Notational difference between open quantum system textbooks.

 Ulrich weiss defines ohmic spectral density as

$$J_{weiss}(\omega) = M \gamma_w \omega$$

While Breuer defines it as

$$J_{br02}(\omega) = \frac{M 2 \gamma_{br02}}{\pi} \omega$$

The point to be notes is, $\pi J_{br02}(\omega) = J_{weiss}(\omega)$, so, the missing factor of $\frac{1}{\pi}$ in Weiss's book can be seen hanging outside in its equations. Also, needless to say, $\gamma_w = 2 \gamma_{br02}$.