PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, $E>100$ GeV) $\gamma$ rays every year since 2002. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet (UV), X-ray and high energy gamma-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling.
In this paper, we discuss the complex spectral and temporal variability observed in PKS 2155-304.
The data discussed include VHE $\gamma$-ray data collected with H.E.S.S. between 2013 and 2016, complemented with multiwavelength (MWL) observations from Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, and the ATOM telescope.
During the period of monitoring, PKS 2155-304 was transitioning from its lower state to the flaring states, and exhibiting different flavors of outbursts.
For the first time, orphan optical flare lasting a few months was observed.
Correlation studies show an indication of correlation between the X-ray and VHE $\gamma$-ray fluxes.
Interestingly, a comparison of optical and X-ray or VHE $\gamma$-ray fluxes does not show global correlation.
However, two distinct tracks in the diagram were found, which correspond to the different flaring activity states of PKS 2155-304.