Comparison of X-ray sources in old open and in globular clusters

Twelve bright (L(x) greater than or similar to 10(36) erg/s) X-ray sources have been detected in globular clusters: they are accreting neutron stars. Five of these are transients. Thirty less luminous sources have been detected in eighteen globular clusters. The luminosity function of these sources does not rise rapidly towards lower luminosities. The sources with L(x) similar to 10(33) erg/s are probably transients in their low state; the least luminous sources currently detected may be cataclysmic variables. The old open cluster M 67 harbours chromospherically active binaries, with L(x) similar to 10(30-31) erg/s. Globular clusters contain fewer of these binaries than estimated by scaling with mass from M 67.