Multiwavelength properties of changing-state active galactic nuclei

I. The evolution of soft excess and X-ray continuum

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² Department of Physics, SRM University-AP, Amaravati, 522240, Andhra Pradesh, India
³ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁵ INAF-Osservatorio Astronomico di Roma, via di Frascati 33, I-00078 Monte Porzio Catone, Italy
⁶ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁷ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁸ Excellence Cluster ORIGINS, Boltzmannsstraße 2, 85748 Garching, Germany
⁹ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D Arica, Chile
¹⁰ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹¹ Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602-3017, USA
¹² Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹³ STAR Institute, Liège Université, Quartier Agora – Allée du six Août, 19c, B-4000 Liège, Belgium
¹⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
¹⁵ Institute of Astronomy, National Tsing Hua University, Hsinchu 300044, Taiwan
¹⁶ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
¹⁷ Hamburger Sternwarte, Universitat Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
¹⁸ Humboldt Research Fellow, Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn D-53121, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 July 2025
Accepted: 8 January 2026

Abstract

Changing-state active galactic nuclei (CSAGNs) exhibit rapid variability; their mass accretion rates can change by several orders of magnitude in a few years. This provides us with a unique opportunity to study the evolution of the inner accretion flow almost in real time. Here we used over 1000 observations to study the broadband X-ray spectra of a sample of five CSAGNs, spanning three orders of magnitude in Eddington ratio (λ_Edd), using phenomenological models to trace the evolution of key spectral components. We derive several fundamental parameters, such as the photon index, soft excess strength, reflection strength, and luminosities of the soft excess and primary continuum. We find that the soft excess and primary continuum emissions show a very strong positive correlation (p ≪ 10⁻¹⁰), suggesting a common physical origin. The soft excess strength does not show any dependence on the reflection parameter, suggesting that in these objects the soft excess is not dominated by a blurred ionized reflection process. On the other hand, the strength of the soft excess is found to be strongly positively correlated with the Eddington ratio (p ≪ 10⁻¹⁰), and we find that the soft excess vanishes below log λ_Edd ∼ −2.5. Moreover, we find a clear V-shaped relation for Γ − λ_Edd, with a break at log λ_Edd = −2.47 ± 0.09. Our findings indicate a change in the geometry of the inner accretion flow at low Eddington ratios, and that the soft excess is primarily produced via warm Comptonization.