The CCD Era and The Future

X-ray CCDs like the ones used in the EPIC-pn CCD camera onboard XMM-Newton [66] provide higher spectral and also spatial resolution compared to proportional counters. It became possible to produce maps in narrower separated energy bands and to resolve some ambiguities in spectral modelling. In Fig. 18.3 we show the first X-ray shadow detected by XMM-Newton. The images have been obtained from three partly overlapping exposures across a part of the Ophiuchus molecular cloud with a strong gradient, so that the series covers the range from on-cloud to off-cloud, in the 0.5 - 0.9 keV band (cf., [18]).

The very dense Bok globule Barnard 68 with an absorbing hydrogen column density exceeding 1023 cm-2 allowed to determine the foreground emission to even higher energies above 1 keV [19].

Higher spectral resolution than possible with CCDs has been obtained using crystal spectrometers aboard the DXS instrument [53], and microcalorimeters flown with rockets [38]. However, these observations suffer from low quantum efficiency and deteriorated spatial or temporal resolution, as the high resolution data have to be rebinned due to low statistics. Therefore, the observations cover larger portions of the sky. Nevertheless, these SXRB spectra show clearly that they are dominated by spectral lines of ionized carbon and oxygen, e.g., Ovii and Oviii), and are thus of thermal origin, though it is hardly possible to describe them by a single isothermal plasma in collisional ionization equilibrium. The already mentioned eROSITA mission will provide a major increase in collecting area at these low energies, a much better spectral response at soft energies compared with ROSAT or XMM-Newton, and a much longer time basis for screening against variable components.

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