LMXB populations in galaxy outskirts: globular clusters and supernova kicks

TL;DR

This study systematically investigates the extended X-ray source populations in early-type galaxy outskirts, revealing halos of neutron star binaries and globular cluster sources, influenced by supernova kicks and galaxy properties.

Contribution

It introduces the first comprehensive analysis of X-ray source halos in galaxy outskirts, linking them to globular clusters and supernova kicks, with detailed observational evidence.

Findings

Extended X-ray source halos are present in most galaxy groups.

Halos are composed mainly of neutron star binaries and globular cluster sources.

Kicked neutron star binaries contribute significantly to the outskirts population.

Abstract

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.