= Proximity effect
{disambiguate=astrophysics}
= Quasar proximity effect
{synonym}
The quasar proximity effect is a reduction of nearby <Lyman-alpha forest> absorption caused by extra <photoionization> from the <quasar>. At fixed density and temperature in <photoionization equilibrium>, an added rate $\Gamma_Q$ changes the neutral fraction by the factor $\Gamma_{\rm bg}/(\Gamma_{\rm bg}+\Gamma_Q)$. A host environment with positive <density contrast> can mask this reduction, and a variable or anisotropic source complicates its interpretation.
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