Metallicity Effect on LMXB Formation in Globular Clusters

TL;DR

This study demonstrates that the higher fraction of low-mass X-ray binaries in metal-rich globular clusters is consistent across a wide range of X-ray luminosities, indicating a fundamental metallicity effect.

Contribution

It provides comprehensive observational evidence that the metallicity effect on LMXB formation is uniform across luminosities and is independent of other cluster properties.

Findings

The metallicity effect is significant at >= 3 sigma in a broad luminosity range.

The ratio of LMXB fractions in metal-rich to metal-poor GCs is approximately 3.4.

The metallicity effect persists across different luminosity regimes, including ULX levels.

Abstract

We present comprehensive observational results of the metallicity effect on the fraction of globular clusters (GC) that contain low-mass X-ray binaries (LMXB), by utilizing all available data obtained with Chandra for LMXBs and HST ACS for GCs. Our primary sample consists of old elliptical galaxies selected from the ACS Virgo and Fornax surveys. To improve statistics at both the lowest and highest X-ray luminosity, we also use previously reported results from other galaxies. It is well known that the LMXB fraction is considerably higher in red, metal-rich, than in blue, metal-poor GCs. In this paper, we test whether this metallicity effect is X-ray luminosity-dependent, and find that the effect holds uniformly in a wide luminosity range. This result is statistically significant (at >= 3 sigma) in LMXBs with luminosities in the range LX = 2 x 10^37 - 5 x 10^38 erg s-1, where the ratio of LMXB fractions in metal-rich to metal-poor GCs is R = 3.4 +- 0.5. A similar ratio is also found at lower (down to 10^36 erg s-1) and higher luminosities (up to the ULX regime), but with less significance (~2 sigma confidence). Because different types of LMXBs dominate in different luminosities, our finding requires a new explanation for the metallicity effect in dynamically formed LMXBs. We confirm that the metallicity effect is not affected by other factors such as stellar age, GC mass, stellar encounter rate, and galacto-centric distance.