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

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.

💡 Research Summary

The authors present a combined Chandra X‑ray and Hubble Space Telescope (HST) study of two low‑core‑density globular clusters, NGC 6144 and E3, with the goal of characterizing faint X‑ray sources within their half‑mass radii and assessing the origin of compact binaries in such environments. Using ACIS‑S observations (≈30 ks for NGC 6144, ≈25 ks for E3) they processed the data with CIAO, applied the wavdetect algorithm at a false‑positive probability of 10⁻⁶, and identified six X‑ray sources inside the half‑mass radius of NGC 6144 and three inside that of E3. By comparing source densities inside and outside the half‑mass radius and employing log N–log S background models, they estimated that four of the NGC 6144 sources and three of the E3 sources are likely background objects, leaving only a handful of cluster‑member candidates (≈2 in NGC 6144, ≤1 in E3).

To discriminate between genuine cluster members and background contaminants, the authors matched the X‑ray positions to deep HST ACS/WFC and WFPC2 images (filters F606W, F814W, and F336W). Astrometric alignment was refined using GAIA DR2 reference stars, achieving sub‑arcsecond accuracy. Within the 95 % X‑ray error circles they identified optical counterparts and examined their colors, magnitudes, and Hα excesses. Several counterparts displayed blue colors and elevated X‑ray‑to‑optical flux ratios (L_X/L_opt ≈10⁻³–10⁻²), consistent with cataclysmic variables (CVs). One source in NGC 6144 exhibited an X‑ray luminosity of ~10³¹ erg s⁻¹, a blue optical color, and possible Hα emission, leading to its classification as a CV candidate. Another NGC 6144 source, with L_X ≈10³⁰ erg s⁻¹ and an optical counterpart lying on the main sequence, matches the properties of an active binary (AB). In E3, one source shows similar X‑ray brightness and blue color, suggesting a CV as well.

The observed numbers of faint X‑ray sources were compared to two predictive frameworks: (1) a collision‑frequency (Γ) scaling, which predicts very few sources in low‑density clusters because stellar encounters are rare, and (2) a mass‑based scaling (N_X ∝ M^α, with α≈0.7), which assumes that the primordial binary population scales with the total cluster mass. The authors find that the mass‑based prediction better matches the data: the detected CV and AB candidates are roughly consistent with the expected number of primordial binaries for the clusters’ masses, whereas the Γ‑based model underestimates the source count by a factor of several. This supports the interpretation that, in low‑density clusters, most faint X‑ray binaries are primordial rather than formed through dynamical encounters.

The paper discusses the implications of these results for binary evolution in globular clusters. In environments with low central densities, mass segregation proceeds slowly and core collapse is delayed, allowing primordial binaries to survive for many gigayears. Consequently, the faint X‑ray source population in NGC 6144 and E3 is dominated by long‑lived CVs and ABs that were present at cluster formation, contrasting with the collision‑induced CVs commonly found in dense cores such as 47 Tuc or NGC 6397. The authors acknowledge limitations, including small-number statistics, uncertainties in background estimates, and incomplete optical coverage, and propose future work involving deeper X‑ray exposures, infrared variability monitoring, and spectroscopic follow‑up to confirm the nature of the candidates and to measure orbital parameters.

In summary, the combined Chandra–HST analysis identifies 1–2 likely cataclysmic variables and one active binary in NGC 6144, and one probable cataclysmic variable in E3. The number of faint X‑ray sources exceeds predictions based on stellar collision rates but aligns with expectations derived from the clusters’ total masses, strongly indicating that the compact binaries in these low‑core‑density globular clusters are of primordial origin. This work highlights the importance of multi‑wavelength studies for disentangling the formation channels of X‑ray binaries across different dynamical regimes in globular clusters.