The accretion of gas on the surface of a neutron star is a source of energy, which can be converted into X-rays radiation. Accretion can ascertain a far greater lifetime of an X-ray star than the radiation of internal energy.
The source of gas could be a cloud of intergalactic gas. If the neutron star is a component of a binary system, the gas could come from the other (non-neutronic) star.
The idea of accretion and evaluation of energetic balance were given by Zel’dovič (1964) and Zel’dovič and Novikov (1964, 1965) and Salpeter (1964). Shortly the idea of accretion was mentioned by Šklovskij (1967).
The motion of gas and radiation spectra idealised case of spheric symmetry are studied now by N. Shakuro and Zel’dovič.