A Detectable Ultra-high-energy Cosmic-Ray Outburst from GRB 221009A

Gamma-ray bursts (GRBs) have been proposed as one of the promising sources of ultra-high-energy cosmic rays (UHECRs), but observational evidence is still lacking. The nearby brightest of all time GRB 221009A, a once in 1000 yr event, is able to accelerate protons to similar to 103 EeV, and then generate extremly energetic neutrons via the photomeson production interaction. Protons arriving at the Milky Way are dominated by neutron-decay-induced protons. The intergalactic magnetic fields would not yield a sizable delay of the >= 10 EeV cosmic rays if its strength is less than or similar to 10-13 G, while Galactic magnetic fields would cause a significant time delay. We predict that a UHECR burst from GRB 221009A would be detectable by the Pierre Auger Observatory and the TAx4, within similar to 10 yr. The detection of such a UHECR outburst will provide the direct evidence for UHECR acceleration in GRBs.