Double-peaked X-ray lines from the oxygen/neon-rich accretion disk in 4U 1626-67

We report on a 39 ks observation of the 7.7 s low-mass X-ray binary pulsar 4U 1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-Ray Observatory. This ultracompact system consists of a disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted companion in a 42 minute binary. We have resolved the previously reported Ne/O emission line complex near 1 keV into Doppler pairs of broadened (approximate to 2500 km s(-1) FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line components are of comparable strength, with blueshifts of 1550-2610 km s(-1) and redshifts of 770-1900 km s(-1). The lines appear to originate in hot (approximate to 10(6) K), dense material just below the X-ray-heated skin of the outer Keplerian accretion disk, or else possibly in a disk wind driven from the pulsar's magnetopause. The observed photoelectric absorption edges of Ne and O appear nearly an order of magnitude stronger than expected from interstellar material and are likely formed in cool, metal-rich material local to the source. Based on the inferred local abundance ratios, we argue that the mass donor in this binary is probably the 0.02 M(circle dot)chemically fractionated core of a C-O-Ne or O-Ne-Mg white dwarf which has previously crystallized.