Fast radio bursts are powerful millisecond flashes of radio emission that arrive from distant galaxies and typically show nearly 100 percent linear polarisation. Because the polarised waves pass through magnetised plasma on their journey, their polarisation angle rotates with radio frequency, an effect known as Faraday rotation that is quantified by the rotation measure. By tracking how this measure changes over time, astronomers can probe conditions in the immediate vicinity of the burst source.

Using the Five-hundred-meter Aperture Spherical Telescope in Guizhou, widely known as the China Sky Eye, researchers monitored a repeating source designated FRB 220529A for roughly 17 months. For most of that period the source appeared relatively unremarkable, emitting bursts whose polarisation and rotation measure remained stable within expected ranges for an active repeater.

Near the end of 2023, the data set revealed an abrupt rise in the rotation measure by more than two orders of magnitude, followed by a rapid decline over about two weeks back to its previous level. The team describes this short-lived magnetic excursion as an RM flare, a brief episode in which the Faraday rotation suddenly spikes before recovering. Such behaviour points to a dense, magnetised plasma clump crossing the line of sight between the source and Earth.

The scientists interpret this transient plasma screen as the product of a coronal mass ejection launched from a companion star in a binary system hosting the fast radio burst engine. In this scenario, a magnetised cloud from the companion moves across the narrow beam from the fast radio burst source, sharply boosting the rotation measure until the cloud disperses or moves clear. The inferred properties of the plasma are consistent with coronal mass ejections from the Sun and other stars in the Milky Way.

Although the companion star itself cannot be directly resolved at such a large distance, its influence becomes visible through meticulous, long term monitoring with the China Sky Eye and complementary observations from Australia's Parkes radio telescope. The binary interpretation provides a natural explanation for the timescale and amplitude of the RM flare without invoking exotic environments or rare intervening structures. It also links fast radio bursts more closely to known stellar processes.

The study supports a unified physical picture in which all fast radio bursts arise from highly magnetised neutron stars known as magnetars. In this framework, binary companions help shape the surrounding plasma and geometry so that some magnetars are more likely to produce frequently repeating radio bursts. Binary interactions may alter how radiation escapes, modulate activity, or occasionally inject additional material that temporarily changes the magnetic conditions, as seen in the RM flare.

The China Sky Eye fast radio burst Key Science Programme, co led by researchers at The University of Hong Kong and collaborating Chinese institutions, has been monitoring repeating sources since 2020 to search for such subtle environmental changes. FRB 220529A emerged as a key target because of its sustained activity and the ability to revisit it over many months, allowing rare transient signatures like the RM flare to be identified with confidence.

The discovery relied on extensive observing campaigns and coordination across facilities, including dedicated key science time on the China Sky Eye, a director's discretionary time program, and principal investigator projects on both the Chinese and Australian telescopes. The collaboration involves scientists from The University of Hong Kong, Purple Mountain Observatory, Yunnan University, the National Astronomical Observatories of the Chinese Academy of Sciences and other partners, bringing together expertise in observation, theory and data analysis.

According to the team, continued long term monitoring of repeating fast radio bursts will be crucial for determining how common binary environments are among these mysterious sources. As more RM flares or related signatures are detected, astronomers expect to refine models of magnetar powered bursts, constrain the range of binary configurations and better understand how stellar activity and plasma structures shape the observed signals. The new result marks a major step toward linking fast radio bursts to concrete astrophysical systems rather than treating them as isolated cosmological flashes.

Research Report: A sudden change and recovery in the magnetic environment around a repeating fast radio burst

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