Title: Low-mass X-ray binaries as tracers for a varying stellar initial mass function in ultra-
compact dwarf glaxies

Abstract:
Ultra compact dwarf galaxies (UCDs) are dense stellar systems at the border between massive 
star-clusters and small galaxies. Recent research has shown that their average optical mass-
to-light (M/L) ratio cannot be explained by stellar populations with a canonical initial stellar 
mass function (IMF). This can be understood if UCDs represent a case of rapid star formation in 
an extremely dense environment, leading to a top-heavy IMF. A top-heavy IMF implies a high 
number of neutron stars (NSs) in an aged stellar population, which could provide the unseen 
mass in UCDs. The NSs can form binary systems with a evolving low-mass stars and thereby 
become visible by X-ray emission. The frequency of such so-called low-mass X-ray binaries 
(LMXBs) depends on the frequency of NSs and thus on the IMF. The notion of a top-heavy IMF 
in UCDs can therefore be tested by modelling the frequency of low-mass X-ray binaries (LMXBs) 
in UCDs and globular clusters (GCs) and comparing it to the observed frequency of LMXBs. The 
observed frequency of LMXBs in UCDs in the Virgo Cluster does indeed support a top-heavy 
IMF in those UCDs. Further information will come from Monte-Carlo simulations of the LMXB-
populations of UCDs and GCs in other galaxy clusters.