Hunting for the nature of the enigmatic narrow-line Seyfert 1 galaxy PKS 2004-447

Abstract

Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) that, in some cases, can harbor powerful relativistic jets. One of them, PKS 2004-447, shows γ-ray emission, and underwent its first recorded multifrequency flare in 2019. However, past studies revealed that in radio this source can be classified as a compact steep-spectrum source (CSS), suggesting that, unlike other γ-ray sources, the relativistic jets of PKS 2004-447 have a large inclination with respect to the line of sight. We present here a set of spectroscopic observations of this object, aimed at carefully measuring its black hole mass and Eddington ratio, determining the properties of its emission lines, and characterizing its long term variability. We find that the black hole mass is (1.5 ± 0.2)×107 M⊙, and the Eddington ratio is 0.08. Both values are within the typical range of NLS1s. The spectra also suggest that the 2019 flare was caused mainly by the relativistic jet, while the accretion disk played a minor role during the event. In conclusion, we confirm that PKS 2004-447 is one of the rare examples of γ-ray emitting CSS/NLS1s hybrid, and that these two classes of objects are likely connected in the framework of AGN evolution.

The Close AGN Reference Survey (CARS)

“…Thus, the inferred central black-hole (BH) masses of NLS1 are small (10 6 -10 8 M , e.g., Zhou et al 2006;Rakshit et al 2017). Although "narrow" broad lines could also be produced by inclined BLR disks in conventional BLS1s (Baldi et al 2016), inclination-independent BH mass estimates (e.g., Du et al 2014Du et al , 2015Wang et al 2014;Pan et al 2018;Berton et al 2021) and the host morphologies of NLS1s (Krongold et al 2001;Järvelä et al 2017Järvelä et al , 2018Berton et al 2019) suggest genuinely low BH masses.…”

A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei—Preliminary Results

“…With the term changing-look AGN (CLAGN), we refer to those sources that can change their classification (e.g., from BLLAC to FSRQ or vice versa) after some dramatic event, such as a change in the accretion or in the jet activity [28][29][30][31][32][33]. Sometimes the jet activity can determine changes in the observational appearance but not a change in classification: for example, the decrease/increase of equivalent width of the emission lines when the continuum increases/decreases because of the jet activity [34][35][36][37] or a shift of the synchrotron peak [38,39]. A word of caution must be set down when comparing optical spectra with very different signal-to-noise ratio (S/N): sometimes, the lack of emission lines or other features might be simply due to a combination of weak lines and low S/N spectra, not to an intrinsic variability of the AGN (e.g., [40]).…”

A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei -- Preliminary Results

“…With the term changing-look AGN (CLAGN), we refer to those sources that can change their classification (e.g., from BLLAC to FSRQ or vice versa) after some dramatic event, such as a change in the accretion or in the jet activity [28,29,30,31,33,32]. Sometimes the jet activity can determine changes in the observational appearance but not a change in classification: for example, the decrease/increase of equivalent width of the emission lines when the continuum increases/decreases because of the jet activity [34,35,36,37] or a shift of the synchrotron peak [38,39]. A word of caution must be set down when comparing optical spectra with very different signal-to-noise ratio (S/N): sometimes, the lack of emission lines or other features might be simply due to a combination of weak lines and low S/N spectra, not to an intrinsic variability of the AGN (e.g., [40]).…”