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 ~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 ~80% at the nominal BNS gravitational-wave detection limit for O4 (190 Mpc), which corresponds to an ~30% improvement compared to the strategy adopted during the previous observing run. For more distant events (~330 Mpc), we reach an ~60% probability of detection, a factor of ~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 ~90% probability of detection out to 330 Mpc, representing an increase of ~20%, while also reducing the total telescope time required to follow up events by ~20%.. Authors: Bom, C. R. [Brazilian Center for Physics Research, Rio de Janeiro (Brazil); Federal Center for Technological Education Celso Suckow da Fonseca, Itaguai, RJ (Brazil). DES Collaboration. et al.] (ORCID:0000000343832969); Annis, J. (ORCID:0000000206093987); Garcia, A. (ORCID:0000000195786322); Palmese, A. (ORCID:0000000260110530); Sherman, N. (ORCID:0000000153990114). DOE Contract: AC02-07CH11359; AC02-76SF00515; AC05-00OR22725. Subjects: 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; 79 ASTRONOMY AND ASTROPHYSICS; astronomical methods; astrostatistics tool; gravitational wave astronomy; transient detection