Target-of-Opportunity Observation Detectability of Kilonovae with WFST

Abstract

Kilonovae are approximately thermal transients, produced by the mergers of binary neutron stars (BNSs) and neutron star (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 sources of cosmic r -process enrichment, and the equation of state of NSs. The 2.5 m Wide-Field Survey Telescope (WFST) operates in 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°, 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 longer wavelengths. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90% (50%) of kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect the 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 ∼30% of kilonovae in the first two nights following the detection of a GW event produced by a BNS during the O4 period.

Keywords

• Gravitational wave astronomy
• Neutron stars