2025A&A...696L..22Z

Broad emission line regions (BLRs) lying in central accretion disks are widely accepted to explain the unique double-peaked broad emission lines in active galactic nuclei (double-peaked BLAGNs). Accepting the accretion disk origin, we theoretically simulated and determined the periodic variations in the central wavelength λ₀ (the first moment) and line width σ (the second moment) of double-peaked broad emission lines. Furthermore, through theoretically simulated periodicities of T_λ0 and T_σ for variations in λ₀ and σ, a periodicity ratio R_fs of T_λ0 to T_σ of about 2 was applied to support the hypothesis that the spiral arms are preferred in BLRs in central accretion disks. Then, we determined the periodic variations in λ₀ and σ and showed them in the known double-peaked BLAGN NGC 1097. This led to the parameter R_fs ∼ 2, which can be applied as a clue to support the hypothesis that the spiral arm structure in disk-like BLRs in the central accretion disk is the most compelling interpretation of the variability in the double-peaked broad Hα in NGC 1097. The results provide clean criteria for testing the accretion disk origins of double-peaked broad emission lines in AGN.