Infrared helium emission lines from Cygnus X-3 suggesting a Wolf-Rayet star companion

CYGNUS X-3 is one of the most luminous X-ray sources in the Galaxy1,2, a bright infrared source3 and a radio source that undergoes huge outbursts4. The system is a binary, presumably a neutron star plus companion, with a 4.79-h orbital period that modulates the X-ray and infrared emission5,6 and that increases on a 600,000-year timescale7,8. Radio observations reveal the presence of a relativistic jet9. The nature of Cyg X-3 has remained unclear, however, in part because the large interstellar extinction3 in its direction prevents optical spectroscopy. Upper limits on spectral features in the near infrared have been reported previously10, but only with recent instrumental improvements have we become able to identify spectral features in the near infrared I and K bands. These are found to be characteristic of Wolf-Rayet stars: strong, broad emission lines of HeI and HeII, but no strong hydrogen lines. These observations strongly suggest the presence of a dense wind in the Cyg X-3 system, and may indicate that the companion is a fairly massive helium star, as had been predicted11 by a model in which the present system is a descendant of a massive X-ray binary. © 1992 Nature Publishing Group.