Jansky 180090 224053647 2008-07-07T02:47:53Z Bkell 32452 avoid two-story fraction in exponent {{otheruses}} In [[radio astronomy]], the '''flux unit''' or '''jansky''' (symbol '''Jy''') is a non-[[SI]] unit of [[electromagnetic radiation|electromagnetic]] [[flux density]] equivalent to 10<sup>&minus;26</sup> [[watt]]s per [[square metre]] per [[hertz]]. The ''flux density'' or ''monochromatic flux'', <math>S</math>, of a source is the integral of the [[spectral radiance]], <math>B</math>, over the source [[solid angle]]: <math>S = \iint_{\mathrm{source}} B(\theta,\phi)\mathrm{d}\Omega</math> The unit is named after the pioneering radio astronomer [[Karl Guthe Jansky|Karl Jansky]], and is defined as: <math>1 \ \mathrm{ Jy} = 10^{-26} \frac{ \mathrm{W} }{ \mathrm{m^2 } \cdot \mathrm{ Hz} }</math> ([[SI]]) <math>1 \ \mathrm{ Jy} = 10^{-23} \frac{\mathrm{erg}}{ \mathrm{sec} \cdot \mathrm{ cm^{2}} \cdot \mathrm{ Hz}}</math> ([[centimetre gram second system of units|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:<ref name="fuk95">{{cite journal | author=M. Fukugita | title=Galaxy Colors in Various Photometric Band Systems | journal=PASP | year=1995 | volume=107 | pages=945–958 | doi=10.1086/133643 }}</ref> <math>F_v \ [\mathrm{\mu Jy}] = 10^{29} \cdot 10^{-(\mathrm{AB}+48.6)/2.5}</math> 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 Janskies, but the [http://adsabs.harvard.edu/abs/1985BICDS..28...49H Haslam et al.] 408&nbsp;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&nbsp;Jy at 159&nbsp;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 (signal processing)|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 meters<sup>2</sup>), and then divided by the detector bandwidth (in hertz). Since 1 W/m<sup>2</sup>/Hz is larger than most real sources, the result is multiplied by 10<sup>26</sup> to get a more appropriate unit for natural astrophysical phenomena.<ref>{{cite web | last = Ask Dr. SETI | title = Research: Understanding the Jansky | url=http://www.setileague.org/askdr/jansky.htm | accessdate = 2007-06-13 }}</ref> ==References== {{reflist}} [[Category:Units of measure]] [[Category:Units of measure in astronomy]] [[Category:Non-SI metric units]] [[da:Jansky]] [[de:Jansky]] [[es:Jansky (unidad)]] [[fr:Jansky]] [[it:Jansky]] [[ja:ジャンスキー]] [[pl:Jansky]]