Abstract
We report the results of a joint \chandra-\hst study of the X-ray binary population in the massive, high-density globular cluster NGC 6388. NGC 6388 has one of the highest predicted X-ray binary production rate of any Galactic cluster. We detected a large population of 61 \chandra sources within the half-mass radius with L$_X > 5 \times 10^{30}$ \ergs. From the X-ray colors, luminosities, (lack of) variability, and spectral fitting, we identify five as likely quiescent low-mass X-ray binaries. Due to the extremely crowded nature of the core of NGC 6388, finding optical identifications to \chandra sources is challenging. We have identified four blue, optically variable counterparts to spectrally hard X-ray sources, evidence that these are bright cataclysmic variables (CVs). One showed variability of 2 magnitudes in V, indicative of a dwarf nova eruption. One other likely CV is identified by its X-ray spectrum (partial covering with high $N_H$) and strong variability, making five likely CVs identified in this cluster. The relatively bright optical magnitudes of these sources put them in the same class as CV1 in M15 and the brightest CVs in 47 Tuc.
Highlights
Studying the relationship between X-ray binaries (XRBs) and globular clusters continues to provide interesting clues into cluster dynamical evolution
The numbers of X-ray sources in a cluster, the quiescent LMXBs (qLMXBs), millisecond pulsars (MSPs), and brighter Cataclysmic variables (CVs), vary directly with this encounter rate, implying strongly that dynamical interactions are the primary source of these populations in globular clusters (Pooley et al 2003; Heinke et al 2003; Pooley & Hut 2006), while fainter active binaries (ABs) seem to be largely primordial in origin (Bassa et al 2004)
Our principal objective was to identify which sources have spectra typical of quiescent low-mass X-ray binary (LMXB), vs. which sources have spectra more commonly associated with cataclysmic variables
Summary
Studying the relationship between X-ray binaries (XRBs) and globular clusters continues to provide interesting clues into cluster dynamical evolution. Since mass segregation causes the more massive populations of binary stars and degenerate remnants (for instance, MSPs) to be more centrally concentrated, these objects will be strongly tied to the dynamical evolution of the cluster. Most significantly, this increases the likelihood of further interactions of binaries in the core which can halt the progress of dynamical relaxation, supporting the core against core collapse (Fregeau 2008). Higher numbers of interactions in cluster cores lead to larger populations of X-ray sources (Pooley et al 2003) This interaction rate can be estimated by the encounter rate, Γ ∝ ρ20rc3/v0 (Verbunt & Hut 1987; Maccarone & Peacock 2011). The numbers of X-ray sources in a cluster, the qLMXBs, MSPs, and brighter CVs, vary directly with this encounter rate, implying strongly that dynamical interactions are the primary source of these populations in globular clusters (Pooley et al 2003; Heinke et al 2003; Pooley & Hut 2006), while fainter ABs seem to be largely primordial in origin (Bassa et al 2004)
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