Glossary
- Accretion Disk: a disk-like structure composed of hot gas swirling around and eventually falling into a black hole. Accretion disks emit X-ray radiation.
- Active Galaxy, Active Galactic Nuclei (AGN): An active galaxy has a bright, compact core called an active galactic nucleus that emits electromagnetic radiation outshining all the other light emitted by the galaxy.
- Airshower:
- Angular Resolution: the smallest angular distance between two distant objects that can be resolved (differentiated) by a telescope. The angular resolution of a ground-based gamma ray telescope is usually around 0.05 degrees, a limit beyond which there can be no improvement due to the physics of gamma-ray air showers.
- Binary System: refers to a situation where two heavy objects, usually stars, orbit one another around their common center of mass. Binary star systems are quite common in the Milky Way galaxy where more stars have a companion than do not. In these systems, the heavier of the two stars evolves more quickly, evetually becoming a white dwarf, neutron star, or black hole. Later, when the lower mass star begins to run out of fuel and expands into a red giant, some of its material may begin to fall onto the heavier, compact star, a process which can give rise to observable X-rays and/or gamma rays.
- Black Hole: A region of space containing so much mass that the fabric of space-time becomes ripped and nothing -- not even light -- can escape its gravitational pull.
- Blazar: a type of active galaxy whose relativistic jets happen to be oriented along our line of sight.
- BL Lacertae: A category of active galaxies . . .
- Broad Line Region (BLR): a region near the central black hole of an active galactic nucleus containing fast-moving clouds of gas. Excited hydrogen atoms in these clouds emit light at a specfic frequency in X-rays. Because the clouds of gas are moving rapidly, this line becomes broadened by the Doppler effect. The width of the broadened line provides astronomers with one method to determine the mass of the central black hole powering the AGN.
- CANGAROO: acronym for "Collaboration of Australia and Nippon (Japan) for a GAmma Ray Observatory in the Outback", an array of air Cherenkov telescopes similar to VERITAS and HESS, located in Australia. CANGAROO Website
- Cascade:
- Compton Gamma Ray Observatory (CRGO): a satellite-borne observatory flown by NASA from 1991 to 2000. The CRGO comprised four instruments, BATSE, OSSE, COMPTEL, and EGRET, which together were sensitive to a range of wavelengths spanning six decades in energy, from 30 keV to 30 GeV.
- Cherenkov radiation: blue light emitted when a relativistic (traveling close to the speed of light in vacuum) charged particle moves through a dense medium (such as air, water, or ice).
- Collection Area: see Effective Area.
- Cosmic Microwave Background Radiation (CMB): electromagnetic radiation which permeates the universe and is thought to be a relic of the big bang. The intensity of CMB radiation peaks at a radio wavelength of 1.9 millimeters.
- Cosmic rays: a steady rain of charged particles such as protons and heavier nuclei (up to iron) as well as photons and neutrinos present throughout the galaxy which impinge on the Earth’s atmosphere, creating cascades of charged particles such as electrons and muons. The origins of cosmic rays are not fully known. Most of them are thought to be accelerated in galactic supernovae, but the origins of the most energetic cosmic rays is still a mystery. Finding the sources of the highest-energy cosmic rays in one of the primary motivations for gamma-ray astronomy.
- Crab: a unit of gamma ray flux based on the steady gamma ray emission observed from the Crab Nebula.
- Crab Nebula: an example of a supernova remnant, the Crab Neubula is the remains of a star that exploded and whose supernova was by Chinese astronomers in the year 1054 A.D. The shock-wave of the supernova explosion is still moving outward and accelerating particles which give rise to very high energy gamma ray emission. The Crab Nebula was the first source discovered to emit very energetic gamma rays based on ground-based observations with the Whipple 10m telescope.
- Cross section: used to compute the probability of a sub-atomic interaction, the cross section is a unit of area corresponding to the effective size of a particle.
- Dark Matter: matter in the Universe which does not visibly radiate and is thus not (yet) observable. Cosmologists estimate that 90% of the mass of the universe is “dark” but the exact form of this matter is the subject of much debate.
- Davies-Cotton Design:
- Declination: one of the coordinates used in the equatorial coordinate system to specifiy the location of an object in the sky. Declination measures the angle north and south of the celestial equator. The other coordinate used in celestial coordinates is Right Ascension.
- Diffuse Flux: refers to a glow of radiation which is not localized, but spread out across a large area of the sky. In cases where point sources of emission are too weak to be detected individually, astronomers can still sometimes detect the overall "glow" caused by the cumulative emission of many objects.
- Effective Area:
- Electromagnetic Spectrum:
- Electron volt (eV): a unit of energy often used for subatomic particles. It is equivalent to the amount of energy gained by an electron after accelerating through one volt of electric potential. Gamma-ray energies are often described in terms of millions of electron volts (MeV), billions of electron volts (GeV), trillions of electron volts (TeV) or higher.
