A detectable ultra-high-energy cosmic ray outburst from GRB 221009A

Gamma-ray bursts (GRBs) have been proposed as one of promising sources of ultra-high-energy cosmic rays (UHECRs), but observational evidence is still lacking. The nearby B.O.A.T. (brightest of all time) GRB 221009A, an once-in-1000-year event, is able to accelerate protons to ∼ 10^3 EeV. Protons arriving at the Milky Way are dominated by neutron-decay-induced protons. The inter-galactic magnetic fields would not yield a sizable delay of the ≥ 10 EeV cosmic rays if its strength is ≲ 10^-13 G, while Galactic magnetic fields would cause a significant time delay. We predict that, an UHECR burst from GRB 221009A would be detectable by the Pierre Auger Observatory and the TA×4, within ∼ 10 years. The detection of such an UHECR outburst will provide the direct evidence for UHECR acceleration in GRBs.