The X-Ray Structure of Centaurus A from 0.1 to 50 keV Observations with EXOSAT

We present the results from four EXOSAT observations of the radio galaxy Centaurus A performed during 1984-1985. Data have been obtained both in the low-energy (< 2 keV) and in the medium-energy (2-50 keV) X-ray energy bands. The emission from the nucleus of the galaxy detected in the medium-energy X-ray band exhibits time variability on all time scales from few minutes to years, with variations of a factor of 5 among the different observations, and in a range of 10% on shorter time scales within individual observations. Short-term variability includes monotonic trends and transient episodes of oscillations at definite periodicities. Small changes have been detected in the spectra of the source obtained at the different epochs. All spectra are well fitted by a power law with energy index α = 0.70 and strong low-energy photoelectric absorption, with an absorbing column N_H_ variable between 1.4 and 1.7 x 10^23^ cm^-2^. An unabsorbed component is observed at low energies in all spectra which can be fitted with a thermal spectrum with T ~ 1.6 keV and is consistent with the integrated emission of the spatially extended sources outside the nucleus. An iron fluorescence line is detected at 6.4 keV, which seems to be constant in absolute intensity in spite of the large variations in the intensity of the continuum emission. The iron K-absorption edge at 7.1 keV is also observed, and the abundance of absorbing iron relative to lighter elements is larger than the cosmic value N_Fe_/N_H_ = 3.3 x 10^-5^ and appears to be correlated with the amount of low-energy absorption. This implies the presence of an absorbing component near the nucleus, which is strongly and variably ionized, depending on the intensity and the spectrum of the ionizing radiation emitted from the nucleus itself. The limits on the location, density, and temperature of the K-absorbing iron, derived from the observed time scale of variability, are consistent with the hypothesis that the absorbing iron is in the hot intercloud gas of a broad-line region similar to that of Seyfert I galaxies or QSOs. If this hypothesis is correct, most of the low-energy absorption is due to the cold clouds in the same region. Concerning long-term variability, the values for the spectral parameters (spectral index and photoelectric absorption) which we have derived for this source, if compared with those obtained from previous observations in the period 1969-1984, suggest a behavior on a time scale of several years which correlates with the sequence of high- and low-intensity states in the X-ray emission. Several spatially extended components have been observed in the low-energy X-ray image of the galaxy. There are some differences with respect to the corresponding image obtained by the Einstein Observatory, and some of these can be attributed to the different energy bands of the two instruments. The emission from the nucleus, which is strongly absorbed below a few keV, is not detected in our low-energy image. This implies that the unabsorbed component observed in the medium-energy spectra does not derive from leakage in the absorbing matter around the nucleus. Other components observed in the low-energy image include the X-ray jet and the galactic ridges, for which, however, our data do not allow us to derive spectral information, and the SW inner radio lobe, which is responsible for about 30% of the emission in our low-energy image, and was not observed in the Einstein HRI image of the same source.