Abstract:
Theory predicts that most intermediate-mass black holes (IMBHs) wander inconspicuously in galactic halos. While direct detection of IMBHs is challenging, the relative motions between IMBHs and distant quasars due to large-scale peculiar velocities induce microlensing variability. As the Einstein radii of IMBHs are comparable to the size of the quasar broad-line region, emission from scales smaller than the broad-line region would appear extraordinarily luminous. Although it is unfeasible to observe the century-long microlensing light curve in its entirety, we present a matched-filtering technique that can distinguish the linear trend of the light curve from the intrinsic, stochastic variability of the quasar. The sensitivity improves with the observation time span, following a T^{−3/2} relation. We discuss strategies to distinguish the lensing signal from long-term quasar variability, noting that microlensing, unlike physically driven, intrinsic variability, exhibits the same amplitude and is synchronized across multiple wavelengths. The differential magnification of the accretion disk relative to the broad-line region, coupled with the lack of magnification of more extended substructures such as the dusty torus or the narrow-line region, provides additional empirical diagnostics. The abundance of quasars and their distances make the occurrence of microlensing events considerable. We argue that searching for the lensing signal using the Large Synoptic Survey Telescope over a 10-year period will yield meaningful constraints on the IMBH number density, and extending the survey by another decade can distinguish between different IMBH formation scenarios.
