Slowly decaying ringdown of a rapidly spinning black hole. II. Inferring the masses and spins of supermassive black holes with LISA
Phys. Rev. D 113, 043020 – Published 9 February, 2026
DOI: https://doi.org/10.1103/kvvw-w3nj
Abstract
Electromagnetic observations reveal that almost all galaxies have supermassive black holes (SMBHs) at their centers, but their properties, especially their spins, are not fully understood. Some of the authors have recently shown [N. Oshita and D. Tsuna, Phys. Rev. D 108, 104031 (2023)] that rapid spins of more than 0.9, inferred for masses around from observations of local SMBHs and cosmological simulations, source long-lived ringdowns that enhance the precision of black hole spectroscopy to test gravity in the near-extreme Kerr spacetime. In this work, we estimate the statistical errors in the SMBH mass-spin inference in anticipation of LISA’s detection of extreme mass ratio mergers. We show that for rapidly spinning SMBHs, more precise mass and spin measurements are expected due to the excitations of higher angular modes. For a near-extremal SMBH of mass merging with a smaller BH with mass ratio at a luminosity distance of more than or around 5 Gpcs, the measurement errors in the mass and spin of the SMBH would be about 10% and 1%, respectively.
