Classify $U(1)$ bundle over $\mathbf{P}^3$, and its topological invariants

Question

I am interested in knowing the classification of the $U(1)$ bundle over the complex projective space $\mathbf{P}^3$. This is effectively a $U(1)$ bundle over the $6$-manifold $M^6$.

What are the possible values of the $$c_1 p_1$$ for that $6$-manifold $M^6$ with $U(1)$ bundle over $\mathbf{P}^3$?

Here $c_1$ is the first Chern class of $U(1)$ bundle, and the $p_1$ is the first Pontryagin class of the tangent bundle of the $6$-manifold?

Can you give a configuration where $\int_{M^6} c_1 p_1=1$ or $\int_{M^6} c_1 p_1=4$?