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Title: Discovery of high-amplitude X-ray variability in the Seyfert-LINER transition galaxy NGC 7589
Authors: Yuan, W.
Komossa, S.
Xu, D.
Osborne, Julian P.
Watson, Michael G.
McMahon, R.G.
First Published: 25-Aug-2004
Publisher: Blackwell Publishing on behalf of the Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, 2004, 353 (3), pp. L29-L33 (5)
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 time-scale of less than 5 yr. 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 (low-ionization nuclear emission-line region) borderline on the active galactic nucleus (AGN) diagnostic diagrams. We classify NGC 7589 as either Seyfert 1.9 or LINER I, in the light of the detection of a broad Halpha 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.
DOI Link: 10.1111/j.1365-2966.2004.08266.x
ISSN: 0035-8711
eISSN: 1365-2966
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2004 RAS. Deposited with reference to the publisher's archiving policy available from the Journal Exclusive Licence Form, which permits posting of the Blackwell Publishing PDF 6 months after publication. The definitive version is available from
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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