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At redshift z = 4.618, GRB 220101A is the most distant gamma-ray burst (GRB)detected by Fermi/LAT to date. It is also a very energetic event, with anequivalent isotropic energy of $3.6\times10^{54}$ erg. We jointly analyzed theFermi/GBM and LAT observations of GRB 220101A with two independent approachesand found a significant spectral break at sub-100 MeV energies during theprompt emission. The fast variability of the emission suggests that thisspectral attenuation is caused by internal opacity to pair creation. Regardlessof the nature of the emission processes assumed in the spectral analysis, weinfer a moderate value for the jet Lorentz factor, $\Gamma\sim110$, and findthat all of the high-energy emission was produced above and near thephotosphere, at a distance of $\sim10^{14}$ cm from the central engine. Wecompare these results with the four other LAT-detected GRBs with similarproperties.

Fermi Constraints on the Ejecta Speed and Prompt Emission Region of the Distant GRB 220101A