Combined Chandra, XMM-Newton, and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808^*

¹ CESR, Observatoire Midi-Pyrénées, Université Paul Sabatier, CNRS, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France e-mail: mathieu.servillat@cesr.fr
² Department of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
³ Instituto de Astrofisica de Canarias, Via Lactea, La Laguna 38200, Santa Cruz de Tenerife, Spain
⁴ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁵ Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA

Received: 13 May 2008
Accepted: 15 August 2008

Abstract

Aims. Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble far ultraviolet observations, we aim to detect and identify the faint X-ray sources belonging to the Galactic globular cluster NGC 2808 in order to understand their role in the evolution of globular clusters.

Methods. We present a Chandra X-ray observation of the Galactic globular cluster NGC 2808. We classify the X-ray sources associated with the cluster by analysing their colours and variability. Previous observations with XMM-Newton and far ultraviolet observations with the Hubble Space Telescope are re-investigated to help identify the Chandra sources associated with the cluster. We compare our results to population synthesis models and observations of other Galactic globular clusters.

Results. We detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core. From statistical analysis, these 16 sources are very likely to be linked to the cluster. We detect short-term (1 day) variability in X-rays for 7 sources, of which 2 fall inside the half-mass radius, and long-term (28 months) variability for 10 further sources, of which 2 fall inside the half-mass radius. Ultraviolet counterparts are found for 8 Chandra sources in the core, of which 2 have good matching probabilities and have ultraviolet properties expected for cataclysmic variables. We find one likely neutron star-quiescent low-mass X-ray binary and 8 cataclysmic variable candidates in the core of NGC 2808. The other sources are cataclysmic variable candidates, but some could possibly be active binaries or millisecond pulsars. We find a possible deficit of X-ray sources compared to 47 Tuc which could be related to the metallicity content and the complexity of the evolution of NGC 2808.