Of Harbingers and Higher Modes: Improved Gravitational-wave Early Warning of Compact Binary Mergers

A crucial component to maximizing the science gain from the multi-messenger follow-up of gravitational-wave (GW) signals from compact binary mergers is the prompt discovery of the electromagnetic counterpart. Ideally, the GW detection and localization must be reported early enough to allow for telescopes to slew to the location of the GW event before the onset of the counterpart. However, the time available for early warning is limited by the short duration spent by the dominant (ℓ = m = 2) mode within the detector's frequency band. Nevertheless, we show that including higher modes—which enter the detector's sensitivity band well before the dominant mode—in GW searches can enable us to significantly improve the early warning ability for compact binaries with asymmetric masses (such as neutron star–black hole (NSBH) binaries). We investigate the reduction in the localization sky-area when the ℓ = m = 3 and ℓ = m = 4 modes are included in addition to the dominant mode, considering typical slew-times of electromagnetic telescopes (30–60 s). We find that, in LIGO's projected "O5" ("Voyager") network with five GW detectors, some of the NSBH mergers, located at a distance of 40 Mpc, can be localized to a few hundred sq. deg. ∼45 s prior to the merger, corresponding to a reduction-factor of 3–4 (5–6) in sky-area. For a third-generation network, we get gains of up to 1.5 min in early warning times for a localization area of 100 sq. deg., even when the source is placed at 100 Mpc.