Year: 2026
Type: preprint
Abstract: We investigate correlations between prominent features in the relativistic X-ray reflection spectrum emitted by an accreting supermassive black hole (SMBH) binary with the underlying properties of the binary system. Model-independent measurements of the relativistic Fe K$α$ line ($\sim$6.4 keV) and the Compton reflection hump ($\sim$20-30 keV) are shown to be useful in constraining binary parameters. We compute 24,570 X-ray reflection spectra from two mini-disks attached to SMBHs at a fixed separation of 100 $GM/c^2$ on circular orbits, by varying its mass ratio, spin parameters, inclination, orbital phase and total mass accretion rate. We find that the location of the blue peak in the relativistic Fe K$α$ is a relatively robust diagnostic of the binary inclination, which could be obtained from a single-epoch X-ray spectrum. Given a few epochs of spectra, one may be able to determine the orbital phase of the binary and place constraints on its mass ratio by monitoring the Fe K$α$ centroid. Of all parameters, SMBH spin effects are most subtle and prone to measurement degeneracies. Some markers of high SMBH spin may nevertheless surface in the composite spectrum due to increased radiative efficiency. The approach developed here can be used to place preliminary constraints on binary parameters before a full parametrized X-ray spectral fitting method is available. It complements gravitational wave measurements by the Pulsar Timing Arrays (PTAs) and the Laser Interferometer Space Antenna by providing independent constraints on binary parameters that may be prone to degeneracy (inclination), or otherwise inaccessible (spin for PTAs).
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: Collaborative Research: Massive Black Hole Binaries as Multimessenger Sources: The X-ray Counterparts to Gravitational Wave Emission, G2430912111, Collaborative Research: Massive Black Hole Binaries as Multimessenger Sources: The X-ray Counterparts to Gravitational Wave Emission, G42542269, FOR 5195: Relativistische Jets in Aktiven Galaxien, G502385185, The Population and Evolution of Dual AGNs at Radio Wavelengths: Predictions for the ngVLA
OpenAlex ID: W7196969259