Can a time-evolving, asymmetric broad line region mimic a massive black hole binary?

¹ Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, I-20126, Milano, Italy
² SISSA – International School for Advanced Studies, Via Bonomea 265, I-34136, Trieste, Italy
³ INAF – Osservatorio Astronomico di Brera, Via Brera 20, I-20121, Milano, Italy
⁴ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126, Milano, Italy
⁵ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100, Como, Italy
⁶ Department of Astronomy and Astrophysics and Institute for Gravitation and the Cosmos, Penn State University, 525 Davey Lab, 251 Pollock Road, University Park, PA, 16802, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 April 2025
Accepted: 23 February 2026

Abstract

Gas within the influence sphere of accreting massive black holes is responsible for the emission of the broad lines observed in optical-UV spectra of unobscured active galactic nuclei. Since the region contributing the most to the broad emission lines (i.e. the broad line region) depends on the active galactic nucleus luminosity, the study of broad line reverberation to a varying continuum can map the morphology and kinematics of gas at sub-pc scales. In this study, we modify a preexisting model for disc-like broad line regions, including non-axisymmetric structures, by adopting an emissivity profile that mimics the observed luminosity-radius relation. This makes our implementation particularly well suited for the analysis of multi-epoch spectroscopic campaigns. After validating the model, we use it to check if strongly non-axisymmetric, single broad line regions could mimic the short time-scale evolution expected from massive black hole binaries. We explore different orientations and anisotropy degrees of the broad line region, as well as different light curve patterns of the continuum to which the broad line region responds. Our analysis confirms that recently proposed algorithms designed to search for massive black hole binaries in large multi-epoch spectroscopic data are not contaminated by false positives ascribed to anisotropic broad line regions around single massive black holes.