Webb finds neutron star in supernova explosion remnant

SN 1987A supernova

The James Webb Space Telescope has found the best evidence yet of neutron star emission at the site of a recently observed supernova.

SN 1987A

The supernova known as SN 1987A originated at a distance of 160 thousand light years from Earth in the Large Magellanic Cloud. SN 1987A was a Type II supernova observed on Earth in 1987, the first supernova to be seen with the naked eye since 1604, before the advent of telescopes. Thus, it provided the astronomical community with a rare opportunity to study the evolution of a supernova and what was left behind from the very beginning.

SN 1987A was a core-collapse supernova, which means that the compacted remnants in its core are expected to have formed either a neutron star or a black hole. Evidence for the existence of such a compact object has been sought for a long time, and while indirect evidence for the presence of a neutron star has been found before, this is the first time that the effects of high-energy radiation from a young neutron star have been detected.

Explosion in real time

Astronomy typically studies processes that take place over a period of at least tens of thousands of years: much longer than the entire history of mankind. Supernovae – the explosive death throes of some massive stars – erupt within hours, and the brightness of the explosion peaks within a few months.

The remnants of the exploded star will continue to evolve at a rapid rate over the next decades. Thus, supernovae provide astronomers with a very rare opportunity to study a key astronomical process in real time.

The supernova SN 1987A was first observed on Earth in February 1987, and its brightness peaked in May of that year (although the distance from Earth means that the supernova flare event occurred about 160 thousand years before). It was the first supernova to be seen with the naked eye since Kepler’s Supernova was observed in 1604.

Observation of the explosion

About two hours before the visible observation of SN 1987A, three observatories around the world observed a neutrino burst lasting only a few seconds. Shortly afterwards, visible light from SN 1987A was observed. These two different observations were associated with the same supernova event and provided important evidence for the theory of how a supernova core collapses.

This theory included the assumption that this type of supernova forms a neutron star or black hole. Since then, astronomers have been searching for evidence of one of these compact objects at the center of the expanding remnant material.

Indirect evidence of a neutron star in the center of the remnant has been found in the last few years, and observations of much older supernova remnants – such as the Crab Nebula – confirm that neutron stars are found in many supernova remnants. However, no direct evidence for the existence of a neutron star in the remnants of SN 1987A (or any other similar recent supernova explosion) has been found so far.

Evidence from Webb

“From the theoretical models of SN 1987A, the 10-second neutrino burst observed just before the supernova explosion suggested that the explosion produced a neutron star or black hole. But we have not observed any convincing sign of such a newborn object from any supernova explosion,” says Claes Fransson of Stockholm University, lead author of this study.

According to him, with the help of Webb, scientists have found direct evidence of radiation caused by a newborn compact object, most likely a neutron star.

Webb began scientific observations in July 2022, and the observations underlying this paper were made on July 16, making the SN 1987A remnant one of the first objects observed by Webb.

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Left: Image of SN 1987A from the Webb 2023 NIRCam (Near Infrared Camera), showing the central structure of the object expanding at a speed of several thousand kilometers per second. The blue region is the densest part of the lumpy ecta, containing heavy elements such as carbon, oxygen, magnesium and iron, as well as dust. The bright “pearl ring” is the result of the collision of the ejecta with a ring of gas ejected about 20,000 years before the explosion. 
Right: The top image shows data obtained in the MRS (medium-resolution spectrograph) mode of the Webb MIRI (mid-infrared instrument). The bottom image shows the data obtained in the NIRSpec mode of the Webb instrument at shorter wavelengths. Spectral analysis of the MIRI results showed a strong signal due to ionized argon from the center of the ejecta surrounding the original site of SN 1987A. The NIRSpec data revealed even more intensely ionized chemical species, including fivefold ionized argon (i.e., argon atoms that have lost five of their 18 electrons). Weak lines of ionized sulfur were also detected with MIRI. Credit: NASA, ESA, CSA, STScI, and C. Fransson (Stockholm University), M. Matsuura (Cardiff University), M. J. Barlow (University College London), P. J. Kavanagh (Maynooth University), J. Larsson (KTH Royal Institute of Technology)

Spectral analysis

The team used the Medium Resolution Spectrograph (MRS) mode of the MIRI instrument, which was helped to develop by members of the same team. MRS is a type of instrument known as an Integral Field Unit (IFU).

IFUs are amazing tools that can simultaneously image an object and capture its spectrum. The IFU generates a spectrum in each pixel, allowing observers to see spectroscopic differences in the object. Analyzing the Doppler shift of each spectrum also allows us to estimate the velocity at each position.

The spectral analysis of the results showed a strong signal due to ionized argon from the center of the ejected material surrounding the original location of SN 1987A.

High-energy radiation

Further observations with another IFU, the NIRSpec (Near Infrared Spectrograph) at shorter wavelengths, allowed the team to detect even more highly ionized chemical species, in particular, fivefold ionized argon (i.e., argon atoms that have lost five of their 18 electrons). To form such ions, high-energy photons are required, and these photons must come from somewhere.

“To create these ions that we observed in the ejection, it was clear that there had to be a high-energy radiation source at the center of the SN 1987A remnant,” Fransson said.

In the article, the scientists analyze various possibilities and conclude that only a few scenarios are likely, and all of them include a newborn neutron star.

Further research

More observations are planned this year with the Webb and ground-based telescopes. The research team hopes that further studies will provide greater clarity on what exactly is happening in the heart of the SN 1987A remnant.

The scientists hope that these observations will stimulate the development of more detailed models, which will eventually allow astronomers to better understand not only SN 1987A, but all nucleus collapse supernovae.


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