Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

¹ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
e-mail: gabriele.ghisellini@inaf.it
² Gran Sasso Science Institute, Viale F. Crispi 7, 67100 L’Aquila, AQ, Italy
³ INFN – Laboratori Nazionali del Gran Sasso, 67100 L’Aquila, AQ, Italy
⁴ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, 64100 Teramo, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ INFN – Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy
⁷ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁸ SISSA, Via Bonomea 265, 34136 Trieste, Italy
⁹ Université degli Studi di Milano-Bicocca, Dip. di Fisica “G. Occhialini”, Piazza della Scienza 3, 20126 Milano, Italy

Received: 3 December 2019
Accepted: 21 January 2020

Abstract

We discuss the new surprising observational results that indicate quite convincingly that the prompt emission of gamma-ray bursts (GRBs) is due to synchrotron radiation produced by a particle distribution that has a low-energy cut-off. The evidence of this is provided by the low-energy part of the spectrum of the prompt emission, which shows the characteristic F_ν ∝ ν^1/3 shape followed by F_ν ∝ ν^−1/2 up to the peak frequency. This implies that although the emitting particles are in fast cooling, they do not cool completely. This poses a severe challenge to the basic ideas about how and where the emission is produced, because the incomplete cooling requires a small value of the magnetic field to limit synchrotron cooling, and a large emitting region to limit the self-Compton cooling, even considering Klein–Nishina scattering effects. Some new and fundamental ingredient is required for understanding the GRBs prompt emission. We propose proton–synchrotron as a promising mechanism to solve the incomplete cooling puzzle.