XMM-Newton observations of the Galactic globular clusters NGC 2808 and NGC 4372^*

CESR, Université Paul Sabatier, CNRS, 9 avenue du Colonel Roche, 31400 Toulouse, France e-mail: mathieu.servillat@cesr.fr

Received: 20 July 2007
Accepted: 28 November 2007

Abstract

Aims.Galactic globular clusters harbour binary systems that are detected as faint X-ray sources. These close binaries are thought to play an important role in the stability of the clusters by liberating energy and delaying the inevitable core collapse of globular clusters. The inventory of close binaries and their identification is therefore essential.

Methods.We present XMM-Newton observations of two Galactic globular clusters: NGC 2808 and NGC 4372. We use X-ray spectral and variability analysis combined with ultra-violet observations made with the XMM-Newton optical monitor and published data from the Hubble Space Telescope to identify sources associated with the clusters. We compare the results of our observations with estimates from population synthesis models.

Results.Five sources out of 96 are likely to be related to NGC 2808. Nine sources are found in the field of view of NGC 4372, none being located inside its half-mass radius. We find one quiescent neutron star low mass X-ray binary candidate in the core of NGC 2808, and propose that the majority of the central sources in NGC 2808 are cataclysmic variables. An estimation leads to 20±10 cataclysmic variables with luminosity above 4.2510³¹ erg s^-1. Millisecond pulsars could also be present in the core of NGC 2808, and some sources outside the half-mass radius could possibly be linked to the cluster.