The origin of the X-ray emission in 3C 273 and 3C 120.

We present an analysis of simultaneous X-ray and ultraviolet observations of the two quasars 3C 273 and 3C 120 which exhibit very similar multi-waveband spectra. This investigation is limited to the long term variations observed in the two spectral bands and to their correlations. In both objects, the X-ray and ultraviolet lightcurves are not coupled and the X-ray spectral slope is not coupled with either of them. However we have obtained an excellent anti-correlation between the slope of the X-ray powerlaw and the logarithm of the ratio of the X-ray to ultraviolet fluxes in both objects. These anti-correlations cannot be spurious as the contribution of the X-ray powerlaw in the ultraviolet band is negligeable. These relations are a strong support for inverse Comptonization models where hot electrons scatter seed ultraviolet photons. The slopes of the observed anti-correlation give an electronic temperature of the order of m_e_c^2^/k_b_. Two X-ray flares do not fit the anti-correlation found in 3C 273. Both appeared a few months before the infrared-optical flares observed in this object. We interpret them as the signatures of the birth of new knots in the radio jet.