The Large Amplitude X-ray Variability in NGC 7589: Possible Evidence for Accretion Mode Transition

TL;DR

This study reports large amplitude X-ray variability in NGC 7589, providing observational evidence for an accretion mode transition from an ADAF to a thin disc, over a timescale of a few years.

Contribution

It presents the first evidence of accretion mode transition in a low luminosity AGN based on multi-wavelength long-term observations.

Findings

X-ray flux varied by a factor of over 50 over 17 years.

Eddington ratio increased from 10^{-3} to 0.13.

Detection of a soft X-ray component consistent with a thin disc.

Abstract

We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 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/XRT and XMM-Newton over 17 years. Results of long-term monitoring data in the UV, optical and infrared bands over ~20 years are also presented. The Eddington ratio increased from $10^{-3}$ to $\sim0.13$, suggesting a transition of the accretion flow from an ADAF 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 ($\sim19^{+15}_{-7}$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 $\sim10^{7}\,M_{\mathrm{Sun}}$. These results may indicate that NGC 7589 had experienced accretion mode transition over a timescale of a few years, suggesting the idea that similar accretion processes are at work for massive black hole and black hole X-ray binaries.