Gravitational Self-Lensing in Populations of Massive Black Hole Binaries
  The community may be on the verge of detecting low-frequency gravitational
waves from massive black hole binaries (MBHBs), but no examples of binary
active galactic nuclei (AGN) have been confirmed. Because MBHBs are
intrinsically rare, the most promising detection methods utilize photometric
data from all-sky surveys. Recently D'Orazio & Di Stefano 2018
(arXiv:1707.02335) suggested gravitational self-lensing as a method of
detecting AGN in close separation binaries. In this study we calculate the
detectability of lensing signatures in realistic populations of simulated
MBHBs. Within our model assumptions, we find that VRO's LSST should be able to
detect 10s to 100s of self-lensing binaries, with the rate uncertainty
depending primarily on the orientation of AGN disks relative to their binary
orbits. Roughly a quarter of lensing detectable systems should also show
detectable Doppler boosting signatures. If AGN disks tend to be aligned with
the orbit, lensing signatures are very nearly achromatic, while in misaligned
configurations the bluer optical bands are lensed more than redder ones.
Whether substantial obscuring material (e.g.~a dusty torus) will be present in
close binaries remains uncertain, but our estimates suggest that a substantial
fraction of systems would still be observable in this case.
