Observational evidence of black hole event horizon from ultra-compact X-ray binaries

Abstract: The event horizon is the most crucial feature that distinguishes black hole (BH) from all other objects in the universe. However, since no radiation can escape from the event horizon, its direct observational evidence remains lacking. Although both are compact objects, the event horizon leads to differences in the radiative behavior between black hole and neutron star (NS) X-ray binaries. Historically efforts to probe the existence of event horizons revealed that the BH X-ray binaries are fainter than NSs with similar orbital periods in their quiescent states. Here, we use a special class of X-ray binaries namely the ultra-compact X-ray binaries (UCXBs), to reveal the existence of the event horizon near stellar-mass black holes. In short-period UCXBs, gravitational radiation dominates the orbital angular momentum loss, allowing us to accurately determine their accretion rates. Based on five UCXBs with orbital periods shorter than 40 min, located in globular clusters, and identified accretors (NS or BH), we obtained their accretion rates. Combining their accretion rates and X-ray luminosities, we calculated the X-ray radiative efficiencies of NS and BH. The radiative efficiency of BH is about two to three orders of magnitude lower than that of NS systems, indicating the accretion energy is swallowed by the BH, thereby indirectly probe the presence of the event horizon.