2020NatAs...4..174B

Gamma-ray bursts (GRBs) are the most energetic electromagnetic sources in the Universe, releasing 1042-1047 J (refs. 1,2) in prompt gamma-ray radiation. Fifty years after their discovery, the physical origin of this emission is still unknown. Synchrotron emission has been an early contender3,4, but was criticized because spectral fits of empirical models suggest too hard a slope of the low-energy power law, violating the so-called synchrotron line-of-death5,6, and for its inefficient extraction of energy when the electrons are not fully cooled, reviving models of photospheric emission7-9. Fitting proper synchrotron spectra10 (rather than heuristic functions) and taking electron cooling into account was shown to work for several GRB spectra10-14. 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 observed by Fermi's Gamma-ray Burst Monitor. Thus, the past exclusion of synchrotron radiation as an emission mechanism derived via the line-of-death was misleading. Our analysis probes the microphysical processes operating within these ultra-relativistic outflows and provides estimates of magnetic field strengths and Lorentz factors of the emitting region directly from spectral fits. The resulting parameter distributions are largely compatible with theoretical spectral15-17 and outflow predictions18. The emission energetics implied by the observed, uncooled electrons remain challenging for all theoretical models.