New study by international team of scientists identifies polarization as
key trait that may reveal the origin of the powerful millisecond-long cosmic radio explosions

Date:
March 19, 2022
Source:
University of Nevada, Las Vegas
Summary:
Nearly 15 years after the discovery of fast radio bursts (FRBs),
the origin of the millisecond-long, deep-space cosmic explosions
remains a mystery. That may soon change, thanks to the work of
an international team of scientists which tracked hundreds of
the bursts from five different sources and found clues in FRB
polarization patterns that may reveal their origin.



FULL STORY ========================================================================== Nearly 15 years after the discovery of fast radio bursts (FRBs), the
origin of the millisecond-long, deep-space cosmic explosions remains
a mystery.


==========================================================================
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.


========================================================================== Story Source: Materials provided by
University_of_Nevada,_Las_Vegas. Original written by Tony Allen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yi Feng, Di Li, Yuan-Pei Yang, Yongkun Zhang, Weiwei Zhu, Bing
Zhang,
Wenbin Lu, Pei Wang, Shi Dai, Ryan S. Lynch, Jumei Yao, Jinchen
Jiang, Jiarui Niu, Dejiang Zhou, Heng Xu, Chenchen Miao, Chenhui
Niu, Lingqi Meng, Lei Qian, Chao-Wei Tsai, Bojun Wang, Mengyao Xue,
Youling Yue, Mao Yuan, Songbo Zhang, Lei Zhang. Frequency-dependent
polarization of repeating fast radio bursts--implications for their
origin. Science, 2022; 375 (6586): 1266 DOI: 10.1126/science.abl7759 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220319081645.htm

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