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 flux density or monochromatic flux, S, of a source is the integral of the spectral radiance, B, over the source solid angle:

${\displaystyle S=\iint \limits _{\text{source}}B(\theta ,\phi )\,\mathrm {d} \Omega .}$

The unit is named after pioneering US radio astronomer Karl Guthe Jansky and is defined as

${\displaystyle 1~\mathrm {Jy} =10^{-26}~\mathrm {W} {\cdot }\mathrm {m^{-2}} {\cdot }\mathrm {Hz^{-1}} }$ (SI)

${\displaystyle 1~\mathrm {Jy} =10^{-23}~\mathrm {erg} {\cdot }\mathrm {s^{-1}} {\cdot }\mathrm {cm^{-2}} {\cdot }\mathrm {Hz^{-1}} }$ (cgs).

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.