X-rays and γ-rays are part of the electromagnetic spectrum with wavelength below 1nm (high energy photons). The study of X-rays and γ-rays from astronomical sources is called X-ray and γ-astronomy.
X-ray emission is expected from astronomical objects that contain extremely hot gases at temperatures from about a million kelvin (K) to hundreds of millions of kelvin (MK). Interaction of relativistic particles (e.g. electrons and protons) with low energy photons as well as matter, usually generate both X-rays and gamma-rays.
Extreme events such as supernovae, behavior of matter under extreme conditions (as in pulsars and blazars) are revealed by electron-positron annihilation, inverse Compton effect and, in some cases, by the decay of radioactive material (gamma-decay). Gamma-ray emission are also created by the presence of natural cosmic “particle accelerators” that are much more powerful with respect to those realized in the laboratories (i.e. LHC).
So, X and γ-astronomy allows for a deeper comprehension of these phenomena. Both X and γ-rays from cosmic sources are mostly absorbed by the Earth’s atmosphere. As a consequence, for the direct detection of cosmic X-rays and γ-rays it is necessary to place the experiment outside of the Earth’s atmosphere by means of a balloon or a satellite. Gamma-ray experiments on satellites usually exploit the e+e- pair production phenomenon. Once the γ photon cross the detector it starts an electromagnetic cascade that is detected by a tracker. This allows the reconstruction of the direction of the primary photon.
At the same time, a calorimeter provides the measurement of its energy. Thanks to these satellites is also possible to reconstruct images of the intensity and direction of the incoming X-ray signals. In the X-ray dedicated telescopes mounted on these satellites, the x-rays are totally reflected by a set of parabolic mirrors and are so focused on a camera.
FIG. 1 On the left: view of the CGRO satellite. On the right: scheme of its instrumentations The observatory was equipped with four instruments for γ observation
Gamma-astronomy made huge progress in ‘90s thanks to the observations of the EGRET instrument mounted on the CGRO satellite. CGRO enabled the discovery of 271 γ-sources with energy above 100 MeV.
The CGRO (Compton Gamma Ray Observatory) was a satellite observatory for the high-energy Universe. It was launched from Space Shuttle Atlantis during STS-37 on April 5th, 1991, and operated until its deorbit on June 4th, 2000. It was, at that time, the heaviest astrophysical payload ever flown at 17,000 kilograms and it made history in gamma-astronomy,
see FIG. 1.
Another important and more recent (launched in 2008) γ-observatory is the Fermi satellite. The GLAST (Gamma Large Area Space Telescope), later called Fermi, is a space observatory for gamma rays which will allow the study of supermassive black holes and pulsars and will search for new physics.
FIG. 2 LEFT: Scheme of the Fermi telescope. RIGHT: Scheme of the LAT instrument.
The Fermi telescope is equipped with 2 instruments: GBM and LAT.
The Gamma-ray Burst Monitor (GBM) is sensitive in the range from 8 keV to 40 MeV. It consists of two sets of scintillators read by PMTs. It was designed to find up to 200 gamma ray bursts per year.
The Large Area Telescope (LAT) is the main instrument of the Fermi telescope. It is an imaging telescope, the next-in-line to EGRET, having double its sensitivity. The LAT instrument is made by a sequence of thin tungsten films (gamma-ray converter) and silicon strip detectors (SSD), FIG. 2. The gamma-ray converter is inserted between two 4 × 4 SSD arrays. The 18 tungsten converter layers and 16 dual silicon tracker planes are stacked in 16 modular "towers", for a total of approx. 9000 SSDs. A gamma-ray enters into the LAT instrument and continues until it interacts with an atom in one of the thin tungsten foils, producing two charged particles: an electron and a positron.
The electron and positron continue to travel through the detector, creating ions in thin silicon strip detectors which are measurable by the readout electronics of the detector. The silicon strips alternate in the X and Y directions, allowing the tracking of the particles in their path through the detector.
FIG. 3 TOP: The NASA SWIFT satellite (on the left) and CHANDRA (on the right). BOTTOM: The European Space Agency XMM-NEWTON satellite.
The first x-ray astronomy dedicated satellite was Uhuru, which in 1979 provided the first x-ray source map.
Thanks to x-ray astronomy, it has been possible to define the characteristics of pulsars and to study the already known collapsed stars (e.g. white dwarves, neutron stars, supernovae and black holes) as well as remnants and active galaxies that accelerate particles thanks to their high magnetic fields.
Currently, the satellites in orbit are XMM-NEWTON, CHANDRA and SWIFT, see FIG 3. They are designed to capture high definition images of x-ray sources.
Thanks to these satellites it is possible to reconstruct images of the intensity and direction of the incoming signals. In the telescopes mounted on the satellites, the x-rays are totally reflected by a set of parabolic mirrors and are so focused on a camera.
FIG. 4 Schematic view of the Chandra HRC.
The Chandra X-Ray Observatory combines the mirrors with four science instruments to capture and probe the X-rays from astronomical sources. The incoming X-rays are focused by the mirrors to a tiny spot on a High-Resolution Camera (HRC) that is one of the two instruments in the focus.
The primary components of the (HRC), FIG. 4, are two Micro-Channel Plates (MCP). A crossed grid of wires detects the electronic signal from the MCPs and allows the position of the original X-ray to be determined with high precision.
FIG. 5 Schematic view of the ACIS detector.
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.
The Chandra Advanced CCD Imaging Spectrometer (ACIS) is the other focal plane instrument. As the name suggests, this instrument is an array of CCDs, see FIG 5.
This instrument is especially useful because it can make X-ray images, whilst at the same time, measure the energy of each incoming X-ray. Thus, scientists can make images 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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