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PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored withthe High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, E>100GeV) $\gamma$ rays every year since 2002. Thanks to the large data setcollected in the VHE range and simultaneous coverage in optical, ultraviolet(UV), X-ray and high energy $\gamma$-ray ranges, this object is an excellentlaboratory to study spectral and temporal variability in blazars. However,despite many years of dense monitoring, the nature of the variability observedin PKS 2155-304 remains puzzling. In this paper, we discuss the complexspectral and temporal variability observed in PKS 2155-304. The data discussedinclude 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, andexhibiting different flavors of outbursts. For the first time, orphan opticalflare lasting a few months was observed. Correlation studies show an indicationof correlation between the X-ray and VHE $\gamma$-ray fluxes. Interestingly, acomparison of optical and X-ray or VHE $\gamma$-ray fluxes does not show globalcorrelation. However, two distinct tracks in the diagram were found, whichcorrespond to the different flaring activity states of PKS 2155-304.

Unraveling The Complex Nature Of The Very High-Energy $\gamma$-Ray Blazar PKS 2155-304