There is growing evidence that the innermost region of Seyfert 2 nucleus is obscured and it is seen only in the scattered light. We observed six Seyfert 2 galaxies, Mrk 3, NGC 1068, NGC 4945, NGC 4388, IRAS 18325-5926 and IRAS 09104+4109, with the X-ray astronomy satellite ASCA. X-ray spectroscopy with high resolution in the wide energy range of 0.5-10 keV, provided considerable new insights to understand the Seyfert 2 phenomena. Their X-ray spectra show striking differences from object to object. It is found that there are three distinct origins of X-ray emission from an obscured central source, as suggested by previous Ginga observations. The second is its small fraction mirrored into our view from scattering matter extended well outside the obscuration. We obtained spectral evidence that the scattering matter is significantly ionized by irradiation of the central source. The third is thermal emission due to intensive star formation (starburst) in the host galaxy. Difference of relative contributions from these components can explain the variety of Seyfert 2 spectra in individual galaxies.
We also found large equivalent width of iron K emission line (EW = 0.3-.5 keV) is a common property of Seyfert 2s' X-ray spectra. Especially when the direct radiation of the central source is completely blocked by surrounding optically thick matter, an iron K line complex consisting of highly ionized lines as well as the cold one at the energy of 6.4 keV is observed on the scattered continuum.
One of our important discoveries is that the starburst component does contribute significantly to the total X-ray emission from Seyfert 2 galaxies. Combined with the spatially resolved X-ray images, extended soft X-ray emission in NGC 1068, NGC 4945 and NGC 4388 are identified with hot plasma (T ~ 106 - several times of 107K) due to the starburst. We propose a possible explanation for the iron K line complex as a combination of fluorescent line from the active nucleus and recombination lines of highly ionized irons from starburst emission.
For an IRAS-selected object IRAS 18325-5926, we performed an optical spectroscopy as well as the X-ray observations to understand the peculiar nature of its nucleus. We find a highly variable X-ray source similar to Seyfert 1s but with moderate absorption and then conclude that the dusty nature generally seen in the IRAS selected objects may lead to the optical type 2 classification otherwise a Seyfert 1 object. A broad iron K emission line is discovered both in the Ginga and ASCA spectra. The resolved line profile can be interpreted as a line produced at inner radii of an accretion disk around a black hole.
From a hyperluminous infrared galaxy IRAS 09104+4109 at an intermediated redshift z = 0.442, we detect a strong Fe K line emission which implies a discovery of the prototype of a new class of Seyfert 2 nuclei emitting at quasar luminosity.