X‐Ray–Extreme‐Ultraviolet Simultaneous Observations of NGC 5548: Fast Extreme‐Ultraviolet Dip without X‐Ray Variation

The bright Seyfert 1 galaxy NGC 5548 was monitored in X-rays by ASCA for 9 days and in EUV by EUVE for 12 days continuously in 1996. The periods of X-ray observations were really simultaneously covered by EUV observations. The 0.1-10 keV combined continuum of X-ray and EUV is well presented by a simple power law with a photon index Gamma = 1.78 plus the Wien part of a disk blackbody with kT = 17 eV. On long timescales of days, the variability of the EUV flux is in good accordance with that of the X-rays. Furthermore, there is a clear positive correlation between C and the 2-10 keV flux. These results are consistent with the scenario that the X-rays are produced by inverse Compton scattering of soft photons in hot electron clouds. On the other hand, focusing on the short-term variability, we found that the EUV flux shows 20,000 s variability with 40% amplitude with neither X-ray flux nor spectral changes when the source is at its maximum flux state. This seems to conflict with the ordinary framework of the inverse Compton model. To explain such fast variability of the EUV flux accompanied by no X-ray change, we suggest that the EUV emission consists of two components; relatively stable disk emission and a variable component that is dominant at EUV flares.