The Binary Black Hole Model for Mrk 231 Bites the Dust

TL;DR

This paper challenges the milli-parsec black-hole binary model for Mrk 231, showing that the observed UV continuum is likely reddened and not intrinsic, and discusses implications for the quasar's spectral energy distribution and structure.

Contribution

The study demonstrates that Mrk 231's UV continuum is consistent with a reddened AGN rather than an intrinsic emission, providing new insights into its spectral energy distribution and structure.

Findings

The UV continuum is likely reddened, not intrinsic.

HeI*/Pbeta ratio can diagnose reddening in quasars.

Far-UV emission is resolved on ~40 parsec scales.

Abstract

Mrk 231 is a nearby quasar with an unusually red near-UV-to-optical continuum, generally explained as heavy reddening by dust (e.g., Leighly et al. 2014). Yan et al. 2015 proposed that Mrk~231 is a milli-parsec black-hole binary with little intrinsic reddening. We show that if the observed FUV continuum is intrinsic, as assumed by Yan et al. 2015, it fails by a factor of about 100 in powering the observed strength of the near-infrared emission lines, and the thermal near and mid-infrared continuum. In contrast, the line and continuum strengths are typical for a reddened AGN spectral energy distribution. We find that the HeI*/Pbeta ratio is sensitive to the spectral energy distribution for a one-zone model. If this sensitivity is maintained in general broad-line region models, then this ratio may prove a useful diagnostic for heavily reddened quasars. Analysis of archival HST STIS and FOC data revealed evidence that the far-UV continuum emission is resolved on size scales of ~40 parsecs. The lack of broad absorption lines in the far-UV continuum might be explained if it were not coincident with the central engine. One possibility is that it is the central engine continuum reflected from the receding wind on the far side of the quasar.