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PH507
Astrophysics
Dr Mark Price
1
Lecture #5: Gravity waves, neutrinos, ground and space-based observatories.
Gravity wave detection.
A gravity wave (or gravitational wave as it should correctly be referred to) is a ripple in
the curvature of the space-time continuum (the enmeshed combination of our three
perceived physical dimensions, plus time) created by the movement of matter. Long
thought to exist, although never yet detected, gravity waves were first hypothesized in
Albert Einstein's general theory of relativity, which predicted that an accelerating mass
would radiate gravitational waves as it lost energy. For example, it would be expected
that two pulsars (celestial bodies that emit radiation in regular pulses) in orbit around
each other should emanate gravity waves as their orbits decay. In accordance with the
First Law of Thermodynamics, which states that neither matter nor energy can be
created or destroyed - although either may be transformed - the energy loss associated
with the orbit's decay is radiated as gravitational waves. According to theory, gravity
waves propagate at approximately the speed of light and pass through matter
unchanged, alternately stretching and shrinking distances on an infinitesimal scale.
Their strength decreases as a function of distance from their source. The study of
gravitational waves could yield an incredible amount of information about the universe
and lead to many practical applications. For example, their ability to pass through
matter unaltered could enable the transmission of a signal over vast distances in space.
Gravitational Wave Detection
Around the world, several countries are constructing gravity wave detectors, highly
sensitive instruments that are expected to be able to detect gravity waves and identify
their sources. Once the projects are in operation, it is expected that the detectors will
work collaboratively. In the United States, the detector project is called LIGO (for Laser
Interferometer Gravitational-Wave Observatory). LIGO researchers hope to establish
the existence of gravitational waves and prove whether or not they actually propagate at
the speed of light and cause the displacement of matter that they pass through. Among
other anticipated outcomes are confirmation of the existence of black holes, and an
enhanced ability to study them and other cosmic phenomena. The LIGO system consists
of suspended weights with mirrored surfaces that can move freely horizontally. If a
gravitational wave were to pass through, the distance between the weights (which is
measured by a laser beam moving back and forth between the mirrors and then
recombined at a photodetector) would be altered.
Researchers from the California Institute of Technology (Caltech) and the
Massachusetts Institute of Technology (MIT) have developed a LIGO prototype
sensitive enough to detect a tiny movement (many times smaller than the diameter of a
single hair) in a test weight 40 meters away. Worldwide, the other gravitational wave
detection projects include a collaborative effort by France and Italy called VIRGO,
another by Germany and Great Britain called GEO 600, a project in Japan called TAMA
300, one in Australia called ACIGA, and NASA's LISA project. In early 2005, the LIGO
project announced a plan to use a million volunteered personal computers to search for a
gravity wave source.
PH507
Astrophysics
Dr Mark Price
2
First generation gravitational wave detector: a Weber bar.
Uses a very large, very cold mass. Gravitational waves passing through the mass will
distort it slightly and change the acoustic signal being passed (via the transducer) into
the bar. Would only be sensitive enough to very large gravitational waves, such as those
produced by a supernova within ~a few hundred light years.
However the next generation of gravity wave detectors use laser interferometry, such as
LIGO in the US. These should be sensitive enough to measure deflections of the order
