Active Galactic Nuclei (AGN) - a term used to describe the central region of a distant galaxy which can appear to be a pointlike source of strong gamma-ray emission. AGN are generally thought to be due to supermassive central black holes accreting nearby matter

BATSE - Burst and Transient Source Experiment, the Compton instrument operating from 15 keV to 1 MeV; the instrument primarily intended to detect bursts, flares, and to monitor long-term emission from other sources 

black hole - a star which has evolved to the point where the self-gravity of the star cannot be balanced by any other nuclear or electromagnetic forces. The result is the complete collapse of the star to a point or singularity

blazar - a type of distant AGN which often appears to be a pointlike source of bright, highly variable radiation

collimator - a structure which is used to narrow the field-of-view of a gamma-ray detector

COMPTEL - the Imaging Compton Telescope, which operates in the 1 - 30 MeV energy range and is useful for imaging and the detection of nuclear lines

Compton scattering - The dominant particle interaction process at MeV energies wherein a photon scatters off of an electron, energizing the electron and changing the trajectory of the photon 

cosmic ray - refers to high-velocity elementary particles such as electrons or protons or atomic nuclei which fill much of interstellar space

COS-B - a European gamma-ray experiment operated from 1975-1982. Sensitive over much the same energy range as the EGRET experiment

cyclotron emission - the gamma-ray emission from electrons in a strong magnetic field. Quantum mechanical effects make this type of emission quantized at discrete energies

diffuse emission - gamma-ray emission which is not confined to a point source. Typically due to the interactions of cosmic rays with interstellar material. Sometimes refers to emission from extended sources such as supernova remnants

EGRET - the Energetic Gamma-Ray Experiment Telescope which operates from 30 MeV to 30 GeV

electron-volt - a unit of electromagnetic energy, sufficient to excite atoms to emit visible light, keV = 1,000 eV, MeV = 1,000 keV, GeV = 1,000 MeV

flux - a detector-independent measure of the brightness of a gamma-ray source

gamma ray - a photon carrying more energy than an x-ray (more than about 50 electron volts) 

gamma-ray burst - brief, intense, random gamma-ray emission from the far reaches of the universe

jet - typically a stream of relativistic particles which flows out from a central source

lightcurve - a time history of emission from a gamma-ray source

light-year - a measure of distance travelled by light in one year?s time = 9.7x1015 meters 

nuclear lines - gamma-ray emission centered on a characteristic energy; caused by interactions of particles with atomic nuclei or by transitions within a nucleus

occultation - the technique of measuring the flux of a source by comparing a detector signal while the source is in the field of view to the signal when the source is hidden or occulted by the Earth 

OSSE - Oriented Scintillation Spectrometer Experiment, the low-energy gamma- ray instrument on the Compton satellite used for detailed study of gamma-ray spectra

pair annihilation - A fundamental interaction between a particle such as an electron and its antiparticle (i.e. a positron) which results in the emission of two photons with the initial pair of particles destroyed. This process dominates other gamma-ray interactions with matter at energies above a few tens of MeV

pair production - the inverse process to pair annihilation where a particle/antiparticle pair are created from two gamma rays 

photon - the fundamental particle of electromagnetic radiation (light). Photons carry energy proportional to their frequency

pulsar - a subclass of neutron stars characterized by a beam of emission which sweeps around as the neutron star rotates, alternately coming into and going out of the field of view of an observer, thus giving rise to a pulsing effect

phototube - an electronic device which converts a photonic signal into an electronic signal. Used in gamma-ray astronomy instrumentation to detect the arrival and energy of an incident photon 

positron - anti-particle of the electron. Capable of mutually annihilating with an electron to emit characteristic radiation around 511 keV

SAS 2 - a pioneering spark-chamber instrument like EGRET launched in 1972

scintillator - a crystal which detects gamma rays by converting the energy into detectable light 

Seyfert - a type of AGN which is dominated by a bright, compact blue nucleus

solar flare - a burst-like emission of radiation from proturbances in the Sun?s outer atmosphere 

spark chamber - a gamma-ray detection device utilizing pair production of the initial photon followed by the detection of a trail of sparks in a grid which follow the trajectory of the resultant electron/positron pair

spectrum - the number of photons a source emits as a function of energy supernova - the endpoint of stellar evolution whereby the star, having exhausted its nuclear fuel explodes, usually leaving behind a compact object such as a black hole or neutron star

synchrotron emission - radiation emitted by charged particles when accelerated by a magnetic field

thermal Brehmstrahlung - the x- and gamma-ray emission process caused by the collisions of particles in a hot plasma ("braking" radiation)

white dwarf - an evolved, compact star which is supported against self-gravity by the repulsive interaction of electrons

x-ray nova - a binary system where episodic accretion from the companion star to the black-hole companion results in  gamma-ray emission which gradually declines over a period of months