A Comprehensive Study of Detectability and Contamination in Deep Rapid Optical Searches for Gravitational Wave Counterparts

Abstract

The first direct detection of gravitational waves (GW) by the ground-based Advanced LIGO/Virgo interferometers is expected to occur within the next few years. These interferometers are designed to detect the mergers of compact object binaries composed of neutron stars and/or black holes to a fiducial distance of ∼ 200 Mpc and a localization region of ∼ 100 deg2. To maximize the science gains from such GW detections it is essential to identify electromagnetic (EM) counterparts. Among the wide range of proposed counterparts, the most promising is optical/IR emission powered by the radioactive decay of r-process elements synthesized in the neutron-rich merger ejecta – a “kilonova”. Here we present detailed simulated observations that encompass a range of strategies for kilonova searches during GW follow-up. We utilize these simulations to assess both the detectability of kilonovae and our ability to distinguish them from a wide range of contaminating transients in the large GW localization regions. We find that if pre-existing deep template images for the GW localization region are available, then nightly observations to a depth of i ≈ 24 mag and z ≈ 23 mag are required to achieve a 95% detection rate; observations that commence within ∼ 12 hours of trigger will also capture the kilonova peak and provide stronger constraints on the ejecta properties. We also find that kilonovae can be robustly separated from other known and hypothetical types of transients utilizing cuts on color (i − z & 0.3 mag) and rise time (trise . 4 days). In the absence of a pre-existing template the observations must reach ∼ 1 mag deeper to achieve the same kilonova detection rate, but robust rejection of contaminants can still be achieved. Motivated by the results of our simulations we discuss the expected performance of current and future wide-field telescopes in achieving these observational goals, and find that prior to LSST the Dark Energy Camera (DECam) on the Blanco 4-m telescope and Hyper Suprime-Cam (HSC) on the Subaru 8-m telescope offer the best kilonova discovery potential.