of one part in 10-21, required to realistically measure graviational waves.
PH507
Astrophysics
Dr Mark Price
3
The AIGO, an hour’s drive from the city of Perth in Western Australia, works in
conjunction with four other gravitational wave detectors located in Italy, Japan and the
United States. The U.S. detector, LIGO (Laser Interferometer Gravitational wave
Observatory), is run by the California Institute of Technology and Massachusetts
Institute of Technology and consists of two sites in Louisiana and Washington. The US
sites are about 3500km apart but effectively operate as a single detector. Scientists hope
that this worldwide network of laser interferometers will catch the waves predicted to be
produced by cataclysmic events, such as collapsing stars and colliding black holes.
The LIGO scientists are confident they'll be able to detect a variety of cataclysms, each
taking place upwards of a hundred million light years away, including colliding neutron
stars, coalescing black holes, and a collapsing star.
Shame nothing has been detected so far!
Next generation detectors: LISA
LISA (Laser Interferometer Space Antenna). Planned for launch in 2015 plans to use
three spacecraft as a giant floating interferometer, with each of the spacecraft a distance
of 5 million kilometres apart. Expected to be a million times more sensitive that current
ground based instruments.
Unfortunately, as of February this year, NASA changed the status of the mission to
“delayed indefinitely”!
PH507
Astrophysics
Dr Mark Price
4
Neutrino Detectors.
The first neutrino detector built was in 1968, and basically consisted of a very large tank
containing 600 tonnes of bleach buried 1.5 km down a mine in South Dakota.
The detector works by observing the interaction of a neutrino with a chlorine atom
(hence the bleach) via the reaction:
37
37
 17
Cl 18
Ar  e 
i.e. a neutrino interacts with a chlorine atom to produce a radioactive isotope of Argon
and an electron.
The tank is left for between 50-100 days, during which time the radioactive Argon
atoms have accumulate within the tank. The argon is then removed, and the amount
counted. This gives us the number of neutrino reactions that have occurred within the
tank.
This gave the first direct evidence of the existence of neutrinos from the Sun, and had
an incidence rate of one neutrino detected every 48 hours!
This early detector could only detect one type of neutrino, the so-called ‘electron
neutrino’. This led to the so-called ‘Solar neutrino problem’, as the detection rate was
only one third of that predicted.
The second generation of neutrino detectors were water based (utilising 3000-8000
tonnes of ultra-pure water), although they were originally constructed to measure proton
decay. Which they never did.
PH507
Astrophysics
Dr Mark Price
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These photomultipliers on the outside of the tank record the Cerenkov radiation emitted
from super-luminal particles that are emitted when the incident neutrino interacts with a
proton.
This type of detector is sensitive to all three neutrino types: the electron neutrino, the
mu neutrino and the tau neutrino.
This then solved the Solar neutrino problem, as it became clear that the neutrinos could
‘oscillate’ between one type and another.
Current state of the art detectors are based in Antarctica and use strings of photomultipliers buried in the deep, transparent Antarctic ice. These detect the blue Cerenkov
radiation emitted when neutrinos interact within the ice in the polar ice cap.
IceCube ( http://icecube.wisc.edu) is currently undergoing initial testing, and when fully
operational will detect interactions within a volume of 1 km3 of ice (equivalent to 1
million tons of water!).
PH507
Astrophysics
Dr Mark Price
6
Ground-based optical, infrared and submm observatories.
JCMT (James Clerk Maxwell Telescope)
Currently the world’s largest submillimetre telescope. Briefly covered in a previous
lecture.
UKIRT (United Kingdom InfraRed Telescope).
