CXOU J121538.2+361921 is the brightest X-ray source in the galaxy NGC 4214 , with an X-ray luminosity of up to 0.7 \times 10 ^ { 39 } \mathrm { erg s ^ { -1 } } .
The observed periodicity of 3.62 hr is interpreted as the orbital period of the system .
It has been suggested that the system is a low-mass helium star with a lower-mass compact companion .
If this idea is correct , then CXOU J121538.2+361921 will evolve into a double neutron star , a binary consisting of a radio pulsar and another neutron star .
In this study we investigate further this possibility .
We find that the X-ray luminosity is consistent with super-Eddington accretion in a helium star-neutron star binary .
The binary is in a state of mass transfer phase which is initiated when the helium-star donor is on the helium shell burning stage .
A donor star with a current mass in the range of around 2.2 – 3.6 \mathrm { M _ { \sun } } is required to explain the observed orbital period .
Helium stars in this mass range are massive enough to collapse in a supernova explosion , making CXOU J121538.2+361921 the immediate progenitor of a double neutron star .