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3.2. Fe-K Emission

Fe-K emission is an important diagnostic in AGN studies since its energy, physical width, and equivalent width (EW) are functions of the physical conditions in the accretion region. In the case of an obscured nuclear region, the EW of an Fe line is expected to be greatly enhanced since the direct continuum is diminished. Although most low-activity galaxies are too faint for Fe-K to detected, in several cases Fe-K is detected and is often complex. For example, the Fe-K EW in NGC 3147 (Ptak et al. 1996), NGC 1365 (Iyomoto et al. 1997), M51 (Terashima et al. 1998a), NGC 4736 (Roberts, Warwick, & Ohashi 1999) and NGC 1052 (Weaver et al. 1999) are high (> 100 eV) relative to Seyfert 1 EW values (~ 100-200 eV; Nandra et al. 1997), consistent with an obscured nucleus. In M81, the Fe-K line appears to contain several components or may be broad, possibly due to an accretion disk (Ishisaki et al 1996; Serlemitsos, Ptak & Yaqoob, 1996).

NGC 4579 (Terashima et al. 1998b; Terashima et al. 2000) is a particularly interesting case where the line was observed to be due to ionized material in a 1995 ASCA observation, but was observed to be due to neutral material in 1998.

(Ionized) Fe-K emission was detected marginally by ASCA in M82 (Ptak et al. 1997) and in M82 and NGC 253 significantly by BeppoSAX (Cappi et al. 1999). This emission stongly suggests that very hot gas (T ~ 108 K) is present in these starburst galaxies. However, the EW of the Fe-K lines is only on the order of 100-200 eV, considerably less than that expected from solar-abundance hot gas (EW ~ 600 eV). Again, the Fe abundance may be depressed in these galaxies, however it is also likely that other sources of continuum are present, diluting the thermal Fe-K emission.