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\section{Introduction}  \label{intro}   Rotation is a key player in the drama that unfolds upon the death of a massive star. The angular momentum (AM) contained in the iron core and overlying layers determines the rotation rate at core collapse (CC), which could have a strong impact on the dynamics of CC and the subsequent supernova \citep[See e.g][]{MacFadyen_1999,Woosley_2002,Woosley_2006,Yoon_2006}. Rotation may help determine the nature of the compact remnant, which could range from a slowly rotating neutron star (NS) to a millisecond magnetar or rapidly rotating black hole \citep[See e.g.][]{heger:00,Heger_2005}. The former may evolve into an ordinary pulsar, while the latter two outcomes offer exciting prospects for the production of long gamma-ray bursts (GRB) and superluminous supernova. In each of these phenomena, rotating central engines are suspected to be the primary source of power \citep{1993ApJ...405..273W,kasen:2010,Metzger_2011}. \citep{1993ApJ...405..273W,Kasen_2010,Metzger_2011}.  Despite rotation being recognized as an important parameter controlling the evolution of massive stars \cite{Maeder_2000}, little is known about the rotation rates of the inner cores of massive stars nearing CC. %Although rapid rotation leaves an imprint in the gravitational wave spectrum produced at core bounce (Ott et al. 2012, Abdikamalov et al. 2014, Klion et al. 2015, Fuller et al. 2015), a gravitational wave detection of a galactic supernovae is likely decades away.  The best observational constraints stem from measurements of the rotation rates of the compact remnants following CC. For instance, a few low-mass black hole X-ray binary systems have been measured to have large spins that can only be accounted for by high spins at birth \citep{Axelsson_2011,Miller_2011,Wong_2012}. However, the rotation rates of young NSs show little evidence for rapid rotation ($P \lesssim 10 \, {\rm ms}$) at birth. The most rapidly rotating young pulsars include PSR J0537-6910 ($P=16\,{\rm ms}$) and the Crab pulsar ($P=33\,{\rm ms}$), whose birth periods have been estimated to be $P_i \lesssim 10\,{\rm ms}$ \citep{marshall:98} and $P_i \sim 19 \, {\rm ms}$ \citep{kaspi:02}, respectively. Many young NSs appear to rotate much more slowly, with typical periods of hundreds of ms \citep{lai:96,gotthelf:13,2015arXiv150107220D}. In general, pulsar observations seem to indicate a broad range of initial birth periods in the vicinity of tens to hundreds of milliseconds \citep{faucher:06,popov:10,gullon:14}. Hence, rapidly rotating young NSs appear to be the exception rather than the rule.