EXTRA-SOFT X-RAY EXCESS OF QUASARS

In a quasar sample with strong soft X-ray emission, we construct a sample of extra-soft X-ray excess quasars in virtue of the relation between the hydrogen column density, NH (IPC), determined by Einstein IPC, and NH (21cm), determined by Galactic 21cm line. The X-ray spectral index a*E in the energy interval 0.3keVE0.5keV is calculated. a*E is obviously larger than the spectral index αE in 0.5keVE3.5keV, showing that 0.3-0.5keV is just the rapidly decreasing part of the right side of big bump from UV to soft X-ray.These results will help establish the composite model for this kind of big bump.