  We report the discovery of a luminous ultra-soft X-ray excess in a radio-loud
narrow-line Seyfert1 galaxy, RXJ1633+4718, from archival ROSAT observations.
The thermal temperature of this emission, when fitted with a blackbody, is as
low as 32.5(+8.0,-6.0)eV. This is in remarkable contrast to the canonical
temperatures of ~0.1-0.2keV found hitherto for the soft X-ray excess in active
galactic nuclei (AGN), and is interestingly close to the maximum temperature
predicted for a postulated accretion disc in this object. If this emission is
indeed blackbody in nature, the derived luminosity
[3.5(+3.3,-1.5)x10^(44)ergs/s] infers a compact emitting area with a size
[~5x10^(12)cm or 0.33AU in radius] that is comparable to several times the
Schwarzschild radius of a black hole at the mass estimated for this AGN
(3x10^6Msun). In fact, this ultra-steep X-ray emission can be well fitted as
the (Compton scattered) Wien tail of the multi-temperature blackbody emission
from an optically thick accretion disc, whose parameters inferred (black hole
mass and accretion rate) are in good agreement with independent estimates using
optical emission line spectrum. We thus consider this feature as a signature of
the long-sought X-ray radiation directly from a disc around a super-massive
black hole, presenting observational evidence for a black hole accretion disc
in AGN. Future observations with better data quality, together with improved
independent measurements of the black hole mass, may constrain the spin of the
black hole.
