M15'S X-RAY BINARY

Alone among the dozens of globular clusters visible in the northern sky, M15 in Pegasus harbors a bright, compact X-ray source. Images obtained several years ago by the Einstein X-ray satellite narrowed the location of the high-energy emitter - known as 4U 2127 + 12 - to within a circle some 7 arc seconds across in the cluster core. And despite the extraordinary concentration of stars there, a visible counterpart, AC 211,  was identified. AC 211 appears bright and variable at ultraviolet wavelengths. It bears little resemblance to ordinary cluster variables like pulsating Cepheids or RR Lyrae stars. Such objects are typically cool and red. In contrast, AC 211 is clearly very hot.

Is that enough to guarantee that AC 211 is the X-ray star? No, but a team of British astronomers led by Phillip A. Charles (Oxford University) discovered ionized-helium emission from the star, indicating the presence of gas heated by X-rays. As in X-ray binaries seen elsewhere in our galaxy, the intesely energetic emission presumably arises when gas from a normal star falls toward a neutron-star companion and heats up to temperatures as high as 10 million degrees Kelvin.

Comparison of the ultraviolet and X-ray light curves indicates that an accretion disk around the neutron star contributes a substantial fraction of the blue light from AC 211. Other properties of this remarkable system have recently come to light. T. Naylor (Oxford University) and co-workers, using sensitive optical spectroscopy, have found an absorption line due to neutral helium. It probably arises in the disk around the compact object. The observed wavelength of the feature shifts slightly back and forth once every 9 hours or so, consistent with the binary's orbital motion derived from ultraviolet and X-ray observations.

Astonishingly, however, the average line-of-sight velocity derived from optical the optical spectra is 150 kilometres per second smaller than that of M15 itself. This suggests that AC 211 is hurtling out of the cluster core. If it maintains its present velocity, it will leave the cluster altogether about 100,000 years from now.