Refined asymptotics of the Riemann-Siegel theta function

Abstract: The Riemann-Siegel theta function $\vartheta(t)$ is examined for $t\to+\infty$. Use of the refined asymptotic expansion for $\log\,\g(z)$ shows that the expansion of $\vartheta(t)$ contains an infinite sequence of increasingly subdominant exponential terms, each multiplied by an asymptotic series involving inverse powers of $\pi t$. Numerical examples are given to detect and confirm the presence of the first three of these exponentials.