Abstract

Contributed Talk - Splinter Relativistic   (MW-2050)

Spin driven relativistic dynamics of supermassive black hole binaries in dense stellar environments

Navonil Saha, Margarita Sobolenko, Peter Berczik and Andreas Just
Astronomisches Rechen-Institut, Universität Heidelberg

The evolution of supermassive black hole (SMBH) binaries in galactic nuclei is governed by a complex interplay between stellar dynamical interactions and relativistic effects, which jointly determine their hardening efficiency and gravitational-wave (GW) signatures. We investigate the role of black hole spin in shaping the orbital evolution, energy exchange, and merger timescales of SMBH binaries embedded in dense stellar environments, focusing on a system with masses (m_1 = 3 imes 10^8 M_odot) and (m_2 = 8 imes 10^7 M_odot) (mass ratio (q approx 3.75)). We employ two complementary approaches: (i) high-resolution direct (N)-body simulations including Post-Newtonian (PN) corrections up to 2.5PN order with spin–orbit and spin–spin couplings, and (ii) controlled three-body scattering experiments using algorithmic regularization including PN corrections up to 3.5PN order. The latter allows us to isolate relativistic effects in individual high-energy encounters, while the former captures the cumulative evolution in realistic galactic nuclei. Across both approaches, we find that SMBH binary evolution is dominated by repeated high-energy scattering events with individual stellar particles, which regulate the transfer of energy and angular momentum and drive binary hardening. Spin-dependent relativistic couplings significantly modify these interactions, altering the efficiency of energy exchange and systematically affecting merger timescales, orbital eccentricity evolution, and precessional dynamics. Higher order PN effects further influence close encounters through relativistic precession and gravitational-wave dissipation, leading to measurable differences in the cumulative hardening rate. Our results demonstrate that spin induced relativistic corrections, combined with scattering dominated dynamics, play a key role in determining the long term evolution of SMBH binaries in terms of the merging timescales, energy exchanges and also the fate of the stellar particles. These effects have direct implications for the expected population of merging SMBHs and their gravitational-wave signatures, providing improved theoretical predictions for future LISA observations.