Gravitational Microlensing Event Statistics for the Zwicky Transient Facility

Abstract

Microlensing surveys have discovered thousands of events with almost all events discovered within the Galactic bulge or toward the Magellanic clouds. The Zwicky Transient Facility (ZTF), while not designed to be a microlensing campaign, is an optical time-domain survey that observes the entire northern sky every few nights including the Galactic plane. ZTF observes $\sim10^9$ stars in g-band and r-band and can significantly contribute to the observed microlensing population. We predict that ZTF will observe $\sim$1100 microlensing events in three years of observing within $10^\circ$ degrees latitude of the Galactic plane, with $\sim$500 events in the outer Galaxy ($\ell \geq 10^\circ$). This yield increases to $\sim$1400 ($\sim$800) events by combining every three ZTF exposures, $\sim$1800 ($\sim$900) events if ZTF observes for a total of five years, and $\sim$2400 ($\sim$1300) events for a five year survey with post-processing image stacking. Using the microlensing modeling software PopSyCLE, we compare the microlensing populations in the Galactic bulge and the outer Galaxy. We also present an analysis of the microlensing event ZTF18abhxjmj to demonstrate how to leverage these population statistics in event modeling. ZTF will constrain Galactic structure, stellar populations, and primordial black holes through photometric microlensing.