Asymmetry in the Spectrum of High-Velocity H2O Maser Emission Features in Active Galactic Nuclei

TL;DR

This paper proposes a model explaining the asymmetry in high-velocity water maser emissions in active galactic nuclei, attributing it to hyperfine splitting effects in the accretion disk, which enhances blueshifted features.

Contribution

It introduces a new mechanism linking hyperfine splitting to spectral asymmetry in water masers within accretion disks of active galactic nuclei.

Findings

Blueshifted features are brighter than redshifted ones in observed sources.

The model explains the asymmetry observed in UGC 3789.

Hyperfine splitting causes gain profile asymmetry in maser emissions.

Abstract

We suggest a mechanism for the amplification of high-velocity water-vapor maser emission features from the central regions of active galactic nuclei. The model of an emitting accretion disk is considered. The high-velocity emission features originate in the right and left wings of the Keplerian disk. The hyperfine splitting of the signal levels leads to an asymmetry in the spectral profile of the water vapor maser line at a frequency of 22.235 GHz. We show that the gain profile asymmetry must lead to an enhanced brightness of the blueshifted high-velocity emission features compared to the redshifted ones. Such a situation is observed in the source UGC 3789.