Zhongli Zhang, Marat Gilfanov, Akos Bogdan
For the first time, we systematically explored the population of discrete X-ray sources in the outskirt of early-type galaxies. Based on a broad sample of 20 galaxies observed with Chandra we revealed over density of X-ray sources in their outskirts. They appear as halos of resolved sources around galaxies, distributing much broader than the stellar light, and extended out to at least ~ 10 re (re is the effective radius). These halos are composed of sources fainter than ~ 5.e38 erg/s, whereas the more luminous sources appear to follow the distribution of 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 cluster, 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: (i) LMXBs located in blue (metal-poor) globular clusters (GCs), which GCs are known to have a broader distribution than the stellar light; (ii) neutron star LMXBs kicked out of the main body of the parent galaxy by the supernova explosion. The available deep optical and X-ray data of NGC 4365 supports this conclusion. For this galaxy we identified 60.1 \pm 10.8 excess sources in the (4-10)re region of which ~ 40% are located in globular clusters, whereas ~ 60% are field LMXBs. We interpret the latter as kicked NS LMXBs. We discuss implications of these results for the natal kick distributions of black holes and neutron stars.
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http://arxiv.org/abs/1211.0399
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