Survey Finds Dark Accelerators

- i have not posted much cosmology related material here lately but i
thought some people might find this interesting.

http://www.universetoday.com/am/publ...s.html?2832005

Survey Finds 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.

Hi Captain Very interesting. Do you think when this source of gamma
radiation cools we might get longer photon wave lengths? I gather these
gamma burst are coming from an area close to our galaxies center bulge.
Would think H.E.S.S would be in orbit like the rest of gamma detectors.
Very clever using the Earth's atmosphere to detect this gamma radiation.
Thank's Captain for showing us this. Bert

Captain! wrote:

- i have not posted much cosmology related material here lately but i
thought some people might find this interesting.

I think the regulars here like Brookski would be much more interested in
cosmetology.