Evidence of Extended Emission in GRB 181123B and Other High-redshift Short GRBs

We study the high-energy properties of GRB 181123B, a short gamma-ray burst (sGRB) at redshift z ≈ 1.75. We show that, despite its nominal short duration with T90 < 2 s, this burst displays evidence of a temporally extended emission (EE) at high energies and that the same trend is observed in the majority of sGRBs at z ≳ 1. We discuss the impact of instrumental selection effects on the GRB classification, stressing that the measured T90 is not an unambiguous indicator of the burst physical origin. By examining their environment (e.g., stellar mass, star formation, offset distribution), we find that these high-z sGRBs share many properties of long GRBs at a similar distance and are consistent with a short-lived progenitor system. If produced by compact binary mergers, these sGRBs with EE may be easier to localize at large distances and herald a larger population of sGRBs in the early universe.