A small fraction of GRBs with available data down to soft X-rays (∼0.5 keV) have been shown to feature a spectral break in the low-energy part (∼1-10 keV) of their prompt emission spectrum. The overall spectral shape is consistent with optically thin synchrotron emission from a population of particles that have cooled on a timescale comparable to the dynamic time to energies that are still much higher than their rest mass energy (marginally fast cooling regime). We consider a hadronic scenario and investigate if the prompt emission of these GRBs can originate from relativistic protons that radiate synchrotron in the marginally fast cooling regime. Using semi-analytical methods, we derive the source parameters, such as magnetic field strength and proton luminosity, and calculate the high-energy neutrino emission expected in this scenario. We also investigate how the emission of secondary pairs produced by photopion interactions and γγ pair production affect the broadband photon spectrum. We support our findings with detailed numerical calculations. Strong modification of the photon spectrum below the break energy due to the synchrotron emission of secondary pairs is found, unless the bulk Lorentz factor is very large (Γ ≳ 103). Moreover, this scenario predicts unreasonably high Poynting luminosities because of the strong magnetic fields (106 − 107 G) that are necessary for the incomplete proton cooling. Our results strongly disfavor marginally fast cooling protons as an explanation of the low-energy spectral break in the prompt GRB spectra.

Hadronic supercriticalities are radiative instabilities that appear when large amounts of energy are stored in relativistic protons. When the proton energy density exceeds some critical value, a runaway process is initiated resulting in the explosive transfer of the proton energy into electron–positron pairs and radiation. The runaway also leads to an increase of the radiative efficiency, namely the ratio of the photon luminosity to the injected proton luminosity. We perform a comprehensive study of the parameter space by investigating the onset of hadronic supercriticalities for a wide range of source parameters (i.e. magnetic field strengths of 1 G−100 kG and radii of 1011−1016 cm) and maximum proton Lorentz factors (103−109). We show that supercriticalities are possible for the whole range of source parameters related to compact astrophysical sources, like gamma-ray bursts and cores and jets of active galactic nuclei. We also provide an in-depth look at the physical mechanisms of hadronic supercriticalities and show that magnetized relativistic plasmas are excellent examples of non-linear dynamical systems in high-energy astrophysics.

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Gamma-ray bursts (GRBs) are intense and short flashes of 𝛾-rays followed by the afterglow, which is a long lasting multi-wavelength emission. Very-high-energy (VHE, 𝐸 > 100 GeV) photons have been recently detected from a couple of GRBs during the afterglow. In this contribution, we numerically investigate the production of VHE photons in GRB afterglows in the context of a leptonic synchrotron self-Compton model. We study the impact of a mixed accelerated electron distribution that is composed of non-thermal (power law) and thermal (Maxwell-Jüttner) components on the gamma-ray afterglow spectra and light curves. When the thermal electron distribution carries most of the total particle energy, the light curves in specific energy ranges may deviate from the standard power-law decay, and the 𝛾-ray spectra may show multiple spectral components attributed to the synchro-Compton process of thermal and non-thermal electrons.

Relativistic hadronic plasmas can become, under certain conditions, supercritical, abruptly and efficiently releasing the energy stored in protons through photon outbursts. Past studies have tried to relate the features of such hadronic supercriticalities (HSC) to the phenomenology of Gamma-Ray Burst (GRB) prompt emission. In this work we investigate, for the first time, HSC in adiabatically expanding sources. We examine the conditions required to trigger HSC, study the role of expansion velocity, and discuss our results in relation to GRB prompt emission. We find multi-pulse light curves from slowly expanding regions (≲ 0.01c) that are a manifestation of the natural HSC quasi-periodicity, while single-pulse light curves with a fast rise and slow decay are found for higher velocities. The formation of the photon spectrum is governed by an in-source electromagnetic cascade. The peak photon energy is approximately $1 \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ MeV for maximum proton energies $(1-10) \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ PeV, while the peak γ-ray luminosities are in the range $(10^{49}-10^{52}) \cdot (\frac{\Gamma }{100})^4$ erg s−1. HSC bursts peaking in the MeV energy band are also copious neutrino emitters with peak energies $\sim 10 \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ TeV and an all-flavour neutrino fluence $\sim 10~{{\%}}$ of the γ-ray one. The hypothesis that long-duration GRBs are powered by HSC could be applied therefore only to the most luminous GRBs observed assuming Bulk Lorentz factors Γ ≤ 100.