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

Abstract

Here, 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 (τγγ): (i) a Band spectrum with τγγ 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 alsomore » at a comparable level from differences in variability time, luminosity, and high-energy photon index. Lastly, 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.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [1]; ORCiD logo [4]; ORCiD logo [5]
  1. Stanford Univ., CA (United States). Hansen Experimental Physics Lab
  2. The Open University of Israel, Raanana (Israel). Department of Natural Sciences; Ben-Gurion University, Beer-Sheva (Israel). Physics Department
  3. The Open University of Israel, Raanana (Israel). Department of Natural Sciences; George Washington Univ., Washington, DC (United States). Department of Physics
  4. Univ. Montpellier (France). Lab. Univers. et Particules de Monpellier
  5. Istituto Nazionale di Fisica Nucleare, Sezione di Trieste (Italy); Universit'a di Trieste (Italy). Dipartimento di Fisica
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1475407
Grant/Contract Number:  
AC02-76SF00515
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 864; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; acceleration of particles; gamma-ray burst: individual (GRB 100724B, GRB 160509A); methods: data analysis; relativistic processes; techniques: spectroscopic

Citation Formats

Vianello, G., Gill, R., Granot, J., Omodei, N., Cohen-Tanugi, J., and Longo, F. The Bright and the Slow—GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV. United States: N. p., 2018. Web. doi:10.3847/1538-4357/aad6ea.
Vianello, G., Gill, R., Granot, J., Omodei, N., Cohen-Tanugi, J., & Longo, F. The Bright and the Slow—GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV. United States. doi:10.3847/1538-4357/aad6ea.
Vianello, G., Gill, R., Granot, J., Omodei, N., Cohen-Tanugi, J., and Longo, F. Wed . "The Bright and the Slow—GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV". United States. doi:10.3847/1538-4357/aad6ea. https://www.osti.gov/servlets/purl/1475407.
@article{osti_1475407,
title = {The Bright and the Slow—GRBs 100724B and 160509A with High-energy Cutoffs at ≲100 MeV},
author = {Vianello, G. and Gill, R. and Granot, J. and Omodei, N. and Cohen-Tanugi, J. and Longo, F.},
abstractNote = {Here, 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 (τγγ): (i) a Band spectrum with τγγ 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. Lastly, 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.},
doi = {10.3847/1538-4357/aad6ea},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 864,
place = {United States},
year = {2018},
month = {9}
}

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