The world’s largest, dedicated infrared telescope facility. Built on the top of the
dormant volcano in Hawaii, at an altitude of almost 4000 metres. Incorporates a 3.8
metre optical mirror (which weighs over 6 tonnes).
Main instruments include:
WFCAM
UIST:
UFTI:
CGS4:
IRPOL:
Michelle:
IRCAM/TUFTI:
Wide field 0.8-2.5 micron camera covering a 0.75 square degree tile in 4
pointings.
1-5um imaging and long-slit grism spectroscopy with R~1500-3500. Also
integral-field spectroscopy and imaging- and spectro-polarimetry.
1-2.5um camera with 1024x1024 pixels; pixel scale 0.09". Imaging
polarimetry and K-band 400 km/s FP also available.
1-5um long-slit grating spectrometer with R ~ 400-40,000.
UKIRT's polarimetry module for use with all instruments except Michelle,
which has its own waveplates.
10-20um imaging and long-slit grating spectroscopy. Echelle spectroscopy
and imaging/spectro-polarimetry also available. Instrument currently at
Gemini
1-5um camera with 256x256 pixels; pixel scale 0.08". L-band imaging
polarimetry also available. IRCAM has been decomissioned.
PH507
Astrophysics
Dr Mark Price
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Gemini.
A rather unique telescope, actually consisting of two telescopes (hence the name), called
‘Gemini North’ and ‘Gemini South’ respectively. Gemini North is (again) located on
the top ot Mauna Kea in Hawaii, and Gemini South is located in the Chilean Andes
mountains.
Both telescopes are nominally identical, and have 8-metre single surface mirrors (as
opposed to the Keck telescopes mutli-segmented mirror).
A picture taken from inside the Gemini North dome (above).
Gemini South, showing the south celestial pole.
PH507
Astrophysics
Dr Mark Price
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Gemini North - list of instruments
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Altair (formerly known as GAOS; facility AO system)
GCAL (facility calibration unit)
GMOS-North (optical multi-object, long-slit and IFU spectrograph and imager)
Michelle (mid-IR imager/spectrometer)
NIFS (near-IR integral field spectrograph)
NIRI (near-IR imager with grism spectroscopy)
TEXES (high resolution mid-IR spectrograph)
Visible
GMOS (multiobject, long-slit and
IFU spectrograph
and imager)
Near-IR
Mid-IR
Other facilities
NIRI (imager with
Michelle (imager/
grism spectroscopy) spectrometer)
Altair (formerly
known as GAOS;
facility AO system)
NIFS (integral field
spectrograph)
GCAL (facility
calibration unit)
TEXES (high
resolution
spectrograph)
A picture showing the 8-metre Gemini North mirror.
PH507
Astrophysics
Dr Mark Price
9
Gemini South - list of instruments
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Acquisition Camera (optical imaging)
bHROS (bench-mounted high-resolution optical spectrograph)
FLAMINGOS-2 (near-IR multi-object spectrograph)
GCAL (facility calibration unit)
GMOS-South (optical multi-object, long-slit and IFU spectrometer and imager)
GNIRS (near-IR spectrograph)
GSAOI (near-IR imager for use with adaptive optics)
NICI (near-IR coronagraphic imager)
Phoenix (high resolution near-IR spectrometer)
T-ReCS (formerly known as MIRI; mid-IR imager and spectrometer)
Gemini South - table of instruments
Visible
Near-IR
GMOS (multiobject, long-slit and
IFU spectrograph
and imager)
Phoenix (high
resolution
spectrometer)
bHROS (highresolution
spectrograph)
GNIRS (long-slit,
cross-dispersed and
IFU spectrograph)
Acquisition Camera
(optical imaging)
NICI
(coronagraphic
imager)
FLAMINGOS-2
(multi-object
spectrograph)
GSAOI (highresolution imager
for use with MultiConjugate Adaptive
Optics)
Mid-IR
T-ReCS (formerly
known as MIRI;
imager and
spectrometer)
Other facilities
GCAL (facility
calibration unit)
PH507
Astrophysics
Dr Mark Price
10
Atacama Large Millimetre Array: ALMA.
If successfully completed, will revolutionise infrared/submillimetre astronomy.
Currently being constructed in the Atacama desert in Chile. A multi-million dollar
venture funded between the US, Europe (through ESO) and Japan.

