# Jansky

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:

$S = iint_\left\{mathrm\left\{source B\left( heta,phi\right)mathrm\left\{d\right\}Omega$

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

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

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

$F_v \left[mathrm\left\{mu Jy\right\}\right] = 10^\left\{29\right\} cdot 10^\left\{-\left(mathrm\left\{AB\right\}+48.6\right)/2.5\right\}$

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 [http://adsabs.harvard.edu/abs/1985BICDS..28...49H 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. [cite web
last = Ask Dr. SETI
title = Research: Understanding the Jansky
url=http://www.setileague.org/askdr/jansky.htm
accessdate = 2007-06-13
]

References

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### Look at other dictionaries:

• Janský — Jansky ist die Einheit für die Strahlungsintensität in der Radioastronomie, siehe Jansky und ist der Name von folgenden Personen Filip Jánský (1922–1987), tschechischer Schriftsteller Jan Janský (1873–1921), tschechischer Arzt, Psychiater und… …   Deutsch Wikipedia

• Jánský — Jansky ist die Einheit für die Strahlungsintensität in der Radioastronomie, siehe Jansky und ist der Name von folgenden Personen Filip Jánský (1922–1987), tschechischer Schriftsteller Jan Janský (1873–1921), tschechischer Arzt, Psychiater und… …   Deutsch Wikipedia

• jansky — [ʒɑ̃ski] n. m. ÉTYM. 1972, in Encycl. Univ.; du nom de Karl G. Jansky, qui mit en évidence des ondes électromagnétiques de source extraterrestre, 1933. ❖ ♦ Astron. Sous multiple, utilisé en radioastronomie et radarastronomie, de l unité de… …   Encyclopédie Universelle

• Jansky — Das Jansky (Jy) ist eine in der Radioastronomie gebräuchliche nicht SI Einheit für die spektrale Flussdichte, d.h. für die von einer Quelle beim Beobachter eintreffende Energie pro Zeit, pro Fläche und pro Frequenzintervall. Sie wurde nach dem… …   Deutsch Wikipedia

• Jansky — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Jansky peut faire référence à : Karl Jansky, découvreur du premier signal radio extraterrestre. Le Jansky, unité utilisée en astronomie mesurant la… …   Wikipédia en Français

• Jansky — janskis statusas T sritis Standartizacija ir metrologija apibrėžtis Spektrinio spinduliuotės srauto tankio matavimo vienetas. Pavadintas Karlo Janskio (Karl Guthe Jansky) – pirmojo žmogaus, atradusio iš kosmoso sklindančias radijo bangas, vardu.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• jansky — janskis statusas T sritis Standartizacija ir metrologija apibrėžtis Spektrinio spinduliuotės srauto tankio matavimo vienetas. Pavadintas Karlo Janskio (Karl Guthe Jansky) – pirmojo žmogaus, atradusio iš kosmoso sklindančias radijo bangas, vardu.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• jansky — (Jy)    a unit used in radio astronomy to measure the strength, or more precisely the flux density, of radio signals from space. In measuring signal strength, it s necessary to take into account both the area of the receiving antenna and the… …   Dictionary of units of measurement

• jansky — /jan skee/, n., pl. janskies. a unit of flux density for electromagnetic radiation, used chiefly in radio astronomy. Abbr.: Jy [after K. JANSKY] * * * …   Universalium

• jansky — noun A non SI unit measuring electromagnetic flux density equal to 10 watts per square meter per hertz. The symbol for jansky is Jy …   Wiktionary

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