Weird Pulsar is Emitting a Mysterious Unexplained Light and Scientists Aren't Sure Why

Astronomers have noticed something unusual around a nearby neutron star, known as RX J0806.4-4123.

Using the Hubble Space Telescope, a team led by Bettina Posselt from Pennsylvania State University, detected a strange emission of infrared light emerging from a region around the snappily-named star, of a kind which has never been seen before.

Neutron stars are some of the strangest objects in the universe. They are produced when massive stars come to their end of their lifecycles and undergo titanic explosions known as supernovas, which blow off the outer layers of material. If the exploding star's mass is insufficient to produce a black hole, the remaining central region collapses under the force of gravity and is squeezed to such an extent that protons and electrons combine to form neutrons.

The resulting objects are very small for stars—usually between 20 and 30 kilometers in diameter—but they are incredibly dense. Despite their diminutive size, they tend to have a mass about 1.4 times that of our Sun. According to NASA, this means that one teaspoonful of a neutron star would weigh a staggering one billion tons.

Due their extremely high density, they also have powerful gravitational fields. In fact, the gravitational field at a neutron star's surface is around 200 billion times that of the Earth's. The stars can also spin incredibly fast, rotating up to several hundred times per second. Some neutron stars, like RX J0806.4-4123 for example, spit out intense beams of radiation, much like interstellar lighthouses. These are known as pulsars.

Neutron stars don't tend to be studied in infrared light because they are much brighter and easier to detect in other wavelengths, using X-rays, gamma-rays and radio waves, for example. But for the latest research the team used Hubble's infrared vision to observe RX J0806.4-4123—and what they found was surprising.

They detected an extended infrared signal that can only be seen in this wavelength. This is unusual, even for the handful of neutron stars that have been observed in infrared wavelengths.

"Such extended infrared emission without any trace of it at other wavelengths—namely X-rays—has so far not been seen before," Posselt told Newsweek.

The team observed an extended area of infrared emissions around the neutron star, the total size of which translates into about 200 astronomical units (approximately 18 billion miles) at the assumed distance of the pulsar, she said.

In a paper published in the Astrophysical Journal, the researchers propose two possible explanations for the mysterious infrared emission. The first is that the infrared light is coming from a disk of material, mostly made up of dust, surrounding the star.

"There could be what is known as a 'fallback disk' of material that coalesced around the neutron star after the supernova," Posselt said in a statement. "Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation."

The second explanation is that there is an energetic wind blowing off the neutron star which interacts with gas in interstellar space, creating a feature known as a "pulsar wind nebula". Pulsar winds are generated when particles are accelerated in the electrical field that is produced by the fast rotation of neutron stars with a strong magnetic field.

This illustration shows a neutron star (RX J0806.4-4123) with a disk of warm dust that produces an infrared signature as detected by NASA’s Hubble Space Telescope. NASA, ESA, and N. Tr’Ehnl (Pennsylvania State University)

"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," Posselt said. "The shocked particles would then emit synchrotron radiation, causing the extended infrared signal 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."

When NASA's upcoming James Webb Space Telescope begins operations, scientists will be able to further explore unexplained phenomena such as the one uncovered by the latest results. The advanced observatory is set for launch in 2021 and will be the successor to Hubble. It will offer unprecedented resolution and sensitivity, enabling it to carry out a broad range of investigations which are beyond the reach of current technology.

"If additional observations confirm the disk, this could be very important for our understanding of the birth parameters of neutron stars and how neutron stars evolve over time," Posselt told Newsweek. "If additional observations confirm the pulsar wind nebula, this is also very interesting because it implies that the accelerated particles around this neutron star have unusually low energies. Studying why this is so could then help us to better understand pulsar wind nebulae in general."

Victoria Kaspi, an astrophysicist at McGill University, who was not involved in the study, told Newsweek that the results as "intriguing. There are not many near-infrared detections of neutron stars and their environments so this source is worthy of attention.

"The interpretation of the emission is unclear, however. The pulsar wind nebula possibility is probably the most conventional, but there is a lot of circumstantial evidence for disks around neutron stars so that possibility should not be discounted."

Hubble, which is jointly run by NASA and the European Space Agency, has been operating for more than 28 years. In that time, it has captured some of the most dramatic and striking images of our universe. While not the first space observatory to be launched, it is both one of the largest and most versatile, providing astronomers with numerous options for observing the cosmos.

This article was updated with comment from Bettina Posselt and Victoria Kaspi.