Gamma-rays are the highest energy, shortest wavelength electromagnetic radiations. Usually, they are thought of as any photons having energies greater than about 100 keV. (It's "gamma ray" when used as a noun, and "gamma-ray" when used as an adjective.)
The techniques needed to detect the highest energy photons have only become available since the late 1960's - a blink of the eye in terms of mankind's involvement in astronomical research. 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 (1,000,000,000,000 eV, where an optical photon has an energy of a few eV) or beyond!
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 what would typically be called spectrometers or photometers in optical astronomy. These 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.
Gamma-ray detectors have come a long way, but the quest for better angular resolution (and therefore source identification) and spectral resolution (for more information on source behavior) is a continuing activity. Gamma-ray detectors are meant to measure the same things detectors at other wavelengths measure, but the challenge of working in this difficult energy range makes more demands on instrument developers than most other fields. Future detectors are beginning to use more advanced solid-state technology to overcome some of these problems and provide large, sensitive detectors which will further establish gamma-ray astronomy as an integral part of astrophysical research.
GREENBELT, MD (Dec. 1, 2009) – NASA's Fermi Gamma-ray Space Telescope has made the first unambiguous detection of high-energy gamma-rays from an enigmatic binary system...
Fermi Telescope Peers Deep into MicroquasarLast Updated on 2009-12-01 11:07:45GREENBELT, MD (Dec. 1, 2009) – NASA's Fermi Gamma-ray Space Telescope has made the first unambiguous detection of high-energy gamma-rays from an enigmatic binary system known as Cygnus X-3. The system pairs a hot, massive star with a compact object -- either a neutron star or a black hole -- that blasts twin radio-emitting jets of matter into space at more than half the speed of light.
FIGURE CAPTION – In Cygnus X-3, an accretion disk surrounding a black hole or neutron star orbits close to a hot, massive star. Gamma rays (purple, in this illustration) likely arise when fast-moving electrons above and below the disk collide with the star's ultraviolet light. Fermi sees more of this emission when the disk is on the far side of its orbit. Credit: NASA's Goddard Space Flight Center
Astronomers call these systems microquasars. Their properties -- strong emission across a... More »
Blast from the Past Gives Clues About Early UniverseLast Updated on 2009-10-29 00:00:00Socorro, NM (Oct. 29, 2009) – Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have gained tantalizing insights into the nature of the most distant object ever observed in the Universe -- a gigantic stellar explosion known as a Gamma Ray Burst (GRB).
FIGURE CAPTION – Gamma-ray bursts longer than two seconds are caused by the detonation of a massive star at the end of its life. Jets of particles and gamma radiation are emitted in opposite directions from the stellar core as the star collapses. This animation shows what a gamma-ray burst might look like up close. Credit: (Credit: NASA/Swift/Cruz deWilde)
The explosion was detected on April 23 by NASA's Swift satellite, and scientists soon realized that it was more than 13 billion light-years from Earth. It represents an event that occurred 630 million years after the Big Bang, when the... More »
Center Of A Galaxy Emits Gamma RaysLast Updated on 2009-10-07 00:00:00Max-Planck Society (Oct. 6, 2009) – Quite a few distant galaxies turn out to be cosmic delivery rooms. Large numbers of massive stars are born in the hearts of these starburst galaxies, and later explode as supernovae. In the remnants they leave behind, particles are accelerated to very high energies. Astrophysicists have now used the H.E.S.S. telescopes to make detailed measurements of the gamma rays from the NGC 253 galaxy. As predicted, these high-energy rays originate from the region of maximum supernova activity close to the centre. (Science Express, September 2009)
FIGURE CAPTION – Heart of a galaxy emitting gamma rays: This image taken with H.E.S.S. shows the heart of the NGC 253 galactic system. The black star marks the optical centre and the white contours indicate the shape of the galaxy. The H.E.S.S. telescope system perceives the centre of the galaxy as a point - as the... More »
Galactic particle accelerator locatedLast Updated on 2009-09-14 00:00:00
ETH, Zurich (Sep. 14, 2009) – An unprecedented measuring campaign has succeeded in precisely defining the place of origin of high-energy gamma radiation in the galaxy Messier 87. This radiation can only be produced by accelerating elementary particles to very high energies in enormous cosmic objects. Now the underlying extreme physical processes and inherent implications can be investigated in more detail.
Our neighbouring galaxy Messier 87 (M87) accelerates elementary particles to extremely high energies - millions of times higher than anything possible with the particle accelerator LHC (Large Hadron Collider) at CERN. These particles contribute to the cosmic radiation that can be measured on earth. For the first time, physicists can now locate exactly where the acceleration of the particles takes place, i.e. right next to the black hole in the centre of the galaxy.
Cosmic... More »
Fermi Gamma-ray Space Telescope - OverviewLast Updated on 2009-06-16 00:00:00
The Universe is home to numerous exotic and beautiful phenomena, some of which can generate almost inconceivable amounts of energy.
Supermassive black holes, merging neutron stars, streams of hot gas moving close to the speed of light ... these are but a few of the marvels that generate gamma-ray radiation, the most energetic form of radiation, billions of times more energetic than the type of light visible to our eyes.
What is happening to produce this much energy? What happens to the surrounding environment near these phenomena? How will studying these energetic objects add to our understanding of the very nature of the Universe and how it behaves?
The Fermi Gamma-ray Space Telescope, formerly GLAST, will open this high-energy world to exploration and help us to answer these questions.
With Fermi, astronomers will at long last have a superior tool to study how black holes, notorious... More »
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