Possible observational signatures of supermassive black hole binaries in their Fe Kα line profiles

Abstract

Here we study the potential observational signatures of supermassive black hole binaries (SMBHBs) in the Fe Kα line profiles emitted from the accretion disks around their components. We simulated the Fe Kα line emission from the relativistic accretion disks using ray tracing method in Kerr metric. The obtained profiles from the SMBHBs are then compared with those in the case of the single supermassive black holes (SMBHs). We considered two models of the SMBHBs: a model when the secondary SMBH is embedded in the accretion disk around the primary, causing an empty gap in the disk, and a model with clearly separated components, where the accretion disks around both primary and secondary give a significant contribution to the composite Fe Kα line emission of a such SMBHB. The obtained results showed that both models of SMBHBs can leave imprints in the form of ripples in the cores of the emitted Fe Kα line profiles, which may look like an absorption component in the line profile. However, in the case of the composite line profiles emitted from two accretion disks, these ripples could have much higher amplitudes and strongly depend on orbital phase of the system, while for those emitted from a disk with an empty gap, the corresponding ripples mostly have lower amplitudes and do not vary significantly with orbital phase. The present day X-ray telescopes are not able to detect such signatures in the observed X-ray spectra of SMBHBs. However this will be possible with the next generation of X-ray observatories, which will also enable application of such effects as a tool for studying the properties of these objects.

Frontiers in accretion physics at high X-ray spectral resolution

“…Such objects in the process of merging are expected to be prime targets for the next generation of space gravitational wave interferometers [82]. If one or both of the component SMBHs are accreting, complex reflection line profiles are expected [83,84]. The combination of electromagnetic line shifts with gravitational wave detections will enable unique constraints on relativitic physics and SMBH growth models via accretionBinary SMBHs that have suffered a recoil of magnitude ∼ 500 km s −1 or more [85] could be easily isolated, if the recoiling SMBH is accreting and shows X-ray line features clearly offset from narrow lines that arise farther out.…”