The bright and the slow - GRBs 100724B and 160509A with high-energy cutoffs at < 100 MeV

We analyze the prompt emission of two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at MeV energies but surprisingly faint at > 100 MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source. Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing a direct estimate of bulk Lorentz factors on the lower end of what is generally observed in Fermi GRBs. Particularly low E_c values may prevent detection by Fermi- LAT, thus introducing a bias in the Fermi- LAT GRB sample against GRBs with low Lorentz factors or variability times.