Broad reprocessed Balmer emission from warped accretion discs

TL;DR

This paper models emission lines from warped accretion discs around black holes to explain asymmetries and features in double-peaked lines observed in active galactic nuclei, highlighting warping as a key factor.

Contribution

It introduces a parametrized model of warped accretion discs to explain complex emission line profiles and their asymmetries in active galactic nuclei.

Findings

Warped discs can produce asymmetric line profiles with blue/red peak ratios less than one.

Disc warping can generate a third peak in emission line profiles.

The model successfully reproduces features observed in SDSS AGN data.

Abstract

Double peaked broad emission lines in active galactic nuclei are generally considered to be formed in an accretion disc. In this paper, we compute the profiles of reprocessing emission lines from a relativistic, warped accretion disc around a black hole in order to explore the possibility that certain asymmetries in the double-peaked emission line profile which can not be explained by a circular Keplerian disc may be induced by disc warping. The disc warping also provides a solution for the energy budget in the emission line region because it increases the solid angle of the outer disc portion subtended to the inner portion of the disc. We adopted a parametrized disc geometry and a central point-like source of ionizing radiation to capture the main characteristics of the emission line profile from such discs. We find that the ratio between the blue and red peaks of the line profiles becoming less than unity can be naturally predicted by a twisted warped disc, and a third peak can be produced in some cases. We show that disc warping can reproduce the main features of multi-peaked line profiles of four active galactic nuclei from the Sloan Digital Sky Survey.