"MAGIC" Reveals The
Transparency Of The Universe
(1 July 2008) Measurements by the
MAGIC Telescope (Major Atmospheric Gamma-ray Imaging Cherenkov Telescope) on La
Palma have shown that the universe is more transparent to high-energy radiation
than previously assumed.
A new publication in "Science" with ETH
Zurich participation describes how measurements of high-energy gamma radiation
from 5.3 billion light years away are yielding new knowledge about the nature
of the universe.
The MAGIC telescope on the Roque de los Muchachos in La Palmas. (courtesy: ETH Zurich)
Astronomers assume that our universe is
approximately 14 billion years old. During this time it has changed greatly,
for example through the formation of new galaxies that emit electromagnetic
waves including light that can be perceived visually. A small fraction of the
latter is absorbed, but the majority is retained in the universe. In this
respect the universe can also be imagined as a background of invisible
electromagnetic radiation. Since high-energy rays interact with low-energy
ones, astronomers assumed for a long time that because of this enormous
universal background radiation, only high-energy rays from very close active
galaxies would be detectable on Earth - the remainder would be
absorbed.
However, a publication that appeared recently in "Science" by
scientists belonging to the "MAGIC Collaboration" (see box) reaches a different
conclusion. The team was the first to succeed in detecting on Earth high-energy
gamma radiation originating from the quasar 3C 279, which is more than 5
billion light-years distant. A quasar is an active galaxy in which an enormous
black hole more than a billion times the mass of the sun radiates gigantic
amounts of energy in various wavelength regions. According to Adrian Biland
from the Institute for Particle Physics (IPP) of ETH Zurich: "The results show
that the radiation background in the universe is distinctly less dense than
expected." Together with seven other ETH Zurich physicists he is a member of
the "MAGIC Collaboration" who are the authors of the publication. According to
calculations by the scientists, the density of the background radiation in the
universe corresponds approximately to the expected radiation intensity of all
the heavenly bodies known from astrophysics. This finding could disprove the
theory that the universe also contains light from objects that are still
entirely unknown and which could have existed in the early universe. According
to Biland: "This means that if heavenly bodies of types at present unknown to
us really did once exist, they must have been of a quite different nature to
the stars we know today. Otherwise their light would still be detectable in
today's radiation background."
Diagrammatic representation of the measurement method for radiation in the (VHE) gamma region. (courtesy: ETH Zurich)
High-energy physics with
potential
Biland and the "MAGIC Collaboration" work in the
relatively young field of astro-particle physics and study "very high-energy"
(VHE) gamma radiation. The original energy of this radiation lies in the range
between 50 giga-electron volts and several tera-electron volts, i.e. radiation
with a wavelength more than a billion times shorter than sunlight. (VHE) gamma
radiation is measured by the MAGIC telescope on the island of La Palma in the
Canaries. This telescope makes the measurements by using the Cherenkov effect,
which put simply is the optical equivalent of a supersonic bang. However the
velocity of the particles causes a flash of light instead of a bang. In other
words charged particles can pass through the atmosphere faster than the
associated electromagnetic radiation can propagate. As a result the
polarisation of the atoms, which is normally symmetrical, can no longer balance
out, and this leads to the emission of what is known as Cherenkov radiation.
When a VHE photon penetrates into the atmosphere it generates a so-called Air
Shower of thousands of electrons and positrons, all of which emit Cherenkov
radiation. These mini-lightning flashes are collected by the 934 aluminium
mirrors of the MAGIC telescope positioned in the cone of this "shower" and are
recorded by a high-performance camera taking two billion images per second, of
which about 1000 are ultimately selected. The main problem is that in addition
to the very rare VHE photons, similar air showers are also generated by cosmic
ray particles, which are more than 10,000 times as frequent, and the actual
measured signal needs to be found and interpreted against this
background.
VHE gamma radiation is not an exceptional
phenomenon
According to Biland, measurements in the VHE gamma
radiation region provide astronomers with a unique opportunity to verify the
laws of astronomy in new areas of energy. Thus the publication in "Science" was
able to disprove the "Blazar Sequence" model, which says that the most
energetic objects are also the brightest. Another opinion prevalent among
astronomers until now was that VHE gamma rays are exceptional astronomical
phenomena. As the data set grew steadily, it became possible within a few years
to identify a whole series of different classes of active galaxies as well as
various supernova remnants in the Milky Way as sources of VHE gamma rays.
Biland says "We even observed sources that had not been discovered by
conventional measurements in any other wavelength region. This means that
objects emitting radiation only in the VHE gamma region also seem to exist - a
phenomenon whose cause is still unknown at the present time."
The
potential of astro-particle physics, especially in the (VHE) gamma region, is
recognised by scientists and research institutions all over the world. A second
MAGIC telescope that will enable stereoscopic observations and thus more exact
measurements will be commissioned on La Palma in September this year. Other
similar installations exist in Namibia (H.E.S.S.), Australia (CANGAROO) and the
USA (VERITAS). In addition the planning procedure is currently underway for the
"Cherenkov Telescope Array" (CTA) project, in which more than 40 European
research institutes are collaborating and which plans to bring into operation
about 100 Cherenkov telescopes. Biland is convinced that "An installation of
this kind allows us an entirely new window into the universe."
Major
Atmospheric Gamma-ray Imaging Cherenkov (MAGIC)
MAGIC is the world's
biggest gamma radiation telescope and is operated by a major collaboration
involving more than 150 researchers from the whole of Europe. The telescope was
commissioned in 2003 on the Roque de los Muchachos ("Rock of the Boys"), the
highest mountain on La Palmas (2500 metres). The telescope, which has a
diameter of 17 metres and a mirror surface area of 236 square metres, makes
measurements only at night but is looked after round the clock by a team of
scientists on the spot.
(source: Samuel Schläfli, ETH
Zurich)
