Gamma-ray bursts are the most energetic electromagnetic sources in the Universe. Their prompt gamma-ray radiation, lasting between a fraction of a second to several thousand seconds, corresponds to an energy release of 10 42 -10 47 J [1,2]. Fifty years after their discovery and several dedicated space-based instruments, the physical origin of this emission is still unknown. Synchrotron emission has been one of the early contenders 3,4 , but was criticized because spectral fits of empirical models (such as a smoothly-connected broken power law or a cut-off power law) suggest too hard a slope of the low-energy power law, violating the so-called synchrotron line-of-death 5,6 , reviving models of photospheric emission 7-9 . Fitting proper synchrotron spectra 10-12 (rather than heuristic functions) was first shown to work for individual GRBs 13,14 , though without tracking electron cooling. When the latter was taken into account, several GRB spectra could be fit successfully 10 . Here we show that idealized synchrotron emission, when properly incorporating time-dependent cooling of the electrons, is capable of fitting~95% of all time-resolved spectra of single-peaked GRBs as measured with Fermi/GBM. The comparison with spectral fit results based on previous empirical models demonstrates that the past exclusion of synchrotron radiation as an emission mechanism derived via the line-of-death was misleading. Our analysis probes the physics of these ultra-relativistic outflows and the microphysical processes which cause them to shine, and for the first time provides estimates of magnetic field strength and Lorentz factors of the emitting region directly from spectral fits. The parameter distributions that we find are grossly compatible with theoretical spectral 15-17 and outflow 18 predictions. The emission energetics remain challenging for all theoretical models. As synchrotron radiation alone can explain the observed emission, it is difficult to reconcile the time scales, efficiencies, and microphysics predicted by relativistic Fermi shock acceleration 19 and the fireball model 20 with the observations. Thus, our modeling of the Fermi/GBM observations provides evidence that GRBs are produced by moderately magnetized jets in which relativistic mini-jets emit optically-thin synchrotron radiation at large emission radii.We perform time-resolved spectral analysis of the prompt spectra of gamma-ray bursts (GRBs) detected with the Fermi Gamma-ray Burst Monitor (GBM). We select a subset of GRBs which exhibit a single contiguous, pulse-like structure which is justified by the assumption that the emission originates from a single dissipation episode 10 . In addition, we require that all GRBs have a measured redshift to ascertain their energetics. This results in a sample of 19 out of the 81 GRBs in the GBM time-resolved spectral catalog 21 . After background modeling, we employ the Bayesian blocks algorithm to create spectral datasets in optimised temporal bins for each of those GRBs, leading to a total of 162 bins, each with a 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.

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.

The localizations of gamma-ray bursts (GRBs) detected with the Gamma-ray Burst Monitor (GBM) onboard the Fermi satellite are known to be affected by significant systematic errors of 3-15 degrees. This is primarily due to mismatch of the employed Band function templates and the actual GRB spectrum. This problem can be avoided by simultaneously fitting for the location and the spectrum of a GRB, as demonstrated with an advanced localization code, BALROG (Burgess et al. 2018). Here, we analyze in a systematic way a sample of 105 bright GBM-detected GRBs for which accurate reference localizations are available from the Swift observatory. We show that the remaining systematic error can be reduced to ∼1-2 • .