A redshifted excess in the broad emission lines after the flare of the γ-ray narrow-line Seyfert 1 PKS 2004−447

Abstract

PKS 2004−447 is a narrow-line Seyfert 1 (NLS1) harbouring a relativistic jet with γ-ray emission. On 2019 October 25, the Fermi-Large Area Telescope captured a γ-ray flare from this source, offering a chance to study the broad-line region (BLR) and jet during such violent events. This can provide insights into the BLR structure and jet interactions, which are important for active galactic nuclei (AGN) and host galaxy co-evolution. We report X-shooter observations of enhancements in the broad line components of Balmer, Paschen, and He I lines seen only during the post-flare and vanishing 1.5 years after. These features are biased redwards up to ∼250 km s−1 and are narrower than the pre-existing broad line profiles. This indicates a connection between the relativistic jet and the BLR of a young AGN, and how γ-ray production can lead to localised addition of broad emission lines.