XMMNewton Observatory

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XMM-Newton Observatory
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Chandra (NASA)
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XMM-Newton Observatory

• The XMM-Newton observatory is equipped with three
  different types of scientific instrument
• EPIC CCD Camera Each telescope has a CCD camera
  at its focus provided by the European Photon
  Imaging Camera (EPIC) consortium. These deliver
  imaging spectroscopy with a resolution of E/DE
  100 at 6 keV over a 30 arc-minute diameter field
  of view. Two types of CCD camera are used which
  have complementary capabilities. One of the three
  XMM-Newton telescopes is fitted with an array of
  pn CCD chips, which emphasise photon throughput
  and overall energy response. The other two
  telescopes focus radiation onto arrays of MOS
  CCDs, which have a smaller physical pixel size
  than the pn CCDs and better sample the
  point-spread function of the telescope.
• Reflection Grating Spectrometer Two of the three
  X-ray telescopes, those which have EPIC-MOS CCD
  cameras, are also equipped with reflection
  gratings that disperse approximately half the
  radiation collected by the telescope onto a
  linear array of 9 CCD chips. This is the
  Reflection Grating Spectrometer, which provides
  X-ray spectroscopy with a resolving power of
  between 100 and 600 in the energy range 0.35-2.5
  keV.
• Optical Monitor A novel feature of XMM-Newton is
  the inclusion of an optical/UV telescope, the
  Optical Monitor This is designed to provide
  multi-wavelength monitoring of variable X-ray
  sources. The OM is co-aligned with the X-ray
  telescopes and will deliver sensitive optical and
  UV imaging data in the wavelength range 1700-6000
  Å on each field that XMM-Newton observes.

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XMM-Newton Observatory
Dispersive spectroscopy in the X-ray domain is a
new technology and XMM is the first satellite to
fly with Reflection Grating Arrays onboard.
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An array of reflection gratings (the RGAs) placed
at the exit of the X-ray mirrors the gratings
reflect roughly half of the X-ray light to an
array of nine detectors offset from the telescope
axis the remaining light passes through
undeflected to the European Photon Imaging Camera
(EPIC) located at the telescope primary focus.
The RGS instruments are following a circle to
ensure that there are no image distortions. This
is called the Rowland circle.
XMM-Newton Observatory
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Each camera consists of an array of 7 Metal Oxide
Semiconductor (MOS) front-illuminated CCD chips
(manufactured by EEV of Chelmsford, UK) of
600x600 pixels each
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The Chandra Advanced CCD Imaging Spectrometer
(ACIS) is one of two focal plane instruments. As
the name suggests, this instrument is an array of
charged coupled devices (CCD's), which are
sophisticated versions of the crude CCD's used in
camcorders. This instrument is especially useful
because it can make X-ray images, and at the same
time, measure the energy of each incoming X-ray.
Thus scientists can make pictures of objects
using only X-rays produced by a single chemical
element, and so compare (for example) the
appearance of a supernova remnant in light
produced by oxygen ions to that of neon or iron
ions. It is the instrument of choice for studying
temperature variations across X-ray sources such
as vast clouds of hot gas in intergalactic space,
or chemical variations across clouds left by
supernova explosions.
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There are two instruments aboard Chandra
dedicated to high resolution spectroscopy the
High Energy Transmission Grating Spectrometer
HETGS and the Low Energy Transmission Grating
Spectrometer LETGS. Each spectrometer is
activated by swinging an assembly into position
behind the mirrors. The assembly holds hundreds
of gold transmission gratings when in place
behind the mirrors, the gratings intercept the
X-rays reflected from the mirrors. These
gratings diffract the intercepted X-rays,
changing their direction by amounts that depend
sensitively on the X-ray energy, much like a
prism separates light into its component colors
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Chandra
The High Resolution Camera (HRC) is one of two
instruments used at the focus of Chandra, where
it detects X-rays reflected from an assembly of
eight mirrors The primary components of the HRC
are two Micro-Channel Plates (MCP). They each
consist of a 10-cm (4-inch) square cluster of 69
million tiny lead-oxide glass tubes that are
about 10 micrometers in diameter (1/8 the
thickness of a human hair) and 1.2 millimeters
(1/20 an inch) long. The tubes have a special
coating that causes electrons to be released when
the tubes are struck by X-rays. These electrons
are accelerated down the tube by a high voltage,
releasing more electrons as they bounce off the
sides of the tube. By the time they leave the end
of the tube, they have created a cloud of thirty
million electrons. A crossed grid of wires
detects this electronic signal and allows the
position of the original X-ray to be determined
with high precision. With this information
astronomers can construct a finely detailed map
of a cosmic X-ray source. The HRC is especially
useful for imaging hot matter in remnants of
exploded stars, and in distant galaxies and
clusters of galaxies, and for identifying very
faint sources.
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The High Resolution Gamma-Ray and Hard X-Ray
Spectrometer (HIREGS) is designed to detect
photons from 20 keV to 18 MeV . The High
Resolution Gamma-ray and Hard X-ray Spectrometer
(HIREGS) is used to detect X-ray and gamma-ray
emissions from solar flares and objects in our
galaxy such as black holes and neutron stars.
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The International Gamma-Ray Astrophysics
Laboratory launched 17 October 2002
First astronomical image (17 Nov 2002) from
Cygnus X-1
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Gamma-ray techniques Gamma-ray astronomy was a
late starter. The techniques needed to detect the
highest energy photons have only become available
in the last 30 years Gamma-rays simply pass
through most materials and thus cannot be
reflected by a mirror like optical or even X-ray
photons. The tools of high-energy physics,
however, are borrowed to detect and characterize
gamma-ray photons and allow scientists to observe
the cosmos up to energies of 1 TeV.
Unfortunately, gamma-ray detectors have to
contend with a large contamination from cosmic
rays. Cosmic rays - elementary particles which
are come from all parts of the sky - often affect
gamma-ray detectors in a similar manner to the
source photons. This background must be
suppressed in order to obtain a pure photonic
signal. This is even more important when you
consider that sources of cosmic gamma-rays are
extremely weak and require long observations,
sometimes several weeks, to get a significant
detection or accurate measurement of a source.
Gamma-ray detectors can be placed in two broad
classes. The first are instruments which are
"light buckets" and focus on a region of the sky
containing the object of interest collecting as
many photons as possible. These types of
detectors typically use scintillators or
solid-state detectors to transform the gamma-ray
into optical or electronic signals which are then
recorded. The second class are detectors which
perform the difficult task of gamma-ray imaging.
Detectors of this type either rely on the nature
of the gamma-ray interaction process such as pair
production or Compton scattering to calculate the
arrival direction of the incoming photon, or use
a device such as a coded-mask to allow an image
to be reconstructed.
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Specific types of Gamma-ray Detectors
Scintillation Detectors, Solid State Detectors,
Compton Scattering, Pair telescopes, Air Cerenkov
detectors.

