Jansky

The flux unit or jansky (symbol Jy) is a non-SI unit of spectral flux density equivalent to 10−26 watts per square metre per hertz. The flux density or monochromatic flux, of a source is the integral of the spectral radiance, over the source solid angle:

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

(SI) (cgs)

The flux density in Jy can be converted to a magnitude basis, for suitable assumptions about the spectrum. For instance, converting an AB magnitude to a flux-density in microjanskys is straightforward:

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 Jy. For extended sources, the surface brightness is often described with units of Jy per solid angle; for example, Far Infra-Red (FIR) maps from the IRAS satellite are in MJy/sr. While 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 K.