We study the reionization histories where ionizing UV photons are emitted
from decaying particles, in addition to usual contributions from stars and
quasars, taking account of the fact that the universe is not fully ionized
until z = 6 as observed by Sloan Digital Sky Survey. Likelihood analysis of the
three-year data from the WMAP (Wilkinson Microwave Anisotropy Probe) severely
constrains the decaying particle scenario.In particular, the decaying particle
with relatively short lifetime is not favored by the polarization data.Comment: 9 pages, 11 figure