X-rays as a cosmic messenger

Sixty years ago, a team led by Riccardo Giacconi discovered the first cosmic Xray source outside our solar system, opening a new window to explore the universe. Unlike the visual sky that our eyes are accustomed to, the“invisible”X-ray universe is alien and glows with compact objects such as black holes, neutron stars, white dwarfs, and hot diffuse gases within clusters of galaxies and galaxies, as well as assorted violent catastrophic events. They manifest the most exotic objects in the universe, as well as extreme physical environments such as the strongest gravity and magnetic fields, and the hottest regions. This article focuses on representative X-ray sources, mainly neutron star and black hole X-ray binaries, supermassive black holes and active galactic nuclei, clusters of galaxies, as well as eruptive events such as gamma-ray bursts, supernovae, and tidal disruption events.