Statistical Properties of the Big Blue Bump in Active Galactic Nuclei

The shape is the most important characteristic of the big blue bump (bbb). By employing the sample from Walter & Fink, and supplementing it with medium energy X-ray data, we have been successful in separating the bbb component from the nonthermal component for the soft X-ray region and thus obtaining the cutoff energy E_cut of the big bump in the XUV region. Our sample contains 30 type I Seyfert galaxies, which is the largest sample size for the determination of the bbb shape. Statistical analyses show a significant anticorrelation between the spectral UV index (b_uv) and E_cut. The thermal emission mechanisms for the bbb include the optically thick thermal radiation from the accretion disk (and its corona region), the optically thin thermal free-free radiation and other models. The statistical properties between b_uv and E_cut, provide an important base for picking and improving a mechanism.