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This Pulsar is Giving Off Weird Infrared Light and We’re Not Sure Why

Researchers believe a disk of dust from a supernova or a pulsar wind nebula could explain the strange energy signal

(Nahks Tr'Ehnl, Penn State)
smithsonian.com

Neutron stars are already considered some of the most peculiar objects in the universe, but now the Hubble Space telescope has found one that is even more odd: it's giving off a bizarre swirling display of glowing infrared light.

Neutrons stars are remnants of exploding stars, or supernovae, pack 1.4 times the mass of our own sun into a body only about 12.4 miles in diameter. They are so dense, a single teaspoon would weigh one billion tons, according to Space.com. When they spin fast enough and emit high-energy electromagnetic radiation, like x-rays, they are known as pulsars.

The particular neutron star in question is called RX J0806.4-4123, and it appears to be emitting lots of infrared light, which could give us new insights into how pulsars form, reports Yasemin Saplakoglu at LiveScience. RX is one of seven x-ray pulsars within 3,300 light years of Earth that astronomers call “The Magnificent Seven.” These seven stars are hotter than astronomers would expect given their age and available energy and rotate more slowly than other pulsars. An international team of astronomers was looking through Hubble data when they noticed that the area around RX was putting off a lot of infrared energy.

“We observed an extended area of infrared emissions around this neutron star…the total size of which translates into about 200 astronomical units (approximately 18 billion miles) at the assumed distance of the pulsar,” says Bettina Posselt of Pennsylvania State and the lead author of the paper in The Astrophysical Journal.

It’s the first time such a large infrared signal has been observed surrounding a pulsar, and it suggests that something more is going on around the dense little star. “The emission is clearly above what the neutron star itself emits—it doesn’t come from the neutron star alone,” Posselt tells Ryan F. Mandelbaum at Gizmodo. “This is very new.”

So if the infrared isn’t coming from the neutron star itself, where is all the energy coming from? The researchers can’t say for sure, but they have a couple of good guesses.

The first suggestion is that the infrared is coming from a fallback disk, or a large disk of dust that formed around the neutron star after its supernova explosion. Posselt tells Saplakoglu at LiveScience that researchers have hypothesized that these disks exist, but have never actually found one. The inner part of the disk, she says, would have enough energy to produce infrared light. It would also explain why RX is hotter and slower than expected, since the disk could have added extra heating to the star and also slowed its rotation.

“If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution,” Posselt says in a NASA release.

The other possible explanation is a phenomenon called a pulsar wind nebula.

Posselt explains in a press release:

A pulsar wind nebula would require that the neutron star exhibits a pulsar wind. A pulsar wind can be produced when particles are accelerated in the electric field that is produced by the fast rotation of a neutron star with a strong magnetic field. As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then radiate synchrotron emission, causing the extended infrared emission that we see. Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting.

Mandelbaum at Gizmodo reports that it’s possible but unlikely that the infrared radiation is coming from a source somewhere behind the pulsar. To find out, the researchers simply need to wait. If the source is associated with the star it will move along with it as it wanders across the sky. If it’s behind it, the pulsar will eventually lose its infrared glow.

And if the source does turn out to be a fallback disk or pulsar wind nebula, researchers will have to wait to learn more about that, too. The researchers have tried to view RX with powerful Earth-based telescopes to get a look at the disk or dust around it, but it was just too faint. Instead, they will need to wait until the long-delayed launch of the next-gen James Webb Space Telescope, the successor to Hubble, which should be able to image the source, revealing if there is a disk or nebula around the star.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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