The BeppoSAX View of the X-Ray Active Nucleus of NGC 4258

BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 10<SUP>41</SUP> ergs s<SUP>-1</SUP>. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5+/-1.2)10<SUP>22</SUP> cm<SUP>-2</SUP> this component is detected at up to 70 keV with a signal-to-noise ratio of >~3 and with a steep power-law energy spectral index of alpha<SUB>E</SUB>=1.11+/-0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E<~2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6+/-0.1 keV and >~1.3 keV. The cooler (L<SUB>0.1-</SUB><SUB>2.4keV</SUB>~10<SUP>40</SUP> ergs s<SUP>-1</SUP>) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with alpha<SUB>E</SUB>=0.2<SUP>+0.8</SUP><SUB>-0.2</SUB>, is L<SUB>0.1-</SUB><SUB>2.4keV</SUB>~710<SUP>39</SUP> ergs s<SUP>-1</SUP>, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.