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

TL;DR

This study uses XMM-Newton and Hubble data to identify and analyze X-ray sources in the globular clusters NGC 2808 and NGC 4372, revealing potential binary systems and their implications for cluster dynamics.

Contribution

First detailed X-ray and UV analysis of NGC 2808 and NGC 4372, identifying candidate binary systems and comparing observations with population models.

Findings

Identified 5 sources likely associated with NGC 2808.

Found 9 sources in NGC 4372's field, none inside its half-mass radius.

Detected a quiescent neutron star candidate in NGC 2808.

Abstract

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. 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. 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 4.25 x 10^31 erg s^-1. Millisecond pulsars could also be present in the core of NGC 2808, and some sources outside of the half-mass radius could possibly be linked to the cluster.