Metallicity effect on low-mass x-ray binary formation in globular clusters

We present comprehensive observational results of the metallicity effect on the fraction of globular clusters (GCs) that contain low-mass X-ray binaries (LMXB), by utilizing all available data obtained with Chandra for LMXBs and Hubble Space Telescope Advanced Camera for Surveys (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 fraction of GCs hosting LMXBs is considerably higher in red, metal-rich, GCs 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 ≥3σ) in LMXBs with luminosities in the range L _X = 2 × 10³⁷ to 5 × 10³⁸ erg s ^-1, where the ratio of GC-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 10³⁶ erg s^-1) and higher luminosities (up to the ULX regime), but with less significance (∼2σ 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.