Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes .
To this end , the long-term variability of the BL Lac object PKS 2155 - 304 is analyzed in the high ( HE , 100 MeV < E < 300 GeV ) and very high energy ( VHE , E > 200 GeV ) \gamma -ray domain .
Over the course of \sim 9 yr of H.E.S.S .
observations the VHE light curve in the quiescent state is consistent with a log-normal behavior .
The VHE variability in this state is well described by flicker noise ( power-spectral-density index \beta _ { \textrm { VHE } } = 1.10 ^ { +0.10 } _ { -0.13 } ) on time scales larger than one day .
An analysis of \sim 5.5 yr of HE Fermi LAT data gives consistent results ( \beta _ { \textrm { HE } } = 1.20 ^ { +0.21 } _ { -0.23 } , on time scales larger than 10 days ) compatible with the VHE findings .
The HE and VHE power spectral densities show a scale invariance across the probed time ranges .
A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established .
These long-term-variability properties are discussed and compared to the red noise behavior ( \beta \sim 2 ) seen on shorter time scales during VHE-flaring states .
The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes , while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection .