Astronomers have found a way to harness clouds of gas in space to make a
natural ‘telescope’ more powerful than any manmade telescope currently
in operation.
And they will use it to peer closer to the edges of black holes than
anyone’s ever seen before.
The phenomenon underlying the technique is confirmed in a recent paper
[10 March] in the Astrophysical Journal by a team from the Australian
research institution CSIRO, the University of Adelaide, and institutions
in The Netherlands.
The paper is based on observations by PhD student Ms Hayley Bignall (now
at the Joint Institute for VLBI in Europe), made with the CSIRO
Australia Telescope Compact Array radio telescope in eastern Australia.
With the new technique researchers will be able to resolve details about
10 microarcseconds across – equivalent to seeing a sugar cube on the Moon,
from Earth. (A microarcsecond is measure of angular size – how big an
object looks. It’s a third of a billionth of a degree.)
“That’s a hundred times finer detail than we can see with any other
current technique in astronomy,” says Ms Bignall. “It’s ten thousand
times better than the Hubble Space Telescope can do.”
“And it’s as powerful as any proposed future space-based optical and
X-ray telescopes.”
Armed with the new technique, astronomers hope to learn a great deal
more about how giant black holes make ‘jets’ of super-hot charged
particles and spit them millions of light-years into space.
“We’ll be able to see to within a third of a light-year of the base of
one of these jets,” says CSIRO’s Dr David Jauncey.
“That’s the ‘business end’ where the jet is made.”
The researchers hope gather basic data such as the size of a jet at its
base, the pattern of magnetic fields there, and how a jet evolves over time.
They are particularly interested in the black holes in quasars. These
are the super-bright cores of distant galaxies.
The new technique uses the same phenomenon that makes stars twinkle –
atmospheric turbulence.
Our Galaxy has an invisible ‘atmosphere’ – a thin gas of electrically
charged particles that fills the space between the stars.
Lumpiness in this gas can act like a set of lenses, focusing and
defocusing radio waves from distant objects, making them appear to
strengthen and weaken. This variation is twinkling or ‘scintillation’.
Objects twinkle only if they look very small on the sky. Quasars are
small enough, so distant that they are mere points of light.
Quasars twinkle more slowly than stars. Any change in less than a day is
considered to be fast. The fastest scintillators have signals that
double or treble in strength in less than an hour.
How fast and how strongly the radio signals vary depends on the size and
shape of the radio source, the size and structure of the gas clouds, the
Earth’s speed and direction as it travels around the Sun, and the speed
and direction in which the gas clouds are travelling.
The researchers have shown that by observing how the variability of the
radio signal changes over the course of a year, they can build up a
two-dimensional picture of the radio-emitting regions of a quasar.
The technique, dubbed ‘Earth-Orbit Synthesis’, was first outlined by Dr
Jean-Pierre Macquart of the University of Groningen and Dr David Jauncey
of CSIRO, in a paper published last year.
The Bignall et al. paper focuses on a rapidly varying quasar called PKS
1257-326, which lies 4 million light-years from Earth.
Ms Bignall found that it shows an annual cycle in its pattern of
scintillation. This had previously been seen in two other quasars,
J1819+3845 and B 0917+624.
But PKS1257-326 is also the first quasar to show a time delay between
the variability recorded at the two frequencies studied (4.8 and 8.6 GHz).
“This is almost certainly because there is a displacement between the
emission at the two frequencies. At the higher frequency we are looking
further down the throat of the radio jet,” says Dr Macquart.
“In angular terms, the offset is about ten microarcseconds. We are
mapping source structure with microarcsecond resolution. We can even
look for changes as matter strays near the black hole and is spat out
along the jets, ” he says.
The signals from PKS1257-326 vary in strength by up to 40% in as little
as 45 minutes. That means that the turbulent gas scattering the signal
lies quite close to Earth – only about 50 light-years away.
An extensive survey for other fast scintillating quasars has turned up
very few. “This suggests that there aren’t many of these ‘scattering
screens’ nearby in the Galaxy,” says CSIRO’s Dr Jim Lovell, a member of
the research team who is also doing a major new search for fast
scintillators.
Images:
http://www.atnf.csiro.au/news/press/images/gascloud/
Publication details
H.E. Bignall, D.L. Jauncey, J.E.J. Lovell, A.K. Tzioumis,
L.Kedziora-Chudczer, J.-P. Macquart, S.J.Tingay, D.P. Rayner and
R.W.Clay. “Rapid Time Variability and Annual Cycles in the
Characteristic Time-Scale of the Scintillating Source PKS 1257-326.”
Astrophysical Journal, March 10, 2003.
The paper can also be downloaded from
http://xxx.lanl.gov/abs/astro-ph/0211451
Researcher contacts:
Ms Hayley Bignall
Joint Institute for VLBI in Europe (in Dwingeloo, The Netherlands)
+31-521-596-520 (office)
bignall@jive.nl
Dr David Jauncey
CSIRO Australia Telescope National Facility (in Canberra)
+61-2-6125-0269 (office),
+61-2-6281-1936 (home)
David.Jauncey@csiro.au
Dr Jean-Pierre Macquart, Kapteyn Institute, University of Groningen,
The Netherlands
+31-50-363-4090 (office)
+31 50 316 8857 (home)
jpm@astro.rug.nl
Dr Jim Lovell
CSIRO Australia Telescope National Facility
+61-2-6125-6715 (office)
+61-2-6287-5527 (home)
or 0412 127 364 (mobile)
Jim.Lovell@csiro.au
