History of High Energy Astrophysics at Leeds

The work in High Energy Astrophysics, which is part of the generic field of Astroparticle Physics, can be traced back some 50 years to the study of cosmic rays initiated in Leeds by Professor J G Wilson. Cosmic ray work was originally undertaken at Leeds for two reasons. Firstly, they provide a route to information about high energy particle physics interactions, at energies well above anything that will ever be reached in a man-made accelerator. Secondly, information of interest to solar physicists and astrophysicists can be sought through understanding features of cosmic rays. In the late 1960s the group was responsible for an experiment to measure primary electrons with an instrument on an early European satellite, ESROII, and operated, as part of a world-wide network, a neutron monitor that was used to study particles from the Sun that are produced in solar flares. This latter work has led to the concept of "space weather predictions" that are important in many aspects of modern communications. In the late 1950s Wilson initiated a major project with scientists from 3 other UK universities to study the highest energy cosmic rays using the technique of extensive air showers. The British National effort in this field was led from Leeds by Wilson until 1976 and from then by Professor Watson. An important strand to the program, which was carried out with a 12 km² array of water-Cherenkov detectors on the Yorkshire Moors, was to search for particles of energy as great as 10²⁰ eV. The number of such particles that reach the Earth is strongly affected by the 2.7 K radiation from the early Universe that was discovered in 1965. When the array was closed in 1987 a small number of events had been detected and a flux of ~ 1 per km² per century was claimed. To study these very rare particles in more detail required the development of much larger detectors and a huge international collaboration. Some members of the high energy astrophysics group became founder members of the Pierre Auger Observatory that is now taking data in Argentina. This instrument, covering ~3000 km² (an area roughly the size of Lancashire), is the work of a consortium of 250 scientists from 15 countries. The studies are expected to continue for another 20 years. As interest in low energy cosmic rays declined, a balloon program was developed to study features of the 2.7 K radiation and to study infra red sources. The latter activity was further explored with the IRAS satellite. A further strand was to study the properties of globular clusters using national telescope facilities. The establishment of astrophysical research efforts in areas other than high energy astrophysics underpinned the undergraduate degree in Physics with Astrophysics that was introduced in the early 1970s. The program available to undergraduates was also supported through an endowment, the Bolton Fund, that has been used to provide 12" and 14" diameter telescopes. Other aspects of the astrophysics program gathered pace and international importance following the move of Professor J. E. Dyson from the University of Manchester in 1996. In the late 1980s some of the members of the cosmic ray group became interested in observing high energy gamma rays. At energies above about 100 GeV, these gamma rays create small showers of electrons and protons and can be detected, at ground level, through the Cherenkov light that is produced in the air. Members of the group were prominent in the VERITAS collaboration that has telescopes in Arizona. This is a new band of the electromagnetic spectrum and study of it is at about the same state as was the study of X-ray astronomy in the 1960s. However, this placed the astrophysics group in a powerful situation. Members of it were well-positioned to study objects from radio wavelengths, using national facilities such as the telescopes at Jodrell Bank, other national telescopes and international satellites, to the very highest energy gamma rays; a range of over 17 orders of magnitude in photon energy. Objects have now been observed that seem to emit most of their power at very high energies and complementary theoretical studies were explored by the group. This work may also link with that on ultra high energy cosmic rays as some of the most energetic sources seen in gamma-rays may also be the sites of production of these particles.