New research from the University of Kansas untangles a decades-old astrophysical puzzle, showing how competing forces — gravity’s pull and magnetospheric plasma — split the radio emissions emanating from the Crab Pulsar, the remnant of a supernova observed by ancient astronomers in 1054 CE, into perfectly spaced ‘stripes.’
This composite image shows the Crab Nebula. The Crab pulsar is in the center of the image. Image credit: X-ray – NASA / CXC / ASU / J. Hester et al.; optical – NASA / HST / ASU / J. Hester et al.
In the year 1054 CE, Chinese astronomers were startled by the appearance of a new star, so bright that it was the brightest object in the night sky, second only to the Moon, and was visible in broad daylight for 23 days. The stellar explosion was also recorded by Japanese, Arabic, and Native American stargazers.
Today, the Crab Nebula is visible at the site of that bright star. Also known as Messier 1, M1, NGC 1952 and Taurus A, it lies approximately 6,500 light-years away in the constellation of Taurus.
The Crab Nebula was first identified in 1731 by English doctor, electrical researcher and astronomer John Bevis and was rediscovered in 1758 by French astronomer Charles Messier. It derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844.
The Crab Pulsar, also known as PSR B0531+21, is the central star in the Crab Nebula.
Because it’s nearby and easily observed, study of the Crab Nebula and Crab Pulsar gives astronomers insight into nebulae, supernovae and neutron stars in general.
“Gravity changes the shape of spacetime,” said University of Kansas Professor Mikhail Medvedev, author of the new study.
“Light doesn’t travel in a straight line in a gravitational field because space itself is curved,” he said.
“What would be straight in flat spacetime becomes curved in the presence of strong gravity. In that sense, gravity acts as a lens in curved spacetime.”
While gravitational lensing has been discussed extensively in the context of black holes, this is the only case where astronomers see a ‘tug-of-war’ between plasma and gravity shaping the observed signal.
“In black hole images, gravity alone shapes the structure,” Professor Medvedev said.
“In the Crab Pulsar, both gravity and plasma act together. This represents the first real-world application of this combined effect.”
“There’s a remarkable pattern in Pulsar’s spectrum,” Professor Medvedev said.
“Unlike ordinary broad spectra — such as sunlight, which contains a continuous range of colors — the Crab’s high-frequency inter-pulse shows discrete spectral bands. If it were a rainbow, it’s as if only specific ‘colors’ appear, with nothing in between.”
This is a mosaic image, one of the largest ever taken by Hubble of the Crab Nebula, a 6-light-year-wide expanding remnant of a star’s supernova explosion. Japanese and Chinese astronomers recorded this violent event nearly 1,000 years ago in 1054, as did, almost certainly, Native Americans. Image credit: NASA / ESA / J. Hester / A. Loll, Arizona State University.
Most pulsar radio emissions are spectrally broader and noisy, not banded so cleanly like the Crab Pulsar.
“The stripes are absolutely distinct with complete darkness between them,” Professor Medvedev said.
“There’s a bright band, then nothing, bright band, nothing. No other pulsar shows this kind of striation. That uniqueness made the Crab Pulsar especially interesting — and challenging — to understand.”
While earlier model could reproduce stripes, the high contrast of the bands actually observed in the Crab Pulsar couldn’t be accounted for.
Indeed, Professor Medvedev recently determined the Crab Pulsar’s plasma matter causes diffraction in the electromagnetic pulses largely responsible for the neutron star’s singular zebra pattern.
But now he has factored in Einstein’s theory of gravity into the mix, finding it plays a pivotal role in the Crab Pulsar’s zebra pattern.
“The previous theoretical model could reproduce stripes, but not with the observed contrast. The inclusion of gravity provides the missing piece,” Professor Medvedev said.
“The plasma in the pulsar’s magnetosphere can be thought of as a lens — but a defocusing lens. Gravity, by contrast, acts as a focusing lens. Plasma tends to spread light rays apart; gravity pulls them inward. When these two effects are superimposed, there are specific paths where they compensate each other.”
The combination of a defocusing magnetospheric plasma and a focusing gravity create in-phase and out-of-phase interference bands of radio-wave intensity that appear as the Crab Pulsar’s zebra stripes.
“By symmetry, there are at least two such paths for the light,” Professor Medvedev said.
“When two nearly identical paths bring light to the observer, they form an interferometer. The signals combine. At some frequencies, they reinforce each other (in phase), producing bright bands. At others, they cancel (out of phase), producing darkness. That is the essence of the interference pattern.”
“There appears to be little additional physics required to explain the stripes qualitatively.”
“Quantitatively, there may be refinements. For example, the current treatment includes gravity in a static, lowest-order approximation.”
“The pulsar is rotating, and including rotational effects could introduce quantitative changes, though not qualitative ones.”
The new study will be published in the Journal of Plasma Physics.
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Mikhail V. Medvedev. 2026. Theory of striped dynamic spectra of the Crab pulsar high-frequency interpulse. Journal of Plasma Physics, in press; arXiv: 2602.16955
