Hunting Gravitational Wave Black Holes with Microlensing

Abstract

Gravitational microlensing is a powerful tool to search for a population of invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and binary BHs at wide orbits that are complementary to gravitational wave observations. By monitoring highly populated regions of source stars like the MW bulge region, one can pursue microlensing events due to these BHs. We find that if BHs have a Salpeter-like mass function extended beyond 30M ⊙ and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of microlensing events at long timescales of the microlensing light curve ≳100 days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as M 2, for such long-timescale events. A monitoring observation of 2 × 1010 stars in the bulge region over 10 yr with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about 6 × 105 BH microlensing events. We evaluate the efficiency of potential LSST cadences for characterizing the light curves of BH microlensing and find that nearly all events of long timescales can be detected.

Finding gravitational-wave black holes with parallax microlensing

“…In our model we assume that massive lens objects with M ≥ 8M arise only from BHs. As shown in Abrams & Takada (2020), long timescale microlensing events with t E 100 days are dominated by BH lenses due to the boosted dependence of the lensing cross section on lens masses, even if BHs are much less abundant than MSs (only 0.0068 BHs per MS). This clearly shows the power of a microlensing observation to search for BHs in the MW.…”

“…To compute the microlensing event rate for a source star in the Galactic bulge, we need to model the spatial and velocity distributions of lens populations in the MW bulge and disk regions. Here we employ the standard models of the MW in Han & Gould (1995) (also see Han & Gould 1996;Niikura et al 2019a;Abrams & Takada 2020), and in this subsection we briefly review the model.…”

“…The differential event rate for BHs is of the order of 10 −11 [event star −1 day −2 ]. As discussed in Abrams & Takada (2020), if we can conduct a monitoring observation of about 10 10 stars in the bulge region over a 10-year time scale, which can be performed with the LSST bulge observation (Street et al 2018), we expect about 3.7 × 10 4 events for BH lenses with t E ∼ 100 days and π E ∼ 10 −2 , which is obtained by N s ×d 2 Γ/dt E d ln π E ×t obs ×t E ∼ 10 10 ×10 −11 ×10 years× 365 days/yr × 100 days 3.7 × 10 4 . Thus an observation by LSST can find many BH events if BHs follow the spatial and velocity distributions of stars, which would be the case if GW BHs originate from massive stars.…”

“…The purpose of this paper is to study the capability of a microlensing observation of stars in the MW bulge region to find and constrain the BH population of ∼ 40 M mass scales in the MW. A GW mass-scale BH generally causes a long-timescale microlensing event with t E 100 days, compared to a stellar mass (∼ 1 M ) lens that is a majority of the population of lenses in the MW (Mao & Paczynski 1996;Wood & Mao 2005;Lu et al 2016;Niikura et al 2019a;Lam et al 2020;Abrams & Takada 2020). However, measuring only the timescale of the microlensing light curve does not allow us to obtain a definite mass estimation of the BH lens because of a severe degeneracy between the lens mass and relative velocity.…”

Lens parameters for Gaia18cbf -- a long gravitational microlensing event in the Galactic plane

“…Annual parallax occurs when the event is long enough for Earth's movement around the Sun to change the position of the observer and produces a distinct asymmetry in the observed light curve (Gould 1992;Alcock et al 1995). Such an effect, combined with a large timescale t E can be used to constrain the mass of the lens and to find microlensing dark remnants candidates (Poindexter et al 2005;Wyrzykowski et al 2016;Wyrzykowski & Mandel 2020;Lam et al 2020;Karolinski & Zhu 2020;Abrams & Takada 2020;Mróz & Wyrzykowski 2021). The longest ever detected microlensing event had a timescale of 640 +68 −54 days (Mao et al 2002) and only a few known events have timescales around 400 days (Wyrzykowski et al 2015;Mróz et al 2020).…”