Astronomers closer to unlocking origin of mysterious fast radio bursts

That may soon change, thanks to the work of an international team of scientists -- including UNLV astrophysicist Bing Zhang -- which tracked hundreds of the bursts from five different sources and found clues in FRB polarization patterns that may reveal their origin. The team's findings were reported in the March 17 issue of the journal Science.

FRBs produce electromagnetic radio waves, which are essentially oscillations of electric and magnetic fields in space and time. The direction of the oscillating electric field is described as the direction of polarization. By analyzing the frequency of polarization in FRBs observed from various sources, scientists revealed similarities in repeating FRBs that point to a complex environment near the source of the bursts.

"This is a major step towards understanding the physical origin of FRBs," said Zhang, a UNLV distinguished professor of astrophysics who coauthored the paper and contributed to the theoretical interpretation of the phenomena.

To make the connection between the bursts, an international research team, led by Yi Feng and Di Li of the National Astronomical Observatories of the Chinese Academy of Sciences, analyzed the polarization properties of five repeating FRB sources using the massive Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Robert C. Byrd Green Bank Telescope (GBT). Since FRBs were first discovered in 2007, astronomers worldwide have turned to powerful radio telescopes like FAST and GBT to trace the bursts and to look for clues on where they come from and how they're produced.

Though still considered mysterious, the source of most FRBs is widely believed to be magnetars, incredibly dense, city-sized neutron stars that possess the strongest magnetic fields in the universe. They typically have nearly 100% polarization. Conversely, in many astrophysical sources that involve hot randomized plasmas, such as the Sun and other stars, the observed emission is unpolarized because the oscillating electric fields have random orientations.

That's where the cosmic detective work kicks in.

In a study the team originally published last year in Nature, FAST detected 1,652 pulses from the active repeater FRB 121102. Even though the bursts from the source were discovered to be highly polarized with other telescopes using higher frequencies -- consistent with magnetars -- none of the bursts detected with FAST in its frequency band were polarized, despite FAST being the largest single-dish radio telescope in the world.

"We were very puzzled by the lack of polarization," said Feng, first author on the newly released Science paper. "Later, when we systematically looked into other repeating FRBs with other telescopes in different frequency bands -- particularly those higher than that of FAST, a unified picture emerged."

According to Zhang, the unified picture is that every repeating FRB source is surrounded by a highly magnetized dense plasma. This plasma produces different rotation of the polarization angle as a function of frequency, and the received radio waves come from multiple paths due to scattering of the waves by the plasma.

When the team accounted for just a single adjustable parameter, Zhang says, the multiple observations revealed a systematic frequency evolution, namely depolarization toward lower frequencies.

"Such a simple explanation, with only one free parameter, could represent a major step toward a physical understanding of the origin of repeating FRBs," he says.

Di Li, a corresponding author of the study, agrees that the analysis could represent a corner piece in completing the cosmic puzzle of FRBs. "For example, the extremely active FRBs could be a distinct population," he says. "Alternatively, we're starting to see the evolutionary trend in FRBs, with more active sources in more complex environments being younger explosions."

The study, "Frequency-dependent polarization of repeating fast radio bursts -- implications for their origin," appeared March 17 in the journal Science. It includes 25 co-authors from 11 institutions and is part of long-running collaboration among institutions. In addition to UNLV and NAOC, collaborating institutions also include Yunnan University, Princeton University, Western Sidney University, Peking University and Green Bank Observatory, USA.