2022ApJ...924...39M

Using ground-based gravitational-wave detectors, we probe the mass function of intermediate-mass black holes (IMBHs) wherein we also include BHs in the upper mass gap at ∼60-130 M_☉. Employing the projected sensitivity of the upcoming LIGO and Virgo fourth observing run (O4), we perform Bayesian analysis on quasi-circular nonprecessing, spinning IMBH binaries (IMBHBs) with total masses 50-500 M_☉, mass ratios 1.25, 4, and 10, and dimensionless spins up to 0.95, and estimate the precision with which the source-frame parameters can be measured. We find that, at 2σ, the mass of the heavier component of IMBHBs can be constrained with an uncertainty of ∼10%-40% at a signal-to-noise ratio of 20. Focusing on the stellar-mass gap with new tabulations of the ¹²C(α, γ)¹⁶O reaction rate and its uncertainties, we evolve massive helium core stars using MESA to establish the lower and upper edges of the mass gap as ≃59_–13⁺³⁴M_☉ and ≃139_–14⁺³⁰M_☉ respectively, where the error bars give the mass range that follows from the ±3σ uncertainty in the ¹²C(α, γ)¹⁶O nuclear reaction rate. We find that high resolution of the tabulated reaction rate and fine temporal resolution are necessary to resolve the peak of the BH mass spectrum. We then study IMBHBs with components lying in the mass gap and show that the O4 run will be able to robustly identify most such systems. Finally, we reanalyze GW190521 with a state-of-the-art aligned-spin waveform model, finding that the primary mass lies in the mass gap with 90% credibility.