High-frequency graviton from inflaton oscillation

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 when the inflaton is weakly coupled and the perturbative reheating picture is valid. Although the signal is typically very small, such a high-frequency tail contains information on inflaton properties such as the inflaton mass as well as the thermal history of the early Universe.