Astroparticle Physics

130 6 Primary Cosmic RaysROSAT HRI2’SNR 1572 (Tycho)MPE 7/90Fig. 6.55Supernova remnant SNR 1572recorded with the HRI detector(High Resolution Instrument) onboard the ROSAT satellite {18}toroidalemissionbrightpulsarhelicaljetFig. 6.56Sketch of X-ray emission from theCrab pulsarbinariesM81 central regionPSPC 20000 sOctober 1992MPE1’M81 with SN 1993JPSPC 27000 sApril 1993Fig. 6.57Sprial galaxy M81 with thesupernova SN 1993J. The imagewas recorded with the PSPCdetector (Position SensitiveProportional Counter) of theROSAT satellite {18}nant SNR 1572 that was observed by the Danish astronomerTycho Brahe shows a nearly spherically expanding shell inthe X-ray range (Fig. 6.55). The shell expands into the interstellarmedium with a velocity of about 50 km/s and it heatsup in the course of this process to several million degrees.The topology of X-ray emission from the Crab Pulsarallows to identify different components: the pulsar itself isvery bright in X rays compared to the otherwise more diffuseemission. The main component consists of a toroidalconfiguration which is caused by synchrotron radiation ofenergetic electrons and positrons in the magnetic field of thepulsar. In addition, electrons and positrons escape along themagnetic field lines at the poles where they produce X raysin a helical wind (see Fig. 6.56).Only six days after the explosion of a supernova in thespiral galaxy M81, ROSAT has measured its X-ray emission.In the right-hand part of Fig. 6.57 the X-ray source SN1993J south of the center of M81 is visible. In an earlier exposureof the same sky region (left-hand part of the figure)this source is absent.A large number of X-ray sources are binaries. In thesebinaries mostly a compact object – a white dwarf, a neutronstar, or a black hole – accretes matter from a nearbycompanion. The matter flowing to the compact object frequentlyforms an accretion disk (see Fig. 5.7), however, thematter can also be transported along the magnetic field lineslanding directly on the neutron star. In such cataclysmicvariables a mass transfer from the companion, e.g., to awhite dwarf, can be sufficient to maintain a permanent hydrogenburning. If the ionized hydrogen lands on a neutronstar also thermonuclear X-ray flashes can occur. Initially,the incident hydrogen fuses in a thin layer at the surface ofthe neutron star to helium. If a sufficient amount of matteris accreted, the helium produced by fusion can achieve suchhigh densities and temperatures that it can be ignited in athermonuclear explosion forming carbon.The observation of thermal X rays from galactic clustersallows a mass determination of the hot plasma and the totalgravitational mass of the cluster. This method is basedon the fact that the temperature is a measure for the gravitationalattraction of the cluster. A high gas temperature –characterized by the energy of the emitted X rays – representsvia the gas pressure the counterforce to gravitation andprevents the gas from falling into the center of the cluster.