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.