Prompt Emission of High Energy Photons from Gamma Ray Bursts

Abstract

Within the internal shock scenario we consider different mechanisms of high energy ($>1$ MeV) photon production inside a Gamma Ray Burst (GRB) fireball and derive the expected high energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high energy emission is dominated by the leptonic component if fraction of shock energy carried by electrons is not very small (e.g. ${\epsilon }_e>10^{-3}$). For very small values of ${\epsilon }_e$ the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime. However, in this case a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For a canonical fireball bulk Lorentz factor (e.g. $\Gamma =400$), emissions above $\sim 10$ GeV are attenuated by two-photon pair production processes. For a fireball with an even higher Lorentz factor, the cutoff energy is higher, and emissions of 10 TeV - PeV due to ${\pi }^0$-decay can also escape from the internal shocks. The flux level is however too low to be detected by current TeV detectors, and these photons also suffer attenuation by external soft photons.

18 pages 10figures, version to be published in MNRAS

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• GRB Radiative Efficiencies Derived from the Swift Data: GRBs vs. XRFs, Long vs. Short
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs
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Cited by in corpus (7)

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• Interpretation and implication of the non-detection of GeV spectrum excess by Fermi gamma-ray Space Telescope in most GRBs
• Prompt TeV Emission from Cosmic Rays Accelerated by Gamma Ray Bursts Interacting with Surrounding Stellar Wind
• Testing Lorentz Violation Using Propagating UHECRs
• A Search for Prompt Very High Energy Emission from Satellite-detected Gamma-ray Bursts using Milagro