... index

The spectral index of a source, $\alpha$, is a measure of how the intensity changes with frequency as given by the power law distribution $S_\nu \propto \nu^{\alpha}$. A steep spectrum means that the intensity of the emission decreases rapidly with increasing frequency.

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...)

1 erg = 10^-7 Joules

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... density

Flux density, $S_\nu$ is the incident flux received at a detector per unit area per unit frequency, at a frequency $\nu$. It has units of Janskys, where 1 Jy = $10^{-26} \mbox{ Wm}^{-2} \mbox{Hz}^{-1}$

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