRAL-ACS from GW 170817/GRB 170817A [11,12].

Although the global picture is fairly clear, the details of the energy dissipation process, the exact radiation mechanism, and the transfer of radiation in the highly dynamical flow remain poorly understood. All of these different processes combine to produce a nonthermal spectrum that is often well described by the Band function [13], an empirical fit to the spectrum featuring a smoothly broken power law. In *νF<sup>ν</sup>* space, which indicates the observed energy flux around the frequency *ν* with *F<sup>ν</sup>* being the spectral flux density, this break manifests as a peak at the mean photon energy h*E*bri ' 250 keV, which also represents the energy at which most of the energy of the burst is released, and the asymptotic powerlaw photon indices below and above the break energy have mean values of h*α*Bandi ' −1 and h*β*Bandi ' −2.3, respectively [14,15]. After decades of spectral modeling of the prompt emission, the basic questions of GRB physics remain unanswered, and it is becoming challenging to advance our understanding with spectral modeling alone.

An exciting opportunity was presented by the claimed detection of high levels of linear polarization, with Π = 80% ± 20%, in GRB 021206 [16]. Although this result had a detection significance of 5.7*σ*, further scrutiny by other works [17,18] cast irrevocable doubts and ultimately refuted the final result. Nevertheless, this one result initiated a vigorous theoretical effort to understand the polarization of prompt GRB emission with the expectation that highly sensitive measurements will be able to resolve many of the outstanding questions of GRB physics. Over the past several years, the number of prompt GRB polarization measurements (in some cases time-resolved) have grown; however, the main results remain inconclusive due to inherent difficulties in obtaining highly statistically significant measurements. Therefore, it is hoped that the next generation of *γ*-ray polarimeters that will be launched in this decade will provide further important clues.

The main objectives of this review were to provide a concise yet comprehensive overview of the current status of theoretical developments as well as observations in the field of prompt GRB polarization and also to highlight the need for developing more sensitive instruments and better analysis tools, which are hoped to yield statistically significant measurements in the coming decade. Many points presented here have also been covered in earlier reviews on the topic e.g., [19–24]. This review begins with a summary of the fundamental questions in GRB physics (Section 2) that can be addressed with measurements of linear polarization along with insights gained from prompt GRB spectral modeling. These include the outflow composition and dynamics (Section 2.1), energy dissipation mechanisms (Section 2.2), radiation mechanisms (Section 2.3), and the angular structure of the outflow (Section 2.4). An overview of *γ*-ray polarimetry is presented in Section 3, which includes the fundamental principles of *γ*-ray polarization measurement (Section 3.1) and a summary of the different detectors that have been used for GRB polarimetry (Section 3.3). The theory of GRB polarization is presented next in Section 4, which covers several topics, such as polarization from uniform (Section 4.1) and structured (Section 4.2) jets with different radiation mechanisms, temporal evolution of polarization (Section 4.3), polarization arising from multiple overlapping pulses (Section 4.4), the most likely polarization for a given radiation mechanism (Section 4.5), and the energy dependence of polarization (Section 4.6). The current status of prompt GRB polarization measurements is presented next, which includes time-integrated (Section 5.1), time-resolved (Section 5.2), and energyresolved (Section 5.3) measurements. The importance of polarization measurements from the other phases of the burst, namely, X-ray flares, reverse-shock emission (optical flash and radio flare), and forward-shock emission, which also probe the properties of the GRB

outflow, is emphasized in Section 6. Finally, Section 7 touches upon the outlook for this decade, which will see the launch of more sensitive instruments (Section 7.1). The predicted performance of some is compared in Section 7.2. This review concludes by offering some suggestions for improvements in the polarization data analysis (Section 7.3) and its theoretical modeling (Section 7.4).
