  We present a deep X-ray observation of the low-luminosity active galactic
nucleus in NGC4258 (M106) using ASCA. The soft X-ray spectrum <2keV is
dominated by thermal emission from optically-thin plasma with kT~0.5keV. The
hard X-ray emission is clearly due to a power-law component with photon index
Gamma=1.8 absorbed by a column density of N_H=8x10^22/cm^2. The power-law is
readily identified with primary X-ray emission from the AGN central engine. We
also clearly detect a narrow iron K-alpha emission line at 6.4keV. No broad
component is detected. We suggest that the bulk of this narrow line comes from
the accretion disk and, furthermore, that the power-law X-ray source which
excites this line emission (which is typically identified with a disk corona)
must be at least 100GM/c^2 in extent. This is in stark contrast to many
higher-luminosity Seyfert galaxies which display a broad iron line indicating a
small 10 GM/c^2 X-ray emitting region. It must be stressed that this study
constrains the size of the X-ray emitting corona rather than the
presence/absence of a radiatively efficient accretion disk in the innermost
regions. If, instead, a substantial fraction of the observed narrow line
originates from material not associated with the accretion disk, limits can be
placed on the parameter space of possible allowed relativistically broad iron
lines. By comparing our data with previous ASCA observations, we find marginal
evidence for a change in absorbing column density through to the central
engine, and good evidence for a change in the AGN flux.
