LMXB populations in galaxy outskirts: globular clusters and supernova
  kicks
  For the first time, we have systematically explored the population of
discrete X-ray sources in the outskirts of early-type galaxies. Based on a
broad sample of 20 galaxies observed with Chandra we detected overdensity of
X-ray sources in their outskirts. The overdensity appears as halos of resolved
sources around the galaxies. These halos are broader than the stellar light,
extending out to at least ~ 10 Re (Re is the effective radius). These halos are
composed of sources fainter than ~5E38 erg/s, whereas the more luminous sources
appear to follow the distribution of the stellar light, suggesting that the
excess source population consists of neutron star binaries. Dividing the galaxy
sample into four groups according to their stellar mass and specific frequency
of globular clusters, we find that the extended halos are present in all groups
except for the low-mass galaxies with low globular cluster content. We propose
that the extended halos may be comprised of two independent components,
low-mass X-ray binaries (LMXBs) located in globular clusters (GCs), which are
known to have a wider distribution than the stellar light, and neutron star
(NS) LMXBs kicked out of the main body of the parent galaxy by supernova
explosions. The available deep optical and X-ray data of NGC 4365 support this
conclusion. For this galaxy we identified 60.1+/-10.8 excess sources in the
4-10 Re region of which ~ 40% are located in GCs, whereas ~ 60% are field
LMXBs. We interpret the latter as kicked NS LMXBs. We discuss the implications
of these results for the natal kick distributions of black holes and neutron
stars.
