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The jansky (symbol Jy, plural ''janskys'') is a non- SI unit of spectral flux density, or
spectral irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
, used especially in
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
. It is equivalent to 10−26
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s per square metre per
hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...
. The ''flux density'' or ''monochromatic flux'', , of a source is the integral of the spectral radiance, , over the source
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...
: :S = \iint\limits_\text B(\theta,\phi) \,\mathrm\Omega. The unit is named after pioneering US radio astronomer
Karl Guthe Jansky Karl Guthe Jansky (October 22, 1905 – February 14, 1950) was an American physicist and radio engineer who in April 1933 first announced his discovery of radio waves emanating from the Milky Way in the constellation Sagittarius. He is considered ...
and is defined as :1~\mathrm = 10^~\mathrm\mathrm\mathrm ( SI) :1~\mathrm = 10^~\mathrm\mathrm\mathrm\mathrm ( cgs). 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 The Third Cambridge Catalogue of Radio Sources (3C) is an astronomical catalogue of celestial radio sources detected originally at 159 MHz, and subsequently at 178 MHz. History 3C The catalogue was published in 1959 by members of the ...
(3C) reports results in janskys. * For extended sources, the surface brightness is often described with units of janskys per solid angle; for example, far-infrared (FIR) maps from the
IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...
satellite are in megajanskys per
steradian The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radian ...
(MJy⋅sr−1). * Although 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 Brightness temperature or radiance temperature is the temperature at which a black body in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity of a grey body object at a frequency \nu. This con ...
; for example the Haslam et al. 408 MHz all-sky continuum survey is reported in terms of a brightness temperature in
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
.


Unit conversions

Jansky units are not a standard SI unit, so it may be necessary to convert the measurements made in the unit to the SI equivalent in terms of watts per square metre per hertz (W·m−2·Hz−1). However, other unit conversions are possible with respect to measuring this unit.


AB magnitude

The flux density in janskys can be converted to a magnitude basis, for suitable assumptions about the spectrum. For instance, converting an
AB magnitude The AB magnitude system is an astronomical magnitude system. Unlike many other magnitude systems, it is based on flux measurements that are calibrated in absolute units, namely spectral flux densities. Definition The ''monochromatic'' AB magni ...
to a flux density in microjanskys is straightforward: :S_v~ mu\text= 10^ \cdot 10^ \cdot 10^ = 10^\tfrac.


dBW·m−2·Hz−1

The linear flux density in janskys can be converted to a decibel basis, suitable for use in fields of telecommunication and radio engineering. 1 jansky is equal to −260  dBW·m−2·Hz−1, or −230 
dBm DBM or dbm may refer to: Science and technology * dBm, a unit for power measurement * DBM (computing), family of key-value database engines including dbm, ndbm, gdbm, and Berkeley DB * Database Manager (DBM), a component of 1987's ''Extended Edi ...
·m−2·Hz−1: :P_ = 10 \log_\left(P_\text\right) - 260, :P_ = 10 \log_\left(P_\text\right) - 230.


Temperature units

The
spectral radiance In radiometry, spectral radiance or specific intensity is the radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of spectral radiance in frequen ...
in janskys per
steradian The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radian ...
can be converted to a
brightness temperature Brightness temperature or radiance temperature is the temperature at which a black body in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity of a grey body object at a frequency \nu. This con ...
, useful in radio and microwave astronomy. Starting with Planck's law, we see : B_ = \frac\frac. This can be solved for temperature, giving :T=\frac. In the low-frequency, high-temperature regime, when h\nu \ll kT, we can use the asymptotic expression: :T\sim \frack\left(\frac+\frac 12\right). A less accurate form is : T_b = \frac, which can be derived from the
Rayleigh–Jeans law In physics, the Rayleigh–Jeans law is an approximation to the spectral radiance of electromagnetic radiation as a function of wavelength from a black body at a given temperature through classical arguments. For wavelength λ, it is: B_ (T) = \ ...
: B_ = \frac.


Usage

The flux to which the jansky refers can be in any form of
radiant energy Radiant may refer to: Computers, software, and video games * Radiant (software), a content management system * GtkRadiant, a level editor created by id Software for their games * Radiant AI, a technology developed by Bethesda Softworks for '' ...
. 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 source An astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe. History In 1932, ...
s have flux densities of the order of 1–100 janskys. For example, the
Third Cambridge Catalogue of Radio Sources The Third Cambridge Catalogue of Radio Sources (3C) is an astronomical catalogue of celestial radio sources detected originally at 159 MHz, and subsequently at 178 MHz. History 3C The catalogue was published in 1959 by members of the ...
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 Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
.
Gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
also carry energy, so their flux density can also be expressed in terms of janskys. Typical signals on Earth are expected to be 1020 Jy or more. However, because of the poor coupling of gravitational waves to matter, such signals are difficult to detect. When measuring broadband continuum emissions, where the energy is roughly evenly distributed across the detector
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, 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·m−2·Hz−1, 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 (mfu) in older astronomical literature.


Orders of magnitude

Note: Unless noted, all values are as seen from the Earth's surface.


References

{{Radio-astronomy Radio astronomy Units of measurement Non-SI metric units Units of measurement in astronomy