ASCA detection of the Fe K edge in the spectrum of the Seyfert 2 galaxy IRAS 04575-7537: a sign of a complex absorber

ASCA X-ray spectral analysis of the Seyfert 2 galaxy IRAS 04575−7537 is presented. The main result is the presence of a significant iron edge at a rest energy of 7.13 −0.16 keV. The spectrum is flat (Γ ∼ 1.5) with a substantial absorption: NH ∼ 1022 cm−2 and does not require any reflection component. There is also evidence of a narrow Fe Kα emission line, whose rest energy (E = 6.35 −0.03 keV) and equivalent width (EW ' 130±50 eV) suggest that the line originates from thick and cold matter. The intensity of the line and, in particular, the depth of the iron edge are too strong to be explained by simple transmission through the measured absorption column density. This strongly suggests that a model more complex than a single absorbed power law is needed. We propose an absorption model that we call “leaky-total absorber” which can explain the spectrum flatness, the iron emission line and the edge absorption feature and has also the advantage of having a straightforward physical interpretation in the framework of Unified models. In this model a thick absorber (NH ∼ 1023 cm−2), possibly associated with broad line region (BLR) clouds, partially (∼ 36%) covers the source continuum with Γ fixed to 1.9 as observed in Sey 1 galaxies. The escaping radiation is then absorbed by a column density NH ∼ 1022 cm−2, which can be attributed to the torus, seen through its rim.