Comparing GC and Field LMXBs in Elliptical Galaxies with deep Chandra and Hubble data

(abridged) We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative ~1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs. In the low luminosity range allowed by our deeper data (LX < 5 x 1037 erg s-1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at ~4# significance at LX < 5 x 1037 erg s-1. As previously reported, these XLFs are consistent at higher luminosities. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of 5-6 x 1037 erg s-1, or a broken power-law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to ~15% of total LMXBs population at ~5x1037 erg s-1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs.

💡 Research Summary

This paper presents a comprehensive statistical study of low‑mass X‑ray binaries (LMXBs) in three nearby, old elliptical galaxies—NGC 3379, NGC 4278, and NGC 4697—using a combined exposure of roughly one megasecond with the Chandra ACIS instrument and deep optical imaging from the Hubble Space Telescope. The authors detect between 90 and 170 X‑ray sources within the D25 isophotal ellipse of each galaxy, yielding a total sample of 187 LMXBs after careful source validation. By cross‑matching the Chandra positions with HST catalogues and applying stringent color‑magnitude criteria, they identify 75 LMXBs that are spatially coincident with confirmed globular clusters (GC‑LMXBs) and 112 that reside in the field (field‑LMXBs).

The central focus of the analysis is the X‑ray luminosity function (XLF) of the two sub‑populations. In the low‑luminosity regime (L_X < 5 × 10³⁷ erg s⁻¹), the authors find a pronounced deficit of GC‑LMXBs relative to field sources. When the three galaxies are co‑added, the incompleteness‑corrected XLFs differ at approximately the 4σ level, indicating a statistically robust separation. By contrast, at higher luminosities (L_X > 5 × 10³⁷ erg s⁻¹) the XLFs of GC and field LMXBs are indistinguishable, reproducing earlier findings that bright LMXBs form efficiently in both environments.

To model the field XLF, the authors explore two viable descriptions: (1) a single power‑law with a localized excess around 5–6 × 10³⁷ erg s⁻¹, and (2) a broken power‑law featuring a low‑luminosity break near the same value. The excess can be interpreted as a contribution from neutron‑star–red‑giant (NS‑RG) binaries, which the authors estimate account for roughly 15 % of the total LMXB population at the break luminosity. The paucity of low‑luminosity GC‑LMXBs, on the other hand, suggests that binaries formed in globular clusters preferentially involve donors that have evolved beyond the main sequence. Such a demographic requires a relatively high initial binary fraction within clusters and efficient dynamical processes (exchange encounters, tidal captures) to assemble and retain these systems.

The paper also discusses the possibility that a fraction of field LMXBs originated in globular clusters and were subsequently ejected, a scenario supported by dynamical simulations of cluster evolution. The authors correct for detection incompleteness, account for distance and metallicity variations among the galaxies, and convert count rates to luminosities using consistent spectral assumptions, thereby ensuring that the observed differences are intrinsic rather than artefacts of observational bias.

Overall, the study provides compelling evidence that GC‑LMXBs and field‑LMXBs have distinct formation histories and evolutionary pathways, especially at low X‑ray luminosities. The results underscore the importance of dense stellar environments in shaping binary evolution and motivate further deep X‑ray surveys combined with high‑resolution optical spectroscopy to characterize donor stars and refine theoretical models of LMXB production in both globular clusters and galactic fields.