X-ray binaries are the systems, which give out strong radiation in the X-rays, and the X-rays are generated while the gas is accreted from the companion star to the compact objects. The central compact objects could be either neutron star or black holes, however, the companion stars most often are normal stars. By definition, the X-ray binaries can be classified as the high mass and low mass X-ray binaries according to the mass of the companion star. If the mass of the companion star is much greater than the solar mass, it is usually called high-mass binaries; otherwise, it is defined as low-mass binaries.
According to the standard stellar evolution theory, if the progenitor mass is greater is between 8 and 20 solar mass, the core of the progenitor will collapse into a neutron star at its end stage. If the initial mass is greater 20ish solar mass, the core will most likely collapse into a black hole, and the dividing line between neutron star and black hole is around 3 solar mass. It is expected that the number of low-mass BHs like 3 or 4 solar mass should be enormous since the number of those progenitors are huge. However, the observed BH mass distribution peaks around 7 solar masses, and no BH with mass less than 5 solar masses has been found. This is so-called the mass-gap problem. This is only one of the puzzles existing in the X-ray binary systems. Besides, how many BH X-ray binaries are there within our Milky Way galaxy? Is there any black hole system within globular clusters?
Therefore, searching for more X-ray binaries and establishing a large sample of them are the foundation of answering those scientific problems. Einstein Probe (EP) has a much better sensitivity compared to the past missions, so it is expected to discover many new X-ray binary systems over its operating period, which will play a key role in helping solve the problems above then.