Scintillators as Gamma-ray Detectors A popular
method for the detection of gamma raysinvolves
the use of crystal scintillators. The general
description of a scintillator is a material that
emits low-energy (usually in the visible range)
photonss when struck by a high-energy charged
particle. When used as a gamma-ray detector, the
scintillator does not directly detect the
gamma-rays. Instead, the gamma-rays produce
charged particles in the scintillator crystals
which interact with the crystal and emit photons.
These lower energy photons are subsequently
collected by photomultiplier tubes (PMTs). When
gamma-rays pass through matter, they can undergo
three basic processes Compton scattering,
photoabsorption, or pair production. Each of
these processes can create high-energy electrons
or anti-electrons (positrons) which interact in
the scintillator as charged particles. By adding
up the energy collected in the surrounding
photomultiplier tubes, we can determine the
energy of the gamma-ray detected.
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Compton scatter telescopes are typically
two-level instruments. In the top level, the
cosmic gamma-ray Compton scatters off an electron
in a scintillator. The scattered photon then
travels down into a second level of scintillator
material which completely absorbs the scattered
photon. Phototubes viewing the two levels can
approximately determine the interaction points at
the two layers and the amount of energy deposited
in each layer.
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By reconstructing the tracks of the charged pair
as it passes through the vertical series of
trackers (e.g. spark chamber) , the gamma-ray
direction and therefore its origin on the sky are
calculated. In addition, through the analysis of
the scattering of the pair (which is an energy
dependent phenomenon) or through the absorption
of the pair by a scintillator detector or a
calorimeter after they exit the spark chamber,
the total energy of the initial gamma-ray is
determined.
The Pair Telescope
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Solid State Gamma-ray Detectors As with
scintillators, these detectors mainly rely on a
photoelectric ionization of the material by the
gamma-ray, but in this case electron/hole pairs
are created in the semiconductor material rather
than electron/ion pairs as in a scintillator.
Using these materials as an imager requires the
used of coded aperture masks or Compton scatter
type configurations. This is another feature
these detectors have in common with
scintillators.
IBIS/ISGRI polycell of 16 CdTe detectors (w/o
ceramic cover)
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The International Gamma-Ray Astrophysics
Laboratory.
INTEGRAL was launched on 17 October 2002
First astronomical image (17 Nov 2002) from
Cygnus X-1
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IMAGING
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SPECTROMETER SPI
Coded aperture mask
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Air Cerenkov Detectors
Gamma-rays interacting in the atmosphere create
what is called an air shower. This describes the
process of the original photon undergoing a pair
production interaction high up in the atmosphere,
creating an electron and positron. These
particles then interact, through bremsstrahlung
and Compton scattering, giving up some of their
energy to creating energetic photons, which in
turn pair produce creating more electrons which
then bremsstrahlung...etc. The result is a
cascade of electrons and photons which travel
down through the atmosphere until the particles
run out of energy. These are extremely energetic
particles which means that they are traveling
very close to the speed of light. In fact, these
particles are traveling faster than the speed of
light "in the medium". The resultant polarization
of local atoms as the charged particles travel
through the atmosphere results in the emission of
a faint, bluish light known as "Cerenkov
radiation, which can be detected by optical
telescopes
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