Although the orbiting Compton Gamma Ray Observatory has been remarkably successful in sensing the universe at wavelengths beyond x-rays, even the powerful instruments on board this mission cannot detect the highest gamma-ray photon energies. Only ground-based gamma-ray telescopes can measure these very-high-energy gamma rays, although indirectly, which reveal a population of distant galaxies displaying unprecedented violence.
In February, 1997, an international collaboration of astronomers working at the Smithsonian's Whipple Observatory in Arizona, saw the gamma-ray signal from one such galaxy, Markarian 501, increase by a factor of ten. European gamma-ray astronomers working in the French Pyrenees and on the Spanish island of La Palma quickly confirmed the high flux level. Although the flux varies greatly, the average level continues to be high. Thus, for the past two months, the previously unremarkable object, Markarian 501, has been the brightest object in the high-energy gamma-ray sky. It dwarfs the emission from nearby supernova remnants such as the Crab Nebula in our own galaxy.
Markarian 501 is a giant elliptical galaxy some 400 million light-years from the earth. It belongs to a sub-class of galaxies that have an active core (generally assumed to be a rotating super-massive black hole) with jets of high speed particles apparently emanating from their poles. Like Markarian 421, the first such object detected by the Whipple group as a very-high-energy gamma-ray source, Markarian 501 is unusual in that the axis of the jet happens to be pointing in our direction.
Because of the shielding effect of the earth's atmosphere, gamma rays must generally be detected by Earth-orbiting gamma-ray telescopes such as Compton. However, if the energy is sufficiently great they can be seen indirectly with sensitive telescopes on mountain tops. Gamma rays are photons with very high energy; the photons seen in these observations have energies that are more than a million million times that of a photon of visible light.
The very-high-energy gamma rays are produced in the interaction of cosmic particles of even greater energy with ambient particles or photons in these jets. The cosmic particles may be electrons or protons and they must be accelerated by processes related to the enormous energy of the black hole to energies in excess of those attained by man-made particle accelerators (the so-called atom smashers such as those at Fermilab or Brookhaven). In these cosmic particle accelerators the particle energies implied by the gamma-ray energies are so great that theoretical astrophysicists are hard-pressed to explain the processes involved. (If the primary particles are protons their energies must be some millions times greater than the observed gamma rays!). The problem is compounded by the extremely short duration of the flares seen in Markarian 421 (one was less than 30 minutes in duration) which imply that the emission regions are very small.
Even more difficult is to explain is how the gamma rays, once produced, can escape from the environs of the jet without interacting with lower energy photons and degrading in energy. Once clear of the galaxy the gamma rays must traverse the vast regions of intergalactic space where they are expected to be absorbed by interacting with infrared radiation from galaxies formed in the early universe. However these observations suggest that the density of infrared photons is less than was previously predicted.
The gamma rays are detected as distinct events as they strike the Earth's upper atmosphere. Their collision with an air molecule generates a cascade of light-emitting particles which can be detected by large optical detectors such as the Whipple Observatory's 10-meter optical reflector.
The Whipple team, which involves scientists from the Smithsonian Astrophysical Observatory, Iowa State University, and Purdue University, as well as University College, Dublin, in Ireland and the University of Leeds in the United Kingdom, pioneered the technique which identifies these gamma rays from various background radiation. The same technique is used by the Armenian-German-Spanish collaboration that operates the HEGRA array of telescopes in the Canary Islands and by the French group that operates the CAT telescope in the French Pyrenees.
The very-high-energy gamma-ray universe will be explored more fully by a new ground-based telescope, VERITAS, proposed for construction in southern Arizona and by the GLAST satellite, a proposed NASA mission, which would extend the sensitivity of CGRO to higher energies.
A series of papers presented at the 4th Compton Science Symposium in Willamsburg, Virginia, describes the recent activity in very-high-energy gamma rays from these galaxies.