Testing Narrow-jet Gamma-Ray Bursts as Sources of Ultrahigh-Energy Cosmic Rays

Gamma-ray bursts (GRBs) have long been considered candidate sources of ultrahigh-energy cosmic rays (UHECRs) due to their large energy release and relativistic outflows. The detection of multi-TeV $γ$-rays from GRB~221009A and its rarity have renewed interest in this connection and motivate considering an additional narrow-jet long GRB population in the local Universe. We investigate whether such a local narrow-jet population can contribute to the observed diffuse UHECR energy spectrum. We also examine the associated cosmogenic neutrino and cascade $γ$-ray emissions to assess the multimessenger viability of this scenario. We fit the observed UHECR spectrum and mass composition data using three source-evolution models: a uniform comoving source emissivity, a standard-jet GRB population tracing the star formation rate (SFR), and a standard + narrow-jet GRB population tracing SFR. We propagate a mixed-composition UHECR injection and calculate the cosmogenic neutrino and cascade $γ$-ray fluxes. The standard + narrow jet model fits the observed UHECR spectrum and composition, with the highest-energy flux dominated by the narrow-jet population confined to $z\le z_{\max,*}\simeq0.36$. This low-redshift dominance lowers the cosmogenic neutrino flux compared to the standard-jet GRB population. For the narrow-jet GRB population, the fit implies a baryon loading factor $ξ_{\rm CR}^{\rm nj}\simeq10$. Such a locally enhanced long-GRB population can therefore explain the highest-energy UHECR flux without violating current multimessenger constraints. Future UHE searches can further probe this scenario through the associated cosmogenic fluxes.