Target of Opportunity Observations Detectability of Kilonovae with WFST

Abstract

Kilonovae are approximately thermal transients, produced by mergers of binary neutron stars (BNSs) and NS-black hole binaries. As the optical counterpart of the gravitational wave event GW170817, AT2017gfo is the first kilonova detected with smoking-gun evidence. Its observation offers vital information for constraining the Hubble constant, the source of cosmic $r$-process enrichment, and the equation of state of neutron stars. The 2.5-meter Wide-Field Survey Telescope (WFST) operates at six bands (u, g, r, i, z, w), spanning from 320 to 925 nm. It will be completed in the first half of 2023, and with a field-of-view diameter of 3 degrees, aims to detect kilonovae in the near future. In this article, considering the influence of the host galaxies and sky brightness, we generate simulated images to investigate WFST's ability to detect AT2017gfo-like kilonovae. Due to their spectra, host galaxies can significantly impact kilonova detection at a longer wavelength. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90\% (50\%) kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect actual efficiency under target-of-opportunity observations, we calculate the total time of follow-up under various localization areas and distances. We find that if the localization areas of most BNS events detected during the fourth observing (O4) run of LIGO and Virgo are hundreds of deg$^2$, WFST is expected to find $\sim$30\% kilonovae in the first two nights during O4 period.