Characterizing low-latency sky localization in multi-detector gravitational-wave networks

Low-latency analyses of gravitational-wave (GW) data from LIGO, Virgo, and KAGRA enable rapid detection of compact binary coalescences (CBCs) and prompt sky localization, essential for electromagnetic follow-up in multi-messenger astronomy. We evaluate the performance and limitations of low-latency sky localization using the BAYESTAR algorithm, and investigate the impact of low-significance Virgo triggers. We injected simulated CBC signals into Gaussian-stationary noise and into Virgo data from the second part of the third LIGO-Virgo observing run (O3b), then reconstructed skymaps across multiple detector network configurations. Localization accuracy was assessed using percentile-percentile plots, the Jaccard index, and the Kullback-Leibler divergence. Skymap posteriors for binary neutron star mergers are statistically self-consistent, with deviations below 3σ, particularly when Virgo is included in the network, whereas skymaps for neutron star–black hole and binary black hole mergers tend to be overconfident. Adding a third detector generally improves accuracy, but the searched area can degrade when Virgo's signal-to-noise ratio (S/N) is low (łeq 5). For high-S/N events, relying on two detectors can mislocalize the source. Excluding Virgo can therefore cause the HL skymap to miss the true location when Virgo has a strong antenna response; in such cases a three-detector configuration is required to recover the correct position and avoid misleading multi-messenger follow-up. We introduce diagnostics to flag problematic skymaps and apply them to O3 public alerts, recovering simulation-predicted trends and flagging a few anomalous morphologies. The results are relevant for improving rapid vetting of GW alerts and guiding observational strategies in multi-messenger astronomy.