 \label{fig:MassiveIGWspin}  Normalized distribution of maximum spin periods in the pre-collapse iron core, $P_{\rm Fe,max}$, due to stochastic spin-up via IGW. The left panel shows the distribution using the pessimistic wave flux (left hand side of equation \ref{eqn:Ewaves}), while the right panel shows the distribution using the optimistic wave flux (right hand side of equation \ref{eqn:Ewaves}). The top axis shows the corresponding maximum NS rotation period, $P_{\rm max,NS}$, assuming conservation of angular momentum during the supernova. For core collapse SNe, $P_{\rm max,NS}$ is most plausibly set during Si-burning (see text), which corresponds to the thick red line. Stochastic IGW spin-up leads to values of $P_{\rm max,NS}$ of tens to hundreds of milliseconds, similar to the initial rotation rates of most pulsars. 