- Energetic Gamma Ray Experiment Telescope (EGRET):
- Energy Threshold:
- Extended Source: Most gamma-ray sources are too far away for us to resolve any details about their physical structure and the resulting image is consistent with point-like emission (like the light from distant stars). However some nearby sources such as supernovae remnants are large enough that gamma-ray observatories can resolve its shape and structure. Any source which has has some angular extent in the sky is called an extended source.
- Extragalactic Background Light (EBL): light in the infrared regine of the EM spectrum which is present throughout the Universe. The EBL is made up of light from stars as well as dust which has been heated by star light. The EBL is important for cosmology because it contains information about the history of star formation since the Big Bang. The EBL is important in gamma-ray astronomy because gamma-rays from a distant source can be partially extinguished due to pair-production interaction with EBL light. The intensity of the EBL is difficult to measure directly, but it can be measured indirectly by calculating how much of the gamma-ray radiation from distant active galaxies disappears on route to the Earth.
- Field of View (FOV): describes the angular size of the patch of the sky seen by a telescope at a given time.
- Flux: the rate at which particles pass through a given area. An example of the units used for flux is particles per square centimeter per second.
- Fred Lawrence Whipple Observatory (FLWO): Located near Amado, Arizona on Mount Hopkins, FLWO is the home to astronomical several observatories including the Whipple 10m telescope. VERITAS will operate at the base-camp of FLWO until 2009.
- Galactic Center: the center of the Milky Way galaxy, located at a distnce of about 26,000 light years in the constellation Sagitarius.
- Galactic Plane: the Milky Way galaxy is flat, like a pancake. Viewed from our perspective, in the plane of the galaxy and about two-thirds of the way out from the center, we see the Milky Way as a band of light stretching across the sky. This is the galactic plane.
- Gamma Ray Burst (GRB): a short blast of gamma-rays from a given location in the sky whose origin is still mysterious. These bursts are observed in equal numbers from every direction in the sky, suggesting the originate outside the Milky Way galaxy. Astronomers think GRBs are caused by massive collapsing stars, or mergers between very heavy objects such as black holes and neutron stars.
- Gamma rays: a type of radiation composed of high energy photons. Gamma-rays are the most energetic type of electromagnetic radiation.
- GeV: a unit of energy equal to 1 billion electron volts.
- GLAST:
- Globular Cluster:
- Hadron: any particle composed of quarks. Example: protons & neutrons
- Hadronic: used to describe a physical process primarily involving hadrons.
- High Energy Stereoscopic System (HESS):
- High Energy Gamma Ray Astronomy (HEGRA) Experiment:
- Imaging Air Cherenkov Technique (IACT): ground-based method of detecting gamma-rays by observing the Cherenkov light emitted by showers of particles produced when the gamma-rays interact in the Earth’s atmosphere.
- Infrared Radiation: electromagnetic radiation composed of photons with less energy than visible light, but more energy than microwaves.
- Interaction: any interplay of sub-atomic particles under the influence a fundamental force of nature and involving the exchange of a mediating particle such as a photon, weak boson, or gluon.
- Inverse Compton Scattering: an interaction between a high-energy electron and a photon in which the electron transfers energy to the photon.
- keV (kilo-electron volt): a unit of energy equal to one thousand electron volts. This unit is often used to describe the energies of X-rays.
- Lepton: fundamental subatomic particles which participate in the weak interaction, including electrons, muons, taus and their associated neutrinos.
- Leptonic: used to describe a physical process primarily involving leptons.
- Light-year (ly): a unit of distance equal to distance traveled by light in one year. One light-year is equivalent to 9.46 x 10^12 kilometers, or about 63,000 times the distance between the Earth and the Sun.
- MAGIC: a large, imaging air-Cherenkov telescope located in La Palma (Canary Islands), Spain. MAGIC is sensitive to medium-energy gamma-rays with energies of 30 GeV and above.
- Magnetic Field:
- Markarian 421, Markarian 501:
- MeV (mega-electron volt): a unit of energy equal to one million electron volts. This unit is often used to describe the energies of high-energy X-rays and low-energy gamma-rays.
- Microwave radiation:: electromagnetic radiation composed of photons carrying less energy than infrared photons but more energy than radio photons. Microwaves are important for studying the early history of the Universe not long after the big bang when the Universe had cooled down sufficiently to become transparent to light.
- MILAGRO:
- Milky Way Galaxy:
- Muon: a heavier counterpart to an electron, and an example of a lepton. A muon is about 207 times more massive than an electron, but has the same electric charge. It has a very short lifetime of just a few microseconds.
- Narrow Line Region (NLR):
- Neutralino:
- Neutrino: electrically neutral, nearly massless subatomic particle involved in certain types of radioactive nuclear decay and subatomic interactions. Neutrinos are leptons and come in three flavors, electron neutrino (νe), muon neutrino (νμ) and tau neutrino (ντ).
