AN X-RAY ILLUMINATED CIRCUMNUCLEAR DISK AT THE CENTER OF NGC-4258

Observations of the angular distribution of the water masers in the nu cleus of NGC 4258 reveal the presence of a thin molecular disk in near ly perfect Keplerian orbit (Miyoshi et al. 1995; Greenhill et al. 1995 ). The observed water maser clumps are distributed in an annulus of in ner radius R-in = 0.13 pc and outer radius R-out = 0.26 pc. NGC 4258 i s also observed to contain a central X-ray source of inferred intrinsi c luminosity is 4 x 10(40) ergs s(-1) over the 2-10 keV energy range ( Makishima et al. 1994). We suggest that the molecular density is too h igh within the inner radius Ri, and the temperature of the gas in the disk is too low to produce maser emission beyond the outer radius of t he annulus. This interpretation implies that the accretion rate is sim ilar to 8 x 10(-4)alpha M. yr(-1). The possibility that the masing reg ion extends beyond the annulus could be ruled out.