An article published in the journal “Science” reports new evidence that the supernova SN 1987A generated a pulsar. A team of researchers used observations conducted with the James Webb Space Telescope to detect the effects of high-energy emissions coming from the pulsar or the pulsar wind nebula, a nebula that surrounds it and is powered by the pulsar. These are confirmations of conclusions reached by other teams of researchers in recent years using observations in other electromagnetic bands.
The image (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)) shows the remnant of the supernova SN 1987A as seen by the James Webb Space Telescope. On the left is a view captured with the NIRCam (Near-Infrared Camera) instrument in 2023 showing the central structure, expanding at a speed of several kilometers per second. The blue region is the densest one of the ejected materials, which contain heavy elements such as carbon, oxygen, magnesium, and iron. The ring surrounding that region is the result of the collision of materials with a ring of gas ejected about 20,000 years before the supernova. On the left, at the top, the remnant seen by the MIRI instrument, and at the bottom by the NIRSpec instrument.
Since 1987, when the supernova SN 1987A became visible on Earth even with the naked eye, astronomers have been trying to locate what is left of the progenitor star within its remnant. The amount of materials that form that remnant, which are now cold and dense when seen from Earth, continues to make a direct view of an object that should be a neutron star or a black hole impossible.
Various studies offered indirect evidence that the object left after supernova SN 1987A is a neutron star, for example, the one in a paper published almost exactly three years ago in “The Astrophysical Journal” that was based on X-ray observations. Now, the James Webb Space Telescope made it possible to conduct infrared observations as well that offer new evidence that it’s a pulsar, a type of neutron star.
There has been great interest in the supernova SN 1987A since its sighting, so observations of its remnant with the James Webb Space Telescope were among the first conducted after the start of its science mission. The researchers used the MIRI (Mid-Infrared Instrument) and NIRSpec (Near-Infrared Spectrograph) instruments to obtain crucial information.
The MIRI instrument was used in its Medium Resolution Spectrograph (MRS) mode, which allows a spectroscopic examination of the observed object to be conducted. In the case of the supernova remnant SN 1987A, the researchers found the chemical signature of ionized argon from the center of the ejecta around the area where the progenitor star was located.
Subsequent observations used the NIRSpec instrument to conduct a spectroscopic examination based on observations conducted at shorter wavelengths. This time, the researchers found chemical signatures of even more heavily ionized elements, in particular, five-times ionized argon, meaning the atoms lost five of their eighteen electrons. High-energy photons are needed to generate those types of ions.
Claes Fransson of Stockholm University, Sweden, the lead author of this study, explained that he and his colleagues looked at various possibilities. What is certain is that there must be a source of high-energy radiation at the center of the SN 1987A supernova remnant but there are several mechanisms that could cause the detected ionization. However, only a few of those possibilities are likely: a central neutron star or a pulsar wind nebula, which still requires the presence of a neutron star.
The picture that emerges by combining the results of this study of the supernova remnant SN 1987A with the ones conducted in recent years in other electromagnetic bands such as X-rays favors the model with the pulsar wind nebula.
Emanuele Greco of the Italian National Institute of Astrophysics in Palermo, who led the 2021 study at X-rays, stated that confirmation of the presence of a neutron star is crucial to understand how these objects evolve in the early years of their life. He also stressed the fact that the one generated by the supernova SN 1987A is the youngest neutron star we know and therefore represents a unique cosmic laboratory of its kind.
The authors of the study conducted with the James Webb Space Telescope intend to continue analyzing the data collected and conducting other observations, also with ground-based telescopes. New information may bring new confirmations to the current conclusions or offer some surprises. They could also help improve theoretical models of supernovae caused by the collapse of a massive star, which generate heavy elements that are scattered throughout interstellar space.
