"Auxiliary" Science with the WFIRST Microlensing Survey
  The Wide Field Infrared Survey Telescope (WFIRST) will monitor $\sim 2$
deg$^2$ toward the Galactic bulge in a wide ($\sim 1-2~\mu$m) W149 filter at
15-minute cadence with exposure times of $\sim$50s for 6 seasons of 72 days
each, for a total $\sim$41,000 exposures taken over $\sim$432 days, spread over
the 5-year prime mission. This will be one of the deepest exposures of the sky
ever taken, reaching a photon-noise photometric precision of 0.01 mag per
exposure and collecting a total of $\sim 10^9$ photons over the course of the
survey for a W149$_{\rm AB}\sim 21$ star. Of order $4 \times 10^7$ stars will
be monitored with W149$_{\rm AB}$<21, and 10$^8$ stars with W145$_{\rm AB}$<23.
The WFIRST microlensing survey will detect $\sim$54,000 microlensing events, of
which roughly 1% ($\sim$500) will be due to isolated black holes, and $\sim$3%
($\sim$1600) will be due to isolated neutron stars. It will be sensitive to
(effectively) isolated compact objects with masses as low as the mass of Pluto,
thereby enabling a measurement of the compact object mass function over 10
orders of magnitude. Assuming photon-noise limited precision, it will detect
$\sim 10^5$ transiting planets with sizes as small as $\sim 2~R_\oplus$,
perform asteroseismology of $\sim 10^6$ giant stars, measure the proper motions
to $\sim 0.3\%$ and parallaxes to $\sim 10\%$ for the $\sim 6 \times 10^6$ disk
and bulge stars in the survey area, and directly detect $\sim 5 \times 10^3$
Trans-Neptunian objects (TNOs) with diameters down to $\sim 10$ km, as well as
detect $\sim 10^3$ occulations of stars by TNOs during the survey. All of this
science will completely serendipitous, i.e., it will not require modifications
of the WFIRST optimal microlensing survey design. Allowing for some minor
deviation from the optimal design, such as monitoring the Galactic center,
would enable an even broader range of transformational science.
