Neon Line Emission in the X-Ray Spectrum of the Pulsar 4U 1626−67

Using the ASCA Observatory we have discovered a strong emission-line complex centered on 1 keV in the X-ray spectrum of the 7 s pulsar 4U 1626-67. The strongest line is identified with hydrogen-like Ne Lyα at 1.008 keV. The strength of the neon emission compared to the expected iron L complex implies a large neon overabundance relative to iron. 4U 1626-67 is a low-mass X-ray binary (LMXRB) system with an orbital period of 2485 s, and is unusual because it is one of a few LMXRB to contain an X-ray pulsar. A hydrogen burning companion star was suggested to explain the X-ray and optical characteristics. Neon is a by-product of helium burning and its overabundance in this system may indicate instead that the companion star is burning (or has burned) helium.