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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:
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.
The flux to which the jansky refers can be in any form of energy. It was created for and is still most frequently used in reference to electromagnetic energy, especially in the context of radio astronomy. The brightest astronomical radio sources have flux densities of the order of one to one hundred janskys. For example, the 3C lists some 300 to 400 radio sources in the Northern Hemisphere brighter than 9 Jy at 159 MHz. This range makes the jansky a suitable unit for radio astronomy.
Gravitational waves also carry energy, so their flux density can also be expressed in terms of janskys. Though gravitational waves have never been directly observed, typical signals on Earth are expected to be Jy or more. However, because of the poor coupling of gravitational waves to matter, such signals are difficult to detect.
It is important to understand the meaning of the per hertz component of the jansky unit. When measuring broadband continuum emissions, where the energy is roughly evenly distributed across the detector bandwidth, the detected signal will increase in proportion to the bandwidth of the detector (as opposed to signals with bandwidth narrower than the detector bandpass). To calculate the flux density in janskys, the total power detected (in watts) is divided by the receiver collecting area (in square meters), and then divided by the detector bandwidth (in hertz). The flux density of astronomical sources is many orders of magnitude below 1 W/(m2·Hz), so the result is multiplied by 1026 to get a more appropriate unit for natural astrophysical phenomena.
The milliJansky, mJy, was sometimes referred to as a milli flux unit (m.f.u.) in the astronomical literature.