Jansky
The jansky (symbol Jy, plural janskys) is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy. It is equivalent to 10−26 watts per square metre per hertz.
The spectral flux density or monochromatic flux, S, of a source is the integral of the spectral radiance, B, over the source solid angle:
S
=
∬
source
B
(
θ
,
ϕ
)
d
Ω
.
{\displaystyle S=\iint \limits _{\text{source}}B(\theta ,\phi )\,\mathrm {d} \Omega .}
The unit is named after pioneering US radio astronomer Karl Guthe Jansky and is defined as
Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources; for example, the Third Cambridge Catalogue of Radio Sources (3C) reports results in janskys.
For extended sources, the surface brightness is often described with units of janskys per solid angle; for example, far-infrared (FIR) maps from the IRAS satellite are in megajanskys per steradian (MJy⋅sr−1).
Although extended sources at all wavelengths can be reported with these units, for radio-frequency maps, extended sources have traditionally been described in terms of a brightness temperature; for example the Haslam et al. 408 MHz all-sky continuum survey is reported in terms of a brightness temperature in kelvin.
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