A theoretical astrophysicist from the University of Kansas may have unraveled a nearly 20-year-old enigma concerning the unusual “zebra” pattern observed in high-frequency radio emissions from the Crab Nebula.
His research has recently been published in Physical Review Letters (PRL).
At the heart of the Crab Nebula lies a neutron star, which has become a 12-mile-wide pulsar, emitting beams of electromagnetic radiation throughout the universe.
“The emission resembles a lighthouse beam, continually sweeping past Earth with each rotation of the star,” explained lead author Mikhail Medvedev, a professor of physics and astronomy at KU. “We’re observing this as pulsed emissions, which typically consist of one or two pulses during each rotation. The pulsar I’m discussing is known as the Crab Pulsar, situated at the center of the Crab Nebula, which is located 6,000 light-years away.”
The Crab Nebula is the remnant of a supernova that was witnessed in 1054.
“Historical accounts, including those from China, describe the sudden appearance of a remarkably bright star in the sky,” the KU researcher mentioned.
However, unlike any other identified pulsar, Medvedev noted that the Crab Pulsar displays a distinctive zebra pattern characterized by unusual band spacing in the electromagnetic spectrum, which correlates with the band frequencies, along with other peculiar traits such as high polarization and stability.
“It’s exceptionally bright across almost all wave bands,” he stated. “This is the only known object that generates a zebra pattern, and it appears solely in a single component of the Crab Pulsar’s emissions. The primary pulse is a broadband pulse, typical of most pulsars, along with other broadband components common to neutron stars. However, the high-frequency interpulse is singular, ranging from 5 to 30 gigahertz—frequencies akin to those of a microwave oven.”
Since the discovery of this pattern in a 2007 study, Medvedev noted it has remained “puzzling” for researchers.
“Various emission mechanisms have been proposed, but none have successfully explained the observed patterns,” he added.
Utilizing data from the Crab Pulsar, Medvedev developed a method employing wave optics to assess the density of the pulsar’s plasma—the “gas” of charged particles (electrons and positrons)—by examining the fringe patterns found in the electromagnetic emissions.
“When an electromagnetic wave interacts with a surface, it doesn’t travel straight through,” Medvedev clarified. “In geometrical optics, shadows caused by obstacles extend indefinitely; if you’re in the shadow, no light reaches you, but outside that area, light is visible. However, wave optics changes this behavior—waves bend around obstacles and interfere, creating a sequence of bright and dark fringes due to constructive and destructive interference.”
This well-known fringe pattern effect originates from consistent constructive interference but exhibits different characteristics when radio waves navigate around a neutron star.
“A typical diffraction pattern would yield evenly spaced fringes if we solely had a neutron star as an obstacle,” the KU researcher explained. “However, the neutron star’s magnetic field generates charged particles creating a dense plasma that varies in density from the star’s surface. As a radio wave travels through the plasma, it moves through less dense areas but is bounced off denser plasma. This reflection varies with frequency—low frequencies reflect at larger distances, creating a more extensive shadow, whereas high frequencies cast smaller shadows, hence the different fringe spacings.”
In this way, Medvedev concluded that the plasma surrounding the Crab Pulsar causes the diffraction in the electromagnetic pulses that are responsible for the neutron star’s unique zebra pattern.
“This model is the first capable of determining those parameters,” Medvedev stated. “By analyzing the fringes, we can infer the density and distribution of plasma in the magnetosphere. This is astounding because these observations enable us to translate fringe measurements into a density distribution of the plasma, essentially allowing us to create an image or perform tomography of the neutron star’s magnetosphere.”
Looking ahead, Medvedev mentioned that his theory could be tested with additional data collected from the Crab Pulsar, along with refinements to account for its intense and unique gravitational and polarization effects. This new insight into how plasma affects a pulsar’s signal could reshape astrophysicists’ understanding of other pulsar phenomena.
“The Crab Pulsar is relatively unique—it’s fairly young by astronomical standards, at about a thousand years old, and remarkably energetic,” he explained. “Yet it’s not alone; we are aware of hundreds of pulsars, including over a dozen that are also young. Known binary pulsars, which have served in testing Einstein’s theory of general relativity, could also be explored using this proposed method. This research could indeed expand our understanding and observational techniques for pulsars, particularly those that are young and highly energetic.”
