Discovery of a quiescent neutron star binary in the globular cluster M 13

We have discovered with XMM-Newton an X-ray source in the core of the globular cluster M 13, whose X-ray spectral properties suggest that it is a quiescent neutron star X-ray binary. The spectrum can be well fitted with a pure hydrogen atmosphere model, with $T_{\infty}=76\pm3$ eV, $R_{\infty}=12.8\pm0.4$ km and an X-ray luminosity of $7.3\pm0.6\times10^{32}$ ergs s-1. In the light of this result, we have discovered a strong correlation between the stellar encounter rate and the number of quiescent neutron stars found in the ten globular clusters observed so far by either XMM-Newton or Chandra. This result lends strong support to the idea that these systems are primarily produced by stellar encounters in the core of globular clusters