ASCA PV observations of the Seyfert 2 galaxy NGC 4388 : the obscured nucleus and its X - ray emission

We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster observed with ASCA during its performance verification (PV) phase. The 0.5–10 keV X-ray spectrum consists of multiple components; (1) a continuum component heavily absorbed by a column density NH ≈ 4×10 cm above 3 keV; (2) a strong 6.4 keV line (equivalent width EW ∼ 500 eV); (3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray emission below 1 keV. The detection of strong absorption for the hard X-ray component is firm evidence for an obscured active nucleus in this Seyfert 2 galaxy. The absorption corrected X-ray luminosity is about 2× 10erg s. This is the first time that the fluorescent iron-K line has been detected in this object; the large EW is a common property of classical Seyfert 2 nuclei. The flat spectrum in the intermediate energy range may be a scattered continuum from the central source. The soft X-ray emission below 1 keV can be thermal emission from a temperature kT ≃ 0.5 keV, consistent with the spatially extended emission observed by ROSAT HRI. However, the low abundance (Z ∼ 0.05Z⊙) and high mass flow rate required for the thermal model and an iron-K line stronger than expected from the obscuring torus model are puzzling. An alternative consistent solution can be obtained if the central source was a hundred times more luminous over than a thousand years ago. All the X-ray emission below 3 keV is then scattered radiation.