Let circle $A$ have center at $A$ and intersect one asymptote of hyperbola $C$ at points $M$ and $N$. If $\angle M A N = 60 ^ { \circ }$, then the eccentricity of $C$ is \_\_\_\_
Let circle $A$ have center at $A$ and intersect one asymptote of hyperbola $C$ at points $M$ and $N$. If $\angle M A N = 60 ^ { \circ }$, then the eccentricity of $C$ is \_\_\_\_