Pre-merger localization of compact-binary mergers with third generation observatories

We present the prospects for the pre-merger detection and localization of binary neutron star mergers with third generation gravitational-wave observatories. We consider a wide variety of gravitational-wave networks which may be operating in the 2030's and beyond; these networks include up to two Cosmic Explorer sites, the Einstein Telescope, and continued observation with the existing second generation ground-based detectors. For a fiducial local merger rate of 300 Gpc$^{-3}$yr$^{-1}$, we find that the Einstein Telescope on its own is able to detect 6 and 2 sources per year at 5 and 30 minutes before merger, respectively, while providing a localization of $<10~\textrm{deg}^2$. A single Cosmic Explorer would detect but be unable to localize sources on its own. A two-detector Cosmic Explorer network, however, would detect 22 and 0.4 mergers per year using the same criteria. A full three-detector network with the operation of dual Cosmic Explorers and the Einstein Telescope would allow for $<1~\textrm{deg}^2$ source localization at 5 minutes before merger for $\sim7$ sources per year. Given the dramatic increase in localization and detection capabilities, third generation observatories will enable the regular observation of the prompt emission of mergers by a broad array of observatories including gamma-ray, x-ray, and optical telescopes. Moreover, sub-degree localizations minutes before merger, combined with narrow-field-of-view high-energy telescopes, could strongly constrain the high-energy pre-merger emission models proposed in the last decade.