Astroparticle Physics

118 6 Primary Cosmic RaysFig. 6.46All-sky survey in the light of γrays {16}possible point sourcesblack holesAmong the discovered active galaxies highly variableblazars (extremely variable objects on short time scales withstrong radio emission) were found which had their maximumof emission in the gamma range. In addition, gammaquasars at high redshifts (z >2) were observed. In this casethe gamma radiation could have been produced by inverseCompton scattering of energetic electrons off photons.Figure 6.46 shows a complete all-sky survey in the lightof γ rays in galactic coordinates. Apart from the galacticcenter and several further point sources, the galactic disk isclearly visible. Candidates for point sources of galactic γrays are pulsars, binary pulsar systems, and supernovae. Extragalacticsources are believed to be compact active galacticnuclei (AGN), quasi-stellar radio sources (quasars), blazars,and accreting black holes. According to common beliefblack holes could be the ‘powerhouses’ of quasars. Blackholes are found at the center of galaxies where the matterdensity is highest thereby providing sufficient material forthe formation of accretion disks arround black holes. Accordingto the definition of a black hole no radiation canescape from it, 8 however, the infalling matter heats up alreadybefore reaching the event horizon so that intensiveenergetic γ rays can be emitted.8 In this context the Hawking radiation, which has no importancefor γ -ray astronomy, will not be considered, since black holeshave a very low ‘temperature’.