Item

Abstract

In the second work of this series, we explore the optimal search strategy for
serendipitous and gravitational-wave-triggered target-of-opportunity
observations of kilonovae (KNe) and optical short-duration gamma-ray burst
(sGRB) afterglows (AGs) from binary neutron star (BNS) mergers, assuming that
cosmological KNe are AT2017gfo-like (but with viewing-angle dependence) and
that the properties of AGs are consistent with those of cosmological sGRB AGs.
A one-day cadence serendipitous search strategy with an exposure time of
$\sim30\,$s can always achieve an optimal search strategy of KNe and AGs for
various survey projects. We show that the optimal detection rates of the
KN-dominated (AG-dominated) events are $\sim0.2/0.5/0.8/20\,$yr$^{-1}$
($\sim500/300/600/3000\,$yr$^{-1}$) for ZTF/Mephisto/WFST/LSST, respectively. A
better search strategy for SiTian is to increase the exposure time. SiTian can
find $\sim5(6000)\,$yr$^{-1}$ KN-dominated (AG-dominated) events. We predict
abundant off-axis orphan AGs may be recorded in the survey database although
not been identified. For target-of-opportunity observations, we simulate the
maximum BNS gravitational-wave (GW) detection rates, which are
$\sim27/210/1800/2.0\times10^5\,$yr$^{-1}$, in the networks of
2nd/2.5th/3rd(Voyager)/3rd(ET\&CE)-generation GW detectors. In the upcoming
2nd-generation networks, follow-up observations with a limiting magnitude of
$m_{\rm limit}\gtrsim22-23\,$mag can discover all EM signals from BNS GW
events. Among these detected GW events, $\sim60\%$ events ($\sim16\,$yr$^{-1}$)
can detect clear KN signals, while AG-dominated events would account for the
other $\sim40\%$ events ($\sim11\,$yr$^{-1}$). In the 2.5th- and
3rd(Voyager)-generation era, the critical magnitudes for the detection of EM
emissions from all BNS GW events would be $\sim23.5\,$mag and $\sim25\,$mag,
respectively.