Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304 - Aharonian, Felix A.; Tibolla, Omar; Wagner, Stefan; Behera, Bagmeet; Hauser, Marcus; Kaufmann, Sarah; Pedaletti, Giovanna; Pühlhofer, Gerd; Quirrenbach, Andreas; Schwemmer, Stephanie; Tam, Pak Hin (Physical review letters, 2008)