Low‐Mass X‐Ray Binaries and Globular Clusters in Early‐Type Galaxies

(Abridged) A high fraction of the Low Mass X-ray Binaries (LMXBs) inearly-type galaxies are associated with globular clusters (GCs). Here, wediscuss the correlations between LMXBs and GCs in a sample of four early-typegalaxies. There is some evidence that the fraction of LMXBs associated with GCs(f_X-GC) increases along the Hubble sequence from spiral bulges to S0s to Es tocDs. On the other hand, the fraction of globular clusters which contain X-raysources appears to be roughly constant at f_GC-X ~ 4%. There is a strongtendency for the X-ray sources to be associated with the optically moreluminous GCs. However, this correlation is consistent with a constantprobability of finding a LMXB per unit optical luminosity; it seems to resultprimarily from the larger number of stars in optically luminous GCs. Theprobability of finding a bright LMXB per unit optical luminosity in the GCs isabout 1.5e-7 LMXBs per L_solar,I for L_X >~ 1e38 erg/s, and rises to about2.0e-7 LMXBs per L_solar,I at lower X-ray luminosities, L_X >~ 3e37 erg/s. Thisfrequency appears to be roughly constant for different galaxies, including thebulges of the Milky Way and M31. There is a tendency for the X-ray sources tobe found preferentially in redder GCs. This seems to indicate that theevolution of X-ray binaries in a GC is affected either by the metallicity orage of the GC, with younger and/or more metal rich GCs having more LMXBs. Thereis a weak tendency for the brightest LMXBs, whose luminosities exceed theEddington luminosity for a 1.4 M_solar neutron star, to avoid GCs. That mayindicate that black hole X-ray are somewhat less likely to be found in GCs, asseems to be true in our Galaxy.Comment: Astrophysical Journal, 595, in press. 44 pages with 16 embedded Postscript figure