VLT/UVES spectroscopy of the O supergiant companion to 4U 1907+09(7)

• Published in: Astronomy and astrophysics (Berlin) Vol. 436; no. 2; pp. 661 - 669
• Main Authors: Cox, N. L. J., Kaper, L., Mokiem, M. R.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.06.2005
• Subjects: ISM: lines and bands; outflows; stars: binaries: close; stars: early-type; stars: winds; X-rays: individual: 4U 1907+09; Stellar winds; Lower bound; Spectroscopy; Accretion; stars: binaries: close; H-alpha lines; Variability; Binary X ray source; Cosmic x-ray sources; Spectral classification; stars: winds, outflows; X-rays: individual: 4U 1907+09; Early type stars; stars: early-type; ISM: lines and bands; X-ray binary stars; Close binary stars; Interstellar line; Supergiant stars; X ray pulsar; Emission line; Interstellar matter; Optical spectrum
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20040511

High-resolution optical spectra of the high-mass X-ray binary 4U 1907+09  covering the wavelength range 4680-10 400 Å indicate that the X-ray pulsar's massive companion is an O8/O9 supergiant with a dense stellar wind. The interstellar atomic lines of $\ion{Na}{i}$ and $\ion{K}{i}$ of this strongly reddened source (${E}_{(B-V)} = 3.45$ mag) are used to set a lower limit to its distance: $d \approx 5$ kpc, excluding the possibility that the massive companion is a Be star. We re-evaluate the system parameters; the new spectral classification of 4U 1907+09's O supergiant companion is discussed in the context of its similarity in X-ray and orbital properties to Wray 15-977/GX 301-2. The Hα line shows variability similar to what is observed in other wind-fed systems. A remarkable feature is the strong He ii 4686 Å emission line, which possibly originates in the accretion flow towards the X-ray pulsar.