About sixty-four 12-metre antennae located at an elevation of 16,400 feet in
Llano de Chajnantor, Chile.

Imaging instrument in all atmospheric windows between 10 mm and 350
microns.

Array configurations from approximately 150 meters to 10 km.
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Spatial resolution of 10 milliarcseconds, 10 times better than the VLA and the
Hubble Space Telescope.
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Able to image sources arcminutes to degrees across at one arcsecond resolution.
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Velocity resolution under 0.05 km/s.
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Faster and more flexible imaging instrument than the VLA.

Largest and most sensitive instrument in the world at millimetre and
submillimetre wavelength.

Point source detection sensitivity 20 times better than the VLA.
One of the driest sites in the world, at an altitude of 5000 metres. Oxygen levels here
are only 60% that at sea level.
First telescope expected to be operational in 2007. Whole array ‘first-light’ in 2015.
PH507
Astrophysics
Dr Mark Price
11
Space-based observatories.
HST: Hubble space telescope.
Probably the most famous space-based telescope.
Lauched from the space shuttle shuttle Discovery in April 1990, Hubble is rapidily
approaching the end of its proposed life cycle. In fact, it was originally envisaged to be
in operation for 15 years.
Incorporates a 2.4 metre (now perfect) mirror and currently has the following
instruments:
1) Wide Field Planetary Camera 2. Consists of 4 separate CCD cameras. Three
wide field cameras, and a narrow field camera for planetary observations.
2) Space Telescope Imaging Spectrograph.A low resolution spectrometer
sensitive to radiation from 115 nm (ultra-violet) – 1000 nm (near infra-red).
3) Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The only
cryogenically cooled instrument on HST. A cooled grating spectrometer
sensitive to the near infra-red between 0.8 and 2.5 microns.
4) Advanced Camera for Surveys. A wide field of view survey camera. Again
sensitive from the near infra-red through to the UV.
5) Fine Guidance Sensors. Used for sensitive astrometry (effectively: measuring
the exact position of objects). Can get positional accuracy of nearby stars down
to 200 micro-arcseconds.
PH507
Astrophysics
Dr Mark Price
12
ISO (Infrared Space Observatory)
Launched in November 1995 carried four instruments working in the near infrared –
submillimetre. Lasted until 1998 when it’s liquid helium ran out.
1) ISOCAM: A CCD camera sensitive to wavelengths of 2.5 – 17 microns
2) ISOPHOT. An infrared photometer sensitive to spot wavelengths between 2.5 –
240 microns.
3) Long Wavelength Spectrometer. Used an array of 10 germanium
photoconductors sensitive to wavelengths from 45 microns – 196 microns.
4) Short Wavelength Spectrometer. Sensitive from 2.4 microns – 45 microns.
Hugely successful, despite serious problems with ionising radiation from the Van Allen
belts. Data from ISO has been used to publish over 1600 scientific papers.
One of it’s most important discoveries was that water ‘vapour’ is ubiquitous in the
galaxy.
PH507
Astrophysics
Dr Mark Price
13
Space Infrared Telescope Facility (SIRTF) (aka Spitzer).
Following on from the success of ISO, NASA launched the last of it’s big observatory
missions in 2003.
Originally called SIRTF, the telescope was renamed to the (rather horrid) sounding
“Spitzer”.
Consists of a 85 cm primary mirror (ISO had a 60 cm one), and three main instrument
clusters.
1) The InfraRed Array Camera (IRAC). Works at four wavelengths simaltanously
in the near infra-red (3.6, 4.5, 5.8 and 8 microns). The two shorter wavelength
channel are 512 x 512 pixel InSb detectors, and the two longer ones are SiAs.
2) InfraRed Spectrograph (IRS). A spectrometer covering four wavelength bands
between 5.3 and 40 microns.
3) Muliband Imaging Photometer (MIPS). Comprised of InSb and Ge:Ga
photoconductors spanning the wavelength band from 24 – 100 microns.
Each detector subsystem is more sensitive than ISO, although the instruments are very
similar.
Spitzer has a nominal predicted lifetime of five years, at which point it’s liquid helium
supply will run out, and the instruments will be rendered useless.
PH507
Astrophysics
Dr Mark Price
14
Kepler: an optical telescope to search for terrestrial sized planets.
Due for launch in 2008, has a proposed four year lifetime.
Scientific objectives.
1. Determine how many terrestrial and larger planets there are in or near the
habitable zone of a wide variety of spectral types of stars.
2. Determine the range of sizes and shapes of the orbits of these planets.
3. Estimate the how many planets there are in multiple-star systems.
4. Determine the range of orbit size, brightness, size, mass and density of shortperiod giant planets.
5. Identify additional members of each discovered planetary system using other
techniques.
6. Determine the properties of those stars that harbour planetary systems.
Detects planets by looking at the minute change in a star’s luminosity when a planet
transits across it. During transit the star’s luminosity dips by about 0.001% and so
Kepler has 42 very sensitive CCD-based photometers which will stare at 100,000 stars
simultaneously.
PH507
Astrophysics
Dr Mark Price
15
Schematic showing the Kepler spacecraft.
Expected results from the full four year mission:

Detection of about 50 planets if most have R ~ 1.0 Re

About 185 planets if most have R ~ 1.3 Re

About 640 planets if most have R ~ 2.2 Re
Or, more likely, some combination of the above.
Note, these are earth-sized planets within a star’s habitable zone, i.e. the area
around a star where liquid water (and therefore life) is likely to form, and persist.
PH507
Astrophysics
Dr Mark Price
16
Things you should know for the exam.
1) Differences between refracting and reflecting telescopes. Advantages and
disadvantages.
2) Infra-red and sub-mm detection methods.
3) Basic CCD operation.
4) How to convert from wavelength -> frequency -> eV.
Resources and further reading.
“Astrophysical Techniques” (4th edition) by C. R. Kitchen.
“Millimetre-wave optics, devices and systems” by J. Lesurf
http://www.wikipedia.co.uk/ (Beware: some info is not correct!).
http://www.nasa.gov/home/ (NASA homepage – lots of links).
http://sci.esa.int/ (European Space Agency homepage – lots of links).