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.
|Journal||Monthly Notices of the Royal Astronomical Society: Letters|
|State||Published - 21 Mar 2018|
Bibliographical notePublisher Copyright:
© 2017 The Author(s).
- Gamma-ray burst: general
- Radiative transfer
- Relativistic processes