For more than two decades, astronomers have been puzzled by the striking pattern of bright, evenly spaced stripes in radio waves from club pulsars, the dense remnants of supernovae recorded by Chinese and Japanese astronomers in 1054.
In 2024, theoretical astrophysicists at the University of Kansas proposed a solution that explains much of this unusual “zebra” pattern. Now, through sophisticated analysis, he has identified gravitational lensing as the final missing ingredient needed to fully explain the phenomenon.
“Gravity changes the shape of space-time,” said Mikhail Medvedev, a professor of physics and astronomy at the University, who will present his findings at the American Physical Society’s 2026 World Physics Summit, to be held from March 15 to 20 at the Colorado Convention Center in Denver.
A related paper has been accepted into the peer-reviewed Journal of Plasma Physics and is currently available on the preprint site arXiv.
“Light does not travel straight in a gravitational field because space itself is curved,” he said. “What would be straight in a flat space-time becomes curved in the presence of strong gravity. In this sense, gravity acts as a lens in a curved space-time.”
A unique cosmic tug of war created by gravity and plasma
Gravitational lensing is well known in black hole research, but Medvedev said this is the first time that gravity and plasma have been observed to work together to form a signal that can be detected from space.
“In images of black holes, gravity is the only thing that shapes the structure,” he says. “In the club pulsar, both gravity and plasma act together. This is the first practical application of this combined effect.”
The Crab pulsar is located at the center of the Crab Nebula in the Perseus arm of the Milky Way, about 6,500 light-years from Earth. Its relatively close distance and clear visibility make it an important object for studying neutron stars, supernova remnants, and nebulae.
Strange signal different from other pulsars
Medvedev describes the pulsar signal as extremely rare. Instead of a continuous spectrum that spreads smoothly across all colors like sunlight, club pulsars produce clearly separated bands.
“There is a remarkable pattern in the pulsar’s spectrum,” Medvedev said. “Unlike a normal broad spectrum, such as sunlight, which contains a continuous color range, the crab’s high-frequency interpulses exhibit discrete spectral bands. If it were a rainbow, it would be as if only certain ‘colors’ appeared, and nothing in between.”
Most pulsars emit radio waves that are noisy and spread across a variety of frequencies. Club pulsars stand out with distinct stripes separated by complete darkness.
“The stripes are perfectly clear, and between the stripes there is complete darkness,” Medvedev said. “There are bright bands and no bright bands, and there are no bright bands at all. No other pulsar exhibits this type of banding. That uniqueness made club pulsars particularly interesting and challenging to understand.”
Gravity provides the missing piece.
Previous versions of Medvedev’s model were able to reproduce the striped pattern, but were unable to match the strong contrast seen in real observations. His research showed that the plasma around pulsars bends and spreads electromagnetic waves through diffraction, helping to form patterns.
We have now accounted for the missing contrast by adding Einstein’s theory of gravity to the model.
“Previous theoretical models were able to reproduce the stripes, but not the observed contrast. Incorporating gravity provided the missing piece,” Medvedev said. “Plasma in a pulsar’s magnetosphere can be thought of as a lens, but it’s a defocused lens. In contrast, gravity acts as a focusing lens. Plasma tends to scatter light rays, and gravity pulls them inward. When these two effects overlap, there are certain paths that compensate for each other.”
Interference fringes create zebra stripes
The interaction between the plasma and gravity creates multiple paths for the pulsar’s radio waves. When these paths align, the waves strengthen and cancel each other, forming a pattern of light and dark bands.
The KU researchers said the combination of defocused magnetospheric plasma and focused gravity creates in-phase and out-of-phase interference bands of radio intensity, which appear as zebra stripes on club pulsars.
“Due to symmetry, there are at least two such paths for light,” he said. “When two nearly identical paths bring light to an observer, they form an interferometer. The signals combine. At some frequencies, they reinforce each other (in phase), producing a bright band. At other frequencies, they cancel (out of phase), producing darkness. This is the essence of an interference pattern.”
New tools to study neutron stars
Medvedev believes that while the core mechanism behind zebra stripes is now mostly understood, further refinement could improve accuracy.
“It seems that little additional physics is needed to explain the stripes qualitatively,” Medvedev said. “Quantitative improvements could be made. For example, current treatments include gravity in a static lowest approximation. The pulsar is rotating, and including rotational effects may result in quantitative changes, but not qualitative changes.”
This new model could provide scientists with a powerful way to study rotating gravitational systems and better understand pulsars, which are usually difficult to visualize directly. It could also help map how matter is distributed around neutron stars, and could even provide clues about the internal structure of neutron stars through gravitational effects.
