The Ultraviolet Spectrum of the Ultracompact X-Ray Binary 4U 1626-67
Abstract
We have obtained Hubble Space Telescope/Space Telescope Imaging Spectrograph low-resolution ultraviolet spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA); 4U 1626-67 is unusual even among X-ray pulsars because of its ultrashort binary period (P=41.4 minutes) and remarkably low mass function (<=1.3×10-6 Msolar). The far-UV spectrum was exposed for a total of 32 ks and has sufficient signal-to-noise ratio to reveal numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin. However, there is evidence that both C I and C IV require additional absorbing gas local to the system. In emission, the usual prominent lines of N V and He II are absent, while both O IV and O V are relatively strong. We further identify a rarely seen feature at ~1660 Å as the O III] multiplet. Our ultraviolet spectra therefore provide independent support for the recent suggestion that the mass donor is the chemically fractionated core of either a CONe or ONeMg white dwarf; this was put forward to explain the results of Chandra high-resolution X-ray spectroscopy. The velocity profiles of the ultraviolet lines are in all cases broad and/or flat topped or perhaps even double peaked for the highest ionization cases of O; in either case, the ultraviolet line profiles are in broad agreement with the Doppler pairs found in the X-ray spectra. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disk; 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
- Publication:
-
The Astronomical Journal
- Pub Date:
- December 2002
- DOI:
- 10.1086/344600
- arXiv:
- arXiv:astro-ph/0205332
- Bibcode:
- 2002AJ....124.3348H
- Keywords:
-
- Stars: Binaries: Close;
- Stars: Circumstellar Matter;
- Stars: Pulsars: Individual: Alphanumeric: 4U 1626-67;
- Stars: Pulsars: Individual: Name: KZ Trianguli Australis;
- Ultraviolet Emission;
- X-Rays;
- Astrophysics
- E-Print:
- 15 pages, 5 figures (color but ok in greyscale), revised version with minor changes