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N. S. Schulz, H. L. Marshall, C. R. Canizares, D. Chakrabarty, J. Lee, D. Dewey (MIT)
We observed the 7.7 s low-mass X-ray binary pulsar 4U 1626-67 with the Chandra High Energy Transmission Grating Spectrometer for 40 ks. This ultracompact system contains a disk-accreting neutron star and a white dwarf companion in a 42 min binary orbit. ASCA previously observed strong X-ray line emission around 1 keV (Angelini et al. 1995), which was interpreted as emission predominantly from hydrogen-like Ne X Lyman-\alpha ions accompanied by broad absorption at 1.4 keV, which suggested an unresolved blend of Ne recombination and H- and He-like Mg absorption edges. Blends with O and Fe line complexes seemed highly probable leading to an inferred overabundance of x6, x2, and x2 relative to solar for Ne, O, and Mg, respectively. Chandra spectra fully resolves these features. We confirm the existence of the Ne X Lyman-\alpha line, the recombination edge as well as the O-K emission. However, the observed lines, predominately from He- and H-like Ne and O and Ne-like Fe ions, are broadened, indicating very high velocities of 5000-6000 km/s. We discuss various possibilities for the origin of these high velocity lines in the context of the geometry and accretion mechanisms of the system. The high velocities indicate that these lines may originate close to the accretion stream onto the neutron star rather than from within the accretion disk.
This work was funded in part by contract SAO SV1-61010 and NASA contract NAS8-39073.