Discovery of high-amplitude X-ray variability in the Seyfert-LINER transition galaxy NGC7589

Abstract

We present the first result of a programme to search for large flux variations in the X-ray sources of the XMM Serendipitous Survey compared to previous ROSAT observations. An increase in X-ray flux by a factor >10 was discovered from the nucleus of the galaxy NGC7589 on a timescale of less than 5 years. The 0.4-10keV XMM spectrum can be approximated by a power-law with photon index of 1.7-1.8, though it seems to flatten above 5keV, suggesting a possible complex model, such as partial covering or disc reflection. A classification based on an analysis of its optical spectrum places NGC7589 in the Seyfert region, but close to the Seyfert-LINER border-line on the AGN diagnostic diagrams. We classify NGC7589 as either Seyfert1.9 or LINERI, in the light of the detection of a broad H_alpha line, which makes NGC7589 an AGN in the low-luminosity regime. We interpret the observed variability in terms of either changes in covering factor of absorbing gas in the AGN, or variability in the intrinsic X-ray luminosity. Should the latter be the case, the inferred Eddington accretion rate increased from the radiatively inefficient accretion dominated regime to a value close to the putative critical value, at which a transition of the accretion mode is supposed to take place. This possibility presents a new prospect of studying accretion physics in the central black holes of external galaxies by direct observing changes of `spectral state', as is common in stellar black hole binary systems.