Scientists were finally able to prove that the neutron star formed from a well-studied supernova, SN 1987A. This breakthrough was made possible by the James Webb Telescope.
A combination of the Hubble image of SN 1987A and the compact high-ionization argon source. The faint blue source at the center was discovered by the NIRSpec instrument on the James Webb Space Telescope. Outside of this is the rest of the supernova, which has the most mass and is expanding. The bright inner “string of pearls” is gas from the star's outer layers that was ejected about 20,000 years before the explosion. The collision between the rapidly expanding supernova remnant and the ring causes hot clumps to appear in the ring. Outside the inner ring are two outer rings, which probably arose at the same time as the inner ring. (Illustration: HST, JWST/NIRSpec, J. Larsson)
Supernovas are the stunning end result when stars eight to ten times the mass of the Sun collapse. In addition to being the most important sources of chemical elements such as carbon, oxygen, silicon and iron, which are essential requirements for life, supernovae can also create the most extreme objects in the universe: Neutron stars And black holes.
In 1987, Supernova 1987A (SN 1987A) exploded in the Large Magellanic Cloud, located near the Milky Way. This was the first time in four centuries that a supernova became visible to the naked eye, giving astronomers an unprecedented close-up image of a supernova explosion. Although SN 1987A is one of the most studied objects in the sky, the question of what remained after the explosion remains unanswered. Was it a compact neutron star or a black hole? The discovery of neutrinos, which are produced in a supernova, indicated that an ultra-compact neutron star should have formed at the center of SN 1987A. But even after three and a half decades of intense observations with the best telescopes, no conclusive evidence of the existence of such a neutron star has been found until now.
Neutron star signals detected
In a study published on February 22, 2024 in the journal Science, an international team of astronomers announced that they had detected signals from a neutron star from the center of the nebula around SN 1987A. Using the James Webb Telescope (JWST), the authors were able to observe spectral lines created either from the hot neutron star or from the so-called pulsar wind nebula around the neutron star.
– Thanks to the remarkable resolution and new instruments of the James Webb Space Telescope, we have been able to examine the center of a supernova and what was created after the explosion for the first time. We now know that there is a built-in source of ionizing radiation, which is likely a neutron star. This was predicted by explosion models and we ran simulations in 1992 suggesting how we might observe this, but only with the James Webb Space Telescope did it become possible. However, the details provided several surprises, says Claes Fransson, a professor in the Department of Astronomy at Stockholm University and the Oscar Klein Center and lead author of the study.
– This is the latest in a series of surprises that this supernova has presented over the years. It was unexpected that the compact object would finally be detected through a very strong argon line, so it was a bit of a surprise that it worked out this way, says Josefin Larsson, a professor in the Department of Physics at KTH and the Oscar Klein Center and colleagues. -Author of the study.
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Supernova (SN) 1987A – the most studied supernova
The star in the Large Magellanic Cloud before the explosion on February 23, 1987 (right) and immediately after the explosion (left). The image shows the massive increase in brightness of the supernova. Credit: David Malin Anglo-Australian Telescope.
SN 1987A is the most studied and best observed supernova ever, and is therefore of particular interest for understanding these objects. The supernova exploded on February 23, 1987 in the Large Magellanic Cloud in the southern sky at a distance of 160,000 light-years. This was the closest supernova explosion since the supernova observed by Johannes Kepler in 1604. For several months, Supernova 1987A could be seen with the naked eye.
SN 1987A is the only supernova ever observed with neutrinos (almost massless particles with very weak interaction with other matter). This was important because it was expected that 99.9% of the massive energy released in this event would be lost in these particles. The remaining 0.1% was sent away as light and kinetic energy. Of the huge number (about 10 to 58) of neutrinos emitted, about 20 have been detected by three different detectors around Earth. SN 1987A was also the first supernova in which the exploding star could be identified from images taken before the explosion (Figure 4). This allowed the star's mass to be determined, which agrees well with theoretical models.
The black hole or neutron star was created
Top row. Leave. Argon II image with MIRI/MRS at compact body speed. Middle: image corresponding to the ring speed. Right: The first image is superposed with the second image, showing how the center is dominated by the compact body. Bottom row: Same for highly ionized argon using the NIRSpec instrument.
Aside from neutrinos, the most interesting consequence of the explosion is the prediction that it will collapse into a black hole or neutron star. These compact remnants were created by the core of the collapsed star, and have a mass of about 1.5 times the mass of the Sun. The remaining mass was pushed away at up to 10% of the speed of light, forming the expanding remnants we can observe today.
Astronomers studying SN 1987A suspect that a neutron star formed after the explosion. The main indicator came from the 10-second duration of the neutrino pulse. But despite additional indications from radio and X-ray observations, no conclusive evidence of a neutron star has been found so far. One important reason is the large amount of dust formed in the years following the explosion. This dust can block most of the visible light from the center, obscuring the compact object at visible wavelengths. Identification of the final product of the explosion was the major remaining unsolved problem for SN 1987A.
The James Webb Telescope made breakthroughs possible
The James Webb Space Telescope can observe light at infrared wavelengths, which can travel more easily through dust that blocks visible light. An international team of astronomers studied SN 1987A using two telescope instruments, MIRI* and NIRSpec. Then they saw a point source in the center of the widespread supernova remnant, emitting light from argon and sulfur ions. Thanks to the precision of the James Webb Space Telescope, and the ability of its instruments to precisely determine the velocity of the emitting source, we know that this point source is very close to the center of the supernova explosion.
While most of the mass of the exploded star is expanding at speeds of up to 10,000 kilometers per second and is thus spread over a large volume, the observed source is still close to the site of the explosion. This is what astronomers would expect for the compact remains after the explosion. The observed spectral lines for argon and sulfur come from ionized atoms, requiring high-energy photons from the compact object. How this could happen as a result of ultraviolet radiation and X-rays from a neutron star was already predicted in 1992 by Roger Chevalier (University of Virginia) and Claes Fransson.
Two possible explanations
Scientists don't see the neutron star directly. Instead, they infer its presence by observing how its radiation affects its surroundings. In their study, the authors discuss two main explanations for the observed spectral lines. They may have been formed by radiation from either a hot, newborn neutron star, which has a surface temperature of more than a million degrees, or from energetic particles accelerating in the strong magnetic field of a rapidly rotating neutron star (also called a pulsar)… This is the same mechanism that causes He spoke about the pulsar at the center of the famous Crab Nebula, the remnant of a supernova observed by Chinese astronomers in 1054.
Both explanatory models lead to similar predictions for the type of spectral lines that are generated. To distinguish between these two models, further observations are needed using the James Webb Space Telescope and ground-based telescopes in visible light, as well as the Hubble Telescope. Regardless, new James Webb Space Telescope observations provide compelling evidence of the presence of a compact object, likely a neutron star, at the center of SN 1987A. The radius of this neutron star is about 10 kilometers, which means that its density is equivalent to the density of the atomic nucleus. One cubic millimeter of this stellar material weighs the equivalent of a giant oil tanker!
In summary, the new JWST observations, combined with previous observations of the exploding star and the neutrinos produced by the explosion, provide a complete picture of this unique object.
The team behind these findings consists of 34 authors from 12 different countries in Europe and the USA. The first author is Claes Fransson, a professor in the Department of Astronomy at Stockholm University and the Oscar Klein Centre.
*MIRI is a tool that researchers at Stockholm University helped develop. Read more
Last updated: February 22, 2024
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