Toward fast targeted gravitational-wave followup search for compact binary coalescences using GstLAL inspiral pipeline

We present a fast, targeted follow-up search for compact binary coalescences that folds EM sky localization into GstLAL's likelihood via priors on inter-detector time and phase (Δt, Δϕ). In practice, this lets us rerank candidates without refiltering, with minimal code changes and compute, while also modestly boosting near-threshold sensitivity. On O3 data with BNS injections at the GW170817 location, the targeted prior delivers a 5–10% gain in sensitive space-time volume at a fixed false-alarm rate versus an all-sky search, recovering more sub-threshold signals. We also report reduced computational costs for rerank-only DAGs and map performance to prior-width choices. We will also present results for targeted tests on injections across various sky locations on real O3/O4 short-GRB triggers. Taken together, these results point to a compute-light route to better low/mid-latency multimessenger searches; our goal is to operationalize this as an automated mid-latency follow-up during O5.