An emission-line object in the core of M15

Optical identification of globular cluster X-ray sources is essential if their evolutionary state and relation to the galactic bulge sources is to be understood. The 10 high-luminosity (>=10³⁶ erg s^-1) X-ray sources associated with globular clusters have very accurate (~1 arc s) positions from the Einstein Observatory survey^1,2. No globular cluster source has yet been optically identified, primarily because all of the bright X-ray sources are located close to the dynamical centre (implying total masses of ~1.5M_solar (ref. 2)), where crowding makes optical photometry and spectroscopy extremely difficult. The only northern cluster containing a bright X-ray source is M15, which has the extremely ultraviolet (U - B = -1.2) and variable star AC211 near the X-ray error circle^3,4. The reported spectrum is that of an A star^5,6, with no emission lines which are normally present in galactic low-mass X-ray binaries. We have now discovered^7,8 strong He II 4,686-Å emission from the region of AC211, and weaker Balmer emission. We conclude that it is an intrinsically high-luminosity (L_x>=10³⁸erg s^-1) X-ray source on the basis of the optical brightness of the star and the He II flux. As the observed L_x is only 6 × 10³⁶ erg s^-1, this implies that the system is at a high inclination and that only scattered X-rays from an extended region are observed. The mass transfer rate required to account for this is very high and places important constraints on the evolutionary state of the mass-losing star which, for its optical brightness, must either be a red giant or horizontal branch star. The location of AC211 questions the accuracy of the Einstein Observatory positions.