  We point out that there is a high-frequency tail of the stochastic
inflationary gravitational wave background that scales as $f^{-1/2}$ with
frequency $f$. This contribution comes from the graviton vacuum fluctuation
amplified by the inflaton coherent oscillation during the reheating stage. It
contains information on inflaton properties such as the inflaton mass as well
as the thermal history of the early Universe.
