Intensity correlation between observations at different wavelengths for Mkn 501 in 1997

The CAT imaging telescope on the site of the former solar plant Th'emis in southern France observed gamma-rays from the BL Lac object Mkn501 above 250 GeV for more than 60 usable hours on-source from March to October 1997. This source was in a state of high activity during all this period. By studying the correlation between the photons of different energies detected by the CAT imaging telescope and by the ASM/RXTE experiment (1.3-12.0 keV) on board the Rossi X-Ray Timing Explorer, we may constrain the mechanisms which could lead to the emission of these photons.