Detailed polarization measurements of the prompt emission of five gamma-ray bursts

Abstract

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution [1, 2, 3] or merging of compact objects [4]. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe [4]; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets [5]. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see [1], however many such measurements suffered from substantial uncertainties, mostly systematic [7, and references therein]. After the first successful measurements by the GAP and COSI instruments [2, 3, 4], here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.POLAR is a dedicated Gamma-ray Burst (GRB) polarization detection experiment onboard China's Tiangong-2 spacelab [11], launched on Sept. 15th, 2016 and stopped operation on March 31, 2017. POLAR detected 55 GRBs with high significance. In order to make statistically significant GRB polarization measurements and yet with negligible systematic errors, we select a subsample of five GRBs for detailed analysis of their polarization properties; please refer to the supplementary information (SI) for the sample selection criteria and the properties of the five selected GRBs. We employ a straight forward χ 2 based analysis, similar to that successfully employed in [4], to study the polarization properties of the five GRBs, while a Bayesian method is employed to accurately determine the credible regions of the measurements. The studies rely on extensive ground and in-orbit calibration data and Monte-Carlo simulations matching the calibration data [12, 13]. Please refer to the methods section for details of the methodology and analysis.In Figure 1, we show the measured modulation curves of the five GRBs integrated over the whole GRB duration, together with the best fitting simulated modulation curves from linear polarization and fitting residuals. All fittings are statistically acceptable with no significant systematic deviations. In Figure 2, we show the 2-D posterior distributions of the five GRBs, i.e., the posterior probability as functions of both polarization angle (PA) and degree (PD). Clearly the measured P...

Time-resolved GRB polarization with POLAR and GBM

“…We find the BALROG derived location much more precise than that of the standard location analysis (see Fig. 1), allowing us to reduce the systematic errors included in the polarization results presented in Zhang et al (2019). Additionally, it has now been shown that the BALROG locations are systematically more accurate (Berlato et al 2019).…”

“…Using this location, spectral and polarization responses were generated for all data types. We note that a standard GBM position 2 exists and, along with their uncertainties, was used for the polarization results presented in Zhang et al (2019), however, the standard localization technique has known systematics and now possess arbitrarily inflated error distributions (Connaughton et al 2015). We find the BALROG derived location much more precise than that of the standard location analysis (see Fig.…”

“…The detection of an evolution in polarization angle within this single pulse GRB could explain the low polarization degrees found for all five GRBs. The results presented in Zhang et al (2019) do not, however allow for a detailed time-resolved study of the remaining four GRBs, nor do they allow determination of the nature of the evolution of the polarization angle in GRB 170114A.…”

“…The POLAR experiment (Produit et al 2018) on board the Chinese space laboratory Tiangong-2 observed 55 GRBs and reported polarization measurements for five of these GRBs (Zhang et al 2019). Time-integrated analysis of these GRBs resulted in strict upper limits on the polarization degrees.…”

Observation data pre-processing and scientific data products generation of POLAR

“…This pipeline can work automatically without manual intervention and will send an email to notify the researchers that new scientific data products have been generated. Up to now, with scientific data products generated by the pipeline of the current version, the in-orbit instrument performance study and calibration have been finished and are presented in Li et al 2018, and the first scientific results of POLAR have been produced and published in Zhang et al 2019 recently. This paper will give a detailed introduction of this pipeline and the main algorithms that are important and specially needed by POLAR.…”