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 ex cess 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 an d is consistent with a very steep power law. The neutral hydrogen abso rbing column is consistent with the Galactic value. The spectral slope is poorly determined if the absorbing column is allowed to vary; alph a = 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 l ack of emission lines and significant variability indicate that the so ft X-ray continuum is not due to emission from a thin thermal plasma o r a leaky neutral or warm absorber. There is marginal evidence that th e spectrum hardens as it brightens, which is consistent with thermal v ariations rather than absorption changes. If this component is the Wie n 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 c os i = 0.50, in order to keep L < L(Edd).