An Etymological Dictionary of Astronomy and Astrophysics
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An X-ray source that appears suddenly in the sky, strongly increases its intensity over a few days, and then declines with a lifetime of several months.

→ transient; → X-ray source.

An X-ray source that is not in the nucleus of a galaxy, and is more luminous than 10³⁹ ergs s^-1, brighter than the → Eddington luminosity of a 10 → solar mass → black hole. In general, there is about one ULX per galaxy in galaxies which host ULXs. The Milky Way contains no such objects. ULXs are thought to be powered by → accretion onto a → compact object. Possible explanations include accretion onto → neutron stars with strong → magnetic fields, onto → stellar black holes (of up to 20 → solar masses) at or in excess of the classical Eddington limit, or onto → intermediate-mass black holes (10³-10⁵ solar masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities up to ~ 10⁴⁰ erg s^-1 (Ciro Pinto et al., 2016, Nature 533, N) 7601).

→ ultraluminous; → X-ray source.

An astronomical object whose dominant mechanism of radiation is through X-ray emission. X-ray sources contain an extremely hot gas at temperatures from 10⁶ to 10⁸ K. They are generated by various physical processes involving high energies, such as accretion on to a compact object, shock waves from supernovae, stellar winds, hot gas in stellar coronae, or hot spaces between galaxies in a cluster. The first celestial X-ray source, after the Sun, to be detected was → Scorpius X-1 by means of rocket flight (Giacconi et al. 1962).

→ X-ray; → source.