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X-ray all-sky survey (HEAO-I)
By the year 2000, X-ray observatories with good imaging capabilities and wide bandpass up to 10 keV will be available (e. g. XMM and AXAF). In principle they will be able to measure directly spectra of a broad distribution of hard X-ray sources (AGNs, SNRs, X-ray binaries etc.). However, for statistical investigations of those objects in their whole variety (e. g. luminosity, absorption) large complete and unbiased source samples are necessary.
The ROSAT all-sky Survey with about 75,000 new X-ray sources over the whole sky could in principle serve as a pathfinder for the future higher-energy observations. However, because of its sensitivity to absorption, the soft X-ray sky provides a biased view of the universe. An all-sky survey in a wide and harder X-ray energy band would therefore be important for the pointed observation programs of AXAF and XMM.
core of NGC 4216 (HST)
It became apparent in the last years that different types of active galactic nuclei (AGN), in particular the Seyfert-1 and Seyfert-2 galaxies, can be simply understood in terms of orientation effects. While in Seyfert-1 galaxies we can look straight into the core region of the AGN, where the energy production occurs, the central engine of Seyfert-2 galaxies is obscured by gas and dust clouds of a thick molecular torus (see e. g. Antonucci 1993 for a recent review). This hypothesis - sometimes called the unified AGN model - has very recently been confirmed in the most dramatic way by direct observations of Seyfert galaxies with the Hubble Space Telescope, which indeed could resolve a dusty torus in the center of an AGN.
The X-ray appearance of an AGN is therefore expected to be strongly dependent on the viewing angle: in Seyfert-2 galaxies, where the direct view is blocked by the thick torus, all soft X-rays are expected to be absorbed by the gas and dust clouds, which can only be penetrated by X-rays with energies above several keV ( N_H > 1024 cm-2 ).
In recent years, through the ROSAT and Ginga observations it also became apparent that there is a whole continuum of different X-ray absorption values, probably governed by the shape of the molecular tori in connection with different orientations. There is also unsurmountable evidence that some of the X-ray absorbing gas is partially ionized (e. g. Nandra and Pounds 1992), so that even soft X-rays can "eat their way" through the torus. This offers the exciting possibility to observe a variety of spectral features with a high energy resolution X-ray instrument, like absorption edges and fluorescence lines in the absorber. In addition the ionization state of the matter should depend on the luminosity of the central machine, so that e. g. quasars should on average be less absorbed than Seyfert galaxies.
Moon and X-ray background (ROSAT)
The extragalactic X-ray background (XRB), discovered about 30 years ago, has been studied extensively with many X-ray experiments, in particular with the satellites HEAO I and HEAO II (see e. g. Boldt 1987) and with ROSAT (e. g. Hasinger et al. 1993). Over the energy range from 3 to about 100 keV its spectrum can be well approximated by an optically thin thermal bremsstrahlung model with kT ~ 40 keV, while at lower X-ray energies a steepening into a new component has been observed.
Studies of the isotropy of the X-ray background give strong support for its extragalactic nature. The XRB is isotropic down to a few percent on practically all observable scales.
Vela and Puppis SNRs (ROSAT)
Soft X-ray emission from galactic sources in the direction of the galactic plane is strongly absorbed by dense clouds of neutral gas which can be seen in the ROSAT all-sky survey. Many X-ray sources which are obscured in the soft band are expected to unveil in a 0.5-10 keV survey. Additional goals are therefore the detection of extended sources with low surface brightness, such as SNR or other hot gas bubbles, as well as the long-term monitoring of bright X-ray sources, e. g. cataclysmic variables and X-ray binaries, with a wide spectrum of time scales (10 s, 90 min, 2 d, 14 d, 0.5 yr, 3 yr).
| Peter Friedrich: pfriedrich@aip.de last changed: 04-Jul-1996 |
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