Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

This thesis addresses the problem of finding and characterizing isolated Galactic stellar-mass BHs via the technique of gravitational microlensing. We present work on both the theoretical and simulation front as well as an observational and modeling perspective.

Prior to 2019, most estimates for the number of BHs in the Milky Way were highly uncertain and dated. This limited our ability to accurately predict and interpret the number of BHs detectable with microlensing. To address this shortcoming, we develop a Milky Way microlensing simulation called PopSyCLE (Population Synthesis for Compact object Lensing Events). Using PopSyCLE, we identify an improved strategy for finding and identifying BHs photometrically, advancing our ability to identify good BH candidates and enable statistical constraints on the BH population to be made.

Although photometric microlensing can statistically characterize BHs, it cannot disentangle the lensing system’s relative proper motion from the lens’s mass. Astrometric microlensing is required to break this fundamental photometric degeneracy to definitively measure a lens’s mass and confirm its BH nature. We analyze 5 archival BH candidate microlensing events by jointly modeling ground-based microlensing survey photometry and Hubble Space Telescope (HST) astrometric observations. We identify one candidate, OB110462, to be a neutron star or low-mass BH (1.6 - 4.4 solar masses), making it the first dark isolated compact object confirmed with a mass measurement. In addition, using the full sample of five objects including non-BH detections and quantifying selection effects, we find this sample to be consistent with 100 million isolated BHs in the Milky Way.

The exact nature of OB110462 was disputed; another group independently found its mass to be 7 solar masses, firmly establishing it as a higher-mass BH. We re-analyze OB110462 using new and re-reduced data, along with updated analysis software. We find that OB110462 is a 6 solar mass BH, making it a solid BH detection. The main reason for finding a higher mass in this work is that the software used in previous work underestimated a positional bias correction, resulting in different astrometric measurements and hence a different mass.

Presently, microlensing is a small and relatively obscure subfield of astronomy. However, this will change with the launch of NASA's next flagship mission, the Nancy Grace Roman Space Telescope. Roman will conduct a microlensing survey of the Galactic Bulge. In addition, other public time-domain surveys will enable searches for microlensing events. In order to enable broader participation and newcomers to the field, we provide some examples of important considerations to take into account when modeling microlensing light curves. We also document, explain, and provide examples of some microlensing notations and conventions not easily found in the literature.

    Cover page: Understanding the Galactic Black Hole Population with Gravitational Microlensing
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2023)
    Microlensing events have historically been discovered throughout the Galactic bulge and plane by surveys designed solely for that purpose. We conduct the first multiyear search for microlensing events on the Zwicky Transient Facility (ZTF), an all-sky optical synoptic survey that observes the entire visible northern sky every few nights. We discover 60 high-quality microlensing events in the 3 yr of ZTF-I using the bulk lightcurves in the ZTF Public Data Release 5.19 of our events are found outside of the Galactic plane (∣b∣ ≥ 10°), nearly doubling the number of previously discovered events in the stellar halo from surveys pointed toward the Magellanic Clouds and the Andromeda galaxy. We also record 1558 ongoing candidate events as potential microlensing that can continue to be observed by ZTF-II for identification. The scalable and computationally efficient methods developed in this work can be applied to future synoptic surveys, such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope, as they attempt to find microlensing events in even larger and deeper data sets.
      Cover page: 60 Microlensing Events from the Three Years of Zwicky Transient Facility Phase OneCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2022)
      Modern surveys of gravitational microlensing events have progressed to detecting thousands per year, and surveys are capable of probing Galactic structure, stellar evolution, lens populations, black hole physics, and the nature of dark matter. One of the key avenues for doing this is the microlensing Einstein radius crossing time (t E) distribution. However, systematics in individual light curves as well as oversimplistic modeling can lead to biased results. To address this, we developed a model to simultaneously handle the microlensing parallax due to Earth's motion, systematic instrumental effects, and unlensed stellar variability with a Gaussian process model. We used light curves for nearly 10,000 OGLE-III and -IV Milky Way bulge microlensing events and fit each with our model. We also developed a forward model approach to infer the t E distribution by forward modeling from the data rather than using point estimates from individual events. We find that modeling the variability in the baseline removes a source of significant bias in individual events, and the previous analyses overestimated the number of t E > 100 day events due to their oversimplistic model ignoring parallax effects. We use our fits to identify the hundreds filling a regime in the microlensing parameter space that are 50% pure of black holes. Finally, we have released the largest-ever catalog of Markov Chain Monte Carlo parameter estimates for microlensing events.
        Cover page: A Reanalysis of Public Galactic Bulge Gravitational Microlensing Events from OGLE-III and -IV
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2019)
        Astrophysical and cosmological observations currently provide the only robust, empirical measurements of dark matter. Future observations with Large Synoptic Survey Telescope (LSST) will provide necessary guidance for the experimental dark matter program. This white paper represents a community effort to summarize the science case for studying the fundamental physics of dark matter with LSST. We discuss how LSST will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational couplings to the Standard Model, and compact object abundances. Additionally, we discuss the ways that LSST will complement other experiments to strengthen our understanding of the fundamental characteristics of dark matter. More information on the LSST dark matter effort can be found at https://lsstdarkmatter.github.io/ .
          Cover page: Probing the Fundamental Nature of Dark Matter with the Large Synoptic Survey Telescope
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2019)
          Astrophysical observations currently provide the only robust, empirical measurements of dark matter. In the coming decade, astrophysical observations will guide other experimental efforts, while simultaneously probing unique regions of dark matter parameter space. This white paper summarizes astrophysical observations that can constrain the fundamental physics of dark matter in the era of LSST. We describe how astrophysical observations will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational interactions with the Standard Model, and compact object abundances. Additionally, we highlight theoretical work and experimental/observational facilities that will complement LSST to strengthen our understanding of the fundamental characteristics of dark matter.
            Cover page: Dark Matter Science in the Era of LSSTCreative Commons 'BY' version 4.0 license
            Top