【#Tech24H】Researchers from Xiamen University and Nanjing University have revealed the physical properties of the first ultracompact X-ray binary discovered outside the Milky Way, opening a new window for exploring the evolution of celestial bodies in distant galaxies. This binary system is located in the Andromeda Galaxy, 2.5 million light-years from Earth, and consists of a stellar-mass black hole and a white dwarf in an extremely tight orbit. Such black-hole–white-dwarf binaries are rare in the universe. This discovery is not only the first ultracompact X-ray binary found beyond our galaxy, but it also boasts the shortest orbital period, the most massive white dwarf, the highest X-ray luminosity, and the strongest gravitational wave radiation among known systems of its kind.