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We analyze the prompt emission of GRB 100724B and GRB 160509A, two of thebrightest Gamma-Ray Bursts (GRBs) observed by Fermi at $\lesssim{\rm MeV}$energies but surprisingly faint at $\gtrsim100\;$MeV energies. Time-resolvedspectroscopy reveals a sharp high-energy cutoff at energies $E_c\sim20-60\;$MeVfor GRB~100724B and $E_c\sim80-150\;$MeV for GRB~160509A. We first characterizephenomenologically the cutoff and its time evolution. We then fit the data totwo models where the high-energy cutoff arises from intrinsic opacity to pairproduction within the source ($\tau_{\gamma\gamma}$): (i) a Band spectrum with$\tau_{\gamma\gamma}$ from the internal-shocks motivated model of Granot et al.(2008), and (ii) the photospheric model of Gill & Thompson (2014). Alternativeexplanations for the cutoff, such as an intrinsic cutoff in the emittingelectron energy distribution, appear to be less natural. Both models provide agood fit to the data with very reasonable physical parameters, providing adirect estimate of bulk Lorentz factors in the range $\Gamma\sim 100-400$, onthe lower end of what is generally observed in Fermi GRBs. Surprisingly, theirlower cutoff energies $E_c$ compared to other Fermi-LAT GRBs arise notpredominantly from the lower Lorentz factors, but also at a comparable levelfrom differences in variability time, luminosity, and high-energy photon index.Finally, particularly low $E_c$ values may prevent detection by Fermi-LAT, thusintroducing a bias in the Fermi-LAT GRB sample against GRBs with low Lorentzfactors or variability times.

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