  The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.
