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 Galaxy 1,2, a bright infrared source 3 and a radio source that undergoes huge outbursts 4. 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 emission 5,6 and that increases on a 600,000-year timescale 7,8. Radio observations reveal the presence of a relativistic jet 9. The nature of Cyg X-3 has remained unclear, however, in part because the large interstellar extinction 3 in its direction prevents optical spectroscopy. Upper limits on spectral features in the near infrared have been reported previously 10, 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 He I and He II, 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 predicted 11 by a model in which the present system is a descendant of a massive X-ray binary.