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PH507
Astrophysics
Dr Mark Price
Lecture 2: The Electromagnetic Spectum and its detection.
Diagram showing the E-M spectrum.
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The E-M spectrum can be grouped into several, broad categories:
1. The radio region. For astronomers this is generally in the frequency
range 20 MHz – 900 GHz.
2. Microwaves. Radio frequencies within the region of 5 GHz – 1 THz
(wavelength= 6 cm – 0.03 cm)
3. Infra-red. Here things get confusing and people generally start talking in
terms of wavelengths, not frequency. General wavelength range of 1
micron (300 THz = 1 x10-6 metres) – 300 microns (1 THz).
4. Visible. Again a change of units from microns to nanometers. A very
small part of the spectrum relatively speaking extending only from 700
nm (red) – 350 nm (violet). Equivalent to 428 THz – 856 THz. Note the
edges of the visible range vary from person to person. People with
artificial lenses can see further into the violet than a person with a
normal lens.
5. Ultra-violet. The region from 350 nm – 10 nm. Another unit is sometimes
used called the Angstrom. 1 Angstrom = 1 x 10-10 metres, it’s symbol is a
capital “A” with a circle above, Å.
6. X-ray. Again, a further change of units from nanometers to electron volts
(eV). Generally from 50 eV – 5000 eV for “soft” X-rays, and >5000 eV for
“hard” X-rays. Thus to convert to frequency/wavelength you have to
use the equation below, where E is the photon energy in electron volts
and e is the charge on an electron (in coulombs).
Ee  hf 
hc

7. Gamma-rays. Again, any photon with an energy > 5000 eV can be called
a gamma-ray. Note, the only distinction between “hard” X-rays and
gamma-rays is their method of production! A photon is referred to as a
gamma ray if it has been produced because of a nuclear reaction (ie.
radioactive decay) and as an X-ray if it was produced due to the
acceleration (or energy transfer) to an electron.
Note in the figure above, there is a small amount of overlap between the
various categories. This is also seen in the way that radiation between, for
example, the radio and the infra-red is detected.
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Astrophysics
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The above shows varying black-body curves for differing temperatures and at
what part of the E-M spectrum the curve is a maximum.
[Q: Does everybody know about blackbody radiation?]
Astronomers use different parts of the E-M spectrum to investigate different
astrophysical processes.
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Radio:
Used to detect individual atomic and molecular species. The table below is a
few of the many hundred different species and their emission (absorption)
frequency.
Molecule
H2
C
12CO
13CO
“
“
“
18
C O
CS
“
HCO+
Freq. (GHz.)
1.4
984.0
230.54
110.19
220.39
330.58
661.19
219.56
97.97
244.94
89.19
Transition
21 cm line
3P0 -> 3P1
2 -> 1
1 -> 0
2 -> 1
3 -> 2
6 -> 5
2 -> 1
2 -> 1
5 -> 4
1 -> 0
As radio telescopes can be constructed to a very large size (greater than a factor
of ~10 large than optical telescopes), they can have a very much higher
resolution (sub-arcsec) than their optical counter-parts. This allows astronomers
to probe the very centre of star forming regions, galaxies etc.
The diagram below is an attempt to illustrate this with a comparison of some
optical images of a several starburst galaxies (taken with the Hubble Space
Telescope) and some radio mapping images of the same object. Note the much
higher resolution in the radio images.
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Microwave:
The technology used to detect microwaves is very similar to that for shorter and
longer wavelengths.
Potential sources of interest:
1. Cosmic microwave background.
2. Solar flares.
3. Pulsars.
4. Active Galactic Nuclei (AGNs).
5. Masers.
The Swedish Odin satellite is a microwave satellite as was COBE (Cosmic
micrOwave Background Explorer), which was the first instrument to measure the
theoretically predicted differences in the cosmic microwave background.
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Infra-red.
Arguably one of the most important regions of the E-M spectrum for
astronomers as it is within the infra-red ragion that the black-body spectrum of
objects such as star formation regions, interstellar dust etc. peaks.
The infra-red region is also further subdivided into the far-infrared (20 um -> 100
um) and the sub-millimetre (100 um -> 300 um).
The sub-millimetre is particularly important for investigating star formation
regions, and will be discuss further later.
Space missions.
IRAS (InfraRed Astronomical Satellite)
ISO (Infrared Space Observatory)
SIRTF (Space InfraRed Telescope Facility)
PLANCK
SPIRE
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ASTRO-F (launched at the end of Feburary this year by the Japanese space
agency, JAXA, the University of Kent had a role in its development).
