The Bright and the Slow -- GRBs 100724B & 160509A with high-energy cutoffs at $lesssim100;$MeV

We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at $lesssim{rm MeV}$ energies but surprisingly faint at $gtrsim100;$MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff at energies $E_csim20-60;$MeV for GRB~100724B and $E_csim80-150;$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 ($tau_{gammagamma}$): (i) a Band spectrum with $tau_{gammagamma}$ 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 a direct estimate of bulk Lorentz factors in the range $Gammasim 100-400$, 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 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.