HEA - History and Background

Gerald Fishman

The history of High-Energy Astrophysics may be said to have begun in 1912 with Hess's discovery of Cosmic Rays showering in the upper reaches of Earth's atmosphere, though what we would regard today as HEA research emerged in the 1960's with the coming of the space programme. Most methods of detecting high-energy photons (which may come from a variety of sources such as the Sun, active galaxies, supernovae and so on) rely on the production of electrons resulting from collision with the incoming high-energy particles. Depending on the energy of these particles, different methods of detection can be used because higher energy levels give rise to different phenomena. Compton Scattering can be detected at energies between 1 - 30 MeV (Mega electron volts, i.e., 1 000 000 to 30 000 000 electron Volts) and is a cascading effect produced by incoming energetic photons interacting with atomic nuclei as in the diagram below.

With higher energies than this, at the intensity of gamma-rays, the phenomenon of pair scattering obtains. This is similar to Compton scattering, but with the production of an electron and its anti-particle, a positron. The production of this pair results in the annihilation of the incoming particle.

Since a great deal of high-energy emission is absorbed (thankfully for us!) by the atmosphere, it follows that any detection facility must be outside it. Therefore there was very little research carried out on specific targets before the space age. Some of the most notable orbiting observatories for high-energy phenomena have been: (where no dates are given, the project has not yet begun operating as of July 2002)

• 1970 - 74: Uhuru
• 1978 - 82: HEAO (High-Energy Astrophysical Observatory) series; some amateur observations used.
• 1990 - c.2010: Hubble Space Telescope
• 1999 - c.2010: Chandra: NASA's highly successful probe, launched by the space shuttle.
• SWIFT: A planned orbiting observatory which will be especially useful as it is able to study objects in more than one wavelength.
• INTEGRAL: Probe to cover a wide range of high-energy emissions (between 15 KeV and 20 MeV)
• GLAST: This probe will use pair-trapping (see above) to study phenomena producing energies above 150 KeV.
• EXIST: Similar to SWIFT, but planned to be 5 times more sensitive.

In addition, there are several other projects in the pipeline whilst not strictly HEA by definition, nevertheless relate to non-visual methods of detection, such as the AMANDA project in Antarctica which will perform neutrino astronomy, and space-based gravitational wave detectors for such phenomena as coalescing black holes. Space is a much better place to observe gravitational waves due to the presence of tides and Earth tremors of all sizes on terra firma.