Detecting electromagnetic counterparts to LIGO/Virgo/KAGRA gravitational wave events with DECam: Neutron Star Mergers

Abstract: With GW170817 being the only multimessenger gravitational wave (GW) event with an associated kilonova (KN) detected so far, there exists a pressing need for realistic estimation of the GW localization uncertainties and rates, as well as optimization of available telescope time to enable the detection of new KNe. For this purpose, we simulate GW events assuming a data-driven, GW-motivated distribution of binary parameters for the LIGO/Virgo/KAGRA (LVK) fourth and fifth observing runs (O4 and O5). We map the binary neutron star (BNS) and neutron star-black hole (NSBH) properties to the optical light curves arising from r-process nucleosynthesis in the ejecta. We use the simulated population of KNe to generate follow-up observing plans, with the primary goal of optimizing detection with the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS). We explore the dependence of KN detectability on the mass, distance, inclination, and spin of the binaries. Assuming that no BNS was detected during O4 until the end of 2024, we present updated GW BNS (NSBH) merger detection rates: $\sim 1-9$ ($2-9$) yr$^{-1}$ in O4, and $13-110$ ($18-110$) yr$^{-1}$ in O5. Of these events, we expect to detect BNS (NSBH) KNe with DECam at a per year rate of: $0-2$ ($0$) in O4, and $2-28$ ($0-3$) in O5, conditional on the uncertainty on the equation of state (EOS) and volumetric rates of the mergers. We expect the majority of BNS detections and also those accompanied by a detectable KN to produce a hypermassive NS remnant, with a significant fraction of the remaining BNSs promptly collapsing to a BH. We release our GW simulations and the depths needed to detect a significant fraction of simulated KNe to enable the astronomical community to use them in their multimessenger campaigns and analyses.