  We report sub-arcsecond resolution X-ray imaging spectroscopy of the low
luminosity active galactic nucleus of NGC 4258 and its immediate surroundings
with the Chandra X-ray Observatory. NGC 4258 was observed four times, with the
first two observations separated by one month, followed over a year later by
two consecutive observations. The spectrum of the nucleus is well described by
a heavily absorbed, hard X-ray power law of variable luminosity, plus a
constant, thermal soft X-ray component. We do not detect an iron K alpha
emission line with the upper limit to the equivalent width of a narrow, neutral
iron line ranging between 94 and 887 eV (90% confidence) for the different
observations. During the second observation on 2000-04-17, two narrow
absorption features are seen with >99.5% confidence at ~6.4 keV and ~6.9 keV,
which we identify as resonant absorption lines of Fe XVIII - Fe XIX K alpha and
Fe XXVI K alpha, respectively. In addition, the 6.9 keV absorption line is
probably variable on a timescale of ~6000 sec. The absorption lines are
analyzed through a curve of growth analysis, which allows the relationship
between ionic column and kinematic temperature or velocity dispersion to be
obtained for the observed equivalent widths. We discuss the properties of the
absorbing gas for both photo and collisionally ionized models. Given that the
maser disk is viewed at an inclination i = 82 degrees, the gas responsible for
the 6.9 keV absorption line may be in an inner disk, a disk-wind boundary layer
or be thermal gas entrained at the base of the jet. The gas which gives rise to
the photoelectric absorption may be the same as that which causes the 6.4 keV
Fe K alpha absorption provided that the gas has a bulk velocity dispersion of a
few thousand km/s. (Abstract abridged).
