EurekAlert, DC

Public release date: 24-Mar-2005

Contact: Julia Maddock
julia.maddock@pparc.ac.uk
44-1-793-442-094
Particle Physics & Astronomy Research Council

Unveiling the high energy Milky Way reveals ‘dark accelerators’

In the March 25th 2005 issue of Science Magazine, the High Energy
Stereoscopic System (H.E.S.S.) team of international astrophysicists,
including UK astronomers from the University of Durham, report results
of a first sensitive survey of the central part of our galaxy in
very high energy (VHE) gamma-rays. Included among the new objects
discovered are two ‘dark accelerators’ – mysterious objects that
are emitting energetic particles, yet apparently have no optical or
x-ray counterpart.

This survey reveals a total of eight new sources of VHE gamma-rays
in the disc of our Galaxy, essentially doubling the number known at
these energies. The results have pushed astronomy into a previously
unknown domain, extending our knowledge of the Milky Way in a novel
wavelength regime thereby opening a new window on our galaxy.

Gamma-rays are produced in extreme cosmic particle accelerators such
as supernova explosions and provide a unique view of the high energy
processes at work in the Milky Way. VHE gamma-ray astronomy is still
a young field and H.E.S.S. is conducting the first sensitive survey
at this energy range, finding previously unknown sources.

Particularly stunning is that two of these new sources discovered
by H.E.S.S. have no obvious counterparts in more conventional
wavelength bands such as optical and X-ray astronomy. The discovery
of VHE gamma-rays from such sources suggests that they may be
‘dark accelerators’, as Stefan Funk from the Max-Planck Institut in
Heidelberg affirms: “These objects seem to only emit radiation in
the highest energy bands. We had hoped that with a new instrument
like H.E.S.S. we would detect some new sources, but the success we
have now exceeds all our expectations.”

Dr Paula Chadwick of the University of Durham adds “Many of the new
objects seem to be known categories of sources, such as supernova
remnants and pulsar wind nebulae. Data on these objects will help us
to understand particle acceleration in our galaxy in more detail; but
finding these ‘dark accelerators’ was a surprise. With no counterpart
at other wavelengths, they are, for the moment, a complete mystery.”

Cosmic particle accelerators are believed to accelerate charged
particles, such as electrons and ions, by acting on these particles
with strong shock waves. High-energy gamma rays are secondary products
of the cosmic accelerators and are easier to detect because they
travel in straight lines from the source, unlike charged particles
which are deflected by magnetic fields. The cosmic accelerators are
usually visible at other wavelengths as well as VHE gamma rays.

The H.E.S.S. array is ideal for finding these new VHE gamma ray
objects, because as well as studying objects seen at other wavelengths
that are expected to be sources of very high energy gamma rays, its
wide field of view (ten times the diameter of the Moon) means that
it can survey the sky and discover previously unknown sources.

Another important discovery is that the new sources appear
with a typical size of the order of a tenth of a degree; the
H.E.S.S. instrument for the first time provides sufficient resolution
and sensitivity to see such structures. Since the objects cluster
within a fraction of a degree from the plane of our Galaxy, they
are most likely located at a significant distance – several 1000
light years from the sun – which implies that these cosmic particle
accelerators extend over a size of light years.

The results were obtained using the High Energy Stereoscopic System
(H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of
four 13 m diameter telescopes is currently the most sensitive detector
of VHE gamma-rays, radiation a million million times more energetic
than the visible light. These high energy gamma rays are quite rare
– even for relatively strong sources, only about one gamma ray per
month hits a square meter at the top of the earth’s atmosphere. Also,
since they are absorbed in the atmosphere, a direct detection of a
significant number of the rare gamma rays would require a satellite
of huge size. The H.E.S.S. telescopes employ a trick – they use the
atmosphere as detector medium. When gamma rays are absorbed in the
air, they emit short flashes of blue light, named Cherenkov light,
lasting a few billionths of a second. This light is collected by the
H.E.S.S. telescopes with big mirrors and extremely sensitive cameras
and can be used to create images of astronomical objects as they
appear in gamma-rays.

The H.E.S.S. telescopes represent several years of construction effort
by an international team of more than 100 scientists and engineers from
Germany, France, the UK, Ireland, the Czech Republic, Armenia, South
Africa and the host country Namibia. The instrument was inaugurated
in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab,
and its first data have already resulted in a number of important
discoveries, including the first astronomical image of a supernova
shock wave at the highest gamma-ray energies.

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From: Emil Lazarian | Ararat NewsPress