The integral \( \int \sqrt{x^2 - a^2} \, dx \) is a standard integral. To solve this, we use the following standard result:
\[
\int \sqrt{x^2 - a^2} \, dx = \frac{x}{2} \sqrt{x^2 - a^2} - \frac{a^2}{2} \ln \left( x + \sqrt{x^2 - a^2} \right) + C,
\]
where \( C \) is the constant of integration.
Conclusion:
The value of the integral is:
\[
\boxed{\frac{x}{2} \sqrt{x^2 - a^2} - \frac{a^2}{2} \ln \left( x + \sqrt{x^2 - a^2} \right) + C}.
\]