An international team of scientists has found a way to use floating clouds of gas in space as natural lenses to probe distant black holes and mysterious objects called quasars.
The thin clouds of electrically charged particles -- called the invisible atmosphere of galaxies -- are enabling the scientists to make observations in much greater detail than is possible with any existing Earth-based or orbiting instruments.
The observations are based on data, first analyzed by Australia's Commonwealth Scientific and Industrial Research Organization, that found the gaseous particle clouds to be rather lumpy. This property permits them to act like lenses that focus and unfocus radio waves from distant objects, making them appear to strengthen and weaken. The effect resembles the way variations in Earth's atmosphere cause distant stars to twinkle.
Because the distances involved are so great, however, the twinkling -- or scintillation, as astronomers call it -- occurs more slowly than with stars. Any change that happens in less than a day is considered fast, and the fastest scintillators emit radio signals that can double or triple in strength in less than an hour.
The frequency of scintillation of a signal depends on several factors: the size and shape of its source; the size and structure, speed and direction of the gas clouds, and Earth's speed and direction as it travels around the sun.
The scientists found that by observing the variability of signals over the course of a year, they could build up detailed images of black holes and quasars -- short for "quasi-stellar objects" -- which remain among the strangest bodies in the universe.
The observational technique, called "Earth-orbit synthesis," can produce images in incredible detail -- about 10 microarcseconds across, or one-third of a billionth of a degree of the night sky, the equivalent of seeing a sugar cube on the moon.
"That's a hundred times finer detail than we can see with any other current technique in astronomy," said Hayley Bignall, a researcher at the Joint Institute for Very Large Baseline Interferometry, in Dwingeloo, The Netherlands, who first compiled the analysis.
"It's 10,000 times better than the Hubble Space Telescope can do," Bignall said, "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," said CSIRO's David Jauncey. "That's the 'business end' where the jet is made."
Bignall has found an annual cycle in the pattern of scintillation of one quasar, with the prosaic name PKS1257-326. This property had been seen earlier in two other quasars, she noted, but PKS1257-326 is the first quasar to show a time delay in the variability recorded at the two radio frequencies she studied.
"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," said team member Jean-Pierre Macquart of the University of Groningen in The Netherlands.
"We are mapping source structure with microarcsecond resolution," Macquart said. "We can even look for changes as matter strays near the black hole and is spat out along the jets."
He added the signals from PKS1257-326 vary in strength by up to 40 percent in as little as 45 minutes. That means the gas clouds scattering the signal lie quite close to Earth -- only about 50 light-years away.
Meanwhile, 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," said CSIRO's Jim Lovell, another team member, who is conducting a search for other fast scintillators.