XMM-Newton X-ray and optical observations of the globular clusters M 55 and NGC 3201

¹ Centre d'Étude Spatiale des Rayonnements, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 04, France e-mail: Natalie.Webb@cesr.fr
² Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK

Received: 7 March 2005
Accepted: 31 August 2005

Abstract

We have observed two low concentration Galactic globular clusters with the X-ray observatory XMM-Newton. We detect 47 faint X-ray sources in the direction of M 55 and 62 in the field of view of NGC 3201. Using the statistical Log N - Log S relationship of extragalactic sources derived from XMM-Newton Lockman Hole observations, to estimate the background source population, we estimate that very few of the sources () in the field of view of M 55 actually belong to the cluster. These sources are located in the centre of the cluster as we expect if the cluster has undergone mass segregation. NGC 3201 has approximately 15 related sources, which are centrally located but are not constrained to lie within the half mass radius. The sources belonging to this cluster can lie up to 5 core radii from the centre of the cluster which could imply that this cluster has been perturbed. Using X-ray (and optical, in the case of M 55) colours, spectral and timing analysis (where possible) and comparing these observations to previous X-ray observations, we find evidence for sources in each cluster that could be cataclysmic variables, active binaries, millisecond pulsars and possible evidence for a quiescent low mass X-ray binary with a neutron star primary, even though we do not expect any such objects in either of the clusters, due to their low central concentrations. The majority of the other sources are background sources, such as AGN.