  The free energy of a closed 3-manifold is a 2-parameter formal power series
which encodes the perturbative Chern-Simons invariant (also known as the LMO
invariant) of a closed 3-manifold with gauge group U(N) for arbitrary $N$. We
prove that the free energy of an arbitrary closed 3-manifold is uniformly
Gevrey-1. As a corollary, it follows that the genus $g$ part of the free energy
is convergent in a neighborhood of zero, independent of the genus. Our results
follow from an estimate of the LMO invariant, in a particular gauge, and from
recent results of Bender-Gao-Richmond on the asymptotics of the number of
rooted maps for arbitrary genus. We illustrate our results with an explicit
formula for the free energy of a Lens space. In addition, using the Painlev\'e
differential equation, we obtain an asymptotic expansion for the number of
cubic graphs to all orders, stengthening the results of Bender-Gao-Richmond.
