High Energy Mystery Lurks At The Galactic Centre

The Galactic Centre harbours a number of potential gamma-ray sources, including a supermassive black hole, remnants of supernova explosions and possibly an accumulation of exotic 'dark matter' particles, each of which should emit the radiation slightly differently. The radiation observed by the H.E.S.S. team comes from a region very near Sagittarius A*, the black hole at the centre of the galaxy. According to most theories of dark matter, it is too energetic to have been created by the annihilation of dark matter particles. The observed energy spectrum best fits theories of the source being a giant supernova explosion, which should produce a constant stream of radiation.

Dr. Paula Chadwick of the University of Durham said, "We know that a giant supernova exploded in this region 10,000 years ago. Such an explosion could accelerate cosmic gamma rays to the high energies we have seen - a billion times more energy than the radiation used for X-rays in hospitals. But further observations will be needed to determine the exact source."

Professor Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council (PPARC) which funds UK involvement in H.E.S.S. said; "Science continues to throw out the unexpected as we push back the frontiers of knowledge." Halliday added "The centre of our Galaxy is a mysterious place, home to exotic phenomena such as a black hole and dark matter. Finding out which of these sources produced the gamma-rays will tell us a lot about the processes taking place in the very heart of the Milky Way."

However, the team's theory doesn't fit with earlier results obtained by the Japanese /Australian CANGAROO instrument or the US Whipple instrument. Both of these have detected high-energy gamma rays from the Galactic Centre in the past (observations from 1995-2002), though not with the same precision as H.E.S.S, and they were unable to pinpoint the exact location as H.E.S.S. has now done, making it harder to deduce the source. These previous results have different characteristics to the H.E.S.S. observations. It is possible that the gamma-ray source at the Galactic Centre varies over the timescale of a year, suggesting that the source is in fact a variable object, such as the central black hole.

The H.E.S.S. team hopes to unravel the mystery with further observations of the Galactic Centre over the next year or two. The full array of four telescopes will be inaugurated on September 29th 2004, see http://www.mpi-hd.mpg.de/hfm/HESS/public/HESS_broschuere_04_c.pdf for further details.

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Notes

The full scientific paper can be seen at http://www.edpsciences.org/articles/aa/abs/2004/37/contents/contents.html

The H.E.S.S. collaborationThe High Energy Stereoscopic System (H.E.S.S.) team consists of scientists from Germany, France, the UK, the Czech Republic, Ireland, Armenia, South Africa and Namibia.

The H.E.S.S. array

Over the last few years, the H.E.S.S. collaboration have been building a system of four telescopes in the Khomas Highland region of Namibia, to study very-high-energy gamma rays from cosmic particle accelerators. The telescopes, known as Cherenkov telescopes, image the light created when high-energy cosmic gamma rays are absorbed in the atmosphere, and have opened a new energy domain for astronomy. The H.E.S.S. telescopes each feature mirrors of area 107 square metres, and are equipped with highly sensitive and very fast 960-pixel light detectors in the focal planes. Construction of the telescope system started in 2001; the fourth telescope was commissioned in December 2003. Observations were being made even while the system was being built, first using a single telescope, then with two and three telescopes. While only the complete four-telescope system provides the full performance, the first H.E.S.S. telescope alone was already superior to any of the instruments operated previously in the southern hemisphere. Among the first targets to be observed with a two-telescope instrument was the Galactic Centre.

Images

Project images at http://www.mpi-hd.mpg.de/hfm/HESS/public/full_images/full_images.htm

For further information, see the project website http://www.mpi-hd.mpg.de/HESS

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.

PPARC's Public Understanding of Science and Technology Awards Scheme provides funding to both small local projects and national initiatives aimed at improving public understanding of its areas of science.