AAS 201st Meeting, January, 2003
Session 54. Compact X-Ray Sources
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

## [54.20] The UV Spectrum of the Ultra-compact X-ray Binary 4U 1626-67

L. Homer, S. F. Anderson, S. Wachter (U. Washington, Seattle), B. Margon (STScI)

We have obtained Hubble Space Telescope/STIS low-resolution ultraviolet spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA). This system is unusual even among X-ray pulsars due to its ultra-short binary period (P=41.4 min) and remarkably low mass-function (<1.3\times10-6Msun). The 32ks far-UV spectrum reveals numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin, although CI and CIV appear to need additional absorbing gas local to the system. Unusual abundance ratios are also indicated by the emission lines; the usual prominent lines of NV and HeII are absent, whilst both OIV and OV are relatively strong and we see the unusual ~1660ÅOIII] multiplet. Our ultraviolet spectra therefore provide independent support for the recent suggestion that the mass donor is the chemically fractionated core of either a C-O-Ne or O-Ne-Mg white dwarf; as proposed to explain the results of Chandra high-resolution X-ray spectroscopy. Furthermore, our ultraviolet line profiles are in overall agreement with the Doppler pairs found in the X-ray spectra, being in all cases broad and/or flat-topped, or even double-peaked. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disc; hence the combination of ultraviolet and X-ray spectral data may provide a rich data set for follow-on detailed models of the disk dynamics and ionization structure in this highly unusual low-mass X-ray pulsar system.

Support for this work was provided by NASA through STScI grant GO-06624.01 and LTSA grant NAG5-7932.

Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.