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

Abstract

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∞ = 76 ± 3 eV, R∞ = 12.8 ± 0.4 km and an X-ray luminosity of 7.3 ± 0.6 × 1032 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.