Crab Pulsar's 'zebra' signal mystery cracked by scientist
The Crab Pulsar, a remnant of a supernova whose burst was visible from Earth in 1054, emits a mysterious radio signal known as the "zebra" pattern. This signal has long intrigued scientists, and it seems they have finally solved its mystery. At least, that's what Mikhail Medvedev from the University of Kansas claims, sharing his explanations in "Physical Review Letters."
12:12 PM EST, December 1, 2024
The chart of the mysterious signal shows bands resembling the zigzag stripes of a zebra. Since the discovery of this phenomenon in 2007, scientists have been trying to understand it, especially since this signal is unique. Nothing else in space emits a similar signal. The latest explanation, proposed by Mikhail Medvedev, an astrophysicist from the University of Kansas, suggests that the "zebra" pattern is an interference effect caused by the diffraction of light on varying plasma densities in the pulsar's magnetosphere.
Mysterious signal from space
"If you have a screen and an electromagnetic wave passes by, the wave doesn’t propagate straight through," explained Medvedev in an interview with ScienceAlert. "In geometrical optics, shadows cast by obstacles would extend indefinitely -- if you're in the shadow, there's no light; outside of it, you see light. But wave optics introduces a different behavior -- waves bend around obstacles and interfere with each other, creating a sequence of bright and dim fringes due to constructive and destructive interference," he said.
The Crab Pulsar is a neutron star formed from the core collapse of a massive star following a supernova explosion. It is located about 20,300 light-years from Earth. It rotates at a speed of about 30 times per second, emitting beams of radio waves resembling a lighthouse's light. Medvedev used extensive observational data of the Crab Pulsar to develop a model based on wave optics. His theory accurately replicates the observed "zebra" pattern, suggesting it is the result of the interaction of radio waves with the plasma and the magnetic field of the pulsar.
"A typical diffraction pattern would produce evenly spaced fringes if we just had a neutron star as a shield," Medvedev stated. "But here, the neutron star's magnetic field generates charged particles constituting a dense plasma, which varies with distance from the star. As a radio wave propagates through the plasma, it passes through dilute areas but is reflected by dense plasma. This reflection varies by frequency: Low frequencies reflect at large radii, casting a bigger shadow, while high frequencies create smaller shadows, resulting in different fringe spacing," Medvedev explained in the interview with ScienceAlert.
The service notes that Medvedev's model can serve as a new tool for measuring plasma density inside the magnetospheres of pulsars and other extreme environments where diffraction patterns occur. Although the Crab Pulsar is unique, there are other places and ways in which this model can be applied.
"The Crab Pulsar is somewhat unique -- it's relatively young by astronomical standards, only about a thousand years old, and highly energetic," Medvedev explained, adding: "But it's not alone; we know of hundreds of pulsars, with over a dozen that are also young. Known binary pulsars, which were used to test Einstein's general relativity theory, can also be explored with the proposed method. This research can indeed broaden our understanding and observation techniques for pulsars, particularly young, energetic ones."
