2006MNRAS.372L...1D

CXOU J121538.2+361921 is the brightest X-ray source in the galaxy NGC 4214, with an X-ray luminosity of up to 0.7x10³⁹erg/s. The observed periodicity of 3.62 h, derived from eclipse timing, 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 this possibility further. 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 which is initiated when the helium-star donor is at the helium shell burning stage. A donor star with a current mass in the range of around 2.2-3.6M_☉ 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.