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Abstract The discovery of a second electromagnetic counterpart to a gravitational wave event represents a critical goal in the field of multi-messenger astronomy. In order to determine the optimal strategy for achieving this goal, we perform comprehensive simulations comparing two potential paths forward: continuing the current LIGO-Virgo-KAGRA (LVK) observing run, O4, versus temporarily shutting down the detectors for upgrades before beginning the next observing run, O5. Our simulations incorporate current O4 instrument sensitivities and duty cycles, as well as projected configurations for O5, while accounting for variables such as binary neutron star merger rates, system properties, viewing angles, dust extinction, and kilonova (KN) observables. Our results indicate that a KN discovery would occur $125_{-125}^{+253}$days (middle 50% interval) sooner in O5 compared to O4, suggesting that extending O4 would lead to faster discovery if the shutdown period between runs is  >4 months. Moreover, for 88% of our simulations, continuing O4 results in earlier KN discovery when compared to the expected two-year shutdown between O4 and O5. Given these findings and the critical importance of avoiding a  >10 yr gap between first and second electromagnetic counterpart discoveries, we suggest LVK consider extending O4 operations for as long as feasible prior to shutting down for critical upgrades. 

ABSTRACT We present a comprehensive, configurable open-source software framework for estimating the rate of electromagnetic detection of kilonovae (KNe) associated with gravitational wave detections of binary neutron star (BNS) mergers. We simulate the current LIGO-Virgo-KAGRA (LVK) observing run (O4) using current sensitivity and uptime values as well as using predicted sensitivites for the next observing run (O5). We find the number of discoverable kilonovae during LVK O4 to be $${ 1}_{- 1}^{+ 4}$$ or $${ 2 }_{- 2 }^{+ 3 }$$, (at 90 per cent confidence) depending on the distribution of NS masses in coalescing binaries, with the number increasing by an order of magnitude during O5 to $${ 19 }_{- 11 }^{+ 24 }$$. Regardless of mass model, we predict at most five detectable KNe (at 95 per cent confidence) in O4. We also produce optical and near-infrared light curves that correspond to the physical properties of each merging system. We have collated important information for allocating observing resources for search and follow-up observations, including distributions of peak magnitudes in several broad-bands and time-scales for which specific facilities can detect each KN. The framework is easily adaptable, and new simulations can quickly be produced in response to updated information such as refined merger rates and NS mass distributions. Finally, we compare our suite of simulations to the thus-far completed portion of O4 (as of 2023, October 14), finding a median number of discoverable KNe of 0 and a 95 percentile upper limit of 2, consistent with no detections so far in O4.