$PN$ is an ordinate of the parabola $y^2 = 4ax$ ($P$ on $y^2 = 4ax$ and $N$ on x-axis). A straight line is drawn parallel to the axis to bisect $NP$ and meets the curve in $Q$. $NQ$ meets the tangent at the vertex in a point $T$ such that $AT = kNP$, then the value of $k$ is (where $A$ is the vertex):