Universe Today
Most objects that astronomers and astrophysicists study have existed for billions of years. Things like supermassive black holes, the Milky Way galaxy, even the Sun and the Earth predate humanity by billions of years.
But not the Crab Nebula. It's the supernova remnant (SNR) of a supernova that exploded about 6,500 years ago. It's light reached Earth in 1054, and the exploding star is called SN 1054. Ancient astronomers recorded its appearance in the night sky, especially Chinese astronomers who called it a "guest star."
The Crab Nebula is one of the most well-studied objects in astronomy. Its striking appearance makes it recognizable to more than just astronomers, and it's been imaged many times in great detail by different telesocopes, including the Hubble. Its Crab Nebula image is like the space telescope's calling card.
There's a lot going on in a complex object like the Crab Nebula, also known as M1 and NGC 1952. It's not only classified as a SNR, it's also a pulsar wind nebula. A central pulsar generates the winds that drive an expanding bubble of high-energy particles outward. It also drives the outward expansion of magnetic fields.
*This image is a combination of optical light from the Hubble (red) and x-ray light from the Chandra Observatory (blue). The red star in the center is the Crab pulsar, and the central part of the image shows the pulsar wind nebula. Image Credits: Optical: NASA/HST/ASU/J. Hester et al. X-Ray: NASA/CXC/ASU/J. Hester et al. - https://hubblesite.org/contents/media/images/2002/24/1248-Image.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=238064*
A typical pulsar emits one radio pulse per rotation. A few have two. In most pulsars, the two appear in different parts of the rotation.
But the Crab Nebula stands out from other pulsars. Its two radio pulses and its high energy pulses appear in the same phase. These pulses look like a zebra pattern in its spectrum, with notable gaps between them, and astrophysicists have struggled to explain why.
New research in the Journal of Plasma Physics explains why the Crab exhibits its unusual zebra stripe pattern. It's titled "Theory of striped dynamic spectra of the Crab pulsar high-frequency interpulse," and the sole author is Mikhail Medvedev. Medvedev is from the Department of Physics and Astronomy at the University of Kansas. This is not Medvedev's first published research into the Crab Nebula and he's been working on understanding these unusual pulses for years.
"This peculiar spectral pattern was first reported in 2007 and subsequently studied in great detail," Medvedev writes. "Despite substantial theoretical efforts over the subsequent fifteen years, no satisfactory mechanism has been proposed to elucidate the ... puzzle."
It all boils down to the Crab's magnetosphere.
Pulsars are highly magnetized neutron stars. Their magnetic fields are compressed just like the neutron star itself. This amplifies them to an extreme degree. They can be one billion times stronger. These extreme magnetic fields dominate almost everything about pulsars. In fact, pulsars are considered as natural laboratories for extreme physics because of their magnetic fields, extreme gravity, and extreme rotations.
Medvedev's research shows that gravity is responsible for the unusual zebra pattern.
“Gravity changes the shape of spacetime,” Medvedev said in a press release.
“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.”
We know this is true because of gravitational lensing. This lensing has been discussed and researched extensively, but not when it comes to neutron stars, according to Medvedev.
“In black hole images, gravity alone shapes the structure,” he said. “In the Crab Pulsar, both gravity and plasma act together. This represents the first real-world application of this combined effect.”
This animation is made of different frequency observations of the Crab Pulsar from five different observatories: the VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.
But there's more to the Crab Nebula's emissions pattern than just zebra-like gaps. It also has high-frequency intergap emissions, and these don't have broad spectra like other pulsars.
“There’s a remarkable pattern in Pulsar’s spectrum,” 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.”
“The stripes are absolutely distinct with complete darkness between them,” 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.”
In his earlier work, the physicist was able to account for the distinct stripes in the pulsar's radio emissions. The neutron star's plasma caused diffractions in its electromagnetic pulses. They were largely responsible for the stripes.
But the high contrast was still unexplained. That changed when he accounted for gravity.
“The previous theoretical model could reproduce stripes, but not with the observed contrast. The inclusion of gravity provides the missing piece,” 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 defocusing lens from the plasma and the focusing lens from the gravity are in a kind of tug-of-war that neither can ever win. The different forces creates both an in-phase and an out-of-phase interference bands of intensity in the radio waves, and that creates the zebra pattern.
“By symmetry, there are at least two such paths for the light,” he 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's still more work to be done even though the model explains the Crab Pulsar's pulses.
"There appears to be little additional physics required to explain the stripes qualitatively,” Medvedev said. “Quantitatively, there may be refinements."
"The pulsar is rotating, and including rotational effects could introduce quantitative changes, though not qualitative ones,” Medvedev said.
Medvedev's work could allow for greater understanding other rotating and gravitational objects, too. Pulsars themselves are difficult to study, and Medvedev's work could advance the study of pulsars in general. For example, the exact source of the pulses from a neutron star is unknown, though the polar regions are under strong consideration. Scientists are still uncertain, in that case, how far above the poles the pulses originate.
"Our model also puts constraints on the source of the pulsar radio emission," Medvedev writes.
