A high fraction of the Low Mass X-ray Binaries ( LMXBs ) in early-type galaxies are associated with globular clusters ( GCs ) .
Here , we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations .
There is some evidence that the fraction of LMXBs associated with globular clusters ( f _ { X - GC } ) increases along the Hubble sequence from spiral bulges ( or spheroids ) to S0s to Es to cDs .
On the other hand , the fraction of globular clusters which contain X-ray sources appears to be roughly constant at f _ { GC - X } \sim 4 % .
There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs .
However , this correlation is consistent with a constant probability of finding a LMXB per unit optical luminosity ; that is , it seems to result primarily from the larger number of stars in optically luminous GCs .
The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5 \times 10 ^ { -7 } LMXBs per L _ { \odot,I } for L _ { X } \gtrsim 1 \times 10 ^ { 38 } erg s ^ { -1 } ( 0.3–10 keV ) and rises to about 2.0 \times 10 ^ { -7 } LMXBs per L _ { \odot,I } at lower X-ray luminosities , L _ { X } \gtrsim 3 \times 10 ^ { 37 } erg s ^ { -1 } .
This frequency appears to be roughly constant for different galaxies , including the bulges of the Milky Way and M31 .
There is a tendency for the X-ray sources to be found preferentially in redder GCs , which is independent of optical luminosity correlation .
This seems to indicate that the evolution of X-ray binaries in a GC is affected either by the metallicity or age of the GC , with younger and/or more metal rich GCs having more LMXBs .
There is no strong difference in the X-ray luminosities of GC and non-GC LMXBs .
There is a weak tendency for the brightest LMXBs , whose luminosities exceed the Eddington luminosity for a 1.4 M _ { \odot } neutron star , to avoid GCs .
That may indicate that black hole X-ray binaries are somewhat less likely to be found in GCs , as seems to be true in our Galaxy .
On the other hand , there are some luminous LMXBs associated with GCs .
There is no clear evidence that the X-ray spectra or variability of GC and non-GC X-ray sources differ .
We also find no evidence for a difference in the spatial distribution of GC and non-GC LMXBs .
Many of these results are similar to those found in NGC 1399 and NGC 4472 by Angelini et al .
and Kundu et al. , respectively .