Blasts from the Past: UHECR Excesses from a Population of GRB 21009A-like Transients

The most prominent feature in the ultra-high-energy cosmic ray (UHECR) sky is a significant excess of events from the vicinity of Centaurus A (Cen A). In this paper we investigate whether the excess may be caused by a past GRB 221009A-like event rather than Cen A itself. If an extraordinary gamma-ray burst (GRB) went off in one of the dwarf galaxies within the Cen A/M83 group, intergalactic magnetic deflections would naturally spread out and delay its cosmic ray signal, making it visible for about 2 times 10 5 years. Scaling this up, we model a local population of these exceptionally bright, narrow-jet transients within the Greisen--Zatsepin--Kuzmin (GZK) horizon. For a local volumetric rate of n 0 4 times10-8 Mpc-3,yr-1 and a jet half-opening angle of 0.8, we estimate 3-7 UHECR excesses across the sky at any given time. Varying the jet opening angle, GZK volume, and visibility time results in a broader range of excess values. It is possible to test this hypothesis by comparing the population predictions against existing and upcoming UHECR anisotropy data obtained by the Pierre Auger Observatory and the Telescope Array.