X-ray Telescopes

The atmosphere is opaque at X-ray wavelengths, see figure 2.1. Therefore X-ray astronomy requires the deployment of detectors above the Earth's atmosphere. Early observations were made with detectors on board balloons or rockets. However, it was with the launch of detectors on board satellites that the field really took off.

Figure 2.1: Transmission of the Earth's Atmosphere as a Function of Wavelength.

The early studies with the Uhuru and HEAO-1 observatories established clusters as extremely luminous X-ray sources ( $L_{{\rm X}} \sim 10^{44}$ erg s$^{-1}$) with extended emission from a thermal emission mechanism, see chapter 3.

Major advances were made with the launch of the Einstein observatory, providing, for the first time, the ability to focus X-rays from bodies other than the Sun. An excellent review of the X-ray astronomy of clusters up to the end of the Einstein era is given by Sarazin (1988). Einstein employed four focal plane instruments, an imaging proportional counter (IPC) providing low resolution imaging ($\sim 1$ arcmin) and spectral capabilities ( $\frac{\Delta\ E}{E} \sim 1$), a high resolution imager with a spatial resolution of $\sim 1$ arcseconds but no spectral resolution, and a solid state spectrometer (SSS) and a focal plane crystal spectrometer (FPCS) both with good spectral resolution but no spatial resolution and poorer sensitivity. A description of the observatory and its capabilities is given by Giacconi et al. (1979).

Einstein allowed serendipitous surveys of clusters of galaxies to be made, which allowed major advances in the understanding of cluster evolution. One such survey with significant implications for the study of cluster evolution was the Einstein Extended Medium Sensitivity Survey (Gioia et al. 1990, Stocke et al. 1991). This survey is still often used to determine the evolution of galaxy clusters due to its relatively high number of high redshift, high luminosity clusters. An analysis based on a sub-sample of the EMSS is presented in section 3.3.

Following the demise of Einstein in 1981 there was a relatively quiet period until the launch of ROSAT (from the German Röntgensatellit) on June 1st, 1990. The majority of the X-ray analysis presented in this thesis was carried out using ROSAT data and hence a description of its instrumentation is given in section 2.1.1.

We are now in the era of X-ray CCD cameras presenting unprecedented spatial and spectral resolution coupled with greater sensitivity than ever. A brief description of the Chandra and XMM satellites is given in section 2.1.2.