The steep soft X-ray spectrum of the highly variable active nucleus in Markarian 478

We present the first moderate-resolution spectrum of the soft X-ray excess in an active galactic nucleus. Mrk 478 was detected in the 70-100 Angstrom spectral region using the Extreme Ultraviolet Explorer (EUVE) spectrometer. The spectrum shows no significant spectral features and is consistent with a very steep power law. The neutral hydrogen absorbing column is consistent with the Galactic value. The spectral slope is poorly determined if the absorbing column is allowed to vary; alpha = 27 +/- 9 (f(v) proportional to v(-alpha)), and the best-fit column density (cm(-2)) is log N-H = 20.6. If the column density is fixed at the Galactic value, log N-H = 20.0, then alpha = 4.70 +/- 0.65. The lack of emission lines and significant variability indicate that the soft X-ray continuum is not due to emission from a thin thermal plasma or a leaky neutral or warm absorber. There is marginal evidence that the spectrum hardens as it brightens, which is consistent with thermal variations rather than absorption changes. If this component is the Wien tail of a blackbody spectrum, then the temperature at the inner edge of the accretion disk must be of order 10(6) K. For an accretion disk model from Sun & Malkan (1989), we require a mass of 1.3 x 10(8) M., an accretion rate of 0.3 M. yr(-1), and a highly inclined disk, with cos i = 0.50, in order to keep L < L(Edd).