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

TL;DR

This study combines Chandra, XMM-Newton, and Hubble observations to identify and analyze faint X-ray sources in the globular cluster NGC 2808, revealing diverse compact objects and their variability, and comparing findings with models and other clusters.

Contribution

It provides a comprehensive multi-wavelength analysis of X-ray sources in NGC 2808, identifying new candidate compact objects and variability patterns, and compares these with models and other clusters.

Findings

Detected 113 X-ray sources, 16 likely associated with the cluster.

Identified 2 cataclysmic variables with ultraviolet counterparts.

Found a possible deficit of X-ray sources compared to 47 Tuc.

Abstract

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. 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. 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 7 cataclysmic variable candidates in the core of NGC 2808. The other 8 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.