Ancillary Science with the WFIRST Microlensing Survey
The Wide Field Infrared Survey Telescope (WFIRST) will monitor ~2 deg2 toward the Galactic bulge in a wide (~1-2 μm) W149 filter at 15-minute cadence with exposure times of ~50s for 6 seasons of 72 days each, for a total ~41,000 exposures taken over ~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 ~109 photons over the course of the survey for a W149AB~21 star. Of order 4x107 stars will be monitored with W149AB<21, and 108 stars with W145AB<23. The WFIRST microlensing survey will detect ~54,000 microlensing events, of which roughly 1% (~500) will be due to isolated black holes, and ~3% (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 ~105 transiting planets with sizes as small as ~2 REarth, perform asteroseismology of ~106 giant stars, measure the proper motions to ~0.3% and parallaxes to ~10% for the 6x106 disk and bulge stars in the survey area, and directly detect ~5x103 Trans-Neptunian objects (TNOs) with diameters down to ~10 km, as well as detect ~103 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 very center of our Galaxy, would enable an even broader range of transformational science.