Abstract
Ultracompact X-ray binary stars usually consist of a neutron star and a white dwarf, two stars bound together by their strong gravity and orbiting each other very rapidly, completing one orbit in less than one hour. Neutron stars are extremely compact remnants of the collapsed cores of massive stars which end their lives in supernova explosions. White dwarfs are the cores of less massive stars like the Sun, which have lost their hydrogen envelopes. With its strong gravity, the neutron star strips off gas from the surface of the white dwarf. This gas, composed of helium, carbon, and oxygen, spirals towards the neutron star in a disk and eventually falls onto its surface. Due to friction, the disk heats up to millions of degrees and becomes a bright source of X-ray radiation, which astronomers observe with detectors in space. Theoretical models predict a population of around a hundred thousand ultracompact X-ray binary stars in the Milky Way, but we see much fewer, and no old ones at all. Analysis of their observed brightnesses suggests that more mass is transferred between both stars than expected, so they evolve faster. Moreover, it appears that mass transfer stops early, after which the binary star systems become invisible in X-rays, explaining the discrepancy. The neutron stars, which are spinning around their axes hundreds of times per second as a result of having gained much mass, eventually become visible in radio as millisecond pulsars. At this point their companions have lost almost all of their mass and have transformed into planetary objects. Possibly, the companions are evaporated entirely by the energetic radiation from the millisecond pulsars, leaving behind solitary millisecond radio pulsars, which would explain the origin of the observed population of these objects.
