4U

The Fourth Uhuru Catalog, commonly abbreviated 4U, is a historical catalog of celestial X-ray sources compiled from observations by the Uhuru X-ray satellite (also known as SAS-1). Published in the early 1970s, it is the fourth and final catalog in the Uhuru series. The 4U catalog lists hundreds of X-ray sources across the sky, including X-ray binaries, pulsars, cataclysmic variables, and active galactic nuclei. Each entry bears a 4U designation, typically followed by approximate right ascension and declination, and includes measured X-ray fluxes and rough positional information. For many sources, the catalog provided the first systematic positions and flux estimates, enabling cross-identification with optical and radio counterparts in subsequent studies.

The 4U catalog played a pivotal role in establishing the population of X-ray sources and mapping the high-energy sky. It informed the search for counterparts and guided follow-up observations with later missions such as HEAO-1, the Einstein Observatory, and ROSAT. However, the positional accuracies and flux calibrations reflect the technical limits of the era, and many 4U sources have since been refined or superseded by later catalogs. Today, 4U designations remain widely used in historical literature and in archives as a record of the first era of all-sky X-ray surveys.