- Neutron Star:
- Non-thermal radiation: electromagnetic radiation produced by a population of particles which are not in thermal contact with each other.
- Optical afterglow: emission of electromagnetic radiation at optical wavelengths originating from the location of a Gamma Ray Burst. The observation of optical afterglows and other evidence allowed astronomers to deduce that Gamma Ray Bursts are a form of extreme supernova explosion.
- Optical Radiation: electromagnetic radiation visible to the human eye. The optical wavelength band extends from about 4000 to 7000 Angstroms (1 Angstrom = 10^-10 m).
- Optical Support Structure (OSS): refers to the rigid, curved structure of steel tubes supporting the VERITAS mirror facets.
- Pair production:: refers to the collision of two photons which produces an electron and positron (anti-particle of the electron). The process is the opposite of matter-antimatter annihilation.
- Photomultiplier Tube (PMT): a detector used to detect small amounts (individual photons) of light.
- Photon:: the term used to describe an individual particle of light. Light exhibits properties of both waves and particles. By studying a phenomenon known as the photo-electric effect, Einstein showed that light consists of individual photons carrying a specific amount of energy that is directly related to the color, or wavelength, of the light.
- Pixel:
- Plasma: a gas comprised of charged particles (such as electrons). Plasma is usually considered a distinct state of matter, and under the right conditions can give rise to several forms of non-thermal radiation.
- Point-source:
- Positioner:
- Positron: the anti-particle of the electron, having an electrical charge of +1.
- Primordial Black Hole:
- Pulsar:
- Pulsar Wind Nebula (PWN):
- Quark: considered elementary or fundamental particles, quarks feel the strong force and are the sub-components of protons, neutrons, and all other hadrons. There are six quarks: down (d), up (u), strange (s), charm (c), bottom [or beauty] (b), and top [or truth] (t).
- Quasar (QSO): short for "quasi-stellar object" (QSO), the very distant objects are now thought to be one type of active galaxy.
- Radio Galaxy:
- Radio Waves:
- Redshift:
- Relativistic: term used to describe an object moving close to the speed of light. An object becomes relativistic when the energy of its motion (kinetic energy) is much greater than the amount of energy stored in the form of its mass, given by Einstein's famous equation, E = mc^2.
- Relativistic Jet: Gigantic structures emanating from active galaxies, composed of very energetic particles and radiation. Although the physical mechanisms responsible for forming relativistic jets are little-understood, the process is thought to be driven by the vast amounts of gravitational energy supplied by a super-massive black hole at the center of the active galaxy.
- Right Ascension: one of the coordinates used in the equatorial coordinate system to specifiy the location of an object in the sky. Right Ascension measures the angle East or West of the prime meridian. The other coordinate used in equatorial coordinates is declination.
- Sensitivity:
- Seyfert Galaxy:
- Spectral Energy Distribution (SED):
- Spectrum: describes the relative abundance of photons or other particles based on a particular quantity. For example, the energy spectrum of a gamma-ray source would show the relative abundance of photons with different energies.
- Supernova:
- Supernova Remnant (SNR):
- SWIFT: a sattelite-borne observatory to be flown by NASA beginning in 2009. The SWIFT observatory will have three instruments: a burst alert telescope to detect gamma-ray bursts, an X-ray telescope, and an ultraviolet telescope.
- Synchrotron Radiation: electromagnetic radiation produced by electrons moving in a magnetic field. Depending on the strength of the magnetic field and the energy of the radiating electrons, synchrotron radiation can be emitted across a large range of photon wavelengths, from infrared to low-energy gamma-rays.
- TeV (tera-electron volt): a unit of energy equal to 1 trillion electron volts.
- Thermal Radiation: electromagnetic radiation produced by an object because of its temperature. The exact spectrum of thermal radiation depends on the temperature of the object. Hotter objects have a thermal spectra peaking at higher photon energies than for cooler objects.
- Threshold: see energy threshold.
- Ultraviolet Radiation: electromagnetic radiation composed of photons with energies greater than visible light, but less energy than X-rays.
- VELA Satellites: twelve spacecraft flown by the United States in the 1960s and 1970s to detect nuclear weapons tests for enforcement of the 1959 partial test ban treaty. Designed to detect the gamma-radiation from the explosions of nuclear weapons, the Vela satellites were the first to detect astrophysical gamma-ray bursts.
- Very Energetic Radiation Imaging Telescope Array System (VERITAS):
- Very High Energy: observational regime involving radiation with particle energies in excess of 100 GeV.
- Whipple 10-m Telescope:
- Weakly Interacting Massive Particle (WIMP): a theoretical form of dark matter. A neutralino is one form of WIMP.
- X-ray Radiation: electromagnetic radiation composed of photons with energies greater than ultraviolet light, but less energetic than gamma-rays.
- X-ray Binary: a binary system observed in X-rays.
