Kavli Affiliate: Luis C. Ho
| First 5 Authors: Zihao Wu, Luis C. Ho, , ,
| Summary:
Recent studies suggest that numerous intermediate-mass black holes (IMBHs)
may wander undetected across the Universe, emitting little radiation. These
IMBHs largely preserve their birth masses, offering critical insights into the
formation of heavy black hole seeds and the dynamical processes driving their
evolution. We propose that such IMBHs could produce detectable microlensing
effects on quasars. Their Einstein radii, comparable to the scale of quasar
broad-line regions, magnify radiation from the accretion disk and broad
emission lines, making these quasars outliers in flux scaling relations.
Meanwhile, the microlensing causes long-term, quasi-linear variability that is
distinguishable from the stochastic variability of quasars through its coherent
multi-wavelength behavior. We develop a matched-filtering technique that
effectively separates the long-term lensing signal from the intrinsic quasar
variability, with sensitivity tripling each time the observational time span
doubles. Moreover, as IMBHs are often surrounded by dense star clusters, their
combined gravitational field produces substantial extended, concentric
caustics. These caustics induce significant variability in optical,
ultraviolet, and X-ray bands over decade timescales, alongside
hour-to-day-scale flux fluctuations in broad emission lines. We predict a
substantial number of detectable events in the upcoming surveys by the Vera C.
Rubin Observatory, considering recent IMBH mass density estimates. Even in the
absence of positive detections, searches for these microlensing signals will
place meaningful constraints on the cosmological mass density of IMBHs,
advancing our understanding of their role in cosmic evolution.
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