Thanks to the Hubble Space Telescope, we all have a vivid image of the Crab Nebula in our minds' eyes. It is the remnant of a supernova explosion that Chinese astronomers recorded in 1056. However, the Crab Nebula is more than just a nebula; it is also a pulsar.
The Crab pulsar pulses in an unusual “zebra” pattern, and an astrophysicist at the University of Kansas believes he has figured out why.
When massive stars explode as supernovae, they leave behind remnants: either a stellar-mass black hole or a neutron star. The latter left behind SN 1054. The neutron star is highly magnetized and rotates rapidly, emitting beams of electromagnetic radiation from its poles. As it rotates, the radiation is intermittently directed towards the Earth, making it visible to us. In this case we speak of a pulsar.
Pulsars are complex objects. They are extremely dense and can pack up to three solar masses of material into a sphere just 30 km in diameter. Their magnetic fields are millions of times stronger than Earth's, they can rotate hundreds of times per second, and their enormous gravity distorts space-time. And their nuclei are basically giant atomic nuclei.
One result of their complexity is their radio emissions, and this is particularly true for the Crab Pulsar.
Pulsars are known for their main pulse (MP), but they also emit other pulses that are more difficult to detect. In 2007, radio astronomers Hankins and Eilek discovered a strange pattern in the Crab pulsar's high-frequency radio emissions. This is the only known pulsar to produce these patterns between the pulsar's main pulse (MP) and intermittent pulse (IP).
“The mean profile of this star is dominated by a main pulse (MP) and an intermediate pulse (IP),” Eilek and Hankins wrote in their paper. However, there are two additional pulses called HFC1 and HFC2 that create the zebra pattern.
This figure shows the average profile of the Crab pulsar over a wide frequency range. MP and IP are shown by dashed lines at pulse phases 70° and 215°. However, between 4.7 and 8.4 GHz the IP is offset from the IP at lower and higher frequencies. This represents the “zebra” pattern of the crab pulsar. Two new high-frequency components also appear (labeled HFC1 and HFC2). Photo credit: Moffett & Hankins 1996.
No one has been able to explain this unusual pattern. However, new research published in Physical Review Letters may finally explain this. The author is Mikhail Medvedev, who specializes in theoretical astrophysics at the University of Kansas. His research is “Origin of Spectral Bands in the Crab Pulsar Radio Emission.”
Medvedev says the Crab pulsar's plasma-filled magnetosphere acts as a diffraction shield to create the zebra pattern. This can explain the bandgap, high polarization, constant position angle and other properties of the emissions.
This figure shows the overall geometry of the Crab pulsar system. The red star is the pulsar. Its emissions pass through the plasma-filled magnetosphere, which acts as a diffraction shield and creates the zebra pattern of pulses. Image source: Medvedev 2024.
A typical pulsar emits radio emissions from its poles, as shown in the figure below. Sometimes they send out two signals per rotation period, a radio signal and a radio frequency signal. They appear in a different phase of rotation, with the higher frequency emission produced outside the cylinder of light, the region where linear velocity approaches the speed of light.
This illustration shows how a standard pulsar emits radio emissions. Electrons and positrons are accelerated through one of the gaps in the magnetosphere. They flow along the open magnetic field lines and emit coherent radio emissions from the poles. Photo credit: National Radio Astronomy Observatory.
But the crab pulsar is different.
“The crab pulsar, on the other hand, is something very special. Its main radio pulse and its intermediate pulse coincide in phase with the high-energy emission, indicating the same emission region,” explains Medvedev.
Medvedev explains that the high frequency interpulse (HFIP) generated by the diffraction effect creates the zebra pattern. “The radio frequency interpulse (HFIP) spectral pattern observed between approximately
?~5 and ?~30 GHz are remarkably different and represent a sequence of emission bands similar to this
“Zebra” pattern,” he writes.
This simple scheme helps explain the diffraction effect. The different colors represent different densities in the plasma field. Regions of the magnetosphere with different densities either rotate with the pulsar or not, contributing to the formation of the zebra pattern in the emissions. Image source: Medvedev 2024.
The model proposed by Medvedev has an additional advantage. He says it can be used to perform tomography on pulsars to reveal more details about their strong magnetospheres.
“The model makes it possible to perform a ‘tomography’ of the pulsar magnetosphere,” he writes.
“We assume that these HFIP properties can also be observed in other pulsars when their radio and high-energy emissions are in phase. This would happen if the radio emission were generated in the outer magnetosphere, as opposed to the “normal” emission from the polar region,” Medvedev explains.
This composite image of the Crab Nebula includes X-rays from Chandra (blue and white), optical data from Hubble (purple), and infrared data from Spitzer (pink). Chandra has observed the crab repeatedly since the telescope was launched in 1999. Image source: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech
Medvedev says his model can also explain the HFC1 and HFC2 in the Crab pulsar's emission spectrum. They are also artifacts of his proposed diffraction model. “We propose that these high-frequency components are the reflections from the magnetosphere of the same source that produce the diffracted HFIP,” he explains.
“Finally, we propose a model that explains the special spectral band structure (the zebra pattern) of the high-frequency intermediate pulse of the radio emission of the Crab pulsar,” writes Medvedev.
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