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We present simultaneous observations by EUVE, ASCA, and RXTE of the type~1Seyfert galaxy NGC 5548. These data indicate that variations in the EUVemission (at $\sim 0.2 $keV) appear to lead similar modulations in higherenergy ($\ga 1 $keV) X-rays by $\sim$10--30 ks. This is contrary to popularmodels which attribute the correlated variability of the EUV, UV and opticalemission in type~1 Seyferts to reprocessing of higher energy radiation. Thisbehavior instead suggests that the variability of the optical through EUVemission is an important driver for the variability of the harder X-rays whichare likely produced by thermal Comptonization. We also investigate the spectralcharacteristics of the fluorescent iron K$\alpha$ line and Compton reflectionemission. In contrast to prior measurements of these spectral features, we findthat the iron K$\alpha$ line has a relatively small equivalent width($W_{K\alpha} \sim 100 $eV) and that the reflection component is consistentwith a covering factor which is significantly less than unity ($\Omega/2\pi\sim 0.4$--0.5). Notably, although the 2--10 keV X-ray flux varies by $\sim \pm25$% and the derived reflection fraction appears to be constant throughout ourobservations, the flux in the Fe~K$\alpha$ line is also constant. This behavioris difficult to reconcile in the context of standard Compton reflection models.Comment: 13 pages, 6 figures, LaTeX, uses emulateapj.sty and apjfonts.sty,  submitted to Ap

SimultaneousEUVE/ASCA/RXTEObservations of NGC 5548