Designing an Optimal Kilonova Search Using DECam for Gravitational-wave Events

We address the problem of optimally identifying all kilonovae detected via gravitational-wave emission in the upcoming LIGO/Virgo/KAGRA observing run, O4, which is expected to be sensitive to a factor of similar to 7 more binary neutron star (BNS) alerts than previously. Electromagnetic follow-up of all but the brightest of these new events will require >1 m telescopes, for which limited time is available. We present an optimized observing strategy for the DECam during O4. We base our study on simulations of gravitational-wave events expected for O4 and wide-prior kilonova simulations. We derive the detectabilities of events for realistic observing conditions. We optimize our strategy for confirming a kilonova while minimizing telescope time. For a wide range of kilonova parameters, corresponding to a fainter kilonova compared to GW170817/AT 2017gfo, we find that, with this optimal strategy, the discovery probability for electromagnetic counterparts with the DECam is similar to 80% at the nominal BNS gravitational-wave detection limit for O4 (190 Mpc), which corresponds to an similar to 30% improvement compared to the strategy adopted during the previous observing run. For more distant events (similar to 330 Mpc), we reach an similar to 60% probability of detection, a factor of similar to 2 increase. For a brighter kilonova model dominated by the blue component that reproduces the observations of GW170817/AT 2017gfo, we find that we can reach similar to 90% probability of detection out to 330 Mpc, representing an increase of similar to 20%, while also reducing the total telescope time required to follow up events by similar to 20%.