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...

Context. Despite over 50 years of research, many open questions remain about the origin and nature of gamma-ray bursts (GRBs). Linear polarization measurements of the prompt emission of these extreme phenomena have long been thought to be key to answering a range of these questions. The POLAR detector was designed to produce the first set of detailed and reliable linear polarization measurements in the 50 − 500 keV energy range. During late 2016 and early 2017, POLAR detected a total of 55 GRBs. The analysis results of 5 of these GRBs have been reported, and were found to be consistent with a low or unpolarized flux. However, previous reports by other collaborations found high levels of linear polarization, including some as high as 90%. Aims. We study the linear polarization for the 14 GRBs observed by POLAR for which statistically robust inferences are possible. Additionally, time-resolved polarization studies are performed on GRBs with sufficient apparent flux. Methods. A publicly available polarization analysis tool, developed within the Multi-Mission Maximum Likelihood framework (3ML), was used to produce statistically robust results. The method allows spectral and polarimetric data from POLAR to be combined with spectral data from the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Neil Gehrels Swift Observatory (hereafter Swift) to jointly model the spectral and polarimetric parameters. Results. The time-integrated analysis finds all results to be compatible with low or zero polarization with the caveat that, when timeresolved analysis is possible within individual pulses, we observe moderate linear polarization with a rapidly changing polarization angle. Therefore, time-integrated polarization results, while pointing to lower polarization, are potentially an artifact of summing over the changing polarization signal and thus washing out the true moderate polarization. We therefore caution against overinterpretation of any time-integrated results inferred herein and encourage the community to wait for more detailed polarization measurements from forthcoming missions such as POLAR-2 and LEAP.

Nanostructured Pt-on-Au electrocatalysts (coded as Pt m ∧Au, m being the atomic Pt/Au ratio), prepared by Pt deposition on Au colloids in two size ranges (Au-I, 10.0 ± 1.2 nm; Au-II, 3.0 ± 0.6 nm) (Zhao and Xu, Phys. Chem. Chem. Phys. 2006, 8, 5106), were employed for the electrooxidation of formic acid (HCOOH) at concentrations of 0.2−3.2 M by cyclic voltammetry. The HCOOH electrooxidation over a Pt shell fully covering Au-I colloids (Pt m ∧Au-I at m > 0.2, Pt dispersion < 20%) occurred mainly in the high potential range (0.6−1.0 V vs SCE). The lowering of m in Pt m ∧Au-I samples resulted in a remarkable increase in the current of HCOOH electrooxidation in the lower potential range (−0.2 to 0.6 V) due to a continued enhancement in the Pt utilization associated with the changes in the Pt-dispersion state. The areal activity (intrinsic activity) of Pt flecks (Pt dispersion > 50%) was 5 times and their mass-specific activity 25 times higher than those of the conventional Pt/C and core@shell structured Pt∧Au-I catalysts. The use of Pt m ∧Au-II for HCOOH electrooxidation produced qualitatively similar results, thus demonstrating a dramatic enhancement of the electrocatalytic activity of Pt m ∧Au/C by reducing the domain size of Pt deposits on Au surfaces. Moreover, the intrinsic activity of Pt flecks in Pt m ∧Au-II was found to be 4 times higher than those in Pt m ∧Au-I, which uncovers that smaller Au particles can serve as a kind of “activity promoter” to their carrying Pt flecks. The current of HCOOH electrooxidation over the Pt m ∧Au catalysts also varied significantly, according to the HCOOH concentration. The highest current was obtained only in an appropriate HCOOH concentration window for a given Pt∧Au/C catalyst.