A binary black hole at the heart of the nearest quasar Markarian 231

Schematic diagram for a BBH–disk (BBH = Binary Black Hole) accretion system. The BBH is assumed to be on circular orbits. The BBH is surrounded by a circumbinary disk, connecting with the mini-disk around each component of the BBH by streams. In between the circumbinary disk and the inner mini-disks, a gap or hole is opened by the secondary SMBH (= SuperMassive Black Hole)

Conclusions from the study “A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231”:

In this paper, we show that various unique features in the optical-to-UV spectrum and the intrinsic X-ray weakness of Mrk 231 can all be well explained, if a pair of SMBHs exists in the core of Mrk 231, with the masses of the primary and the secondary SMBHs as ∼ 1.5 × 10⁸M_⊙ and 4.5 × 10⁶M_⊙, respectively. The existence of a BBH in Mrk 231 is compatible with its disturbed morphology and tidal features, which indicates a merger event in the past. (Note that the secondary SMBH is rather low in mass; however, it should be able to sink down to the center because the stars initially associated with it enhance the dynamical friction.) The semimajor axis of this BBH is ∼ 590 AU, about 190 times of the Schwarzschild radius of the primary SMBH, and its orbital period is just ∼ 1.2 year, relatively short among the few known BBH candidates, which makes it an ideal system to study the dynamics of BBH systems. Such a BBH emits gravitational wave on tens of nanohertz, and the change rate of its orbital period due to gravitational wave radiation is about 40 seconds per orbit. This BBH might be a target for gravitational wave studies in future.

The orbit of such a BBH system decays on a timescale of a few times of 10⁵ year due to gravitational wave radiation and the torque of the circumbinary disk, which is not too small compared with the lifetime of quasars (a few times 10⁷ to 10⁸ year). The majority of quasars are believed to be triggered by mergers of galaxies and consequently involve mergers of SMBHs, and those BBH systems with mass ratio in the range of a few percent to 1 may lead to a notch in the optical- to-UV continuum emission if their semimajor axes are in the range of a few hundreds to about one thousands gravitational radii, which correspond to orbital decay timescales of 10⁵–10⁶ year. Therefore, the occurrence rate of active BBH systems, with deficits in the optical-to-UV emission, may be roughly a few thousandths to about one percent among quasars. Our analysis of Mrk 231 demonstrates the feasibility of finding BBH systems by searching for the deficits in the optical-to-UV emission among the spectra of quasars, a new method proposed by a number of authors.