Two CSIRO scientists working in an international team have helped show that a mysterious source of cosmic radio bursts lies in an extreme environment, perhaps in close proximity to a massive black hole, or within a nebula of unprecedented power.
The results have been presented at the American Astronomical Society's winter meeting in Washington, DC, and appear on the cover of the journal Nature on 11 January.
Dr George Heald (CSIRO) and Dr Charlotte Sobey (CSIRO and the Curtin University node of the International Centre for Radio Astronomy Research) were called on to explain unusual properties of a ‘fast radio burst’ source called FRB121102.
Fast radio bursts (FRBs) last for just milliseconds and come from deep space. They were first discovered with CSIRO’s Parkes radio telescope in 2007 and about 25 are now known. Their physical origin remains a mystery.
FRB121102 is the only burst that has been seen to repeat. Being able to observe it more than once let astronomers pinpoint its home – a dwarf galaxy three billion light-years away – last year.
In this new research, an international team led by Daniele Michilli and Dr Jason Hessels (ASTRON & University of Amsterdam) and Dr Andrew Seymour (Arecibo Observatory) used two of the world’s largest radio telescopes – the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia – to show that the radio bursts from FRB121102 are strongly polarised.
Polarisation is a property of light and radio waves. When polarised light travels through a plasma with a magnetic field it becomes ‘twisted’. Measuring the degree of twist shows us how strong the magnetic field is.
The CSIRO scientists are experts in interpreting these kinds of observations.
The amount of twisting observed in FRB 121102’s radio bursts is among the largest ever measured in a radio source.
“Detecting the bursts at higher radio frequencies than ever before has enabled us to measure the immense amount of twisting imparted on the source's considerably polarised light by the extreme environment within its galaxy,” Dr Sobey said.
The researchers conclude that the bursts are passing through an exceptionally strong magnetic field in a dense plasma (a hot, ionised gas).
“The only known sources in our galaxy that are twisted as much as FRB 121102 are in the Galactic Centre, which is a dynamic region near a massive black hole. Maybe FRB 121102 is in a similar environment in its host galaxy,” said Daniele Michilli, PhD candidate at the University of Amsterdam and ASTRON, the Netherlands Institute for Radio Astronomy.
“However, the twisting of the radio bursts could also be explained if the source is located in a powerful nebula or supernova remnant,” he said.
“Despite the immense amount of twisting, we found it remarkable that the measurements were surprisingly 'clean' - that is, not complex," Dr Head said. "Still, the amount of twisting seems to vary by at least ten per cent over a half-year timescale."
Further monitoring of FRB 121102 should help determine if the source is indeed living dangerously near a black hole.
With a number of widefield radio telescopes now coming online, more FRBs are expected to be discovered in the coming year, and astronomers are poised to answer more fundamental questions about their nature and origin.
‘An extreme magneto-ionic environment associated with fast radio burst source FRB121102', Nature 553, 182–185 (11 January 2018) doi:10.1038/nature25149