Gamma-ray bursts as cool synchrotron sources

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 contender^3,4, 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-death^5,6, and for its inefficient extraction of energy when the electrons are not fully cooled, reviving models of photospheric emission^7–9. Fitting proper synchrotron spectra¹⁰ (rather than heuristic functions) and taking electron cooling into account was shown to work for several GRB spectra^10–14. 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 spectral^15–17 and outflow predictions¹⁸. The emission energetics implied by the observed, uncooled electrons remain challenging for all theoretical models.