Preferential Accretion onto the Secondary Black Hole Strengthens Gravitational Wave Signals

Julia M. Comerford, Joseph Simon

First listed 2025-10-07 | Last updated 2025-10-07

Abstract

Pulsar timing arrays have recently found evidence for nanohertz gravitational waves that are consistent with being produced by a cosmological population of binary supermassive black holes (SMBHs). However, the amplitude of this gravitational wave background is larger than predicted from theoretical and empirical models of SMBH binary populations. We investigate preferential accretion onto the secondary, less massive SMBH of the binary as a potential solution to this discrepancy. We carry out the first observationally-based analysis of the effect of preferential accretion on the SMBH binary population, and we find that preferential accretion onto the secondary SMBH increases the binary SMBH mass ratio, causing many minor galaxy mergers to lead to major SMBH mergers. The fraction of SMBH mergers that are major mergers increases by a factor of 2-3 when preferential accretion is included. Further, we find that only a small amount of preferential accretion (10% total SMBH mass growth) is needed to bring the predicted gravitational wave background amplitude into agreement with observations. Preferential accretion has an even larger effect on gravitational wave signals detected by LISA, which will probe SMBH binaries at higher redshifts where the environment is more gas-rich, and can also help explain the rapid build up of overmassive black holes at high redshifts observed by the James Webb Space Telescope. It also shortens the time to the first detection of an individual SMBH binary emitting continuous waves. Preferential accretion strengthens the gravitational wave signals produced by any binary embedded in a circumbinary disk, including LIGO sources.

Short digest

ApJ-accepted, this paper delivers the first observationally anchored test of how preferential accretion onto the secondary SMBH reshapes binary demographics and gravitational-wave output. Using SDSS close-pair merger rates and empirical SMBH scaling inputs, the authors show that modest differential growth raises mass ratios, converting many minor galaxy mergers into major SMBH mergers and boosting the major-merger fraction by a factor of 2–3. Only ~10% total SMBH mass growth onto the secondary brings the predicted nanohertz gravitational-wave background into agreement with PTA measurements and accelerates the first continuous-wave binary detection. The effect is even stronger for LISA-era binaries in gas-rich, higher‑z environments, offering a path toward the rapid build-up of overmassive black holes seen by JWST.

Key figures to inspect

• Mass-ratio evolution: before/after preferential accretion q-distributions or q vs accreted-mass fraction—verify how many systems cross the paper’s adopted major-merger threshold and quantify the shift in the median q.
• GWB amplitude impact: predicted background amplitude versus level of preferential accretion—identify where the curve intersects the PTA-inferred amplitude band and check that ≈10% total SMBH growth suffices.
• Major-merger fraction: comparison of fractions with and without differential accretion, possibly binned by host mass or redshift—confirm the stated 2–3× boost and where it is most pronounced.
• LISA population implications: strain–frequency or SNR distribution for massive binaries with/without preferential accretion—inspect the enhancement at higher redshift where gas fractions are larger.
• PTA continuous-wave prospects: timeline or detection probability versus observing time under different accretion assumptions—see how preferential accretion advances the first resolvable source.