Evaluating KAGRA upgrade scenarios for multimessenger observations of binary neutron stars

Binary neutron star mergers are key targets for multimessenger astronomy, motivating future upgrades of gravitational-wave detectors. For KAGRA, both broadband sensitivity improvements that increase the binary neutron star detection range, and high-frequency optimizations targeting neutron-star physics are under consideration. We present a computationally efficient framework to evaluate the multimessenger performance of detector upgrades by combining Fisher-matrix estimates of localization area and localization volume with detector duty factors and binary neutron star merger rates. We apply this framework to proposed KAGRA upgrade scenarios within the LIGO-Virgo-KAGRA network. For identical sources, the high-frequency upgrade improves sky localization by about 20% compared with the broadband option. However, when detection rates are taken into account, the broadband upgrade yields a larger number of well-localized events. Despite its shorter binary neutron star range than the other detectors, the inclusion of KAGRA increases the number of events localized within 10 cubed Mpc cubed volume by about 60%. These results provide a quantitative framework for evaluating future detector upgrades from the perspective of multimessenger observations.