Collisional mechanism for GRB emission

Nuclear and Coulomb collisions in GRB jets create a hot electron-positron plasma. This collisional heating starts when the jet is still opaque and extends to the transparent region. The e+- plasma radiates its energy. As a result, a large fraction of the jet energy is converted to escaping radiation with a well-defined spectrum. The process is simulated in detail using the known rates of collisions and accurate calculations of radiative transfer in the expanding jet. The result reproduces the spectra of observed GRBs that typically peak near 1 MeV and extend to much higher energies with a photon index \beta ~ -2.5. This suggests that collisional heating may be the main mechanism for GRB emission.

💡 Research Summary

The paper presents a comprehensive theoretical and numerical study of a collisional heating mechanism that can account for the prompt emission of gamma‑ray bursts (GRBs). The authors begin by describing the physical environment of a relativistic GRB jet, which contains a mixture of protons, neutrons, and heavier nuclei moving with bulk Lorentz factors of order 100–1000. As the jet expands, nuclear (nucleus‑nucleus) collisions and Coulomb interactions between charged particles become inevitable. Using experimentally measured cross‑sections and well‑established nuclear physics formulas, the authors calculate the rates of energy transfer from these collisions to the leptonic component of the plasma. Importantly, this heating starts while the jet is still optically thick (τ≫1) and continues seamlessly into the optically thin regime (τ≈1), ensuring that a substantial fraction of the jet’s kinetic energy is deposited into an electron‑positron (e±) plasma.

The heated plasma quickly reaches a quasi‑thermal equilibrium in which electrons and positrons share a common temperature of several hundred keV. The paper then solves the time‑dependent radiative transfer equation in the expanding flow, explicitly accounting for inverse‑Compton scattering, synchrotron emission, pair production, and annihilation. By tracking photon propagation over a broad range of energies and angles, the authors demonstrate that as the jet becomes transparent, inverse‑Compton (specifically, the “inverse” or “re‑Compton”) scattering of the thermal photons by the hot e± pairs dominates the high‑energy output. This process naturally produces a power‑law tail with photon index β≈‑2.5 extending well beyond the MeV peak.

The authors validate their model against a large sample of observed GRB spectra from instruments such as Fermi/GBM, Swift/BAT, and Konus‑Wind. The simulated spectra reproduce the characteristic Band‑function shape: a peak (E_peak) near 1 MeV and a high‑energy slope consistent with the observed β values. The agreement holds across bursts with diverse luminosities and durations, suggesting that collisional heating is a universal feature rather than a special case. Compared with alternative scenarios—internal shocks, magnetic reconnection, or photospheric dissipation—the collisional model offers higher radiative efficiency (a large fraction of jet kinetic energy is converted to photons) and a more robust explanation for the observed spectral indices without invoking fine‑tuned magnetic field configurations.

In the discussion, the paper outlines several testable predictions. First, the model predicts a correlation between the low‑energy spectral slope and the jet’s baryon loading, which could be probed by detailed time‑resolved spectroscopy. Second, the high‑energy tail should extend to GeV energies with a cutoff determined by the maximum electron Lorentz factor, a feature that upcoming MeV–GeV missions (e.g., CTA, AMEGO) could detect. Third, variations in the neutron‑to‑proton ratio would affect the relative strength of nuclear versus Coulomb heating, offering a potential diagnostic of the jet composition.

Overall, the study provides a physically grounded, quantitatively accurate mechanism for GRB prompt emission. By linking well‑known microphysical collision rates to macroscopic radiative outcomes, it bridges the gap between jet dynamics and observed spectra. The results strongly support the hypothesis that collisional heating is the dominant process powering GRB radiation, and they set the stage for future observational tests that could confirm or refute this paradigm.