Inside an expanding network glowing from a massive explosion, a dead star is transmitting radio light pulses towards Earth.
This is the Crab Pulsar, and within its radio emissions lies a strange signal that has puzzled astronomers for many years. Known as the zebra pattern, it resembles the odd spacing of wavelengths when represented in a graph, similar to the wavy stripes of a zebra. Nothing else in space emits in this manner, and astronomers have been searching for an explanation since this pattern was first observed nearly two decades ago.
According to expert Mikhail Medvedev from the University of Kansas, it is a diffraction pattern created by the diffraction of light by varying plasma densities within the magnetosphere of this pulsar.
A strange signal in the radio emissions of the Crab Pulsar. (Illustrative image).
“If there is a screen and electromagnetic waves pass through, those waves do not travel straight,” Medvedev explains.
The Crab Pulsar is the remnant of a supernova explosion that occurred 6,200 light-years away, visible in Earth’s sky in 1054 BC. It was the spectacular death of a giant star that expelled material outward in a “violent sneeze.” The core of the star – no longer supported by the external pressure of fusion – collapsed under gravitational forces, forming a neutron star.
These extremely dense objects are quite small, with the heaviest objects weighing 2.3 times that of the Sun, compressed into a sphere only 20 km wide. Pulsars are a type of neutron star that emits beams of radio waves from their poles. As the star spins at an astonishing rate, these beams sweep over Earth like the light from a lighthouse.
The Crab Pulsar has a rotation period of about 33 milliseconds, meaning it pulses approximately 30 times per second. Astronomers have been studying this pulsar since its discovery in the 1960s.
The diffraction interference pattern resembling zebra stripes of the Crab Pulsar. (Image: Hankins & Eilek).
Medvedev discovered that: “The interaction between plasma and magnetic fields creates a diffraction interference pattern resembling wavy zebra stripes.”
“A typical diffraction pattern would create evenly spaced fringes if we only had a neutron star acting as a shield,” Medvedev stated.
“However, here, the magnetic field of the neutron star generates charged particles that form a dense plasma, varying with distance from the star. When radio waves pass through the plasma, they traverse through less dense regions but are reflected by the denser plasma. This reflection varies by frequency: lower frequencies reflect at larger radii, creating larger shadows, while higher frequencies create smaller shadows, resulting in different fringe spacings.”
“The Crab Pulsar is quite unique – it is relatively young by astronomical standards, only about a thousand years old, and has high energy,” Medvedev said. He added: “This research could truly expand our understanding and observational techniques regarding pulsars, especially young and dynamic pulsars.”
