Subphotospheric emission from short gamma-ray bursts: Protons mold the multi-messenger signals

TL;DR

This study uses advanced simulations to show that hadronic processes below the photosphere significantly influence the photon and neutrino signals in short gamma-ray bursts, revealing new insights into their multi-messenger signatures.

Contribution

First comprehensive simulation coupling relativistic MHD with particle transport to explore subphotospheric emission in sGRBs, highlighting the role of hadronic processes.

Findings

Photon spectra are significantly distorted from the Wien distribution.

Hadronic interactions below the photosphere produce detectable neutrino fluxes.

Non-thermal photon emission can originate from previously neglected subphotospheric hadronic processes.

Abstract

The origin of the observed Band-like photon spectrum in short gamma-ray bursts (sGRBs) is a long-standing mystery. We carry out the first general relativistic magnetohydrodynamic simulation of a sGRB jet with initial magnetization $\sigma_0 = 150$ in dynamical ejecta from a binary merger. From this simulation, we identify regions along the jet of efficient energy dissipation due to magnetic reconnection and collisionless sub-shocks. Taking into account electron and proton acceleration processes, we solve for the first time the coupled transport equations for photons, electrons, protons, neutrinos, and intermediate particles species up to close to the photosphere (i.e., up to $1 \times 10^{12}$ cm), accounting for all relevant radiative and cooling processes. We find that the subphotospheric multi-messenger signals carry strong signatures of the hadronic interactions and their resulting particle cascades. Importantly, the spectral energy distribution of photons is significantly distorted with respect to the Wien one, commonly assumed below the photosphere. Our findings suggest that the bulk of the non-thermal photon spectrum observed in sGRBs can stem from hadronic processes, occurring below the photosphere and previously neglected, with an accompanying energy flux of neutrinos peaking in the GeV energy range.