We present simultaneous observations by EUVE, ASCA, and RXTE of the type~1 Seyfert galaxy NGC 5548. These data indicate that variations in the EUV emission (at $sim 0.2 $keV) appear to lead similar modulations in higher energy ($ga 1 $keV) X-rays by $sim$10--30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type~1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also investigate the spectral characteristics of the fluorescent iron K$alpha$ line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K$alpha$ line has a relatively small equivalent width ($W_{Kalpha} sim 100 $eV) and that the reflection component is consistent with a covering factor which is significantly less than unity ($Omega/2pi sim 0.4$--0.5). Notably, although the 2--10 keV X-ray flux varies by $sim pm 25$% and the derived reflection fraction appears to be constant throughout our observations, the flux in the Fe~K$alpha$ line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.
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