Characteristics of He II Proximity Profiles
  The proximity profile in the spectra of z~3 quasars, where fluxes extend
blueward of the He II Lya wavelength 304 (1+z) A, is one of the most important
spectral features in the study of the intergalactic medium. Based on the HST
spectra of 24 He II quasars, we find that the majority of them display a
proximity profile, corresponding to an ionization radius as large as 20 Mpc in
the source's rest frame. In comparison with those in the H i spectra of the
quasars at z~6, the He II proximity effect is more prominent and is observed
over a considerably longer period of reionization. The He II proximity zone
sizes decrease at higher redshifts, particularly at z > 3.3. This trend is
similar to that for H I, signaling an onset of He II reionization at z~4.
  For quasar SDSS1253+6817 (z=3.48), the He II absorption trough displays a
gradual decline and serves a good case for modeling the He II reionization. To
model such a broad profile requires a quasar radiation field whose distribution
between 4 and 1 Rydberg is considerably harder than normally assumed. The UV
continuum of this quasar is indeed exceptionally steep, and the He II
ionization level in the quasar vicinity is higher than the average level in the
intergalactic medium. These results are evidence that a very hard EUV continuum
from this quasar produces a large ionized zone around it.
  Distinct exceptions are the two brightest He II quasars at z~2.8, for which
no significant proximity profile is present, possibly implying that they are
young.
