Reprocessed emission from warped accretion discs induced by the Bardeen-Petterson effect

TL;DR

This paper models the reprocessed emission lines from warped accretion discs affected by the Bardeen-Petterson effect, revealing complex line profiles that could explain observed triplet-peaked lines.

Contribution

It introduces a numerical model of warped accretion discs and predicts their emission line profiles, linking disc warping to observable spectral features.

Findings

Warped discs twist by about π radians before flattening.

Line profiles depend weakly on source radius between 3r_g and 10r_g.

Triplet-peaked profiles may indicate warped discs in observations.

Abstract

The broad Balmer emission-line profiles resulting from the reprocessing of UV/X-ray radiation from a warped accretion disc induced by the Bardeen-Petterson effect are studied. We adopt a thin warped disc geometry and a central ring-like illuminating source in our model. We compute the steady-state shape of the warped disc numerically, and then use it in the calculation of the line profile. We find that, from the outer radius to the inner radius of the disc, the warp is twisted by an angle of $\sim\pi$ before being flattened efficiently into the equatorial plane. The profiles obtained depend weakly on the illuminating source radius in the range from $3r_{g}$ to $10r_g$, but depend strongly on this radius when it approaches the marginally stable orbit of an extreme Kerr black hole. Double- or triplet-peaked line profiles are present in most cases when the illuminating source radius is low. The triplet-peaked line profiles observed from the Sloan Digital Sky Survey may be a {"}signature" of a warped disc.