Discovery of Strongly-lensed Gravitational Waves - Implications for the
  LSST Observing Strategy
  LSST's wide-field of view and sensitivity will revolutionize studies of the
transient sky by finding extraordinary numbers of new transients every night.
The recent discovery of a kilonova counterpart to LIGO/Virgo's first detection
of gravitational waves (GWs) from a double neutron star (NS-NS) merger also
creates an exciting opportunity for LSST to offer a Target of Opportunity (ToO)
mode of observing. We have been exploring the possibility of detecting strongly
lensed GWs, that would enable new tests of GR, extend multi-messenger astronomy
out to $z\gtrsim1$, and deliver a new class of sub-millisecond precision
time-delay constraints on lens mass distributions. We forecast that the rate of
detection of lensed NS-NS mergers in the 2020s will be $\sim0.1$ per Earth
year, that the typical source will be at $z\simeq2$, and that the
multiply-imaged kilonova counterpart will have a magnitude of ${\rm
AB}\simeq25.4$ in $g/r/i$-band filters - i.e. fainter than the sensitivity of a
single LSST WFD visit. We therefore advocate (1) creating a flexible and
efficient Target of Opportunity programme within the LSST observing strategy
that is capable of discovering sources fainter than single-visit depth, and (2)
surveying the entire observable extragalactic sky as rapidly as possible in the
WFD survey. The latter will enable a very broad range of early science that
relies on wide survey area for detection of large samples of objects and/or
maximizing the fraction of sky over which reference imaging is available. For
example, it will enable prompt discovery of a uniform and all-sky sample of
galaxy/group/cluster-scale lenses that will underpin LSST strong-lensing
science. This white paper complements submissions from DESC, SLSC, and TVSSC,
that discuss kilonova, GW, and strong lensing.
