Astroparticle Physics

6.4 X-Ray Astronomy 129In 1959 the first X-ray telescope was built (R. Giacconni,Nobel Prize 2002) and flown on an Aerobee rocketin 1962. During its six minutes flight time it discovered thefirst extrasolar X-ray sources in the constellation Scorpio.The observation time could be extended with the first X-raysatellite UHURU (meaning ‘freedom’ in Swahili), whichwas launched from a base in Kenya 1970. Every week inorbit it produced more results than all previous experimentscombined.In the course of time a large number of X-ray satelliteshas provided more and more precision information aboutthe X-ray sky. The satellite with the highest resolution up to1999 was a common German–British–American project: theROentgen SATellite ROSAT (see Fig. 6.54). ROSAT measuredX rays in the range of 0.1upto2.5 keV with a Woltertelescope of 83 cm diameter. As X-ray detectors, multiwireproportional chambers (PSPC) 12 with 25 arc second resolutionand a channel-plate multiplier (HRI) 13 with5arcsecond resolution were in use. One of the PSPCs was permanentlyblinded by looking by mistake into the Sun. Thesecond PSPC stopped operation after a data-taking periodof 4 years because its gas supply was exhausted. Since thenonly the channel-plate multiplier was available as an X-raydetector. Compared to earlier X-ray satellites, ROSAT hada much larger geometrical acceptance, better angular andenergy resolution, and a considerably increased signal-tonoiseratio: per angular pixel element the background ratewas only one event per day.In a sky survey ROSAT has discovered about 130 000 X-ray sources. For comparison: the earlier flown Einstein ObservatoryHEAO 14 had only found 840 sources. The mostfrequent type of X-ray sources are nuclei of active galaxies(≈ 65 000) and normal stars (≈ 50 000). About 13 000galactic clusters and 500 normal galaxies were found to emitX rays. The smallest class of X-ray sources are supernovaremnants with approximately 300 identified objects.Supernova remnants (SNR) represent the most beautifulX-ray sources in the sky. ROSAT found that the Vela pulsaralso emits X rays with a period of 89 ms known fromits optical emission. It appears that the X-ray emission ofVela X1 is partially of thermal origin. The supernova rem-12 PSPC – Position Sensitive Proportional Chamber13 HRI – High Resolution Imager14 HEAO – High Energy Astronomy Observatoryextrasolar X raysFig. 6.54Photograph of the X-ray satelliteROSAT {18}130 000 discoveredX-ray sourcesHEAO