A mystery lurking at the centre of our own Milky Way galaxy – an object
radiating high-energy gamma rays – has been detected by a team of UK
astronomers working with international partners. Their research,
published today (September 22nd) in the Journal Astronomy and
Astrophysics, was carried out using the High Energy Stereoscopic System
(H.E.S.S.), an array of four telescopes, in Namibia, South-West Africa.

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.

Notes for Editors

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. collaboration

The 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.

Contacts

Dr Paula Chadwick
Department of Physics
University of Durham
Phone: +44 191 334 3560
Email: p.m.chadwick@durham.ac.uk

Julia Maddock
PPARC Press Office
Tel +44 1793 442094
Mobile +44 7901 514 975
Email: Julia.maddock@pparc.ac.uk

Keith Seacroft
Durham Press Office
Tel +44 191 334 6077
Email Keith.seacroft@durham.ac.uk

Images

High resolution Galactic centre pictures at
http://www.pparc.ac.uk/Nw/gr_images.htm

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.