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Astron. Astrophys. 322, 49-52 (1997)

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An X-ray illuminated circumnuclear disk at the center of NGC 4258

Xinwu Cao and D.R. Jiang

Shanghai Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, P.R. China

Received 3 September 1996 / Accepted 15 November 1996


Observations of the angular distribution of the water masers in the nucleus of NGC 4258 reveal the presence of a thin molecular disk in nearly perfect Keplerian orbit (Miyoshi et al. 1995; Greenhill et al. 1995). The observed water maser clumps are distributed in an annulus of inner radius [FORMULA] =0.13 pc and outer radius [FORMULA] =0.26 pc. NGC 4258 is also observed to contain a central X-ray source of inferred intrinsic luminosity is [FORMULA] ergs s-1 over the 2-10 keV energy range (Makishima et al. 1994). We suggest that the molecular density is too high within the inner radius [FORMULA] and the temperature of the gas in the disk is too low to produce maser emission beyond the outer radius of the annulus. This interpretation implies that the accretion rate is [FORMULA] [FORMULA]  yr-1. The possibility that the masing region extends beyond the annulus could be ruled out.

Key words: accretion, accretion disks – galaxies: nuclei

Send offprint requests to: Xinwu Cao (cxw@center.shao.ac.cn)

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998