Brightness in Radio Astronomy
General Terms
The brightness of celestial sources can be expressed in several different ways.
The terms used by astronomers for these quantities include:
- Energy - the total radiative energy emitted by a source over
some range of wavelengths (or frequencies) during some time interval, measured
in joules, ergs, etc., e.g., the total photon energy yield of a typical
supernova explosion is roughly 1044 joules (or
2 × 1029 megatons of TNT!).
- Luminosity or Intrinsic Brightness - the energy emitted from a
source in all directions per unit time, often measured in watts (joules per
second), e.g, the luminosity of the Sun over all wavelengths is about
3.846 × 1026 W. (Physicists use the
term power to describe this quantity.)
- Flux or Apparent Brightness - power passing through a unit area,
useful for measuring energy coming from a distant, relatively compact object,
like the Sun. (Warning: physicists use the term intensity to describe
this quantity, and flux to describe field strength summed over an area!)
- Specific Flux - flux per unit wavelength or frequency, e.g.,
in watts per square meter per hertz [W m-2 Hz-1].
- Integrated Flux - the sum of specific flux over a range of
wavelengths or frequencies, e.g., in watts per square meter
[W m-2]. (Optical astronomers sometimes express the
logarithm of integrated flux in units of magnitudes.)
- Intensity or Surface Brightness - flux passing through a unit
solid angle, like a square degree or steradian, useful for measuring energy
coming from part of an extended object, like part of our Galaxy. (Warning:
physicists use the term irradiance to describe this quantity,
and intensity to describe what astronomers call flux!)
- Specific Intensity - intensity per unit wavelength or
frequency, e.g., in watts per square meter per hertz per steradian
[W m-2 Hz-1 sr-1].
- Integrated Intensity - the sum of specific intensity over a
range of wavelengths or frequencies, e.g., in watts per square meter per
steradian [W m-2 sr-1]. (Optical
astronomers sometimes express the logarithm of integrated intensity in
units of magnitudes per square arcsecond.)
Special Terms
Radio astronomers use some terms and units for a couple of the above
quantities that may be unfamiliar even to optical astronomers!
- Flux Density is specific flux explicitly in per-frequency terms
and is measured in janskys. These units are named for Karl Jansky, who
first detected extraterrestrial radio emission in the 1930s, and are defined
as: 1 Jy =
10-26 W m-2 Hz-1.
(Since the Sun is a relatively bright radio source, Solar radio astronomers
use larger solar flux units (sfu), where 1 sfu = 104
Jy = 10-22 W m-2 Hz-1.)
- Brightness Temperature is a proxy for specific intensity and is
measured in kelvins, which are like degrees Celsius but are counted up
from absolute zero (0.00 K = -273.15°C = -459.67°F). The
brightness temperature is the temperature needed for a blackbody
(perfect thermal radiator) to produce the same specific intensity as the
observed source. (Such idealized objects do not reflect any light but only
emit it.) This does not mean that the radiation from any given
source is from a blackbody -- in fact a lot of radio emission is
from non-thermal mechanisms
-- but it is equal to the physical
temperature for purely thermal sources, and it is directly proportional to the specific
intensity at low frequencies (under 10 GHz for the coldest interstellar
clouds, with a higher limit for warmer objects):
Iν = 2 ν2 k Tb / c2
(h ν << k T , so typical photon energy is much less than typical thermal energy per particle),
where
Iν = specific intensity
[W m-2 Hz-1 sr-1] ,
Tb = brightness temperature [K] ,
T = physical temperature [K] ,
ν = frequency [Hz] ,
c = the speed of light in a vacuum = 2.998 × 108 m s-1 ,
k = Boltzmann's constant = 1.381 × 10-23 J K-1 ,
and
h = Planck's constant = 6.626 × 10-34 J s
.
- Conversion: If the specific intensity of a source
Iν is constant within a radio telescope's "beam" -- the
solid angle Ω in steradians over which it is sensitive to
radiation -- then its flux density Sν is related to its intensity
and brightness temperature as
Sν = 2 Iν Ω
= 2 ν2 k Tb Ω / c2
(h ν << k T)
,
where the effective solid angle of an elliptical Gaussian beam is
Ω = π θA θB / [4 ln(2)]
,
and θA and θB are the full widths at
half power of the beam's major and minor axes in radians.
How Much is a Jansky?
Celestial sources of radio emission are much fainter than most human sources of
radio waves, whether intentional (broadcast stations, cell phones) or otherwise
(power lines, microprocessors, etc.). It is for this reason that radio
astronomers seek observing sites far from population centers: to minimize
potential interfering signals, just as optical astronomers seek to avoid "light
pollution". To understand just how faint natural radio sources are, and what a
jansky really measures, it is useful to make a quantitative comparison with
more familiar "anthropogenic" radio signals.
FM radio broadcast stations in the
United States typically have 100 kilowatts of effective radiated power (ERP),
which includes gain factors from the transmitting antenna design (most
radiation goes out horizontally but is equally distributed in azimuth, with a
gain of perhaps 5 to 10 times that of an isotropic radiator, so the equivalent
isotropic radiated power is only 10-20 kW). The range of such stations is
usually about 50 miles = 80 km. The broadcast power will be reduced by some
form of inverse-square law, even though it isn't isotropic. For simplicity,
let's ignore any propagation effects and assume the arriving power density
(APD) is given by an isotropic pattern modified by the gain, with the receiver
in the direction of maximum gain. In this case,
APD = ERP / (4 π d2)
where d = the distance from transmitter to receiver.
