Gamma-rays are the most energetic (highest frequency, shortest wavelength) form of electromagnetic radiation.
These photons are produced terrestrially (on Earth) through nuclear decay reactions as well nuclear fusion and nuclear fission experiments. But there are also astronomical sources of gamma-rays, created by the most violent interactions in the Universe.
Energy range of Gamma-rays
Gamma-rays could be defined simply as any radiation with an energy greater than the most energetic X-rays. This would place the lower bound of gamma-rays at about 120 kilo-electron volts (keV), the the literature will sometimes place the lower limit at about 100 keV.
These energies correspond to a lower frequency limit of about 1019 Hertz (Hz) and wavelengths less than 10 picometers (pm).
Strictly speaking, the upper bound of gamma-rays is unknown. Since they are the most energetic photons detected, the theoretical energy limit is determined by whatever mechanical process can produce the highest energies. However, there have been gamma-rays detected that, as so far, do not have a known mechanism.
Creation of Gamma-rays
There are many natural processes that produce gamma-rays, most notably of which is nuclear decay. However, gamma-rays are also produced by other nuclear processes such as nuclear fusion, nuclear fission and matter-antimatter annihilation.
It is this feature, the production of gamma-rays through interaction of nucleonic material, that was used as the defining characteristic between X-rays and gamma-rays. (X-rays are typically produced through electron de-excitation and acceleration.) Specifically the lower energy bound for photon production by nuclear decay is about 120 keV, the canonical value for the lower gamma-ray energy threshold.
But while production of gamma-rays is associated with nucleonic interactions, the field of high -energy astronomy has expanded the possibilities for production of the highest energy radiation.
Importance in Astronomy
There are several known source types that will produce gamma-rays.
- Pulsars and Magnetars: Pulsars, and highly magnetized neutron stars known as magnetars, are the result of a Type II supernova. As these very compact stellar remnants spin at high rates (some nearing 1000 revolutions per second), beams of high energy radiation are emitted from out near the light-cylinder (where the tangential velocity of the co-rotating magnetic field reaches the speed of light). Locally, a pulse of gamma-rays is detected at every time the radiation beam sweeps past our direction. Gamma-rays have been detected from many of the most energetic pulsars (usually indicative of young neutron stars) as well as some millisecond pulsars. (Millisecond pulsars are objects are a special class of pulsars that have been "spun-up" by a companion star at the end of their life, and now rotate faster than when they were first created.) The gamma-rays detected from pulsars tend to be lower energy photons and therefore only detectible by space based gamma-ray observatories (see below), but recently the Crab Pulsar was detected at energies above 100 GeV, making it the first pulsar to be detected by ground based observatories.
- Supernova Remnants and Pulsar Wind Nebulae: Related to pulsars, supernova remnants are the glowing hot gas that was once ejected from a star as its core collapsed into a neutron star or black hole. As the initial shock wave from the supernova propagates the outer envelope of the star into the interstellar medium, it will energize the surrounding gas. The gas will then radiate energy at wavelengths ranging from Radio to gamma-ray. Similarly once the pulsar has formed at the center of the supernova remnant it will begin to dump massive amounts of energy into the space once occupied by the main sequence star. As this "wind" drives material outwards, the resulting Pulsar Wind Nebula (or PWN) can emit intense gamma-rays.
- Gamma-ray Bursts: These events are the most powerful in the Universe. Over a few seconds a gamma-ray bust can release most energy than our Sun will during its entire lifetime. It is believed that there are three different types of Gamma-ray Bursts (GRBs). One is caused by the collapse of a supermassive star. (These events are sometimes called hypernovae.) But there is some indication that colliding neutron stars, or in some models colliding black holes, are also responsible for some types of GRBs. Finally, magnetars may also occupy a class of gamma-ray bursts, but these would be lesser events and are usually classified as gamma-repeaters.
- Active Galaxies (AGN) and Quasars: Events relating to supernovae are the primary gamma-ray sources in our galaxy. However, other galaxies, specifically Active Galaxies and Quasars, are known to be sources of gamma-rays as well. These galaxies contain supermassive black holes that, as they accrete material they collimate hugh jets that can dwarf the host galaxy. When these jets are directed at Earth, they can provide some of the strongest steady sources of gamma-rays in the night sky, despite the fact that they can be at incredible distance from Earth.
Types of Gamma-ray Detectors in Astronomy
Because of the opacity of our atmosphere, gamma-rays usually do not penetrate down to the surface of the Earth. However, there are techniques that can be employed that search for shockwaves created indirectly by interactions of gamma-rays in our atmosphere. These ground based detectors, known as Atmospheric Cherenkov Telescopes (ACTs), complement the space based observatories as they detect gamma-rays at different energies.
- Space Based Gamma-ray Observatories: The most conventional, but also the most expensive, these orbiting telescopes search from gamma-rays with energies just above that of X-rays (actually many of these instruments also detect Hard X-rays) up to about 300 GeV. They are limited by their size, they have to fit on a rocket after all, and as a result - cost. Launched in June, 2008 the Fermi Gamma-ray Satellite is the most advanced and most successful space based gamma-ray observatory to date.
- Ground Based Gamma-ray Observatories: Ground based gamma-ray observatories have to overcome the fact that gamma-rays can not penetrate very far into the atmosphere before they interact with matter. To do so, they search not for gamma-rays themselves, but instead the by product of their atmospheric interactions. The gamma-ray will pair-produce into an electron and positron pair. Traveling at superluminal speeds (relative to the atmospheric medium) they emit optical shock waves known as Cherenkov radiation. This faint blue light is detected by ground based telescopes, such as the VERITAS array in southern Arizona, and used to reconstruct the path of the gamma-ray.