An 8.5 Hour X-Ray Period in the M15 X-Ray Source 4U 2129+12

Photometric and spectroscopic periods have been reported recently for AC 211, the variable ultraviolet excess star in the core of the globular cluster M15. AC 211 is the proposed optical counterpart of the high-luminosity globular cluster X-ray source 4U 2129+12. The author has searched the HEAO A-1 data base and discovered periodic modulations of the X-ray source at a period of 8.66±0.08 hr; this period is consistent with the optical period. The X-ray light curve has a pulse fraction of 24%. The discovery of X-ray modulation at the optical period confirms the identification of AC 211 as the optical counterpart of 4U 2129+12. The large X-ray modulations are consistent with the proposal that X-rays from the compact object are shielded by an extended accretion disk and only X-rays scattered from an accretion disk corona are observed.