  BeppoSAX observed NGC4258 on December 1998, when its 2-10 keV luminosity was
\~1E41 erg/s. 100% variability is observed in the 3-10 keV band on timescales
of a half a day while 20% variability is observed on timescales of one hour.
The nuclear component is visible above 2 keV only, being obscured by a column
density of (9.5+/-1.2)E22 cm-2; this component is detected up to 70 keV with
S/N>=3 and with the steep power law energy index of 1.11+/-0.14. Bremsstrahlung
emission for the 2-70 keV X-ray luminosity, as expected in ADAF models with
strong winds, is ruled out by the data. The ratio between the nuclear radio and
the X-ray luminosities is <=1E-5, similar to that of radio quiet AGN. X-ray
variability and spectral shape, radio to X-ray and NIR to X-ray luminosity
ratios suggest that the nucleus of NGC4258 could be a scaled-down version of a
Seyfert nucleus. The soft (E<=2keV) X-ray emission is complex. There are at
least two thermal-like components, with T1=0.6+/-0.1 keV and T2>=1.3 keV. The
cooler (L(0.1-2.4keV)=1E40 erg/s) component is probably associated with the
jet, resolved in X-rays by the ROSAT HRI. The second component, which can be
modeled equally well by an unobscured power law model, has L(0.1-2.4keV)~7E39
erg/s, consistent with that expected from discrete X-ray sources in the host
galaxy. NGC4258 and other maser AGNs show strong nuclear X-ray absorption. We
propose that this large column of gas might be responsible for shielding the
regions of maser emission from X-ray illumination. So a large column density
gas may be a necessary property of masing AGNs.
