A new class of Fermi/LAT data has been recently introduced. It is characterized by an event selection looser than the standard classes, and it allows for the recovery of a large effective area for the study of transient sources such as Gamma-ray Bursts, especially between ~20 and 100 MeV. Using this new LAT Low-Energy class (LLE) we observe that the spectrum above the peak energy of the nuFnu spectrum (Epeak) is not well described by a power law, as instead commonly observed in GRBs.

A very good description of the data can be recovered by adding an exponential cutoff, with a cutoff energy of few tens of MeV. The presence of such spectral cutoffs has been hypothesized by previous works to explain the paucity of detections from Fermi/LAT. One of the two GRBs is very bright in the LLE data, which allows us to measure in the same energy range both the minimum variability time scale of the burst and the spectral cutoff. To our knowledge it is the first time that the variability can be studied at such high energies. Interpreting the cutoff as due to pair production, and using the minimum variability time scale, we provide an estimate for the bulk Lorentz factor of the jet responsible for the GRB.