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Quasars
Quasars (quasi-stellar objects) are objects of immense power output emitting 100 to 1000 times the amount of radiation as our own Milky Way galaxy. Because they are so bright, quasars are some of the most distant objects we can see in the Universe. Quasars are also variable on the order of a few days, which means that the source of radiation must be contained in a volume of space of a few light-days across. As a result they look stellar, i.e. point-like, even through the largest telescopes. However they are believed to be embedded in the central regions of galaxies.
Quasars are multi-wavelength emitters, with roughly equal energy output from far-infrared through to X-ray wavelengths. The study of X-ray emission from quasars provides a powerful diagnostic of matter in the quasar environment and along the line of sight from earth. The combination of X-ray and UV constraints is extremely powerful in determining the physical conditions in the absorbing gas. In particular, the associated metal line absorbers in quasar UV spectra show up as a "warm absorber" in the soft X-ray. The combined UV and X-ray constraints have demonstrated that the absorbing gas is highly ionised and outflowing. High-redshift, compact, radio-loud quasars show strong soft X-ray absorption possibly related to a surrounding medium which also confines the radio source.
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