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Long-term monitoring of PKS 2155$-$304 with ATOM and H.E.S.S.: investigation of optical/$γ$-ray correlations in different spectral states

In this paper we report on the analysis of all the available optical and very high-energy $γ$-ray ($>$200 GeV) data for the BL Lac object PKS 2155$-$304, collected simultaneously with the ATOM and H.E.S.S. telescopes from 2007 until 2009. This study also includes X-ray (RXTE, Swift) and high-energy $γ$-ray (Fermi-LAT) data. During the period analysed, the source was transitioning from its flaring to quiescent optical states,and was characterized by only moderate flux changes at different wavelengths on the timescales of days and months. A flattening of the optical continuum with an increasing optical flux can be noted in the collected dataset, but only occasionally and only at higher flux levels. We did not find any universal relation between the very high-energy $γ$-ray and optical flux changes on the timescales from days and weeks up to several years. On the other hand, we noted that at higher flux levels the source can follow two distinct tracks in the optical flux-colour diagrams, which seem to be related to distinct $γ$-ray states of the blazar. The obtained results therefore indicate a complex scaling between the optical and $γ$-ray emission of PKS 2155$-$304, with different correlation patterns holding at different epochs, and a $γ$-ray flux depending on the combination of an optical flux and colour rather than a flux alone.