Disentangling the Black Hole Mass Spectrum with Photometric Microlensing Surveys

Abstract

From the formation mechanisms of stars and compact objects to nuclear physics, modern astronomy frequently leverages surveys to understand populations of objects to answer fundamental questions. The population of dark and isolated compact objects in the Galaxy contains critical information related to many of these topics, but is only practically accessible via gravitational microlensing. However, photometric microlensing observables are degenerate for different types of lenses, and one can seldom classify an event as involving either a compact object or stellar lens on its own. To address this difficulty, we apply a Bayesian framework that treats lens type probabilistically and jointly with a lens population model. This method allows lens population characteristics to be inferred despite intrinsic uncertainty in the lens class of any single event. We investigate this method’s effectiveness on a simulated ground-based photometric survey in the context of characterizing a hypothetical population of primordial black holes (PBHs) with an average mass of 30M ⊙. On simulated data, our method outperforms current black hole (BH) lens identification pipelines and characterizes different subpopulations of lenses while jointly constraining the PBH contribution to dark matter to ≈25%. Key to robust inference, our method can marginalize over population model uncertainty. We find the lower mass cutoff for stellar origin BHs, a key observable in understanding the BH mass gap, particularly difficult to infer in our simulations. This work lays the foundation for cutting-edge PBH abundance constraints to be extracted from current photometric microlensing surveys.

Astrometric Microlensing by Primordial Black Holes with the Roman Space Telescope

“…Assuming a monochromatic mass spectrum is the standard in the literature, but the relaxation of this assumption can alter PBH constraints and change simulated sensitivities (e.g., Green 2016;Carr et al 2017;Green & Kavanagh 2021). Future work could explore relaxing the monochromatic mass spectrum assumptions and deriving constraints using the methods of Perkins et al (2024).…”

“…In addition to predicting the number of astrometric PBH microlensing events for the GBTDS, it is also important to investigate how distinguishable PBH lenses are from the astrophysical lens population (stars, WD, NS, and SOBHs) because this will affect the quality of PBH population constraints that can be obtained (Perkins et al 2024). There are a variety of tools that can quantify separability; here we will use two methods that contain complementary information.…”

“…Figure 11 shows the estimated constraints that can be derived from this work, alongside other current PBH constraints in f DM -m PBH . To derive rigorous, population-level constraints would require modeling an inhomogeneous Poisson process within a hierarchical framework (e.g., Perkins et al 2024) and is beyond the scope of this work.…”

“…In the absence of performing a computationally expensive, fully hierarchical framework to derive PBH population constraints possible with the GBTDS (e.g., Perkins et al 2024), we instead rely on limiting cases and approximations to estimate them. As outlined in Section 4.4, we consider two limiting cases:…”

“…A subsequent high-cadence microlensing survey toward M31 placed upper bounds on f DM ≈ 0.1%-1% at substellar (10 −14 -10 −9 )M e mass ranges (Niikura et al 2019a). Most recently, observation toward the Galactic bulge from the Optical Gravitational Lensing Experiment (OGLE; Udalski et al 2015;Mróz et al 2017) has placed constraints on f DM ≈ 1%-10% for a mass range of (10 −6 -10 −3 )M e (Niikura et al 2019b), and methods to robustly disentangle PBH and astrophysical lens populations using photometric microlensing surveys toward the Bulge have been developed (e.g., Perkins et al 2024).…”