Astroparticle Physics

132 6 Primary Cosmic RaysFig. 6.59Collision of two merging galacticclusters, each containing hundredsof galaxies. The site of thiscatastophic event is Abell 754 at adistance of about 9 millionlight-years. The photo shows acoded pressure map of this region,where the galaxies themselves areconfined around the white spotswhich correspond to regions ofhigh pressure, followed bydecreasing pressure as one goesaway from the centers {20}residing at the centers of distant galaxies may be difficult tofind since they are hidden deep inside vast amounts of absorbingdust so that only energetic X rays or γ rays can escape.Already now these new X-ray satellites have observedgalaxies at high redshifts emitting huge amounts of energyin the form of X rays, far more than can ever expected to beproduced by star formation. Therefore, it is conjectured thatthese galaxies must contain actively accreting supermassiveblack holes.Even though presently known classes of sources willprobably dominate the statistics of Chandra and XMM Newtondiscoveries, the possibility of finding completely newand exciting populations of X-ray sources also exists.A very recent result is the observation of the collisionof two galactic clusters in Abell 754 at a distance of about9 million light-years in the light of X rays. These clusterswith millions of galaxies merge in a catastrophic collisioninto one single very large cluster.On the other hand, the X-ray mission ASTRO-E, startedearly in the year 2000, had to be abandoned because thebooster rocket did not carry the satellite into an altituderequired for the intended orbit. The satellite presumablyburned up during reentry into the atmosphere.The diffuse X-ray background which was discovered rel-atively early consists to a large extent (75%) of resolvedextragalactic sources. It could easily be that the remainingdiffuse part of X rays consists of so far non-resolved distantX-ray sources.The largest fraction of the earlier unresolved X-raysources are constituted by active galactic nuclei and quasars.A long-term exposure of ROSAT (40 hours) revealed morethan 400 X-ray sources per square degree.diffuse background X raysFig. 6.60X-ray emission from the Moonrecorded with the PSPC detectoron board of ROSAT. The dark sideof the Moon shields the cosmicX-ray background {18}iron line in X-ray spectraX rays from the MoonIn the spectra of many X-ray sources the iron line(5.9 keV) is observed. This is a clear indication that eitheriron is synthesized directly in supernova explosions or the Xrays originate from older sources whose material has beenprocessed through several stellar generations.A surprising result of the most recent investigations wasthat practically all stars emit X rays. A spectacular observationwas also the detection of X rays from the Moon. However,the Moon does not emit these X rays itself. It is ratherreflected corona radiation from our Sun in the same way asthe Moon also does not shine in the optical range but ratherreflects the sunlight (Fig. 6.60).