The bandwidth (BW) allocated to 1 FM station is 200 kHz.
Let's assume the signal strength is uniform across this
bandwidth. For the above parameters, the flux density at a receiver 80
km from the station -- near the edge of its effective range -- in the
optimal-gain path will be:
Sν
= APD / BW
= ERP / (4 π d2 BW)
= 105 W / [4 × 3.14 × (8 × 104 m)2 × (2 × 105 Hz)]
= 6.2 × 10-12 W m-2 Hz-1
= 6.2 × 1014 Jy
where 1 Jy = 10-26 W m-2 Hz-1 as noted above.
For comparison, most radio astronomy sources have signal strengths of a few
Jy or less. The Sun, which is the brightest celestial source at most
frequencies, has a flux density of about 106 - 108 Jy at
1 GHz, depending on whether there is surface activity (flares, etc.) or
not. The brightest supernova remnant, Cassiopeia A, is about 3000 Jy
at 1 GHz but a whopping 20,000 Jy at 100 MHz (the FM broadcast
band), because it's a highly nonthermal (synchrotron) source -- as is Solar
activity at these frequencies (Cas A is intrinsically much brighter than
the Sun but appears fainter because it's a lot
farther away). The faintest 1.4 GHz sources in recent large-scale radio
surveys like the NRAO-VLA Sky Survey are a few milli-janskys. Newer, deeper
surveys like the Evolutionary Map of the Universe project are targeting sources
at the 50 μJy (50 micro-janskys) level, which is about 100 times fainter
than the NVSS, or 60 million times fainter than Cas A. As you
might surmise, such detections require that there is no significant
interference from nearby radio broadcast stations!
It's also noteworthy that the brightness contrast between radio-frequency
interference and radio astronomical sources is much greater than that
between optical light pollution and most optical astronomical sources!
Inner-city skies (when clear) can be up to 100 times (5 magnitudes) brighter
than the darkest night skies far from any artificial light sources, reducing
the number of visible stars from thousands to dozens. But as indicated above,
a stray radio broadcast can easily be a million times brighter than the
Sun at radio wavelengths, and a trillion times brighter than more
"ordinary" radio sources! The contrast in the latter case is similar to that
between the optical brightness of the Sun and the 3rd-magnitude stars that fill
in many of the fainter parts of prominent constellations in the night sky.
How "Hot" is the Sky?
The Universe does not have a single physical temperature (if it did, then life
could not exist, according to the laws of thermodynamics). Instead, it
contains a mixture of hot and cold objects, running the gamut from a few
kelvins to billions of kelvins. All things produce radiation of one sort or
another over a range of frequencies, which can be characterized in flux,
intensity, or brightness temperature terms.
At radio frequencies, the major types of radiation are:
- Continuum radiation from changes in speed or direction of free
charged particles in space, primarily electrons, which includes:
- Thermal continuum from free electrons curving past positive
ions (bremsstrahlung radiation), as occurs in stars, planets, and
interstellar clouds heated by starlight; this is characteristic of anything
in some form of thermal equilibrium, including blackbodies
- Non-thermal continuum from electrons moving in magnetic fields
(cyclotron emission, which becomes synchrotron emission for
relativistic motion), as occurs in pulsars, supernova remnants, and near
supermassive black holes in the cores of active galaxies
- Spectral lines from discrete changes in the quantum
configurations of atoms or molecules, for example, the bound electron in a
neutral hydrogen atom changing its quantum spin from the same direction as the
proton's to the opposite direction, which has a slightly lower energy state,
and emitting a 1.4 GHz (21 cm) photon. (Most spectral lines arise
in "thermal" situations, but a few like masers can be considered
"nonthermal".)
The radio sky includes a wide variety of sources of all of the above types,
whose relative contributions vary with direction, frequency, and time. Thus,
just as the Universe lacks a uniform physical temperature, the sky lacks a
uniform brightness temperature. But if discrete sources like stars, galaxies,
and other objects of small angular extent are excluded, the general diffuse
"background" has:
- a very low brightness temperature of just a few kelvins in the
"microwave window" (roughly 1-100 GHz), where the cosmic microwave
background (CMB) can best be observed
- a steeply-rising brightness temperature at lower frequencies, reaching
thousands of kelvins at the lowest observed frequencies (around
10-100 MHz), where synchrotron emission from our Galaxy predominates
- increased brightness at high frequencies in the infrared/sub-millimeter
(THz) regime, where interstellar dust, heated by starlight, produces a
different kind of Galactic background -- but here the definition of brightness
temperature above breaks down, since photon energies become too large (see
above).
So how "hot" the sky appears varies, and at low frequencies, it has nothing
to do with real temperature, except in special cases like the CMB. Below a few
hundred MHz, the brightness temperature of the sky is very warm indeed, but
above a GHz or so, where one can see the CMB, the sky is truly "cold" by human
standards -- much colder than the ground in fact, or any person who happens to
step in front of a radio telescope!
References
Steven Gibson
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