Possible observational signatures of SMBHBs in their Fe K$\alpha$ line profiles

TL;DR

This study investigates how supermassive black hole binaries (SMBHBs) might be identified through distinctive ripples in their Fe Kα line profiles, which could be detectable with future X-ray observatories.

Contribution

The paper models Fe Kα line profiles from SMBHBs using relativistic ray tracing, revealing potential observational signatures like ripples that differ from single SMBH profiles.

Findings

SMBHBs can produce ripples in Fe Kα line cores.

Ripples vary with orbital phase in composite disk models.

Current telescopes cannot detect these signatures yet.

Abstract

Here we study the potential observational signatures of supermassive black hole binaries (SMBHBs) in the Fe K$\alpha$ line profiles emitted from the accretion disks around their components. We simulated the Fe K$\alpha$ 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 K$\alpha$ 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 K$\alpha$ 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.