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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 (≈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 (≈106 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☉ chemically fractionated core of a C-O-Ne or O-Ne-Mg white dwarf which has previously crystallized.