A spectral decomposition of the variable optical, ultraviolet and X-ray continuum of NGC 5548

We present an analysis and decomposition of the broad-band optical/UV/X-ray/g-ray spectrum of the Seyfert 1 galaxy NGC 5548. The spectrum consists of an average of simultaneous optical/IUE/Ginga observations accompanied by ROSAT and CGRO/OSSE data from non-simultaneous observations. We show that the overall optical/UV/X-ray/g-ray spectrum can be deconvolved into three basic continuum components: a cool multitemperature blackbody, a hard thermal Comptonized component and an EUV/soft X-ray component well described by a thermal Comptonization continuum. Assuming that the optical/IUE spectrum comes from a cold disc, the maximum disc temperature is very well constrained by the data to be kT disc 3:2 0:2 0:2 eV. This rules out models explaining the soft excess as a far tail of the disc spectrum. We show that the soft excess inferred by the data requires a separate continuum component, which is consistent with thermal Comptonization in optically thick (t 30), warm (0:3 keV) plasma. This Comptonization component contains a significant fraction of the source energy. The plasma parameters of the hard continuum (t 2, kT HC 50 keV) are consistent with those suggested for the average spectrum of Seyferts.