Eccentricity enables the earliest warning and localization of gravitational waves with ground-based detectors

The early and precise localization of gravitational waves (GWs) is pivotal in detecting their electromagnetic (EM) counterparts, especially for binary neutron stars (BNS) and neutron star -black hole binaries (NSBH). In this letter, we pioneer the exploration of utilizing the higher harmonic modes induced by the eccentricity of compact binaries to localize GWs with ground -based detectors even before the quadrupole baseline l = 2 mode enters the detector band. Our theoretical analysis marks a first in proposing a strategy for gaining the earliest possible warning and maximizing preparation time for observing pre and/or postmerger EM counterparts. We simulate three typical binaries from GWTC-3 with eccentricities ranging from 0.05 to 0.4. Our results reveal that the third -generation (3G) detectors (low -frequency cutoff f 0 = 5 Hz) can accumulate sufficient signal-to-noise ratios through higher modes before the onset of the baseline l = 2 mode entry into the band. Notably, relying solely on the higher modes, the 3G detector network ET & thorn; 2CE achieves an average localization on the order of 1 -10 2 deg 2 around 1-1.8 hours before the merger of a GW170817-like BNS, and 10 -10 3 deg 2 approximately 18-30 minutes prior to the merger of a GW200115-like NSBH. A 100 deg 2 localization is attainable even 2-4 hours prior to a BNS merger. Moreover, in the near face -on orientations which are generally more favorable for EM counterpart detection, the localization can be further improved.