Odd infrared emission makes this neutron star special

(Credit: Getty Images)

An unusual infrared emission that the Hubble Space Telescope detected from a nearby neutron star could indicate that the pulsar has features never seen before, researchers report.

The observation could help astronomers better understand the evolution of neutron stars—the incredibly dense remnants of massive stars after a supernova. A paper describing the research and two possible explanations for the unusual finding appears in the Astrophysical Journal.

“This particular neutron star belongs to a group of seven nearby X-ray pulsars—nicknamed ‘the Magnificent Seven’…”

“This particular neutron star belongs to a group of seven nearby X-ray pulsars—nicknamed ‘the Magnificent Seven’—that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” says lead author Bettina Posselt, associate research professor of astronomy and astrophysics at Penn State.

“We observed an extended area of infrared emissions around this neutron star—named RX J0806.4-4123—the total size of which translates into about 200 astronomical units (or 2.5 times the orbit of Pluto around the Sun) at the assumed distance of the pulsar,” Posselt says.

This is the first neutron star in which researchers have seen an extended emission only in the infrared. The researchers suggest two possibilities that could explain the extended infrared emission the Hubble Space Telescope saw. The first is that there is a disk of material—possibly mostly dust—surrounding the pulsar.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” says Posselt. “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. If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

Illustrated GIF showing a neutron star with a circum-pulsar disk. If seen at the proper angle the scattered emission from the inner part of the disk could produce the extended infrared emission astronomers observed around the neutron star RX J0806.4-4123. (Credit: Nahks Tr’Ehnl, Penn State)

The second possible explanation for the extended infrared emission from this neutron star is a “pulsar wind nebula.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” says Posselt. “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,” Posselt explains.

Illustrated GIF showing a neutron star with a pulsar wind nebula produced by the interaction of the pulsar wind and the oncoming interstellar medium. A pulsar wind nebula could explain the extended infrared emission astronomers observed. Such an infrared-only pulsar wind nebula is unusual because it implies a rather low energy of the accelerated particles.(Credit: Nahks Tr’Ehnl, Penn State)

Although researchers generally study neutron stars in radio and high-energy emissions, such as X-rays, this study demonstrates that they can gain new and interesting information about neutron stars by studying them in the infrared. Using the new NASA James Webb Space Telescope, due to launch in 2021, astronomers will be able to further explore this newly opened discovery space in the infrared to better understand neutron star evolution.

Additional researchers are from Penn State; Sabanci University in Instanbul, Turkey; and the University of Arizona. NASA, the Scientific and Technological Research Council of Turkey, the U.S. National Science Foundation, Penn State, the Penn State Eberly College of Science, and the Pennsylvania Space Grant Consortium supported the research.

Source: Penn State