Spaceflight Now, FL
Sept 22 2004

High energy mystery lurks at the galactic centre
PARTICLE PHYSICS AND ASTRONOMY RESEARCH COUNCIL

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

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.