Jansky

In radio astronomy, the flux unit or jansky (symbol Jy) is a non-SI unit of electromagnetic flux density 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:

$S\; =\; iint\_\{mathrm\{source\}\}\; B(theta,phi)mathrm\{d\}Omega$

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

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

$1\; mathrm\{\; Jy\}\; =\; 10^\{-23\}\; frac\{mathrm\{erg\}\}\{\; mathrm\{sec\}\; cdot\; mathrm\{\; cm^\{2\}\}\; cdot\; mathrm\{\; Hz\}\}$ (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:

$F\_v\; [mathrm\{mu\; Jy\}]\; =\; 10^\{29\}\; cdot\; 10^\{-(mathrm\{AB\}+48.6)/2.5\}$

Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources. For extended sources, the brightness within a single telescope beam can more conveniently be described in terms of a brightness temperature (since this is independent of the telescope's gain). For example, the Third Cambridge Catalogue of Radio Sources (3C) specifies brightnesses in janskys, but the Haslam et al. 408 MHz all-sky continuum survey gives a map in terms of brightness temperature.

Usage

The brightest natural radio sources have flux densities of the order of one to one hundred janskys, which makes the jansky a suitable unit for radio astronomy. For example, the 3C lists some 300 to 400 radio sources in the Northern Hemisphere brighter than 9 Jy at 159 MHz.

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 meters²), and then divided by the detector bandwidth (in hertz). Since 1 W/m²/Hz is larger than most real sources, the result is multiplied by 10²⁶ to get a more appropriate unit for natural astrophysical phenomena.

References