Observable Consequences of Merger-Driven Gaps and Holes in Black Hole Accretion Disks

TL;DR

This paper explores how gaps or holes in black hole accretion disks caused by binary interactions produce observable signatures in AGN spectra, enabling detection and characterization of black hole binaries.

Contribution

It introduces a method to infer black hole binary properties from spectral features, particularly the gap width and mass ratio, using observable spectral slopes.

Findings

Spectral energy distribution shows a decrement due to gaps in accretion disks.

The slope change in the broken power-law correlates with gap width and mass ratio.

AGN with inner cavities are predicted to be optically bright but X-ray faint.

Abstract

We calculate the observable signature of a black hole accretion disk with a gap or hole created by a secondary black hole embedded in the disk. We find that for an interesting range of parameters of black hole masses (~10^6 to 10^9 solar masses), orbital separation (~1 AU to ~0.1 pc), and gap width (10 to 190 disk scale heights), the missing thermal emission from a gap manifests itself in an observable decrement in the spectral energy distribution. We present observational diagnostics in terms of power-law forms that can be fit to line-free regions in AGN spectra or in fluxes from sequences of broad filters. Most interestingly, the change in slope in the broken power-law is almost entirely dependent on the width of gap in the accretion disk, which in turn is uniquely determined by mass ratio of the black holes, such that it scales roughly as q^(5/12). Thus one can use spectral observations of the continuum of bright active galactic nuclei to infer not only the presence of a closely separated black hole binary but also the mass ratio. When the black hole merger opens an entire hole (or cavity) in the inner disk, the broad band SED of the AGN or quasar may serve as a diagnostic. Such sources should be especially luminous in optical bands but intrinsically faint in X-rays (i.e., not merely obscured). We briefly note that viable candidates may have already been identified, though extant detailed modeling of those with high quality data have not yet revealed an inner cavity.