In order to fully understand how objects like stars and nebulae work, we need to get a complete picture of how they behave. To accomplish this we need to observe them in all wavelengths across the electromagnetic spectrum. While fields of astronomy such as radio astronomy, optical astronomy and X-ray astronomy have been around for decades, only recently has gamma-ray astronomy come into its own.
History of Gamma-ray Astronomy
Like most other types of radiation, gamma-rays were predicted to exist long before they were actually detected. The early work of scientists such as Eugene Feenberg, H. Primakoff, Sachio Hayakawa, I.B. Hutchinson and Philip Morrison in the late 40s and 50s led to the conclusion that certain processes in the Universe, such as supernova explosions, should produce extremely high energy radiation. But actually detecting these photons turned out to be very difficult.
Gamma-rays have extremely high energy; so much so that they are almost completely absorbed in the atmosphere. It was initially thought that only detectors placed in space would be able to measure these photons. And it was in fact high altitude balloon detectors and space satellites that provided the first detections of gamma-rays. The first experiment to do so, the Explorer 11 satellite in 1961 only accumulated around 100 gamma-rays. Clearly, in order to do science it would take more sophisticated detection technology.
While detectors continued to be built, they met with little success. And it wasn't until the SAS-2 and COS-B missions in the 1970s that the technology advanced enough that the instruments could detect enough gamma-rays to make significant statements about their origins. Then, in 1980 the Solar Maximum Mission was launched and detected gamma-rays from solar flares. These early missions gave scientists hope that more sophisticated instruments could be built to explore the high energy nature of the Universe.
The Next Evolution of Space-Based Gamma-ray Detectors
One of the main problems in doing gamma-ray astronomy is that the photons that we are trying to detect are of an extremely high energy. Just as X-rays will penetrate the skin of a person (making them useful for medical imaging), gamma-rays will penetrate even more substantial material. In order to detect the photons, they need to first interact with a material and cause them to "decay" into charged particles. Making a detector big enough to capture enough gamma-rays is difficult, not to mention trying to get the instrument into space. As a result, space based detectors are limited to the range of gamma-ray energies that they can detect.
The latest, and most sophisticated, space-based gamma-ray detector to be launched was the Fermi Gamma-ray Space Telescope in June of 2008. Fermi can detect photons in the 10 keV - 300 GeV range. (Though Fermi does not collect enough photons at the 300 GeV threshold to be useful science data in most cases.) This is considered the low energy gamma-ray band. It is impractical to build a larger detector (which would be needed to begin to detect photons of even higher energy) as it would be both cost prohibitive and be physically and technologically difficult to put anything larger into orbit.
Probing Very High Energy Gamma-ray Sources Using Ground-Based Detectors
Instead, a new technique must be used to detect photons of even higher energies. In 1968 construction of the The Mount Hopkins Observatory (later renamed the Fred Lawrence Whipple Observatory) was completed. The Whipple observatory was the first of the Imaging Atmospheric Cherenkov Telescopes (IACTs).
Observatories utilizing the IACT technique rely on the fact that very high energies are absorbed in the Earth's atmosphere. Specifically, as a gamma-ray enters the atmosphere it will interact with an atmospheric molecule and convert all of it's energy into mass in the form of an electron and positron (the anti-particle of the electron). But the charged particles will be traveling faster than the speed of light in air (while the speed of light in a vacuum is the cosmic speed limit, particles can temporarily travel faster than the speed of light in a given medium).
As the charged particles slow down they emit an optical boom (in a similar way that a fighter jet produces a sonic boom when it travels faster than the speed of sound). This optical boom, known as Cherenkov radiation, is a very faint blue light that penetrates down to the surface of the Earth. The Whipple observatory was designed to reflect the Cherenkov light (using a 10-meter optical reflector) into a camera.
After a couple decades of perfecting the technique (mostly by refining the camera design and technology), the Whipple telescope detected very high energy gamma-rays from the Crab Nebula in 1989. Ever since technology has continued to advance, and the latest generation of ground-based gamma-ray telescopes are showing that objects all across the Universe emit gamma-rays.
Among the current generation of IACTs, operating between 50 GeV and 50 TeV, the VERITAS, HESS, MAGIC II and CANGAROO III arrays currently have the greatest sensitivity. As we continue to probe the gamma-ray Universe we learn more about how objects such as black holes, supernova remnants and neutron stars work.
