2025ApJ...994...17D

On 2023 March 7 the Fermi Gamma-ray Burst Monitor observed the second-highest-fluence gamma-ray burst (GRB) ever, GRB 230307A. With a duration beyond 100 s, GRB 230307A contained a multitude of rapidly varying peaks, and was so bright it caused instrumental effects in the GBM detectors. The high fluence of this burst, (6.02 ± 0.02) × 10^–3 erg cm^–2, prompted rapid follow-up across the electromagnetic spectrum including the discovery of an associated kilonova. GRB 230307A is one of a few long GRBs with an associated compact merger origin. Three main temporal regions of interest are identified for fine time-resolution spectral analysis: triggering pulse, main emission, and late emission, and the parameter evolution is traced across these regions. The high flux of the burst allowed for the statistical preference of a more complex, physically motivated model, the Double Smoothly Broken Power Law, over typical spectral fitting functions for GRBs. From this model the evolution of the parameters was found to be in accordance with those expected for synchrotron radiation in the fast-cooling regime. Additionally, it was found that the flux experiences a steep decline in late time intervals, a feature which is often attributed to high-latitude emission, which follows the dissipation episodes. Furthermore, GRB 230307A was found to have one of the highest inferred bulk Lorentz factors of Γ = 1600. GRB 230307A is a noteworthy burst in terms of flux alone, but additionally provides a unique insight into the possible temporal and spectral characteristics of a new long merger class of GRBs.