THE VARIABILITY AND SPECTRUM OF NGC-5548 IN THE EXTREME-ULTRAVIOLET

We have measured the light curve and spectrum of NCC 5548 obtained wit h the Extreme Ultraviolet Explorer (EUVE) during a period when the gal axy was also intensively monitored with the Nubble Space Telescope, th e International Ultraviolet Explorer, and ground-based telescopes. NGC 5548 was observed with EUVE for a total of about 20 days spread over a period of 2 months. The broadband extreme ultraviolet (EUV) light cu rve showed several factor of 2 variations on 0.5 day timescales and on e factor of 4 decrease over 2 days. The normalized variability index w as smaller than in the EUVE observation of Mrk 478. Cross-correlation of the EW light curve with the contemporaneous ultraviolet light curve indicates that the EUV and UV/optical variations are, to within the l imitations of these data, simultaneous, although the amplitude in the EUV is twice that in the UV. The shape of the EUVE spectrum is consist ent with a gradual steepening from the UV through the soft X-rays. Con trary to reports by others, we do not detect any emission lines in the EUV spectrum that would arise in optically thin spectral models. We b elieve that the reported line detections were, in fact, due to a low-l evel fixed pattern in the detector background. Models of the spectrum may be constrained by the time-averaged flux in the broadband detector combined with the weakly constrained spectral shape. A thermal model best describes the EUVE spectrum and its variations.