Serendipitous Discoveries of Kilonovae in the LSST Main Survey:
  Maximising Detections of Sub-Threshold Gravitational Wave Events
  We investigate the ability of the Large Synoptic Survey Telescope (LSST) to
discover kilonovae (kNe) from binary neutron star (BNS) and neutron star-black
hole (NSBH) mergers, focusing on serendipitous detections in the Wide-Fast-Deep
(WFD) survey. We simulate observations of kNe with proposed LSST survey
strategies, paying particular attention to cadence choices that are compatible
with the broader LSST cosmology programme. We find that if all kNe are
identical to GW170817, the baseline survey strategy will yield 58 kNe over the
survey lifetime. If we instead assume a representative population model of BNS
kNe, we expect to detect only 27 kNe. However, we find the choice of survey
strategy significantly impacts these numbers and can increase them to 254 kNe
and 82 kNe over the survey lifetime, respectively. This improvement arises from
an increased cadence of observations between different filters with respect to
the baseline. We then consider the ability of the Advanced LIGO/Virgo (ALV)
detector network to detect these BNS mergers. If the optimal survey strategy is
adopted, 202 of the GW170817-like kNe and 56 of the BNS population model kNe
are detected with LSST but are below the threshold for detection by the ALV
network. This represents, for both models, an increase by a factor greater than
4.5 in the number of detected sub-threshold events over the baseline survey
strategy. Such a population of sub-threshold events would provide an
opportunity to conduct electromagnetic-triggered searches for signals in
gravitational-wave detector data and assess selection effects in measurements
of the Hubble constant from standard sirens, e.g., related to viewing angle
effects.
