Letter to the Editor

Periodic variations in the first and second moments of broad Balmer emission lines from central accretion disks

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, GuangXi University, No. 100, Daxue road, Nanning 530004, P.R. China

^⋆ Corresponding author: xgzhang@gxu.edu.cn

Received: 14 February 2025
Accepted: 7 April 2025

Abstract

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.