The electrostatic potential in a charged spherical region of radius $r$ varies as $V=a r^3+b$, where $a$ and $b$ are constants. The total charge in the sphere of unit radius is $\alpha \times \pi a \mathrm{o}_0$. The value of $\alpha$ is $\_\_\_\_$ . (Permittivity of vacuum is $\mathrm{o}_{\mathrm{o}}$ )