Parabola

$S=y^{2}-4 a x=0, S^{\prime}=y^{2}+a x=0$ are two parabolas and $P(t)$ is a point on the parabola $S^{\prime}=0$. If $A$ and $B$ are the feet of the perpendiculars from $P$ on to coordinate $2 x_{4}$ and $A B$ is a tangent to the parabola $S=0$ at the point $Q\left(t_{1}\right)$, then $t_{1}=$