Microlensing Events in the Galactic Plane Using the Zwicky Transient Facility

TL;DR

This study uses ZTF data to identify 60 microlensing events in the Galactic plane, revealing longer timescales and promising potential for studying dark objects like black holes and exoplanets within the Galactic disk.

Contribution

First large-scale survey of microlensing events in the Galactic plane using ZTF, demonstrating the feasibility and scientific value of this approach.

Findings

60 candidate microlensing events identified

Event rate decreases exponentially with Galactic longitude

Average Einstein timescale is about 60 days, longer than towards the bulge

Abstract

Microlensing is a powerful technique to study the Galactic population of "dark" objects such as exoplanets both bound and unbound, brown dwarfs, low-luminosity stars, old white dwarfs, neutron stars, and almost the only way to study isolated stellar-mass black holes. The majority of previous efforts to search for gravitational microlensing events have concentrated towards high-density fields such as the Galactic bulge. Microlensing events in the Galactic plane have the advantage of closer proximity and better constrained relative proper motions, leading to better constrained lens mass estimates at the expense of a lower optical depth compared to events towards the Galactic bulge. We use the Zwicky Transient Facility (ZTF) Data Release 5 (DR5) compiled from 2018--2021 to survey the Galactic plane in the region of $|b| < 20^\circ$. We find a total of 60 candidate microlensing events including three that show a strong microlensing parallax effect. The rate of events traces Galactic structure, decreasing exponentially as a function Galactic longitude with scale length $\ell_0 \sim 37^\circ$. On average, we find Einstein timescales of our microlensing events to be about three times as long ($\sim60$ days) compared to those towards the Galactic bulge ($\sim20$ days). This pilot project demonstrates that microlensing towards the Galactic plane shows strong promise for characterization of dark objects within the Galatic disk.