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Session 36 - Extragalactic X-Ray Sources.
Display session, Tuesday, January 14
The X-ray properties of the type 2 Seyfert galaxy NGC 4388 have been investigated using simultaneous observations by ROSAT and ASCA in 1993 and an ASCA observation in 1995. It was found that the hard X-ray flux had dropped by a factor of two between these observations and that the profile and strength of the Fe K\alpha emission line had also changed.
The overall X-ray spectrum of NGC 4388 between 0.5 and 10.0 keV is well described by a flat power law (\Gamma = 1.5 \pm 0.3) absorbed by material with a covering factor of 0.95 and a large column density, N_H = (3.1 \pm 0.8) \times 10^23 cm^-2. The Fe K\alpha emission line feature in the 1993 ASCA spectra can be modeled with a complex of broad and narrow components at energies consistent with highly ionized Iron. However, the Fe K\alpha emission line in the 1995 spectra has only a single, narrow component and is consistent with emission due to neutral Iron.
The soft X-ray flux of the July 1993 ASCA observation is consistent with the flux observed during a June - July 1993 ROSAT PSPC observation. When the spectra from both satellites are modeled simultaneously, the greater soft X-ray sensitivity of the ROSAT PSPC indicated that an additional thermal component to the X-ray emission is present (kT \sim 0.6 keV).
The drop in continuum flux and accompanying fall in Fe K\alpha line intensity over a period of \sim 2 years indicates that the material responsible for the fluorescent emission lies within \sim 0.6 pc of the continuum source.
These hard X-ray spectral properties are similar to those seen for the prototype obscured Seyfert 1 galaxies NGC 1068 and Mkn 3. There is also evidence for further emission features in the ASCA spectra reminiscent of the reflection dominated hard X-ray spectra of NGC 6552 and the Circinus galaxy. Such low significance spectral features have also been seen in the ASCA spectra of NGC 1068 and Mkn 3.
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