Measure of a Bisecting Angle

Geometry Level 3

Circle $\Gamma$ with center $O$ is tangential to $PA$ at $A$ . Line $PO$ extended intersects $\Gamma$ at $B$ (i.e. $PO < PB$ ). $C$ is a point on $AB$ such that $PC$ bisects $\angle APB$ . What is the measure (in degrees) of $\angle PCA$ ?