Decades-long quest ends as the landmark observatory detects signs of the 1987 blast’s central neutron star.
This image of supernova 1987A was created by combining a photo from the Hubble Space Telescope with data from one of JWST’s instruments, which detected a signature of the neutron star (blue) at its core. Credit: J. Larsson
The James Webb Space Telescope (JWST) has solved a decades-old mystery about one of the most famous explosions of a star in history.
Astronomers used the observatory to finally spot signs of an ultradense ‘neutron star’ lurking in the explosion’s core in a galaxy that orbits the Milky Way. Light from the explosion reached Earth 37 years ago this week, in a supernova that revolutionized modern astrophysics by providing an up-close look at how stars die.
But despite years of studying this blast, known as supernova (SN) 1987A, astronomers had not been able to detect what was left behind: maybe a black hole, which can sometimes be formed, or perhaps a neutron star, as many predicted?
“It’s something that’s been searched for ever since the explosion,” says Patrick Kavanagh, an astrophysicist at Maynooth University in Ireland, and a member of the team reporting the discovery today in Science1. “And now we’ve found it.”
JWST did not observe the neutron star directly, because it remains obscured behind a veil of dust from the explosion. But the telescope detected light coming from argon and sulfur atoms that had been ionized, or electrically charged, by radiation blazing from the long-sought neutron star.
“This is a very plausible case for seeing the effects of the neutron star we all expected,” says Robert Kirshner, an astronomer and executive director of the TMT International Observatory in Pasadena, California, who has studied the supernova for decades. “There have been hints and allegations [before], but nothing as direct as this.”
An ‘elusive nugget’
The supernova astonished scientists when it appeared in February 1987 in the Large Magellanic Cloud galaxy, which is around 50,000 parsecs (160,000 light years) from Earth. The first sign that something had happened was a wave of the ghostly particles known as neutrinos, which washed over Earth and triggered neutrino detectors around the world. Within hours, a ‘new’ star blazed bright enough to be visible to the naked eye. It was the closest and brightest supernova observed since 1604, at the dawn of the age of the telescope.
Over the years, astronomers watched as rings of gas and dust expanded outwards from the site of the explosion, usually growing dimmer but sometimes brightening when various ejected materials collided. The world’s most powerful telescopes — including JWST’s predecessor, the Hubble Space Telescope — tracked the evolution of the explosion. Studies of SN 1987A ultimately led to many discoveries about stellar evolution, such as how dying stars expel the chemical elements forged in their hearts into space.
Supernova 1987A (at centre of image) is located in the Large Magellanic Cloud, a neighbouring galaxy.Credit: NASA, ESA, Robert P. Kirshner (CfA, Moore Foundation), Max Mutchler (STScI), Roberto Avila (STScI)
But nobody had ever been able to spot the ember that was left behind — an “elusive nugget”, Kirshner calls it — when the original star blew up. Theory suggests that the original star exploded in the most common type of supernova, in which a large star (one that’s at least eight times the mass of the Sun) runs out of hydrogen, helium and other elements to sustain its nuclear fusion, and thus collapses and explodes.
One outcome of such a supernova is to leave behind a black hole. But early observations of SN 1987A, such as the wave of neutrinos, suggested that it should have given rise to a neutron star, which can be just 20 kilometres across but is so dense that a teaspoonful weighs millions of tonnes. Astronomers have found several tantalizing hints of this outcome using other telescopes, but none have yielded a solid conclusion, meaning that other possibilities were still on the table2,3.
Enter JWST, which launched in late 2021 and can observe celestial bodies at different wavelengths and higher resolution than can many other telescopes. In July 2022, in some of its first scientific observations, the powerful space telescope observed SN 1987A for nine hours. Two of its cutting-edge instruments provided unprecedented insights into what was happening at the heart of the exploded star. “The data were really superb quality, much better than I had imagined,” says team member Josefin Larsson, an astrophysicist at the KTH Royal Institute of Technology in Stockholm.
The strongest evidence so far
The JWST observations revealed the fingerprint of ionized argon and sulfur gas triggered by the central neutron star. The finding is “the strongest observational evidence so far” for the presence of a neutron star in SN 1987A, says Mikako Matsuura, an astrophysicist at Cardiff University, UK. She won’t go so far as to call it conclusive, but says that “JWST is really an amazing telescope to deliver such a discovery”.
Now astronomers will shift their focus to better understanding the neutron star and how it evolves over time. Lead author Claes Fransson, an astrophysicist at Stockholm University, and his colleagues have new observations from JWST, including some taken just this week, and plan to look for more details, such as whether the neutron star is enveloped by powerful magnetic fields.
