We report the discovery of a luminous ultra-soft X-ray excess in a radio-loud narrow-line Seyfert 1 galaxy , RX J1633+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 \sim 0.1–0.2 keV 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 } \times 10 ^ { 44 } { erg s ^ { -1 } } ) infers a compact emitting area with a size ( \sim 5 \times 10 ^ { 12 } cm or 0.33 AU in radius ) that is comparable to several times the Schwarzschild radius of a black hole at the mass estimated for this AGN ( \sim 3 \times 10 ^ { 6 } M _ { \odot } ) .
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 .