Radio Signals from the Edge of Extreme Stars

Radio waves could be generated at distances once thought impossible, near the fastest-rotating stars in the universe

To the point

• Astronomers analysed the radio and gamma-ray emission of nearly 200 extremely fast rotating pulsars.

• One-third of these millisecond pulsars show radio signals coming from two or more separate regions. Some of the isolated radio pulses line up perfectly with the emission of gamma-rays.

• The authors suggest that millisecond pulsars produce radio waves not just close to their surfaces, but also in a region far out, where magnetic fields sweep around at nearly the speed of light to keep up with the star’s rotation.

Pulsars are ultra-dense, rapidly spinning, and highly magnetised remnants of dead stars. They act like cosmic lighthouses, sending out regular pulses of radio waves and sometimes gamma rays in beams that sweep across the sky. A special class called millisecond pulsars spins hundreds of times per second and is among the most precise clocks in the Universe. For decades, astronomers believed that a pulsar’s radio signals are only produced close to the star’s surface, near its magnetic poles. The new study, published in the current issue of Monthly Notices of the Royal Astronomical Society, challenges that long-held idea.

An unexpected discovery

Michael Kramer from the Max Planck Institute for Radio Astronomy (MPIfR) in Germany and Simon Johnston from Australia’s national science agency, CSIRO, analysed radio observations of nearly 200 millisecond pulsars and compared them with gamma-ray data. The duo discovered something striking in this large data set: About one-third of millisecond pulsars show radio signals coming from two or more completely separate regions, with emission free gaps in between. In comparison, this behaviour occurs in only about 3% of slower rotating pulsars. Even more striking, many of these isolated radio pulses line up perfectly with gamma-ray flashes detected by NASA’s Fermi satellite — suggesting that both signals are produced in the same extreme region of space.

A surprising conclusion

To explain these patterns, the authors propose that millisecond pulsars produce radio waves in two very different places: one close to the star’s magnetic poles, as traditionally assumed, and another in a swirling “current sheet” just beyond the so-called light cylinder. Located farther out than the magnetic poles, the light cylinder marks the boundary where magnetic fields sweep around at nearly the speed of light to keep up with the star’s rotation. Depending on the observer's perspective on the pulsar, one sees radio emission from either near the surface, from far out, or from both regions. This gives rise to the unusual, broken-up radio profiles that puzzled astronomers for years. The “current sheet” of charged particles is already thought to be responsible for gamma-ray emission. The alignment of radio waves and gamma-rays can be explained through this shared place of origin. 

Exciting prospects and open questions

This discovery has several important consequences: More pulsars may be detectable than previously thought, because radio emission may not be limited to a narrow cone from close to the magnetic poles. Instead, it spreads over a wider range of directions. The finding also helps explain why astronomers often struggle to interpret the polarisation (orientation) of radio waves from millisecond pulsars. Furthermore, it suggests that nearly all gamma-ray millisecond pulsars also emit radio waves, even if those signals may be faint or difficult to detect. This raises new challenges for theory: Scientists now need to explain how stable radio pulses can be generated so far away from the star, in an extreme and turbulent environment.

“Millisecond pulsars are key tools for studying gravity, dense matter, and even gravitational waves. Understanding where their signals come from — and why they look the way they do — is essential for using them as precision instruments”, explains Michael Kramer. Co-author Simon Johnston adds: “This study shows that these tiny, fast-spinning stars are even more complex and surprising than we thought, broadcasting from both their surfaces and from the very edge of their magnetic reach.”