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 \sim 35 { ~ { } M } _ { \odot } , indicating a cluster age \sim 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 } \gtrsim 40 { ~ { } M } _ { \odot } ) 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 } \lesssim 20 { ~ { } M } _ { \odot } 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 { ~ { } M } _ { \odot } , under favorable conditions , to form neutron stars .
If the very high observed binary fraction of massive stars in Westerlund 1 ( \gtrsim 70 \% ) 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 .