An 8.5 hour X-ray binary in the globular cluster M15

Abstract

The variable and blue emission-line star, AC211, in the core of M15 has been shown to be the optical counterpart of the bright (∼3×10 36 erg s −1) X-ray source on the basis of its colour, variability and period. The 8.5 hour binary period was found by optical spectroscopy and photometry /1.2/ and subsequently detected in archival X-ray data /3/. The low-amplitude X-ray variations are in stark contrast to the 1 mag optical variations and, together with the low L x/L opt ratio, support the view that AC211 is an accretion disc corona system. High resolution, time resolved optical spectroscopy shows strong, broad and variable HeII 4686Å emission and several HeI absorption lines. The HeI lines are substantially blueshifted (150 km s −1) with respect to the mean cluster velocity, and display a velocity modulation of 40 km s −1 at the binary period. A neutron star may be accomodated as the compact object only if the companion star is <0.4 M ®. Extensive CCD photometry /4/ reveals a large (1.6 mag) amplitude in U, but there are occasions when the source is remarkably constant. This may be related to vertical disc structure variability as seen in other LMXBs, but accounting for the extraordinary systemic velocity is much more difficult.