X-ray sources in globular clusters

We show that the bright (⁠|$L_x\gtrsim10^{36}\text {erg s}^{-1}$|⁠) X-ray sources in globular clusters of our Galaxy are limited to luminosities |$L_x\lesssim10^{37.3}\text {erg s}^{-1}$|⁠, whereas the sources in the galactic disc range to |$\sim 10^{38.4} \text {erg s}^{-1}$|⁠. The latter limit also applies to the globular-cluster sources of M31. We investigate two possible explanations for this difference, viz. a dependence of the mass-transfer rate |$\dot M$| on the secondary mass in low-mass X-ray binaries, and a dependence of |$\dot M$| on metallicity. The first explanation can only hold if |$\dot M$| increases by a factor ∼5 between secondary masses of |$\sim 0.8\enspace M_\odot$| and |$\sim 0.9\enspace M_\odot$|⁠. This steep increase is difficult to reconcile with the observed flatness of the X-ray luminosity distribution. Secondly, the observations of the globular clusters in M31 show that the mass-transfer rate does not depend strongly on the metallicity. Concerning the less luminous X-ray sources (⁠|$L_x \lt 10^{34.5}\text {erg s}^{-1}$|⁠) we argue that those with |$10^{33} \text {erg s}^{-1}\lt L_x \lt 10^{34.5}\text {erg s}^{-1}$| are probably too bright to be cataclysmic variables. They may be confined to the cluster cores, and we propose that they are quiescent soft X-ray transients. The existence of a class of moderately bright quiescent soft X-ray transients would also explain that sources in this luminosity interval have been observed in the galactic bulge in the Einstein galactic survey.