Spectroscopy of the M 15 X-ray source : discovery of binary motion and an unusual systemic velocity.

Abstract

The authors present the first phase-resolved optical spectroscopy of AC 211, the optical counterpart of the X-ray source in the core of M15. The spectrum is dominated by the variable and complex He II λ4686 emission, but the authors have also found He I in absorption. By following the motion of He I λ4471 they have discovered the binary period to be 9.1±0.5 hr, with a semi-amplitude of 40±15 km s^-1. If this is interpreted as reflecting the orbital motion of the compact object, either the primary is 10 - 14 M_sun;, or the secondary is a very light (<0.4 M_sun;) giant. But, most remarkably, the authors find that the mean He I velocity is blueshifted by ≡150±10 km s^-1 with respect to the rest of the cluster.