- What Are Gamma Rays?
Gamma-ray astronomy uses various techniques to observe, either directly or indirectly, photons that have energies above approximately 1.2 x 105 eV (about 2 x 10-14 Joule). This corresponds to a wavelength of around 10-11m and a frequency around 3 x 1019 Hz.
Compare this with green visible light, which has a wavelength around 500 x 10-9m, a frequency of 6 x 1014 Hz, and energy around 2.5 eV or 4 x 10-19 J. That is, a gamma-ray photon carries at least 50,000 more energy than the light you see with your eyes...
Thus, gamma-rays are much more energetic than visible-light photons, and the physical processes that are required to produce such high-energy photons are themselves correspondingly more energetic. This provides constraints on the type of processes that are capable of producing gamma-rays, and so detecting and studying gamma-rays from sources gives us another window onto the nature of these sources.
- How Do We Detect Them?
An incoming gamma-ray photon (red), and the associated Cerenkov light pool (blue). The Cerenkov light is detected by specialised ground-based telescopes.
Gamma-rays can be observed via ground-based and space-based detectors.
Satellite-borne detectors can detect gamma-rays directly; for example, the BATSE satellite incorporated counters which measured directly the interaction between gamma-rays and the material in the detectors. Thus, the gamma-ray itself was measured by those detectors.
Ground-based detectors use indirect methods, such as the atmospheric Cerenkov air shower method. Here, when the gamma-ray enters Earth's atmosphere, it is travelling faster than the speed of light in air, and emits Cerenkov radiation.
In essence, this consists of visible or near-visible photons, and we look for these Cerenkov photons. Thus, we do not observe the gamma-ray directly, but the results of its interaction with the Earth's atmosphere.
- What Do We Do in HEAG?
The MAGIC telescope on La Palma undergoing calibration on a foggy night.
Whilst MAGIC is not used by HEAG members, this image well illustrates the optical layout of such telescopes in general.
Staff and students in HEAG utilise both space-based and ground-based techniques. We have a major collaboration with the HESS gamma-ray telescope system in Namibia.
We are also involved with planning and development of the Cerenkov Telescope Array.
Our research is directed at determining the observational characteristics of the sources of the gamma-rays, and elucidating the physical process that produce such high-energy photons.
In particular for galactic gamma-ray sources within our own Milky Way galaxy, those gamma-rays can arise from the interaction between cosmic ray particles and the various components of our galaxy's interstellar medium (such as gas clouds), so a major effort is the examination of that interaction.
Thus, we also undertake research at other wavelengths, particularly in the radio and sub-millimetre regimes, and also examining the nature of the galactic magnetic field.
Please see the section on Radio research for more information on this aspect of what we do.
For more information, please contact Associate Professor Gavin Rowell.