Low mass X-ray binaries and globular clusters in early-type galaxies

Chandra observations have allowed the detection of a large number of low mass X-ray binaries (LMXBs) in early-type galaxies. Comparisons to catalogs of globular clusters (GCs) from Hubble Space Telescope observations have shown that a high fraction of the LMXBs in early-type galaxies are associated with GCs. The fraction of LMXBs associated with globular clusters increases along the Hubble sequence from spiral bulges 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 (∼4% for L_X≳,10³⁸ ergs/s, ∼10% for L_X≳10³⁷ ergs/s). There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. There is a trend for the X-ray sources to be found preferentially in redder, more metal-rich GCs, which is independent of optical luminosity correlation. The relative role of formation of LMXBs in GCs and in situ formation in the field is uncertain. One of the best ways to study this is to compare the spatial distribution of GC-LMXBs, field LMXBs, GCs, and optical light in the galaxies. Theoretical models and results of fits to the observed distributions are presented. Our multiple observations of NGC 4365 and NGC 4697 over several years allow us to study the variability of LMXBs. We have detected very luminous X-ray flares from three sources in NGC 4697 with durations of ∼70 to ∼3000 seconds, which have no clear analogue in our Galaxy. One suggestion is that these are due to micro-blazars; X-ray binaries with accreting black holes with jets which are pointed in our direction.