Finding gravitational-wave black holes with parallax microlensing

Abstract

The LIGO-Virgo gravitational-wave (GW) observation unveiled the new population of black holes (BHs) that appears to have an extended mass spectrum up to around 70M , much heavier than the previously-believed mass range (∼ 8M ). In this paper, we study the capability of a microlensing observation of stars in the Milky Way (MW) bulge region to identify BHs of GW mass scales, taking into account the microlensing parallax characterized by the parameter π E ∝ M −1/2 (M is the mass of a lens), which is a dimension-less quantity defined by the ratio of the astronomical unit to the projected Einstein radius. First, assuming that BHs follow the same spatial and velocity distributions of stars as predicted by the standard MW model, we show that microlensing events with long light curve timescales, t E 100 days, and small parallax effects, π E ∼ 10 −2 , are dominated by BH lenses compared to stellar-mass lenses. Second, using a Markov chain Monte Carlo analysis of the simulated light curve, we show that BH lens candidates are securely identified on individual basis, if the parallax effect is detected or well constrained to the precision of a percent level in π E . We also discuss that a microlensing event of an intermediate-mass BH of ∼ 1000M , if it occurs, can be identified in a distinguishable way from stellar-mass BHs.

Possible evidence of QCD axion stars in HSC and OGLE microlensing events

“…[24] used the publicly-available 2622 microlensing events, obtained from the 5-year OGLE observation of ∼ 5 × 10 7 stars in the Galactic bulge region, to discuss the populations of lensing objects. The OGLE events revealed a distinct population of 6 ultra-short timescale microlensing events ([0.1, 0.3] day scales) compared to the other majority of events that have a smooth distribution of microlensing timescales and are explained by standard populations of lensing objects such as brown dwarfs, main-sequence stars and white dwarfs in the standard model of the MW [also see 24,[26][27][28][29]. Niikura et al [13] discussed that these ultra-short timescale events can be explained by compact objects such as primordial black holes that have the same spatial and velocity distributions as predicted in the MW halo model, if the compact objects are in the range of Earth mass scale (∼ 10 −6 M ).…”

Detectability of Wandering Intermediate-Mass Black Holes in the Milky Way Galaxy from Radio to X-rays

“…Additionally, the passage of these sources along the line of sight with a background star will create a micro-lensing event, which could be detected with the use of extensive stellar surveys, such as Gaia (see, e.g., Toki & Takada 2021). Other techniques, such as the symmetric achromatic variability (Vedantham et al 2017), could be used to detect IMBHs via milli-lensing.…”