Gravitational Microlensing in the Era of All-Sky Surveys

November 6, 2025

When: November 6, 2025 3:30PM

Where: LCO Downstairs Conference Room

UC Berkley

Gravitational microlensing provides a unique opportunity to probe the mass distribution of stars, black holes, and other objects in the Milky Way. Historically, microlensing events have been discovered primarily in the Galactic bulge by surveys designed solely for that purpose. As we enter the age of visible all-sky surveys, such as that of the Zwicky Transient Facility (ZTF) and Vera C. Rubin Observatory (Rubin), we gain the ability to probe microlensing events throughout the Galaxy. This allows us to analyze galactic structure and how the stellar and black hole mass functions vary across the Galaxy. I will present our analysis of optimal survey strategies, pipelines to discover microlensing events among a billion other lightcurves, and population simulations to physically interpret survey results. For Rubin, we have evaluated 360+ Operation Simulations to find their impact on microlensing discovery and characterization. Microlensing discovery eﬀiciency is dominated by observing footprint, where more time spent looking at regions of high stellar density including the Galactic bulge, Galactic plane, and Magellanic clouds, leads to higher discovery and characterization rates. We also present a multiyear search for microlensing events with ZTF. We discovered 52 microlensing events in the 3 yrs of ZTF-I using the bulk lightcurves in the ZTF Public Data Release 5. 11 of our events are found outside of the Galactic plane (∣b∣ ≥ 10°), significantly increasing the number of previously discovered events in the stellar halo. To interpret these results we have improved our population simulation tool, PopSyCLE (Population Synthesis for Compact-object Lensing Events) to include binary objects which are typically neglected. Inclusion of binaries in PopSyCLE has brought simulated results into closer agreement with observations from the OGLE survey, and we find that > 50% of our simulated events include a binary lens or source system. Through this work we have shown that open-access, all-sky surveys will be powerful tools for probing galactic structure, exoplanets, and black holes through microlensing.

Natasha Abrams

Natasha Abrams is an astrophysics PhD student and Berkeley Fellow at UC Berkeley, searching for black holes via microlensing. She is working with Prof. Jessica Lu on photometric microlensing with large surveys, astrometric microlensing, and understanding how binaries impact our statistical understanding of the events we discover. Natasha Abrams completed her Bachelor of Arts at Harvard University in 2021, graduating magna cum laude with highest honors in Astrophysics and Physics. As an undergraduate, she primarily worked with Prof. Christopher Stubbs on microlensing with the Vera C. Rubin Observatory and searching for black hole-variable star binaries via the Light Travel Time Effect. Outside of research, she enjoys choral singing, playing board games, and reading fantasy books.

Seminar