The Bright and the Slow - GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV

We analyze the prompt emission of GRB 100724B and GRB 160509A, 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 at energies E _c ∼ 20-60 MeV for GRB 100724B and E _c ∼ 80-150 MeV for GRB 160509A. 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 (τ _γγ): (i) a Band spectrum with τ _γγ from the internal-shocks-motivated model of Granot et al. (2008) and (ii) the photospheric model of Gill & Thompson (2014). 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 an estimate of bulk Lorentz factors in the range Γ ∼ 100-400, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies E _c compared to other Fermi/LAT GRBs arise not only predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, 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.