  We obtain a formal integral solution to the 3+1 D Boltzmann Equation in
relaxation time approximation. The gradient series obtained from this integral
solution contains exponentially decaying non-hydrodynamic terms. It is shown
that this gradient expansion can have a finite radius of convergence under
certain assumptions of analyticity. We then argue that, in the relaxation time
model, proximity to local thermal equilibrium is not necessary for the system
to be described by hydrodynamic equations.
