2020MNRAS.492.2335L

We report the discovery of large amplitude X-ray variability in the low-luminosity active galactic nucleus NGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/X-Ray Telescope and XMM-Newton over 17 yr. Results of long-term monitoring data in the UV, optical, and infrared bands over ∼20 yr are also presented. The Eddington ratio λ_Edd increased from 10^–3 to ∼0.13, suggesting a transition of the accretion flow from an advection dominated accretion flow to a standard thin accretion disc. Further evidence supporting the thin disc in the high-luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component (∼19⁺¹⁵_–7 eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of ∼10⁷M_☉. These results may indicate that NGC 7589 had experienced accretion mode transition over a time-scale of a few years, suggesting the idea that similar accretion processes are at work for massive BH and BH X-ray binaries.