The black hole. ... The black holes are generated in the result of a collapse of giant stars with the mass more than three Sun's masses. ... And moreover the gas near the neutron star or the black hole will get hot and will become a source of high energetic electromagnetic radiation in X-rays and gamma range. ... In the majority of double systems, being the sources of X-rays radiation, the invisible component mass does not exceed the two solar masses, so it is a neutron star. ...