Astroparticle Physics

124 6 Primary Cosmic Raysballoon experimentsrocket flightsof the Earth’s atmosphere, however, is too large to allow extraterrestrialX rays to reach sea level. In the keV energyregion, corresponding to the brightness maximum of mostX-ray sources, the range of X rays in air is 10 cm only. To beable to observe X rays from astronomical objects one thereforehas to operate detectors at the top of the atmosphereor in space. This would imply balloon experiments, rocketflights, or satellite missions.Balloon experiments can reach a flight altitude of 35 to40 km. Their flight duration amounts to typically between 20to 40 hours. At these altitudes, however, a substantial fractionof X rays is already absorbed. Balloons, therefore, canonly observe X-ray sources at energies exceeding 50 keVwithout appreciable absorption losses. In contrast, rocketsnormally reach large altitudes. Consequently, they can measureX-ray sources unbiased by absorption effects. However,their flight time of typically several minutes, before they fallback to Earth, is extremely short. Satellites have the big ad-vantage that their orbit is permanently outside the Earth’satmosphere allowing observation times of several years.In 1962 X-ray sources were discovered by chance whenan American Aerobee rocket with a detector consisting ofthree Geiger counters searched for X rays from the Moon.No lunar X radiation was found, but instead extrasolar Xrays from the constellations Scorpio and Sagittarius wereobserved. This was a big surprise because it was known thatour Sun radiates a small fraction of its energy in the X-rayrange and – because of solid angle arguments – one did notexpect X-ray radiation from other stellar objects. This is becausethe distance of the nearest stars is more than 100 000times larger than the distance of our Sun. The brightness ofsuch distant sources must have been enormous compared tothe solar X-ray luminosity to be able to detect them with detectorsthat were in use in the sixties. The mechanism whatmade the sources Scorpio and Sagittarius shine so bright inthe X-ray range, therefore, was an interesting astrophysicalquestion.satellitesdiscoveryof cosmic X-ray sourcesbright X-ray sources6.4.2 Production Mechanisms for X RaysThe sources of X rays are similar to those of gamma rays.Since the energy spectrum of electromagnetic radiation usuallydecreases steeply with increasing energy, X-ray sourcesoutnumber gamma-ray sources. In addition to the processesalready discussed in Sect. 6.3 (gamma-ray astronomy) like