JultikaUniversity of Oulu repository

Abstract

The aim of this thesis is to make a literature review of the mass distribution of neutron stars and black holes in X-ray binaries. X-ray binaries are binary systems which contain a stellar remnant and a main sequence star. The stellar remnant accretes matter from the donor, a main sequence star. This matter forms an accretion disk around the remnant and as the matter spirals in toward the surface of the remnant it becomes so energetic that it radiates its gravitational potential energy away as X-rays. There are next to no objects between the lightest known black holes and heaviest neutron stars making their distribution uneven. The so-called lower mass gap is between 2.5 M⊙ and 5.0 M⊙. This was not expected since the stellar masses have a smooth distribution, thus there must be something afecting the formation of neutron stars and black holes. The causes for this mass gap are not yet well known but as more data is gathered from X-ray binary systems we have come up with few explanations for this gap. In general, we either do not understand the situation completely or then there is some bias introduced from the observations as stellar remnants are hard to detect if they are not located inside a binary system. Recent data about the masses of neutron stars and black holes are explored and one can clearly see the mass gap.