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We report the discovery of a luminous ultra-soft X-ray excess in a radio-loudnarrow-line Seyfert1 galaxy, RXJ1633+4718, from archival ROSAT observations.The thermal temperature of this emission, when fitted with a blackbody, is aslow as 32.5(+8.0,-6.0)eV. This is in remarkable contrast to the canonicaltemperatures of ~0.1-0.2keV found hitherto for the soft X-ray excess in activegalactic nuclei (AGN), and is interestingly close to the maximum temperaturepredicted for a postulated accretion disc in this object. If this emission isindeed 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 theSchwarzschild 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 asthe (Compton scattered) Wien tail of the multi-temperature blackbody emissionfrom an optically thick accretion disc, whose parameters inferred (black holemass and accretion rate) are in good agreement with independent estimates usingoptical emission line spectrum. We thus consider this feature as a signature ofthe long-sought X-ray radiation directly from a disc around a super-massiveblack hole, presenting observational evidence for a black hole accretion discin AGN. Future observations with better data quality, together with improvedindependent measurements of the black hole mass, may constrain the spin of theblack hole.

X-ray observational signature of a black hole accretion disc in an active galactic nucleus RXJ1633+4718