A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231

TL;DR

This paper proposes that the quasar Mrk 231 hosts a supermassive binary black hole with a 590 AU separation and a 1.2-year orbit, inferred from its unique optical-UV spectrum, offering a new method to detect such systems.

Contribution

The study presents the first interpretation of Mrk 231's spectrum as emission from a binary black hole system, introducing a novel spectral signature for identifying sub-parsec BBHs.

Findings

Spectral features consistent with a BBH in Mrk 231

Estimated orbital period of ~1.2 years

Proposed a new method for detecting BBHs via optical-UV deficits

Abstract

Supermassive binary black holes (BBHs) are unavoidable products of galaxy mergers and are expected to exist in the cores of many quasars. Great effort has been made during the past several decades to search for BBHs among quasars; however, observational evidence for BBHs remains elusive and ambiguous, which is difficult to reconcile with theoretical expectations. In this paper, we show that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and an orbital period of ~1.2 year. The flat optical and UV continua are mainly emitted from a circumbinary disk and a mini-disk around the secondary black hole (BH), respectively; and the observed sharp drop off and flux deficit at wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the secondary BH migrating within the circumbinary disk. If confirmed by future observations, this BBH will provide a unique laboratory to study the interplay between BBHs and accretion flows onto them. Our result also demonstrates a new method to find sub-parsec scale BBHs by searching for deficits in the optical-to-UV continuum among the spectra of quasars.