Astroparticle Physics

6.4 X-Ray Astronomy 123bursters all reside in our galaxy or in the nearby MagellanicClouds. Most of these objects could be identified with youngsupernova remnants. These ‘soft gamma-ray repeaters’ appearto be associated with enormous magnetic fields. If sucha magnetar rearranges its magnetic field to reach a morefavourable energy state, a star quake might occasionally occurin the course of which γ bursts are emitted. The γ -rayburst of the magnetar SGR-1900+14 was recorded by sevenresearch satellites on August 27, 1998. From the observedslowing down of the rotational period of this magnetar oneconcludes that this object possesses a superstrong magneticfield of 10 11 Tesla exceeding the magnetic fields of normalneutron stars by a factor of 1000.With these properties γ bursters are also excellent candidatesas sources of cosmic rays. It is frequently discussedthat the birth or the collapse of neutron stars could be associatedwith the emission of narrowly collimated particle jets. Ifthis were true, we would only be able to see a small fractionof γ bursters. The total number of γ bursters would then besufficiently large to explain the observed particle fluxes ofcosmic rays. The enormous time-dependent magnetic fieldswould also produce strong electric fields in which cosmicrayparticles could be accelerated up to the highest energies.soft gamma-ray repeatermagnetars6.4 X-Ray Astronomy“Light brings the news of the universe.”Sir William Bragg6.4.1 IntroductionX rays differ from γ rays by their production mechanismand their energy. X rays are produced if electrons are deceleratedin the Coulomb field of atomic nuclei or in transitionsbetween atomic electron levels. Their energy rangesbetween approximately 1–100 keV. In contrast, γ rays areusually emitted in transitions between nuclear levels, nucleartransformations, or in elementary-particle processes.After the discovery of X rays in 1895 by Wilhelm ConradRöntgen, X rays were mainly used in medical applicationsbecause of their high penetration power. X rays withenergies exceeding 50 keV can easily pass through 30 cm oftissue (absorption probability ≈ 50%). The column densityproduction of X raysenergy dependenceof X-ray absorption