Changing-look Narrow-Line Seyfert 1s?

Abstract

Two major challenges to unification schemes for active galactic nuclei (AGN) are the existence of Narrow-Line Seyfert 1s (NLS1s) and the existence of changing-look (CL) AGNs. AGNs can drastically change their spectral appearance in the optical (changing their Seyfert type) and/or in the X-ray region. We illustrate the CL phenomenon with our multi-wavelength monitoring of NGC 2617 and discuss its properties compared with NLS1s. There are few examples of CL NLS1s and the changes are mostly only in the X-ray region. It has been proposed that some of these could be cases of a tidal-disruption events (TDE) or supernova events. If BLRs have a flat geometry and NLS1s are seen face-on then we have to see CL cases only if the orientation of the BLR changes as a result of a TDE or a close encounter of a star without a TDE. If NLS1s include both high Eddington accretion rate and low-inclination AGNs then a significant fraction of NLS1s could be obscured and would not be identified as NLS1s. CL cases might happen more in such objects if dust sublimation occurs following a strong increase in the optical luminosity.Revisiting narrow-line Seyfert 1 galaxies and their place in the Universe -NLS1 Padova

Changing-Look Narrow-Line Seyfert 1 Galaxies, their Detection with SVOM, and the Case of NGC 1566

“…Only few NLS1-type galaxies have so far been identified as CL AGN (Table 1) 2 . This Section provides a brief overview of their properties, and critically discusses the need to distinguish from other phenomena like type II SNe in dense media and (very rare) stellar tidal disruption events.…”

“…Several models have been suggested to explain CL AGN. Some have explored extinction variability, for instance through dusty material crossing our line of sight or alternatively by dust destruction in response to an outburst [2]. The majority of theoretical models has focused on short-term changes in the accretion rate (on the time scale of months-decades), for instance due to disk instabilities or other mechanisms (e.g., the radiation pressure disk instability, the Hydrogen ionization instability, magneticpressure-supported disks, magnetic flux inversion, instabilities in warped disks at large radii, or shock formation in highly precessing disks ) (e.g., [3][4][5][6][7][8][9][10][11][12][13][14][15]) or have explored the possibility of tidal disk interactions in binary SMBH systems [16].…”

Instability of Circumnuclear Gas Supply as an Origin of the “Changing-look” Phenomenon of Supermassive Black Holes

“…We argue that the Bondi accretion scenario is helpful for explaining the rare detection of the CL phenomenon of narrowline Seyfert 1 galaxies (NLS1s). To date, there are only five CL-NLS1s (e.g., Oknyansky et al 2018;Frederick et al 2019;MacLeod et al 2019;Hon et al 2022;Xu et al 2024). Taking into account their small SMBHs (M BH ∼ 10 7 M e ) and high accretion rate close to the Eddington limit (e.g., Boroson 2002;Grupe 2004;Xu et al 2012; see Komossa 2008 for a review), the rarity of CL-NLS1s could be explained in terms of the proposed Bondi accretion scenario because of the very long CL cycle time (∼10 3 yr) predicted by Equation (3).…”

On the Time Scales of Optical Variability of AGN and the Shape of Their Optical Emission Line Profiles

“…The variability is most significant in case of some "changing look"(CL) objects, such as NGC 5548 [9], NGC 4151 [7], and some other examples see, (e.g., [20,21]). Some of them appear to be periodic, such as for example NGC 5548, NGC 4151, and a few more periodic CL candidates; see [7,9,22].…”