Ripple effects and oscillations in the broad Fe Kα line as a probe of massive black hole mergers

Abstract

When a sufficiently massive satellite (or secondary) black hole is embedded in a gas disk around a (primary) supermassive black hole, it can open an empty gap in the disk. A gap-opening secondary close to the primary will leave an imprint in the broad component of the Fe Kα emission line, which varies in a unique and predictable manner. If the gap persists into the innermost disk, the effect consists of a pair of dips in the broad line which ripple blue-ward and red-ward from the line centroid energy respectively, as the gap moves closer to the primary. This ripple effect could be unambiguously detectable and allow an electromagnetic monitoring of massive black hole mergers as they occur. As the mass ratio of the secondary to primary black hole increases to q 0.01, we expect the gap to widen, possibly clearing a central cavity in the inner disk, which shows up in the broad Fe Kα line component. If the secondary stalls at 10 2 r g in its in-migration, due to low co-rotating gas mass, a detectable ripple effect occurs in the broad line component on the disk viscous timescale as the inner disk drains and the outer disk is dammed. If the secondary maintains an accretion disk within a central cavity, due to dam bursting or leakage, a periodic 'see-saw' oscillation effect is exhibited in the observed line profile. Here we demonstrate the range of ripple effect signatures potentially detectable with Astro-H and IXO/Athena, and oscillation effects potentially detectable with XMM-Newton or LOFT for a wide variety of merger and disk conditions, including gap width (or cavity size), disk inclination angle and emissivity profile, damming of the accretion flow by the secondary, and a mini-disk around the satellite black hole. A systematic study of ripple effects would require a telescope effective area substantially larger than that planned for IXO/Athena. Future mission planning should take this into account. Observations of the ripple effect and periodic oscillations can be used to provide an early warning of gravitational radiation emission from the AGN. Once gravitational waves consistent with massive black hole mergers are detected, an archival search for the FeKα ripple effect or periodic oscillations will help in localizing their origin.

On the High-Energy Neutrino Emission from Active Galactic Nuclei

“…The orbital motion of the secondary BH opens a gap in the disk, and it affects the line profile. The authors of [70] have simulated the FeKα line of the accretion disk of a SMBH in a 10, 20, 50, 90 Schwarzschild radii separated binary (with 10 per-cent error on the separation), and their simulations resulted ripples on the theoretical line-shape induced by such orbital motion, which do not appear on the unperturbed iron line. Considering the closeness of these systems they could be possible sources of strong GW bursts over human lifetime.…”

A reduced orbital period for the supermassive black hole binary candidate in the quasar PG 1302-102?

“…Iron K α lines. Because the binary separation can be reduced below ∼ < 100RS, FeK α lines generated at such small separations can have characteristic binary-related features, such as 'missing wings' (due to the central cavity), or 'see-saw oscillations' of the red and blue wings (due to Doppler-shifting of the emission from minidiscs; McKernan et al 2013). These may be detectable with the upcoming Astro-H mission (Takahashi et al 2014).…”

Detection of radial velocity shifts due to black hole binaries near merger

“…In the limit of a b of a few tens of rg, we should expect the line profile to differ from the simple model below (see e.g. Sesana et al (2012); McKernan et al (2013) and references in the latter for a discussion of the wide range of possible configurations including secondary disks, gaps and cavities). However most FeKα emission may originate within ∼ 20 − 30rg of the primary and very tight binaries (a b of tens of rg) will be much rarer than those at wider separations (a b of a few hundred rg).…”

“…Reynolds & Nowak 2003, for a review). See McKernan et al (2013) (and references therein) for a discussion of how the broad FeKα line component can be used to probe configurations of gas in the inner accretion disk and therefore test for possible MBH binaries. The narrow core of the FeKα line observed in AGN (e.g.…”