Observing intermediate-mass black holes and the upper--stellar-mass gap with LIGO and Virgo

TL;DR

This paper investigates the detection and characterization of intermediate-mass black holes and the stellar-mass gap using LIGO and Virgo gravitational-wave data, including Bayesian analysis, stellar evolution modeling, and re-analysis of GW190521.

Contribution

It provides new estimates of the black hole mass gap edges, assesses the precision of source parameter measurements with upcoming detectors, and re-analyzes GW190521 with advanced waveform models.

Findings

Mass of heavier IMBH component can be constrained within 10-40% at SNR 20.

Mass gap edges are estimated as approximately 59-139 solar masses with uncertainties.

O4 run can identify most IMBHBs with components in the mass gap.

Abstract

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 $\sim 60-130~M_\odot$. Employing the projected sensitivity of the upcoming LIGO and Virgo fourth observing (O4) run, we perform Bayesian analysis on quasi-circular non-precessing, spinning IMBH binaries (IMBHBs) with total masses $50\mbox{--} 500\, M_\odot$, 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 $2\sigma$, the mass of the heavier component of IMBHBs can be constrained with an uncertainty of $\sim 10-40\%$ at a signal-to-noise ratio of $20$. Focusing on the stellar-mass gap with new tabulations of the $^{12}\text{C}(\alpha, \gamma)^{16} \text{O}$ reaction rate and its uncertanties, we evolve massive helium core stars using \MESA\, to establish the lower and upper edge of the mass gap as $\simeq$\,59$^{+34}_{-13}$\,$M_{\odot}$ and $\simeq$\,139$^{+30}_{-14}$\,$M_{\odot}$ respectively, where the error bars give the mass range that follows from the $\pm 3\sigma$ uncertainty in the $^{12}\text{C}(\alpha, \gamma) ^{16} \text{O}$ 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 re-analyse GW190521 with a state-of-the-art aligned-spin waveform model, finding that the primary mass lies in the mass gap with 90\% credibility.