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Abstract

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 $<1~\textrm{deg}^2$ 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.