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December 03, 2002

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The Australia Telescope Compact Array (ATCA), where data about quasar
PKS 0405-385 was gathered.
Copyright CSIRO
High Resolution Version

A team of radio astronomers led by Professor Barney Rickett of UCSD's Jacobs School of Engineering has explained wild fluctuations in radio waves in one of the cosmos' most energetic, mysterious, and puzzling of objects, a distant quasar located about 10 billion light years from earth.

In the process, Rickett and team are also shedding new light on the topography of our astronomical neighborhood, having located a layer of interstellar material starting about 70 light years from earth, perhaps a remnant of a super nova explosion.

The research, to be published December 10 in the Astrophysics Journal, attributes the rapid variations in radio density from quasar PKS 0405-385 to interactions with this thin layer of material. The detected scintillations match precisely the pattern to be expected from such interference.

Initial explanations assumed the fluctuations were due to rapid intrinsic changes in the quasars themselves, and led some astronomers to construct exotic theories, which are now no longer needed.

Rickett and team believe that the quasar appears to scintillate because radio waves it emits interact with the interstellar medium, or ISM, composed of ionized gas particles in the Milky Way, in much the same way starlight is made to twinkle by turbulence in earth's atmosphere. Rickett and team's conclusions are similar to those reached by two other astronomers examining the other quasar observed with a rapidly fluctuating energy signature, J1819+3845, west of the Northern Hemisphere star Vega. Last summer, Ger De Bruyn of the Netherlands Foundation for Research in Astronomy and Jane Dennett-Thorpe of the University of Amsterdam reported that this quasar's observed pattern is due to interactions with the interstellar medium. The research on both of the objects has also led to a re-evaluation of their temperatures and sizes now consistent with the prevailing theory that the energetic jets radiated by quasars are powered by matter falling into massive black holes at the core of distant galaxies.

Rickett, who discovered the phenomena of interstellar scintillation in 1969, is professor of Electrical and Computer Engineering at the Jacobs School. His team included Lucyna Kedziora-Chudczer and David L. Jauncey, both of the Australia National (telescope) Facility. Kedziora-Chudczer and Jauncey recorded the fluctuations from PKS 0405-385 in 1996, at the Compact Array of the Australia Telescope in Northern New South Wales. The data sparked a controversy that threatened to turn quasar theory on its head. Kedziora-Chudczer was shocked to find that the object brightened and faded by 50 percent in less than an hour, much faster than any known quasar.

By focusing on the rapid (intra-day) fluctuations of the radio waves from PKS 0405-385, Rickett and team were able to come up with a better estimate of its size viewed from earth, finding it no greater than 30 micro arc seconds. This angular size is 50 times smaller than can be resolved by the biggest of earth's radio telescopes and implies a linear size at origin of only one light year across.

With this information, Rickett was able to back out a new estimate for the quasar's radiation temperature about 2x10^13 degrees Kelvin, or about three billion times hotter than the sun. That falls just within the bounds that current physics would predict for a quasar but is 10 to 100 times cooler than had been deduced by previous methods.

Another intriguing outcome of research are details about our own region in space. The fluctuations from PKS 0405 385 indicate that there is a turbulent layer of gas starting about 70 light years from earth, perhaps providing evidence to support a theory that the sun rests in "cavity" carved out by a super nova. If this theory is correct, the material disturbing PKS 0405 385's radio signals marks the edge of a "bubble" of particles expelled by the super nova.


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