Multi-Messenger Modeling of Low-Luminosity Gamma-Ray Bursts

TL;DR

This work develops a time-averaged leptohadronic model for low-luminosity GRBs and fits seven LL GRBs using multiwavelength prompt-emission data and AMES, employing MCMC to infer posterior distributions of energy partitions, notably the cosmic-ray loading factor $ξ_p$. The analysis reveals diverse physical regimes across events, with $ξ_p$ spanning $0.2$–$1.6$ and SBO-like bursts showing the strongest potential for neutrino production. PCA clustering indicates sub-populations corresponding to IC-dominated, cascade-dominated, and synchrotron-dominated emission, highlighting physical diversity within LL GRBs. Neutrino fluence predictions remain below current IceCube limits but become testable with next-generation detectors and stacking, underscoring LL GRBs as a meaningful, if subdominant, component of the high-energy neutrino sky and motivating real-time multi-messenger searches and future MeV to GeV facilities.

Abstract

Low-luminosity gamma-ray bursts (LL GRBs), a subclass of the most powerful transients in the Universe, remain promising sources of high-energy astrophysical neutrinos, despite strong IceCube constraints on typical long GRBs. In this work, a novel approach is introduced to study a sample of seven LL~GRBs with their multi-wavelength observations to investigate leptohadronic processes during their prompt emission phases. The relative energy densities in magnetic fields, non-thermal electrons, and protons are constrained, with the latter defining the cosmic-ray (CR) loading factor. Our results suggest that LL~GRBs exhibit diverse emission processes, as confirmed by a machine-learning analysis of the fitted parameters. Across the seven LL~GRBs, we find the posterior medians of the CR loading factor in the range of $ξ_p \sim 0.2$--$1.6$. GRB~060218 and GRB~100316D, the lowest-luminosity bursts ($L_{γ, \rm iso} \sim 10^{46}$-$10^{47}\rm~erg~s^{-1}$) consistent with the shock-breakout (SBO) scenario, yield the highest CR loading factor and therefore are expected to produce neutrinos more efficiently. Our model predicts the expected number of neutrino signals that are consistent with current limits but would be detectable with next-generation neutrino observatories. These results strengthen the case for LL~GRBs as promising sources of high-energy astrophysical neutrinos and motivate real-time searches for coincident LL~GRB and neutrino events. Next-generation X-ray and MeV facilities will be critical for identifying more LL~GRBs and strengthening their role in multi-messenger astrophysics.

Figures (11)

• Figure 1: Schematic of massive star deaths. Left: Choked jet model with orphan neutrino emission and no gamma rays. Middle: Shock breakout model for LL GRBs, producing neutrinos and photons. Right: Emerging jet scenario for typical GRBs and LL GRBs, with neutrino emission accompanied by photons.
• Figure 2: Individual fits of GRB 171205A and GRB 050826. Fermi-LAT upper limits are shown as red arrows, and Swift-BAT data are shown in black. The total predicted photon spectra at the posterior median parameter values are shown as solid pink lines with 1 $\sigma$ uncertainty bands, and contributions from hadronic processes are indicated by dashed purple and brown lines. Model predicted neutrino spectra at the posterior median parameter values are shown as solid blue lines, with shaded regions representing 1 $\sigma$ uncertainties. Timescales are calculated using posterior median parameter values listed in Table \ref{['tab:best_fit_params']}.
• Figure 3: Same as Fig. \ref{['fig:17_05']}, but for GRB 190829A and GRB 201015A. The predicted photon and neutrino spectra, as well as physical timescales, are calculated using the posterior median parameter values listed in Table \ref{['tab:best_fit_params']}.
• Figure 4: Same as Fig. \ref{['fig:17_05']}, but for GRB 060218, GRB 100316D, and GRB 120422A.
• Figure 5: PCA projection onto the first two principal components of the seven LL GRBs based on the 68% confidence region of their post-burn-in MCMC samples for seven fitted parameters. Each colored cloud represents the 1$\sigma$ distribution of a GRB's fitted parameters, with individual points corresponding to sampled parameter sets and the posterior median parameter values marked by black circled spots.