Hybrid Strategy for Coordinated Interstellar Signaling: Linking the Galactic Center and Extragalactic Bursts

Abstract The search for extraterrestrial intelligence (SETI) is largely limited by the vastness of the signaling parameter space. The concurrent signaling scheme offers a framework in which civilizations can coordinate their transmission and reception by referring to a common astrophysical event. Building on this idea, I propose a hybrid strategy that combines the Galactic center as a spatial reference with an extragalactic burst as a temporal marker. If such a scheme is indeed employed, the sky area to be surveyed in SETI could be reduced by more than 2 orders of magnitude, based solely on existing astronomical data. I examine records of three types of extragalactic bursts (supernovae, neutron star mergers, and gamma-ray bursts, GRBs) to identify suitable temporal markers. Among them, GRB 221009A is particularly notable due to its high fluence and favorable sky location.