paper

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

arXiv:1706.01481 · doi:10.3847/1538-4357/aad6ea

Abstract

We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at energies but surprisingly faint at MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff at energies MeV for GRB~100724B and 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 (): (i) a Band spectrum with 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 , on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies 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 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.

Submitted to ApJ

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