The Variability and Spectrum of NGC 5548 in the Extreme Ultraviolet

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Abstract
We have measured the light curve and spectrum of NGC 5548 obtained with the Extreme Ultraviolet Explorer (EUVE) during a period when the galaxy was also intensively monitored with the Hubble Space Telescope, the 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 curve showed several factor of 2 variations on 0.5 day timescales and one factor of 4 decrease over 2 days. The normalized variability index was smaller than in the EUVE observation of Mrk 478. Cross-correlation of the EUV light curve with the contemporaneous ultraviolet light curve indicates that the EUV and UV/optical variations are, to within the limitations of these data, simultaneous, although the amplitude in the EUV is twice that in the UV. The shape of the EUVE spectrum is consistent with a gradual steepening from the UV through the soft X-rays. Contrary to reports by others, we do not detect any emission lines in the EUV spectrum that would arise in optically thin spectral models. We believe that the reported line detections were, in fact, due to a low-level 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.