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 NGC 7589 on a timescale of less than 5 years .
The 0.4–10 keV XMM spectrum can be approximated by a power-law with photon index of 1.7–1.8 , though it seems to flatten above 5 keV , suggesting a possible complex model , such as partial covering or disc reflection .
A classification based on an analysis of its optical spectrum places NGC 7589 in the Seyfert region , but close to the Seyfert–LINER border-line on the AGN diagnostic diagrams .
We classify NGC 7589 as either Seyfert 1.9 or LINER I , in the light of the detection of a broad H \alpha line , which makes NGC 7589 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 .