Visible
By far the most common form of astronomy, it’s main scientific uses are in solar
system/planetary studies, studying the dynamics of stars and solar physics.
The visible “colours” of stars are also used to classify their spectral type (O, B, G
etc.).
Space Missions.
HST (Hubble Space Telescope).
NGST (Next Generation Space Telescope – shelved ?).
Ultra-violet
There are two primary sources of UV continuum radiation. Active Galactic
Nuclei (AGNs) emit nonthermal UV. This light is produced as part of the
radiation from the material in the surroundings of a massive black hole. The
second class of UV emitters, massive stars, emit thermal UV light. In their case
the energy of the radiation is a measure of their surface temperature.
Ultra-violet radiation is emitted by plasmas with temperatures >100,000K .
Space missions:
UVE (UltraViolet Explorer)
EUVE (Extended UltraViolet Explorer)
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X-ray / gamma-ray
X-rays/gamma-rays are emitted by extremely hot gas with a temperature in
excess of 1 x 106 K, and thus are only found in regions of extremely high
gravitational fields such as those around neutron stars and black holes.
The above is an X-ray image of the moon. The “glow” is from X-ray
fluorescence of the moon’s surface. The interesting part of the image is the dark
side of the moon showing less dots. Thus the dots are not just noise, but are
actually X-ray sources.
Gamma rays are generally confined to emission by so-called “gamma ray
bursters” and the reason for these bursts remains unknown, but it is speculated
that it is due to supernovae exploding in distant galaxies, or material infall into
a black-hole.
Space missions:
ROSAT
CHANDRA
INTEGRAL
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E-M RADIATION DETECTION METHODS.
Radio detection techniques.
Heterodyning.
This is probably one of the most sensitive methods of E-M detection, and allows
very high resolution spectrography to be used (Q=Δf/f ~ 1/10000).
It uses the non-linear properties of a mixing element to produce harmonics of a
known frequency (referred to the a local oscillator), with an unknown
frequency (emitted by the object of interest).
A schematic of a typical heterodyne system is shown below.
How does it work ?
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It relies on the non-Ohmic behaviour of the mixing element. A resistor obeys
Ohm’s law:
V  IR
However, a non-linear device (such as a tungsten light bulb) does not, and may
have an I-V characteristic such as the one shown below. Such an I-V
characteristic is typical of a diode.
Where:
V  0 :
I 0
(reverse direction)
V  0:
I V2
(forward direction)
Now imagine we have two input signals: one at a known frequency from the
local oscillator, V LO , and one from the source, VS .
Thus:
MAX
V LO  V LO
sin (2f LO t )
and similarly:
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VS  VSMAX sin (2f S t )
Therefore the total voltage seen at the diode, VTOT , is simply:
VTOT = VLO  VS
and therefore the diode current, I, is given by
I  [ VTOT ] 2
I  [ VLO  VS ] 2
MAX
sin (2f LO t ) + VSMAX sin (2f S t ) ] 2
 [ VLO
Using the trignometric identities:
sin 2 ( x ) 
1
[1 – cos ( 2x )]
2
sin(a) sin(b) =
1
[cos(a-b) – cos(a+b)]
2
we find that the diode current
I  cos ( 2 f LO ) + cos ( 2 f S ) + cos  f LO  f S  +  f LO  f S 
And thus it can be seen that we have additional components at frequencies of:
2 f LO ,2 f S ,  f LO  f S ,  f LO  f S 
For the vast majority of purposes we are only interested in the difference
frequency or (more commonly) the Intermediate Frequency (IF) given by:
IF =  f LO  f S 
All the unwanted frequencies are filtered out and the IF frequency is passed on
to the pre-amplifier and finally to the spectrometer.
The usefulness of this can be illustrated if we wanted to look at the neutral
carbon transistion line at 820 GHz. This frequency is far too high to measure
using current electronics. Therefore, we setup a local oscillator to output a
known signal at, say, 821 GHz. This produces an IF frequency of 1 GHz which
can be easily amplified and measured.
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Astrophysics
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Upper and lower sidebands.
One of the side effects of using this technique is the existence of sidebands. In
the example above I used a local oscillator frequency of 821 GHz. If I had a
contaminating source in my telescope beam which happened to be emitting at
822 GHz, I would also see a contribution from that source in my IF passband.
This can be illustrated with a diagram.
Real devices: SIS junctions.
