Polarimetric insights into a potential binary supermassive black hole system in Mrk 231

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
² Univ. Lyon, Univ. Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, F-69622 Villeurbanne, France

^⋆ Corresponding author: julie.biedermann@astro.unistra.fr

Received: 7 January 2025
Accepted: 22 April 2025

Abstract

Context. Markarian 231 (Mrk 231) is one of the brightest ultraluminous infrared galaxies (ULIRGs) known to date. It displays a unique optical-ultraviolet (optical-UV) spectrum, characterized by a strong and perplexing attenuation in the near-UV, associated with a sudden polarization peak.

Aims. The aim of this study is to clarify the puzzling nature of Mrk 231’s spectrum by examining the potential existence of a binary supermassive black hole (SMBH) system in its core. To this end, we combined photometric information used in prior studies, with archival polarimetric data to provide a more thorough approach to the quasar’s structure and emission mechanisms. In particular, we evaluated the binary SMBH model as a potential explanation for the near-UV cutoff, while exploring its polarization response, for this source, for the first time.

Methods. Building on previous spectro-photometric modeling, we investigated the hypothesis that the core of Mrk 231 may host a binary SMBH system. In this scenario, the accretion disk of the primary, more massive SMBH is responsible for the optical-UV spectrum. The disk of the secondary, less massive SMBH, would be expected to essentially emit in the far-UV. We applied this model to archival photometric and polarimetric data of Mrk 231 and tried to obtain the best fit possible. To support our findings, we performed radiative transfer calculations to determine the spatial disposition of each main component constituting Mrk 231.

Results. We find that a binary SMBH model can reproduce both the observed flux and polarization of Mrk 231 remarkably well. We infer that the core potentially hosts a binary SMBH system, with a primary SMBH of about 1.6 × 10⁸ M_⊙ and a secondary of about 1.1 × 10⁷ M_⊙, separated by a semimajor axis of ∼146 AU. The secondary SMBH drives a degree of polarization of ∼3% between 0.1 and 0.2 μm, with a corresponding polarization position angle of about 134°, which is consistent with scattering within an accretion disk. The primary SMBH and the structure around it are responsible for a degree of polarization of ∼23% between 0.3 and 0.4 μm with a corresponding polarization position angle of about 96°, that is possibly attributed to scattering within the quasar’s wind. Finally, our model predicts the existence of a second peak in polarized flux in the far-ultraviolet, a telltale signature that could definitively prove the presence of a binary SMBH.

Conclusions. These results strongly support the hypothesis of a binary SMBH in Mrk 231 and emphasizes the need for new far-ultraviolet spectropolarimeters to clearly detect the existence of subparsec binary SMBHs in nearby quasars.