Analysis of the high-energy emission of the BL Lac PKS 2155-304 with Fermi-LAT data

Abstract: Some of the most interesting objects in the Universe are Active Galactic Nuclei. In the centre of an active galaxy is a supermassive black hole that accretes matter from the surrounding galaxy. In the process, not yet fully understood, some of the matter is ejected in two jets, perpendicular to the plane of the galaxy. The energy of the particles in the jets are extremely high, sometimes over 1019 eV. The features of an active galaxy can be very different depending on from which angle it is viewed. This means that some astronomical objects that earlier seemed to be very heterogeneous might be only different manifestations of the same type of object, namely active galactic nuclei. This thesis introduces some of these different objects. The unifying theory is described. Ways of detecting the high-energy radiation and two important instruments, H.E.S.S. and Fermi-LAT are described. Three studies of the BL Lac PKS 2155-304, an active galactic nucleus that points its jet almost straight at Earth, are made using Fermi-LAT data. The conclusion of the studies is that the source is variable at least in the time scale of days and that in order to gather further information about these objects simultaneous multi-wavelength surveys have to be done.