As for actually seeing the neutron star through a telescope, the dust will have to clear out more. “As the supernova expands,” Fransson says, “the dust and gas blocking the light to the centre will get thinner and thinner, so that we will be able to see the central region easier.”
doi: https://doi.org/10.1038/d41586-024-00528-4
Quelle: nature
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GOTCHA: FIRM EVIDENCE FOR A NEUTRON STAR IN SUPERNOVA 1987A
After decades of debate, James Webb Space Telescope observations provide firm evidence of a neutron star inside the 1987A supernova remnant.
Combination of a Hubble Space Telescope image of SN 1987A and the compact argon source. The faint blue source in the centre is the emission from the compact source detected with the JWST/NIRSpec instrument. Outside this is the stellar debris, containing most of the mass, expanding at thousands of km/second. The inner bright “string of pearls” is the gas from the outer layers of the star that was expelled about 20,000 years before the final explosion. The is the fast debris are now colliding with the ring, explaining the bright spots. Outside of the inner ring are two outer rings, presumably produced by the same process as formed the inner ring. The bright stars to the left and right of the inner ring are unrelated to the supernova. Hubble Space Telescope WFPC-3/James Webb Space Telescope NIRSpec/J. Larsson
Supernova 1987A left a neutron star behind, according to new observations from the James Webb Space Telescope (JWST). It’s what astronomers have assumed ever since the stellar explosion was first observed on February 23, 1987. However, conclusive evidence turned out to be elusive.
Now, a team led by Claes Fransson (Stockholm University) says they’ve cinched the case. “I think it is fair to say that this marks the discovery of a neutron star,” comments Fransson’s Stockholm colleague Dennis Alp, who previously studied the supernova but was not involved in the new research.
A massive star that runs out of nuclear fuel blows its outer layers out into space, forming an expanding supernova remnant. But the core of the star collapses in on itself. Depending on the core mass, this leads to either a neutron star — a superdense ball of nuclear particles more massive than the Sun but no larger than some 25 kilometers (16 miles) across — or a black hole.
At a distance of some 168,000 light-years, SN1987A was the nearest supernova observed in recent history. The detection of neutrinos produced by the blast suggested the formation of a neutron star, but the ultra-compact object remains hidden by gas and dust in the inner parts of the supernova remnant. “Dust absorbs a lot of the radiation,” says Fransson. “However, JWST observes in the infrared, where the absorption by dust is at a minimum.”
Webb’s sensitive mid- and near-infrared spectrographs have now detected emission lines of highly ionized argon and sulfur atoms (atoms that have lost up to five electrons) from the very center of the remnant, indicating the presence of a nearby energetic source of X-rays. According to Fransson, the only possible source is a hot, young neutron star, which has a surface temperature of 2 million to 3 million degrees and radiates in high-energy X-rays. The results appear in the February 23rd Science, on the supernova’s 37th anniversary .
Interestingly, the JWST results may indicate that the neutron star is racing through space at a velocity of a few hundred kilometers per second, as the argon and sulfur emission region is slightly offset from the original explosion center. The emission lines are blueshifted, indicating it’s moving in our direction. Such natal kick velocities are a well-known phenomenon of neutron stars, resulting from a slight asymmetry of the supernova explosion.
At left is JWST's near-infrared image of Supernova 1987A, released in 2023. The image at top right shows singly ionized argon, captured at mid-infrared wavelengths. The image at bottom right shows near-infrared light from multiply ionized argon. NASA, ESA, CSA, STScI, Claes Fransson (Stockholm University), Mikako Matsuura (Cardiff University), M. Barlow (UCL), Patrick Kavanagh (Maynooth University), Josefin Larsson (KTH)
Evidence for the existence of a neutron star built up slowly. In 2019, observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed a blob of warm dust, possibly heated by a neutron star.
Two years later, a team led by Emanuele Greco (then at the University of Palermo, Italy) found X-ray evidence for a pulsar wind nebula — a flow of charged particles, accelerated by the powerful magnetic field of a rapidly spinning neutron star. However, they could not exclude an alternative explanation: The X-rays they observed could also be produced by shocks in the glowing ring of gas surrounding the exploded star.
Thanks to the high angular resolution of JWST, Fransson’s team is now sure that those ionizing X-rays must originate very close to the explosion site. However, they still can’t distinguish whether the X-rays come from the surface of the neutron star itself or from a pulsar wind nebula around the star. “Both models can reproduce our observations,” Fransson says.
“The exciting news is that there is now evidence that indeed there is a neutron star, perhaps surrounded by a pulsar wind nebula,” says Jacco Vink (University of Amsterdam), who was not involved in the study. Vink notes that the new JWST observations are still indirect evidence, like the earlier results from Greco and his colleagues. “But they support each other,” he says. “The fact that the emission is observed close to the center looks already quite convincing.”
According to Vink, a firm direct proof would entail detecting radio or X-ray pulsations from the neutron star, or seeing an X-ray point source. “At this moment the central regions of SN1987A are still shielded by a lot of dust,” he says, “but in time this will likely be done.”
