ESPRESSO reveals a single but perturbed broad-line region in the supermassive black hole binary candidate PG 1302–102

In this work, we present a new, fully Bayesian analysis of the highest-resolution optical spectrum of the supermassive black hole (SMBH) binary candidate PG 1302−102, obtained with ESPRESSO at the VLT (R ≃ 138 000). Our methodology, based on robust Bayesian model selection, reveals the presence of multiple narrow emission lines at the expected redshift of the source and confirms (for Hβ) and detects (for Hγ) the clear presence of redshifted broad components. Additionally, we have discovered a “very broad” and, if it is associated with the Hβ, “very redshifted” component at λ ≃ 5000 Å. We evaluate two scenarios for explaining the observed broad emission line (BEL) features in PG 1302−102. In the case in which the redshifted BEL asymmetry arises from the orbital motion of a putative binary, our measurements coupled with simple estimates of the broad-line region (BLR) sizes suggest that the individual black hole BLRs are either settled in a single BLR or in the process of merging and, therefore truncated and highly disturbed. Alternatively, in the scenario of a single SMBH, we explain the distorted emission of the BELs with a nonsymmetric distribution of the BLR clouds; namely, a thin disk with a spiral perturbation. This BLR configuration is statistically preferred over any empirical multi-Gaussian fit and simultaneously explains the asymmetric emission of the Hβ and Hγ close to the bulk of the line and any additional excess (or the lack of it, in the case of the Hγ) at much longer wavelengths. The physical origins of the perturbation are still unclear and a connection with the possible presence of a black hole binary cannot be ruled out. Given the growing evidence from theoretical and observational works demonstrating the common presence of disturbed BLRs in active galactic nuclei, we argue that an origin related to self-gravitating instabilities may be more plausible.