The
brightness of stars varies as we look at them (they twinkle) because
turbulence in Earth's atmosphere acts like many small lenses, sometimes
bending the stars light away from us and sometimes towards us.
"Stars twinkle, but planets don't" is a well known phrase. The
planets don't twinkle because they are much closer and their size is
larger than the size of the turbulent patches in the Earth's atmosphere.
The space between the stars is often thought of as being empty, but it
isn't. On average there is 1 atom per cubic centimetre and this
is the galaxy's atmosphere (called the interstellar medium). The
interstellar medium is also turbulent and affects radio waves in much
the same way that the Earth's atmosphere affects light waves and causes
some to radio sources to twinkle (called scintillation). Only the
smallest radio sources scintillate, pulsars in our Galaxy and some
distant quasars. The scintillation causes the intensity of these
distant quasars to vary on a timescale of hours to days. The
timescale is determined by both the structure of the quasar and the
properties of the interstellar medium in the direction of the quasar.
A quasar produces 100's of times more energy than our entire galaxy,
from a region about the size of our solar system. They are
powered by stars, gas and dust being drawn into giant blackholes with
masses millions of times that of our Sun. However, all quasars
are a very long way away, the nearest is more than 800 million
light-years away, their size is about one millionth of an
arcsecond. To put this in perspective, one millionth of an
arcsecond is the size of a pinhead on the Moon as seen from the
Earth. Studying the variations of quasars produced by
scintillation is the only method we have for studying quasars in such
fine detail.
As part of a project funded by the Australian Research Council, a team
consisting of astronomers from the University of Tasmania and the
Australia Telescope National Facility have been using the Ceduna radio
telescope since March 2003 to continuously monitor a small number of
distant quasars. This unique experiment has already yielded a
number of surprises, including sudden changes in the size and timescale
of scintillation in some sources and the presence of multiple
timescales in others. One indication of the success of this
project is that groups in the USA and Japan are in the process of
setting up copy-cat experiments.
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Dr
Simon Ellingsen:
Dr Giuseppe Cimo:
Cliff
Senkbeil:
Prof Peter McCulloch:
Steve Carter:
Collaborators:
Dave Jauncey
& Jim Lovell (ATNF)
Lucyna
Kedziora-Chudczer (Sydney)
Hayley Bignal
(JIVE)
J-P. Macquart
(NRAO/Caltech)
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Intraday Variability