Webb discovers neutron star within supernova remnant

by Martijn Luinstra

Using the joint NASA, European Space Agency, and Canadian Space Agency James Webb Space Telescope (JWST), an international team of astronomers has found evidence of a neutron star in the remnants of a recent supernova. The supernova, called SN 1987A, was first observed on Feb. 24, 1987, and occurred 160,000 light-years from Earth in the Large Magellanic Cloud. It was the most recent supernova in the Local Group of galaxies and the first one to have been observable with the naked eye since Kepler’s Supernova in 1604.

At the end of the life of a large star — a star that is at least eight times more massive than the mass of the Sun — its core is no longer able to resist the forces of gravity and collapses in on itself. This causes the star’s outer layers to be expelled in a tremendous explosion called a supernova. Depending on the size of the core, the event either leaves behind a neutron star or a black hole if the star is big enough.

A few hours before SN 1987A, a burst of minute subatomic particles called neutrinos was detected. This indicated that the supernova left behind a compact object, like a neutron star or a black hole, but did not provide enough evidence to conclusively exclude one or the other. The recent study provided the first direct evidence of the presence of a neutron star in the supernova remnant.

A combination of a Hubble Space Telescope image showing SN 1987A with the argon source observed by Webb. (Credit: Hubble Space Telescope WFPC-3/James Webb Space Telescope NIRSpec/J. Larsson)

“From theoretical models of SN 1987A, the 10-second burst of neutrinos observed just before the supernova implied that a neutron star or black hole was formed in the explosion,” said lead author Claes Fransson of Stockholm University. “But we have not observed any compelling signature of such a newborn object from any supernova explosion. With Webb, we have now found direct evidence for emission triggered by the newborn compact object, most likely a neutron star.”

Webb observed the supernova remnant on July 16, 2022, shortly after the telescope started science observations. The telescope collected data using its Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) instruments, which allowed the scientists to take an image of the object and capture a spectrum for each pixel in the image all at once.

These spectra show the intensities of different frequencies of light in the signal. By analyzing these, astronomers can draw conclusions about the chemical composition of different parts of the object.

“It was so exciting looking at the JWST observations of SN 1987A for the first time,” said co-author Patrick Kavanagh of Maynooth University, Ireland. “As we checked the MIRI and NIRSpec data, the very bright emission from argon at the center of SN 1987A jumped out. We knew immediately that this was something special that could finally answer the question on the nature of the compact object.”

Webb’s spectra showed that the argon atoms had been ionized. In other words, the atoms had been stripped of some of their electrons. To find out what type of compact object was hiding behind the dust left by the explosion, the team considered different explanations of what could have ionized these atoms. They modeled multiple scenarios and found two that matched Webb’s observations.

One possible scenario is that the atoms have been ionized by ultraviolet and X-ray radiation emitted by a cooling neutron star. In this scenario, the surface of the neutron star would be more than one million Kelvin 35 years after the explosion. A similar neutron star has been found in the supernova remnant Cassiopeia A.

Optical image of SN 1987A taken by the Hubble Space Telescope in 2022. The white contours mark the argon emission observed by NIRSpec. (Credit: Fransson et al.)

The other explanation is that radiation emitted from a so-called pulsar wind nebula has ionized the argon atoms. Such a nebula is created by a rapidly spinning neutron star, or pulsar, dragging charged particles from the dust cloud around. A pulsar like this has also been found in the Crab Nebula.

Limitations in the model prevent the astronomers from determining which scenario is more likely. However, both scenarios require the presence of a neutron star, so they did finally reveal what’s at the core of the remnants of SN 1987A.

“The mystery over whether a neutron star is hiding in the dust has lasted for more than 30 years and it is exciting that we have solved it,” said co-author Mike Barlow of University College London. “Supernovae are the main sources of chemical elements that make life possible – so we want to get our models of them right. There is no other object like the neutron star in Supernova 1987A, so close to us and having formed so recently. Because the material surrounding it is expanding, we will see more of it as time goes on.”

Fransson et al. published their findings in the journal Science on Feb. 22, 2024.

(Lead image: SN 1987A as imaged by Webb’s Near-Infrared Camera. The cut-outs show the light from ionized argon captured by the MIRI instrument (top right) and the NIRSpec instrument (bottom right). (Credit: NASA, ESA, CSA, STScI, Claes Fransson (Stockholm University), Mikako Matsuura (Cardiff University), M. Barlow (UCL), Patrick Kavanagh (Maynooth University), Josefin Larsson (KTH))

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