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We report the discovery of GRB 221009A, the highest flux gamma-ray burst everobserved by the Fermi Gamma-ray Burst Monitor (GBM). This GRB has continuousprompt emission lasting more than 600 seconds which smoothly transitions toafterglow visible in the GBM energy range (8 keV--40 MeV), and total energeticshigher than any other burst in the GBM sample. By using a variety of new andexisting analysis techniques we probe the spectral and temporal evolution ofGRB 221009A. We find no emission prior to the GBM trigger time (t0; 2022October 9 at 13:16:59.99 UTC), indicating that this is the time of promptemission onset. The triggering pulse exhibits distinct spectral and temporalproperties suggestive of the thermal, photospheric emission of shock-breakout,with significant emission up to $\sim$15 MeV. We characterize the onset ofexternal shock at t0+600 s and find evidence of a plateau region in theearly-afterglow phase which transitions to a slope consistent with Swift-XRTafterglow measurements. We place the total energetics of GRB 221009A in contextwith the rest of the GBM sample and find that this GRB has the highest totalisotropic-equivalent energy($\textrm{E}_{\gamma,\textrm{iso}}=1.0\times10^{55}$ erg) and second highestisotropic-equivalent luminosity($\textrm{L}_{\gamma,\textrm{iso}}=9.9\times10^{53}$ erg/s) based on redshiftof z = 0.151. These extreme energetics are what allowed us to observe thecontinuously emitting central engine of GBM from the beginning of the promptemission phase through the onset of early afterglow.

Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow