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Optical spectra of the carbon-oxygen accretion discs in the ultra-compact X-ray binaries 4U 0614+09, 4U 1543-624 and 2S 0918-549

G. Nelemans·P. G. Jonker·T. R. Marsh·M. van der Klis
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Abstract

We present optical spectra in the range 4600 -- 8600 A for three low-mass X-ray binaries which have been suggested to belong to the class of ultra-compact X-ray binaries based on their X-ray spectra. Our spectra show no evidence for hydrogen or helium emission lines, as are seen in classical X-ray binaries. The spectrum of 4U~0614+09 does show emission lines which we identify with carbon and oxygen lines of CII, CIII, OII and OIII. While the spectra of 4U 1543-624 and 2S 0918-549 have a lower signal-to-noise ratio, and thus are more difficult to interpret, some of the characteristic features of 4U 0614+09 are present in these spectra too, although sometimes they are clearly weaker. We conclude that the optical spectra give further evidence for the ultra-compact nature of these X-ray binaries and for their donor stars being carbon-oxygen white dwarfs.

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G. Nelemans, P. G. Jonker, T. R. Marsh, M. van der Klis. 2003-12-01. Optical spectra of the carbon-oxygen accretion discs in the ultra-compact X-ray binaries 4U 0614+09, 4U 1543-624 and 2S 0918-549. https://doi.org/10.1111/j.1365-2966.2004.07486.x

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