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 merger rate of 300 Gpc$^{-3}$yr$^{-1}$, we find that the Einstein Telescope on its own is able to detect 6 (2) sources per year 5 (30) minutes before merger and provide 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 (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 $