Nearly 15 years after the discovery of fast radio bursts (FRBs), the origin of the millisecond-long cosmic explosion in deep space remains a mystery.
That may soon change, thanks to the work of an international team of scientists including UNLV astrophysicist Bing Zhang, who tracked hundreds of outbursts from five different sources and found them at the FRB pole. Clues that may shed light on its origin have been found in the pattern of chemistry.The team’s findings were published in the March 17 issue of the journal science.
FRBs generate 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 polarization direction. By analyzing the polarization frequencies of FRBs observed from various sources, the scientists revealed similarities in repeating FRBs that point to complex environments near the source of the burst.
“This is an important step in understanding the physical origin of FRBs,” said Zhang, UNLV Distinguished Professor of Astrophysics, who co-authored the paper and contributed to a theoretical explanation of the phenomenon.
To link these outbursts, an international team of researchers led by Yi Feng and Li Di of the National Astronomical Observatory of the Chinese Academy of Sciences analyzed the polarization properties of five repeating FRB sources using a huge five-hundred-meter aperture Spherical Radio Telescope (FAST) and Robert C. . Byrd Green Bank Telescope (GBT). Since FRBs were first discovered in 2007, astronomers around the world have turned to powerful radio telescopes like FAST and GBT to track the bursts and find clues about where they came from and how they were created.
Although still considered mysterious, it is widely believed that the source of most FRBs is magnetars, extremely dense city-sized neutron stars with the strongest magnetic fields in the universe. They usually have nearly 100% polarization. In contrast, in many astrophysical sources involving hot random plasmas, such as the Sun and other stars, the observed emission is unpolarized because the oscillating electric fields have random directions.
This is where the work of cosmic detectives begins.
In a study the team originally published last year nature, FAST detected 1,652 pulses from the active repeater FRB 121102. Even though the bursts from the source were found 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 world’s largest single-disk radio telescope.
“We are very confused by the lack of polarization,” said Feng, lead author of the new release science Paper. “Later, when we systematically studied other repeating FRBs with other telescopes in different frequency bands — especially those above FAST, a unified picture emerged.”
According to Zhang, the unifying picture is that each repeating FRB source is surrounded by a highly magnetized dense plasma. Such plasmas produce different rotations of polarization angles as a function of frequency, and the received radio waves come from multiple paths due to the scattering of the waves by the plasma.
When the team considered only one tunable parameter, Zhang said, multiple observations revealed the frequency evolution of the system, the depolarization toward lower frequencies.
“Such a simple explanation, with only one free parameter, may represent an important step in the physical understanding of the origin of repeating FRBs,” he said.
Li Di, corresponding author of the study, agrees that the analysis could represent one corner of the FRB cosmic puzzle. “Extremely active FRBs, for example, may be a distinct group,” he said. “Alternatively, we’re starting to see an evolutionary trend toward FRBs, where more active sources in more complex environments are younger blasts.”
The study, “Frequency-dependent polarization of repeating fast radio bursts – implications for their origin,” was published March 17 in the journal science. It includes 25 co-authors from 11 institutions and is part of a long-term collaboration between institutions. In addition to UNLV and NAOC, partner institutions include Yunnan University, Princeton University, Western Sydney University, Peking University and the Green Bank Observatory.