Universe Today
March 28 2005
Survey Finds Dark Accelerators
Summary – (Mar 28, 2005) A team of European astronomers has found
some unusual objects in the central part of our galaxy which are
emitting very high-energy gamma rays. What’s strange, though, is
these objects are invisible in the optical and X-ray spectra. So what
they are is a complete mystery. Not only that, but these objects are
also quite large; possibly on the order of several light years
across. The observations were done using the High Energy Stereoscopic
System (H.E.S.S.) telescopes in Namibia.
Full Story – 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.
Original Source: PPARC News Release