| Feb 22, 2024 |
James Webb telescope detects traces of neutron star in iconic supernova
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(Nanowerk News) Supernovae are the spectacular end result of the collapse of stars more massive than 8-10 times the mass of the sun. Besides being the main sources of chemical elements such as carbon, oxygen, silicon, and iron that make life possible, they are also responsible for creating the most exotic objects in the universe, neutron stars and black holes.
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In 1987, supernova 1987A (SN 1987A) exploded in the Large Magellanic Cloud, which is located near the Milky Way. It was the first time in four centuries that a supernova became visible to the naked eye, giving astronomers an unprecedented close-up of a supernova explosion. Although SN 1987A is one of the most studied objects in the sky, the question of what was left after the explosion remains unanswered. Did it become a compact neutron star or a black hole? The detection of neutrinos, which are produced in the supernova, indicated that a super compact neutron star should have formed at the center of the SN 1987A. But even after three and a half decades of intensive observations with the best telescopes, no conclusive evidence of such a neutron star has been found, until now.
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| Combination of a Hubble image of SN 1987A and the compact highly ionized argon source. The faint blue source in the centre was detected by the NIRSpec instrument on JWST. Outside of this is the rest of the supernova, which contains the most mass and is expanding at thousands of kilometers every second. The inner bright "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 gives rise to the heated clumps in the ring. Outside the inner ring are two outer rings, which probably arose at the same time as the inner ring was formed. The bright stars to the left and right of the inner ring are unrelated to the supernova. (Image: HST, JWST/NIRSpec, J. Larsson)
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In a study published in the journal Science ("Emission lines due to ionizing radiation from a compact object in the remnant of Supernova 1987A"), an international team of astronomers announced that they had detected signals from a neutron star from the centre of the nebula around SN 1987A. Using the James Webb Telescope (JWST), the authors were able to observe spectral lines that had either been created from the hot neutron star or from a so-called pulsar wind nebula around the neutron star.
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“Thanks to the fantastic resolution and the new instruments at JWST, we have been able to examine the centre of the supernova and what was created after the explosion for the first time. We now know that there is a compact source of ionizing radiation there, which is likely a neutron star. This was predicted by the explosion models and we did simulations in 1992 that indicated how to observe this, but it was only with JWST that it became possible. However, the details offered several surprises,” says Claes Fransson, professor at the Department of Astronomy, Stockholm University and the Oskar Klein Centre and the lead author of the study.
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“This is the latest in a series of surprises that this supernova has offered 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 fun that it turned out this way,” says Josefin Larsson, professor at the Department of Physics, KTH Royal Institute of Technology and the Oskar Klein Centre and co-author of the study.
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Supernova (SN)1987A – the most studied supernova
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SN 1987A is the most studied and best observed supernova of all and therefore of particular importance to understanding these objects. Exploding on February 23, 1987 in the Large Magellanic Cloud in the southern sky at a distance of 160,000 light years, it was the closest supernova explosion since the supernova observed by Johannes Kepler in 1604. For several months, it was possible to see SN 1987A with the naked eye.
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SN 1987A is the only supernova that has been observed via its neutrinos (almost massless particles with an extremely weak interaction with other matter). This was important because 99.9 percent of the enormous energy released in this event was predicted to be lost in these particles. The remaining 0.1 percent was sent away in the form of light and kinetic energy. Of the huge number (about 1058) of neutrinos emitted, about 20 were detected by three different detectors around the Earth. SN 1987A was also the first supernova in which the exploding star could be identified from images taken before the explosion. This allowed the mass of the star to be determined, which agreed well with theoretical models.
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Black hole or neutron star was created
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Apart from the neutrinos, the most interesting consequence of the explosion is the prediction that it will collapse into a black hole or neutron star. This compact remnant was created by the collapsed star's core, and has a mass about 1.5 times that of the Sun. The remaining mass was pushed away at up to 10 percent of the speed of light, forming the expanding remnant we can observe today.
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The astronomers studying SN 1987A suspected that a neutron star had formed after the explosion. The main indication came from the neutrino pulse's duration of 10 seconds. But despite further indications from radio and X-ray observations, no conclusive evidence for a neutron star has been found until now. An important reason is the large amount of dust that formed in the years after the explosion. This dust can block most of the visible light from the center, obscuring the compact object at visible wavelengths. Identifying the final product of the explosion was the main remaining unsolved problem for SN 1987A.
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James Webb Space Telescope made breakthroughs possible
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The James Webb Space Telescope (JWST) can observe light at infrared wavelengths, which can more easily travel through the dust that blocks visible light. An international team of astronomers studied SN 1987A using two of the telescope's instruments, MIRI and NIRSpec. They then saw a point source in the centre of the widespread supernova remnant, emitting light from argon and sulfur ions (see Figs. 1+3). Thanks to JWST's resolution, and the ability of its instruments to accurately determine the velocity of the emitting source, we know that this point source is very close to the centre of the supernova explosion.
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While most of the exploding star's mass is expanding at up to 10,000 km/second and has therefore been spread over a large volume, the observed source is still close to the explosion site. This is what astronomers expect for the compact remnant after the explosion. The observed spectral lines from argon and sulfur come from ionized atoms, requiring high-energy photons from the compact object. How this can happen as a result of the ultraviolet and X-ray radiation from a neutron star was already predicted in 1992 by Roger Chevalier (University of Virginia) and Claes Fransson.
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Two possible explanations
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The scientists do not see the neutron star directly. Instead, they infer its existence 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 created due to the radiation from either the hot, newborn neutron star, which has a surface temperature of more than a million degrees, or from energetic particles accelerated in the strong magnetic field of the rapidly rotating neutron star (which is also called a pulsar). This is the same mechanism that takes place around the pulsar at the centre of the famous Crab Nebula, which is the remnant of a supernova observed by Chinese astronomers in 1054.
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Both of these explanatory models result in similar predictions for what kind of spectral lines are created. To distinguish between these two models, further observations with JWST and ground-based telescopes in visible light, as well as the Hubble telescope, are therefore required.
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Regardless, the new JWST observations provide compelling evidence for the existence of a compact object, likely a neutron star, at the centre of SN 1987A. The radius of such a neutron star is approximately 10 km, which means that the density is as great as in an atomic nucleus. One cubic millimeter of such stellar matter weighs about as much as a supertanker!
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In summary, the new JWST observations, along with previous observations of the exploding star and the neutrinos created in the explosion, provide a complete picture of this unique object.
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The team behind these results consists of 34 authors from 12 countries in Europe and the USA. First author is Claes Fransson, professor at the Department of Astronomy at Stockholm University and the Oskar Klein Centre.
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