Time-resolved GRB polarization with POLAR and GBM

Abstract

Context. Simultaneous γ-ray measurements of γ-ray burst spectra and polarization offer a unique way to determine the underlying emission mechanism(s) in these objects, as well as probing the particle acceleration mechanism(s) that lead to the observed γ-ray emission. Aims. We examine the jointly observed data from POLAR and Fermi-GBM of GRB 170114A to determine its spectral and polarization properties, and seek to understand the emission processes that generate these observations. We aim to develop an extensible and statistically sound framework for these types of measurements applicable to other instruments. Methods. We leveraged the existing 3ML analysis framework to develop a new analysis pipeline for simultaneously modeling the spectral and polarization data. We derived the proper Poisson likelihood for γ-ray polarization measurements in the presence of background. The developed framework is publicly available for similar measurements with other γ-ray polarimeters. The data are analyzed within a Bayesian probabilistic context and the spectral data from both instruments are simultaneously modeled with a physical, numerical synchrotron code. Results. The spectral modeling of the data is consistent with a synchrotron photon model as has been found in a majority of similarly analyzed single-pulse gamma-ray bursts. The polarization results reveal a slight trend of growing polarization in time reaching values of ∼30% at the temporal peak of the emission. We also observed that the polarization angle evolves with time throughout the emission. These results suggest a synchrotron origin of the emission but further observations of many GRBs are required to verify these evolutionary trends. Furthermore, we encourage the development of time-resolved polarization models for the prompt emission of gamma-ray bursts as the current models are not predictive enough to enable a full modeling of our current data.

Neutron star mergers and how to study them

“…Then, the on-set of afterglow emission occurs when the external shock develops as the jet interacts with the surrounding media. This figure is courtesy of Dan Kocevski (private communication) et al 2019; Burgess et al 2019). Continued advancement in these studies is a promising method to understand the prompt emission mechanism of GRBs.…”

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

“…Therefore, here we report an upper limit on polarisation fraction as 37% and 50% at 1.5σ (86%) and 2σ (90%) confidence level respectively, for one parameter of interest. The low polarisation fraction across the emission episode can be due to (i) varying polarisation angle as a function of time within the episode, despite high polarisation being high (Götz et al 2009;Zhang et al 2019;Burgess et al 2019b;Sharma et al 2019) or (ii) the observed emission within 1/Γ is inherently weakly polarised (Toma et al 2009, also see section 6.4).…”

“…Linear polarisation measurement of the prompt emission of GRBs is a powerful tool, in addition to spectral analysis, to strongly constrain the radiation process, geometry of the emitting region and the composition of the GRB outflow (Covino & Gotz 2016). Currently, polarisation measurement of prompt emission has been reported for only a handful of cases (Wigger et al 2004;Willis et al 2005;McGlynn et al 2007;Götz et al 2009;McGlynn et al 2009;Yonetoku et al 2011Yonetoku et al , 2012Götz et al 2013Götz et al , 2014Zhang et al 2019;Burgess et al 2019b;Chand et al 2018Chand et al , 2019Sharma et al 2019;Chattopadhyay et al 2019; for a recent review see McConnell 2017). Linear polarisation has also been measured in early optical afterglows of a few GRBs (Wijers et al 1999;Covino et al 2002;Greiner et al 2003;King et al 2014;Gorbovskoy et al 2016;Troja et al 2017;Laskar et al 2019).…”

“Double-tracking” Characteristics of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

“…After the launch of Fermi in 2008, with the spectral data of higher quality, the former two patterns are confirmed to be dominated: "hard-to-soft" for about two-thirds and "flux-tracking" for about one-third (e.g., Lu et al 2012;Yu et al 2018). The physical origin of these E p evolution patterns still remain unsolved, though some scenarios have been proposed in the literature (e.g., Liang et al 1997;Ryde & Svensson 1999;Medvedev 2006;Zhang 2011;Zhang & Yan 2011;Deng & Zhang 2014;Uhm & Zhang 2014Oganesyan et al 2018Oganesyan et al , 2019Uhm et al 2018;Burgess et al 2019).…”