Assessing the Impact of Binary Systems on Microlensing: Adding Binary Systems to SPISEA and PopSyCLE

Natasha Abrams  ✧  University of California, Berkeley, CA, USA

Gravitational microlensing provides a unique opportunity to probe the mass distribution of stars, black holes, and other objects in the Milky Way. However, population simulations are necessary to interpret results from photometric microlensing surveys. The contribution from binary objects is often minimized or neglected in these simulations despite the high percentage of binary systems and the potential microlensing holds to probe binaries. In order to simulate the population effects of binary and multiple systems on microlensing, we add resolved binary parameters based on Duchêne & Kraus (2013) to Stellar Population Interface for Stellar Evolution and Atmospheres (SPISEA), software that simulates stellar clusters. We then inject these multiples into Population Synthesis for Compact-object Lensing Events (PopSyCLE), a package that simulates Milky Way microlensing surveys. When making OGLE style cuts, we find that 27% of events have a multiple-lens and single source, 23% have a single lens and a multiple-source, and 16% have a multiple-lens and a multiple-source. This suggests that stellar and compact object binary source and even binary lens-binary source models should be included more frequently in event analysis. Since we do not yet include binary disruption, the black holes are all in binaries. Therefore, the higher masses of black holes leads them to have a higher average mass increase than stars, and the fraction of events with Einstein crossing time \(> 120\) days caused by black holes increases from 22% to 41%. The mean Einstein crossing time shifts from 19.6 days for single events only to 27.9 days for singles and multiple events, after cutting obvious binary events with multiple peaks and high asymmetry. The Einstein crossing time distribution of PSPL-like events (i.e. single and multiple events that are not obvious binaries from light curves) is now well aligned with that from Mróz et al. (2017), indicating that multiple systems had been a significant missing piece between simulations and reality.