About 5% of all known neutron stars are members of a binary system. The formation and evolution scenario of binary neutron stars is a rather exotic and complicated process.
The companion stars may be either ordinary stars, white dwarfs or other neutron stars. According to modern theories of binary evolution it is expected that neutron stars also exist in binary systems with black hole companions.
Such binaries are expected to be prime sources for emittinggravitational waves. Neutron stars in binary systems often emit X-rays which is caused by the heating of material (gas) accreted from the companion star.
Material from the outer layers of a (bloated) companion star is sucked towards the neutron star as a result of its very strong gravitational field.
As a result of this process binary neutron stars may also coalesce into black holes if the accretion of mass takes place under extreme conditions.
It has been proposed that coalescence of binaries consisting of two neutron stars may be responsible for producing short gamma-ray bursts. Such events may also be responsible for creating all chemical elements beyond iron, as opposed to thesupernova nucleosynthesis theory.