An Etymological Dictionary of Astronomy and Astrophysics
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A cloud of ionized gas within → active galactic nuclei (AGN) that causes absorption at → soft X-ray wavelengths. Warm absorbers were first suggested by Halpern (1984) to explain Einstein data of the quasar MR 2251-178. They are dubbed "warm" absorbers as they imply gas at temperatures of 10⁴-10⁵ K; the gas is → photoionized, not collisionally ionized. High resolution observations of warm absorbers have shown that they are outfowing. See also → cold absorber (Ceri Ellen Ashton, 2005, A Study of Warm Absorbers in Active Galactic Nuclei, Thesis, Mullard Space Science Laboratory Department of Space and Climate Physics University College London ).

→ warm; → absorber.