Cosmology with the Wide-Field Infrared Survey Telescope -- Synergies
  with the Rubin Observatory Legacy Survey of Space and Time
  We explore synergies between the space-based Wide-Field Infrared Survey
Telescope (WFIRST) and the ground-based Rubin Observatory Legacy Survey of
Space and Time (LSST). In particular, we consider a scenario where the
currently envisioned survey strategy for WFIRST's High Latitude Survey (HLS),
i.e., 2000 square degrees in four narrow photometric bands is altered in favor
of a strategy that combines rapid coverage of the LSST area (to full LSST
depth) in one band. We find that a 5-month WFIRST survey in the W-band can
cover the full LSST survey area providing high-resolution imaging for >95% of
the LSST Year 10 gold galaxy sample. We explore a second, more ambitious
scenario where WFIRST spends 1.5 years covering the LSST area. For this second
scenario we quantify the constraining power on dark energy equation of state
parameters from a joint weak lensing and galaxy clustering analysis, and
compare it to an LSST-only survey and to the Reference WFIRST HLS survey. Our
survey simulations are based on the WFIRST exposure time calculator and
redshift distributions from the CANDELS catalog. Our statistical uncertainties
account for higher-order correlations of the density field, and we include a
wide range of systematic effects, such as uncertainties in shape and redshift
measurements, and modeling uncertainties of astrophysical systematics, such as
galaxy bias, intrinsic galaxy alignment, and baryonic physics. Assuming the
5-month WFIRST wide scenario, we find a significant increase in constraining
power for the joint LSST+WFIRST wide survey compared to LSST Y10 (FoM(Wwide)=
2.4 FoM(LSST)) and compared to LSST+WFIRST HLS (FoM(Wwide)= 5.5 FoM(HLS)).
