X-Ray Reflection Signatures of Supermassive Black Hole Binaries

Abstract We investigate the presence of supermassive black hole (SMBH) binary signatures and the feasibility of identifying them through X-ray reflection spectra. The X-ray emitting region is modeled as a set of two mini-disks bound to the individual SMBHs separated by 100 GM / c 2 and the spectra calculated as a function of the mass, mass ratio, and total accretion rate of the binary. The X-ray reflection features are strongly influenced by the accretion-inversion phenomenon expected in SMBH binaries, which results in a wide range of ionization conditions in the two mini-disks. These are imprinted in the resulting composite spectra and the double-peaked and time-variable relativistic Fe K α line profiles. To test whether these features can be used as evidence for the presence of an SMBH binary, we fit mock 100 ks observations with a single active galactic nucleus (AGN) model. For a 10 9 M ⊙ binary targeted by pulsar timing arrays (PTAs), at z = 0.1, the single AGN model clearly fails to fit the data, while at z = 1, the fit is acceptable but unable to converge on the SMBH spin. For a 10 6 M ⊙ binary, a progenitor of a Laser Interferometer Space Antenna (LISA) source, spectral fitting is only possible at z = 0.1, with the outcomes similar to the PTA binary at z = 1. We also find that PTA binaries can be expected to show a distinct X-ray spectral variability in multiepoch observations, whereas for LISA precursors, orbital averaging results in the loss of spectral variability signatures.