Gravitational-wave detectability range informed by external messengers

A rapid estimate of gravitational-wave (GW) detectability associated with astronomical transients is crucial for optimizing multi-messenger follow-up strategies and for constraining the physical origin of the transient itself. We introduce the targeted detectability range (TDR), which is designed to evaluate with minimal computational effort the detectability of compact binary coalescences under the hypothesis of an association with an external messenger, such as an electromagnetic or neutrino signal. Unlike the standard GW range, which is based on averaged source parameters, the TDR incorporates prior information from observations of the external messenger, including sky localization, inclination constraints, and physically motivated bounds on the component masses. We report the TDR of all short- and long-duration gamma-ray bursts that were observed during the first three observing runs of Advanced LIGO and Advanced Virgo. The method was validated through a systematic comparison with the 90$%$ exclusion distances provided by modeled targeted GW searches. In the absence of a coincident detection by all-sky all-time GW searches, the TDR provides a rapid and quantitative constraint on a possible merger origin of the astrophysical source. Its low-latency implementation and public availability enable a timely prioritization of follow-up observations and optimized allocation of observational resources, with a direct impact on the physical interpretation of astronomical transients.