Gotcha: Firm Evidence for a Neutron Star in Supernova 1987A

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 clinched 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.

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.”