The total amount of energy emitted by an object in the form of X-rays per unit time.X-ray luminosity is the total amount of energy an astronomical object emits in the form of X-rays per unit time. It is a fundamental observable in high-energy astrophysics, used to study phenomena where matter is heated to millions of degrees or accelerated to relativistic speeds. This includes accreting black holes and neutron stars, supernova remnants, the hot gas in galaxy clusters, and the coronae of stars like the Sun.
The first X-ray source beyond the Sun, Scorpius X-1, was discovered in 1962 by Riccardo Giacconi and colleagues using a sounding rocket. This discovery launched the field of X-ray astronomy and led to Giacconi's Nobel Prize in Physics in 2002.
X-rays are produced when charged particles, typically electrons, are accelerated or decelerated rapidly. In astrophysical settings, this often happens when gas falls into a deep gravitational potential well (like a black hole or neutron star), heating up to temperatures of millions of kelvins, or when electrons spiral in strong magnetic fields. The luminosity is measured by collecting X-ray photons with space-based telescopes, since Earth's atmosphere blocks them. The total energy per second is then calculated by integrating the flux over the source's distance and the observed energy range.
X-ray luminosity is used to identify and classify compact objects, measure the mass of galaxy clusters, and probe the properties of the hot interstellar medium. It is also a key diagnostic in surveys for active galactic nuclei and X-ray binaries.
The X-ray luminosity of the Sun is about 10^27 erg/s, but a single accreting black hole in a binary system can outshine the Sun's X-ray output by a factor of a million or more.
