Massive unseen compact companions to hot subluminous stars

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Neutron stars and stellar-mass black holes are the remnants of massive stars ending their lifes in supernova explosions. Detecting these exotic objects is usually only possible in a binary, when matter is transferred from the luminous star to the compact object and bright X-rays are emitted from the strongly heated accreted material. In a detached binary, however, they can only be detected from the Dopper-reflex motion of the visible star. In our pilot study of detached hot subdwarf binaries we took advantage of their orbital synchronisation and found that at least three out of our sample of 51 must have very massive, but invisible companions, i.e. neutron stars or even black holes. This fraction is unexpectedly large because these objects are considered to be rare. This new connection between hot subdwarfs, which are numerous in our Galaxy, and massive compact objects may lead to a tremendous increase in the number of known neutron stars and black holes. However, our analysis suffers from selection effects which can not be quantified yet. This can only be achieved by a much larger sample. Therefore, we have initiated a large survey to search for hot subwarfs with massive companions in a much bigger space volume than before. High sensitivity to massive companions is ensured by the survey s efficient target pre-selection strategy. The smoking gun will be a high-amplitude, short-period binary system, because it will exclude any possibility of doubt about the nature of the companion. The results will shed light on the population of neutron stars and black holes as companions in detached binaries and their evolutionary link to the X-ray binary population. The project will also reveal the properties of the halo population of hot subdwarfs and allow the Galactic gravitational potential to be probed.