X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3

TL;DR

This study uses Chandra and Hubble observations to analyze X-ray and optical sources in low core density globular clusters NGC6144 and E3, suggesting many sources are primordial binaries rather than collision products.

Contribution

First detailed multi-wavelength analysis of X-ray sources in low core density globular clusters, indicating primordial origin of compact binaries.

Findings

Detected 1-2 likely cataclysmic variables in NGC6144

Identified 1 active binary in NGC6144

Number of faint X-ray sources aligns with mass-based predictions

Abstract

We report on the Chandra X-ray Observatory and Hubble Space Telescope observation of two low core density globular clusters, NGC6144 and E3. By comparing the number of X-ray sources inside the half-mass radius to those outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among which 4 are expected to be background sources; 3 X-ray sources are also found within the half-mass radius of E3, of which 3 is expected to be background source. Therefore, we cannot exclude that all our sources are background sources. However, combining the results from X-ray and optical observations, we found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be cataclysmic variables and that 1 source in NGC6144 is an active binary, based on the X-ray and optical properties. The number of faint X-ray sources in NGC6144 and E3 found with Chandra and HST is higher than a prediction based on collision frequency, but is closer to that based on mass. Our observations strongly suggest that the compact binary systems in NGC6144 and E3 are primordial in origin.