China Focus: Chinese astronomers unveil birthplace environment of mysterious cosmic burst

BEIJING, Jan. 31 (Xinhua) -- Through a systematic, multi-year monitoring with China's Five-hundred-meter Aperture Spherical radio Telescope (FAST), Chinese scientists have, for the first time, witnessed the year-to-year evolution of a fast radio burst (FRB), revealing that it originates in a young supernova remnant.

This discovery, made by a research team composed of scientists from Central China Normal University, Tsinghua University, Yunnan University and the National Astronomical Observatories of the Chinese Academy of Sciences, was recently published in the academic journal Science Bulletin.

Li Di, a chair professor at Tsinghua University and former chief scientist of FAST, said that FRBs, first discovered by humanity in 2007, are the most intense and extremely frequent mysterious bursts in the radio frequency band of the universe. Unraveling the origin and physical mechanisms of FRBs is a research frontier, holding opportunities for major breakthroughs in astronomy and physics.

These millisecond-long eruptions, emanating from the distant cosmos, can release in an instant the energy the Sun radiates in an entire year. Most FRBs are seen once and fall silent, or exhibit long dormancy, severely hampering any study of their native environments, Li explained.

In 2019, Li Di's team used FAST to discover the world's first persistently active FRB, coded FRB 20190520B, which remains active without interruption. Subsequent coordinated observations using multiple international ground and space-based facilities have pinpointed its "home" to a metal-poor dwarf galaxy located about 3 billion light-years away.

Over the ensuing four years, a team led by Niu Chenhui, a professor at Central China Normal University, conducted follow-up observations of FRB 20190520B using FAST, confirming that it is the only FRB known to remain active across multiple years. This unprecedented stable activity provides a valuable "natural laboratory" for studying the evolution and origin of FRBs.

The researchers systematically analyzed over 400 bursts from this source and found that a key parameter, the Dispersion Measure (DM), of FRB 20190520B exhibited a steady and unusually rapid decrease over the four years, indicating that the FRB signals are traveling through a dense environment that is expanding and thinning over time.

The observations strongly imply that the burst engine is a young magnetar embedded within a supernova remnant. As the supernova shell expands, the plasma density decreases, naturally leading to the observed sustained decline in DM.

The team's model indicates that the observed signals originate from a remnant only 10 to 100 years old after the supernova explosion, making it an "extremely young" remnant on cosmic timescales.

"Young magnetars within supernova remnants have long been considered candidates for FRB origins, yet direct evidence was lacking," said Li. "Our work captures, for the first time, the systematic evolution of an FRB across years, not only confirming its supernova origin but also opening a new pathway to infer the cradle of such bursts through long-term monitoring."

The scientists predict that long-term DM decreases should be a common trait for young, active repeating FRBs. FAST's unmatched sensitivity gives it a singular advantage in detecting weak, persistent signals, positioning it as the cornerstone for testing this prediction and systematically unveiling where FRBs are born.

"This work provides the most compelling evidence to date that the birth sites of some FRBs are indeed the death sites of their massive parent star. The new result provides an exciting hint at the mysterious origin of at least a sub-class of repeating fast radio bursts," said Liam Connor, a professor at Harvard University.

Located in a naturally deep and round karst depression in southwest China's Guizhou Province, FAST has a reception area equal to 30 standard football fields. As the world's largest single-dish radio telescope, FAST started formal operations in January 2020 and was officially opened to the world in March 2021. ■