Structure and Kinematics of the Broad-Line Region in Selected Changing-Look AGN
by Martin W. Ochmann
Date of Examination:2024-03-12
Date of issue:2024-10-18
Advisor:Prof. Dr. Wolfram Kollatschny
Referee:Prof. Dr. Wolfram Kollatschny
Referee:Prof. Dr. Ansgar Reiners
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Abstract
English
In this thesis, the results of three variability studies conducted on three individual active galactic nuclei (AGN) are presented. Each of these AGN belongs to the class of changing-look (CL) AGN, which are characterized by their change of spectral classification, switching between Type I and Type II and associated sub-types. The first chapter introduces the paradigm of AGN and reviews their internal structure with a particular focus on the broad-line region (BLR). The basic concepts of AGN variability campaigns are explained, the concept of reverberation mapping (RM) is introduced, and some current challenges – relevant to this thesis – related to BLR and CL AGN research are outlined. The second chapter presents the results of a densely sampled, optical spectroscopic variability campaign of Mrk 926 with the HET in 2010. Mrk 926 was observed during a drastic phase of decline, during which the optical luminosity decreased by more than 50 % within 2.5 months. The spectra reveal promptly varying outer Balmer satellite components between ±5000 km/s and ±13 000 km/s . By means of 1D and 2D reverberation mapping, the BLR was identified to have a ring-like structure with radii of 26.5 and 33.5 light-days for Hβ and Hα, respectively. This results in a mass of the central supermassive black hole (SMBH) of (1.1 ± 0.2) × 10⁸ M_⊙ and an AGN inclination of i ∼ 50° . The SMBH mass gives a low Eddington ratio L/L_{Edd} of Mrk 926, decreasing from 8 % to 3 % during the campaign. The power-law index of the RMS spectrum of the campaign is consistent with predictions from the Shakura & Sunyaev thin accretion disk (AD) model, indicating that reddening by dust is not the cause for the drop in continuum luminosity. The kinematic signature of the outer Balmer satellite components suggests an origin in a region distinct from the rest of the BLR, probably a hollow outflow cone or a small-scale jet. The third chapter presents the results of a multi-wavelength, photometric as well as spectroscopic variability campaign of IRAS 23226-3854 with XMM-Newton, NuSTAR, Swift and SALT during a CL outburst in 2019. The X-ray and optical continuum increased by a factor of 5 and 3, respectively, and IRAS 23226-3854 transitioned to a Type-I Seyfert galaxy. The Hβ and Hα profiles show no major variations and are double-peaked, indicating an origin in a rotating disk-like BLR. X-ray and Balmer line absorption indicate the presence of outflows with velocities of v ∼ 36 000 km/s and v ∼ 10 000 km/s , respectively. The X-ray spectrum is unabsorbed, suggesting that the changes in continuum flux are caused by changes in accretion rate. The mass of the SMBH is 1.7 × 10⁸ M_⊙ , as determined in our earlier study of this source. Over the course of the entire campaign from 1997 to 2022, the shape of the Balmer emission line profiles changes independently of the continuum intensity, suggesting internal changes in the BLR structure. The fourth chapter presents results of an optical spectroscopic variability campaign of NGC 1566 taken with VLT/MUSE and SALT before, during, and after its CL outburst in 2018. The profiles of O I λ8446 and the near-IR Ca II triplet lines are asymmetric and double-peaked with widths of FWHM ∼ 1920 km/s . This is the first time that double-peaked O I λ8446 and Ca II triplet lines in AGN are reported in the literature, and small-scale features in the profiles suggest the detection of BLR inhomogeneities. The profiles are well modeled by emission from a relativistic eccentric accretion disk with an inclination of i ∼ 8° plus an additional component, likely originating from above or further out in the BLR disk. The modeled size of the disk is in agreement with established reverberation mapping scaling relations. The Hβ profiles can be reconstructed from the double-peaked profiles by convolving them with a Lorentzian with a width of 900 km/s , suggesting a vertical stratification of the BLR with respect to scale-height dependent turbulence. A blueward drift of the Hβ profiles during the onset of the CL event might indicate the onset of an accretion disk wind. Except for this wind, the observed kinematics of BLR remain unchanged based on the largely constant FWHM(Hβ) of ∼ 2200 km/s . Based on the FWHM of Hβ, the stellar dispersion σ_∗ = 98^{+12}_{−9} km/s , and results from previous studies, the SMBH mass is estimated as (5.3 ± 2.7) × 10⁶ M_⊙. In the fifth chapter, the results of the variability studies are discussed and put into context of current AGN research. All investigated CL AGN exhibit disk- or ring-like BLR structures. Outflows and winds were detected in IRAS 23226-3854 and NGC 1566. The studies suggest that obscuration is not responsible for the changing-look phenomenon in these AGN , and that the cause for (apparent) changes in the BLR can be increasing continuum luminosity or structural changes in the BLR on dynamical time scales. Densely sampled multi-wavelength variability campaigns of ongoing CL transitions are crucial to study the CL phenomenon in more detail in the future.
Keywords: active galactic nuclei; galaxies; supermassive black holes; accretion; broad-line region; changing-look; transient event; reverberation mapping; spectroscopy; MRK 926; IRAS 23226-3854; NGC 1566