Synchronized Stars Power Cosmic Radio Laser

Researchers have helped unravel the mystery of why certain pairs of stars pulse with regular, long-period bursts of radio waves. The particular class of objects they observed come in pairs that always include a compact dead star, called a white dwarf, locked in orbit with an M dwarf, a red star smaller than our Sun.

In recent years, astronomers have been puzzled by the fact that a handful of these stellar pairs pulse with radio waves every few minutes over spans of hours. These minute-long gaps between the bursts are much longer than the rapid radio pulses emanating from spinning dead stars, called pulsars, which repeat within mere seconds.

Scientists suspected that the long-period radio bursts were somehow tied to the synchronized motion of the white dwarf–M dwarf binary stars, but questions remained about how the process worked. Now, using supercomputer simulations, Caltech researchers have laid out a clear picture for how the interacting white dwarf and M dwarf binaries power beams of intense radio light that shoot into space.

"The white dwarf binaries can act like scaled-up planetary radio engines—orbital motion through a strong magnetic field can power bright coherent radio bursts," says Yici Zhong, a Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech from 2024 to July of 2026 and the lead author of a paper on the findings published in The Astrophysical Journal Letters. Zhong works in the group of the study's principal investigator, Elias Most, an assistant professor of theoretical astrophysics and a William H. Hurt Scholar at Caltech.

The team's supercomputer simulations specifically detail how a known mechanism called electron cyclotron maser instability (ECMI) causes the radio bursts. ECMI occurs throughout universe, around stars and planets, and even in Earth's auroras. It involves electrons spiraling through magnetic fields in such a way that produces radio emission. ECMI is most famously behind intense radio bursts that have been observed between Jupiter and its moon Io beginning in 1955.

In 1969, Caltech's Peter Goldreich, the Lee A. DuBridge Professor of Astrophysics and Planetary Physics, Emeritus, and the late Donald Lynden-Bell, formerly of University of Cambridge, solved a piece of the Jupiter–Io puzzle, proposing that a powerful current is generated between the two bodies as Io sweeps through Jupiter's magnetic field. The researchers argued that the orbital motion resulted in a million-ampere tube-like flow of electric current flowing between Io and Jupiter's magnetosphere, a prediction later confirmed by direct satellite imaging. Other researchers would later demonstrate how these electrical currents power the radio waves.

The new study from Zhong and Most shows how this same mechanism powers radio bursts in the orbiting white dwarf and M dwarf pairs. In these systems, the white dwarf and M dwarf are roughly the same mass, but the white dwarf—a burnt-out cinder leftover from a dying star that was once like our Sun—is smaller than the M dwarf. A handful of these systems are known to produce the long-duration radio bursts; in fact, one of them, called GLEAM-X J0704-37, was confirmed by alum Antonio Rodriguez (PhD '25) to be powered by such a system.

As the stellar duo circle around each other once every two hours, a powerful current is generated to fuel the ECMI, as with Jupiter and Io. Electrons in the current become unstable relative to the ECMI and a "laser" of radio beams, also referred to as a maser, is produced.

"The maser is always on," Zhong explains. "We just see it when it comes around, hence the observed pulses."

Most adds, "The electrons become collectively unstable and start dancing around magnetic field lines in unison like a Viennese waltz."

The researchers say their numerical simulations gave them a more complete look at what happens between the stars than what was possible before. The simulations predicted that the radio emission would be polarized like the glare of light reflected off a lake. The simulations also showed that the ECMI mechanism can be up to 10 times more efficient at producing radio signals than previously believed.

"The results confirm that Peter Goldreich's and Donald Lynden-Bell's theory about Jupiter and Io is applicable beyond planets in our solar system," Most says. "And we show that the mechanism is 10 times more efficient than was previously thought."

The paper titled "Unraveling the Emission Mechanism Powering Long Period Radio Transients from Interacting White Dwarf Binaries via Kinetic Plasma Simulations" was partially supported by the US National Science Foundation.