A Superconductor-Insulator-Superconductor (or SIS) tunnel junction is an
electronic device consisting of two superconducting electrodes separated
by a very thin insulating barrier. Electron tunnelling across this barrier
gives rise to a very non-linear current-voltage characteristic. The diagram
below shows I-V characteristics for three SIS junctions comprised of different
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superconducting alloys manufactured at the University of Kent. The table
below tabulates the superconducting energy gap for various superconducting
alloys and the corresponding frequency.
Alloy composition Superconducting energy gap (meV) Frequency (GHz.).
Pb:Bi - Pb:Bi
3.30
798
Pb:Au:In - Pb:Bi
2.90
701
Pb:Au:In - Pb:Au
2.65
641
Nb - AlO2 - Nb
2.8
677
It is this non-linear I-V characteristic that gives rise to the strong
frequency mixing essential for good heterodyne receivers.
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A SIS junction is formed from evaporated metal films deposited onto a thin
quartz substrate. The SIS junctions manufactured at the University of Kent are
made by the so-called, 'Dolan bridge' method. A Dolan Bridge is a suspended 3D bridge made of photoresist. A scanning electron micrograph of such a
structure is shown below. The metals are thermally evaporated within a
vacuum chamber and the junction is formed by in the 'shadow' region under
the bridge. To operate properly, the junction must be cooled to very low
temperatures, typically to the temperature of liquid helium (4.2 K).
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The image below is a scanning electron micrograph of a SIS junction made at
the University of Kent. The actual junction area itself is approximately 1
micron2 and is outlined in yellow.
Junctions have been made at the University of Kent with areas of less than 0.3
microns2. This is important as we need the area to be as small as possible to
reduce the parasitic capacitance of the junctions and to optimise the value of the
omega*R*C product to a value of approximately 4. Where omega=2 x pi x
frequency, R is the normal state resistance of the junction, and C is the
junctions’ capacitance. It has been ascertained that an omega*R*C product of 4
gives optimum receiver performance. Thus when operating at a frequecy of,
say, 850 GHz it is important to reduce the capacitance of the junction as much
as possible.
One other important consideration is the superconducting energy gap of
the alloy used. Superconducting materials have an energy gap which is
analogous to the energy gap of a semi-conductor. As the photon energy of the
detected radiation increases above the energy gap of the supercondutor the
Cooper pairs start to break-up and the superconductor very rapidly starts to
become 'lossy' - it's conductivity starts to approach that of it's room temperature
conductivity - even if the material is still well below it's critical temperature.
The table above gives the energy gap of some alloys used to make SIS junctions,
their energy gap and the corresponding frequency at which the Cooper pairs
start to break up. It should be noted that although SIS junctions can still be used
above this frequency the ohmic losses of the material rapidly start to
compromise the overall performance of the receiver. The Nb-AlO2-Nb values
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are included to illustrate that Pb:Bi alloys have a higher energy gap and are thus
better suited for very high frequency work.
Infrared detection.
A very important region of the E-M spectrum for astronomers, as this is where
the black-body curve peaks for objects with temperatures between 10K and
100K, such as regions of star and planetary formation.
Unfortunately, it is also the region where an astronomer’s view of the universe
is impaired by the atmosphere.
Atmospheric attenuation above Mauna Kea (Hawaii) under average
conditions.
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As can be seen, the atmosphere is totally opaque within this region, and thus to
investigate within this band we need to put detectors above the atmosphere.
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A world class observatory: The James Clerk Maxwell telescope.
With a diameter of 15m the James Clerk Maxwell Telescope (JCMT) is the
largest astronomical telescope in the world designed specifically to operate in
the submillimeter wavelength region of the spectrum. The JCMT is used to
study our Solar System, interstellar dust and gas, and distant galaxies. It is
situated close to the summit of Mauna Kea, Hawaii, at an altitude of 4092m. It is
a jointly funded telescope between the UK and Canada.
This is an interesting telescope as it works in a part of the E-M spectrum which
crosses from the radio into the infra-red.
Below are five instruments which have been used, or are currently being used
on the JCMT.
1. Receiver A3 (230 GHz). Heterodyne radio receiver.
2. Receiver B3 (345 GHz). Heterodyne radio receiver.
3. Receiver W (460 and 690 GHz). Heterodyne radio receiver.
4. SCUBA (Submillimetre Common User Bolometer Array) working at 850
um and 450 um.
5. THUMPER (Two Hundred Micron PhotomeER) working at 200 microns
using a gallium doped germanium photoconductor.
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Astrophysics
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