The effect of pair cascades on the high-energy spectral cut-off in gamma-ray bursts

Ramandeep Gill, Jonathan Granot

Research output: Contribution to journalArticlepeer-review


The highly luminous and variable prompt emission in gamma-ray bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum is still uncertain, making it hard to determine the outflow's bulk Lorentz factor Γ. GRBs with spectral cut-off due to pair production (γγ → e+e-) at energies Ec ≳ 10MeV are extremely useful for inferring Γ. We find that when the emission region has a high enough compactness, then as it becomes optically thick to scattering, Compton downscattering by non-relativistic e±-pairs can shift the spectral cut-off energy well below the self-annihilation threshold, Esa = Γ mec2/(1 + z). We treat this effect numerically and show that Γ obtained assuming Ec = Esa can underpredict its true value by as much as an order of magnitude.

Original languageEnglish
Pages (from-to)L1-L5
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
StatePublished - 21 Mar 2018

Bibliographical note

Publisher Copyright:
© 2017 The Author(s).


  • Gamma-ray burst: general
  • Opacity
  • Plasmas
  • Radiative transfer
  • Relativistic processes


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