The Bright and the Slow—GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2018
- DOI:
- 10.3847/1538-4357/aad6ea
- arXiv:
- arXiv:1706.01481
- Bibcode:
- 2018ApJ...864..163V
- Keywords:
-
- acceleration of particles;
- gamma-ray burst: individual: GRB 100724B;
- GRB 160509A;
- methods: data analysis;
- relativistic processes;
- techniques: spectroscopic;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Submitted to ApJ