THE COMPLEX VARIABLE SOFT-X-RAY SPECTRUM OF NGC-5548

We present ROSAT 0.2-2 keV monitoring data on NGC 5548 taken over the period 1992 December-1993 January, when the source was at its historic al maximum in X-ray flux. The data show a soft excess and partially io nized absorber modifying the high-energy power-law (a similar to 0.9) spectrum. The major variability during the campaign is a strong flare event over 8 d, in which the soft excess brightens without a correspon ding change in the power-law intensity. This is incompatible with the simple reprocessing models which are currently popular, where the soft excess is produced from thermalized hard X-ray emission illuminating an accretion disc. Given the high source luminosity, greater than or e qual to 20.1 L(Edd), it seems likely that the soft excess is dominated by intrinsic emission from the accreting material. While the data do not allow the ionization of the absorbing material to be constrained i n each single spectrum, a comparison of the brightest spectrum with th at from a long exposure taken 2d later and which is a factor of 2 fain ter shows a significant decrease in the ionization. This implies a den sity of greater than or equal to 5 x 10(5) cm(-3) for the warm materia l, and hence a distance of less than similar to 10(18) Cm from the cen tral source.