Webb Finds Clues of Neutron Star at Heart of Supernova Remnant

The NASA/ESA/CSA James Webb Space Telescope has found the best evidence yet for emission from a neutron star at the site of a recently observed supernova. The supernova, known as SN 1987A, occurred 160 000 light-years from Earth in the Large Magellanic Cloud. SN 1987A was observed on Earth in 1987, the first supernova that was visible to the naked eye since 1604 — before the advent of telescopes.

It has offered astronomers a rare opportunity to study the evolution of a supernova and what was left behind, from the very beginning. SN 1987A was a type II, core-collapse, supernova, meaning that the compacted remains at its core are expected to have formed either a neutron star or a black hole. Evidence for such a compact object has long been sought. Indications for the presence of a neutron star has previously been found, but this is the first time that the effects of high energy emission from the young neutron star have been detected.

Astronomy typically involves the study of processes that take place over at least tens of thousands of years, far longer than all of human recorded history. Supernovae — the explosive final death throes of some massive stars — blast out within hours, and the brightness of the explosion peaks within a few months. The remains of the exploding star will continue to evolve at a rapid rate over the following decades. Thus, supernovae offer a very rare opportunity to study a key astronomical process in real time.

The supernova SN 1987A was first observed on Earth in February 1987 and its brightness peaked in May that year (although its distance from Earth means that the supernova event actually took place about 160 000 years before). It was the first supernova that could be seen with the naked eye since Kepler's Supernova in 1604.

About two hours prior to the visible light observation of SN 1987A, three observatories around the world saw a burst of neutrinos lasting a few seconds. The neutrino burst shortly before visible light from SN 1987A was linked to the same supernova event. This provided important clues to refine our understanding of core-collapse supernovae, Scientists suspected that this type of supernova would form a neutron star or a black hole.

Astronomers have searched for evidence for one of these compact objects at the centre of the expanding remnant material ever since. Indications for the presence of a neutron star at the centre of the remnant has been found in the past few years. Observations of much older supernova remnants — such as the Crab Nebula — confirm that neutron stars are found in many of these remnants. However, no direct evidence of a neutron star in the aftermath of SN 1987A (or any other such recent supernova explosion) had been observed, until now.

Claes Fransson of Stockholm University, and the lead author on this study, explains: “From theoretical models of SN 1987A, the ten-second burst of neutrinos observed just before the supernova implied that a neutron star or black hole was formed in the explosion. But we have not observed any compelling signature of such a newborn object from any supernova explosion. With Webb, we have now found direct evidence for emission triggered by the newborn compact object, most likely a neutron star.”

Webb began science observations in July 2022, and the observations behind this work were taken on 16 July, making the SN 1987A remnant one the first objects observed by Webb. The team used the Medium Resolution Spectrograph (MRS) mode of Webb’s MIRI instrument, which the members of the same team helped to develop. The MRS is a type of instrument known as an Integral Field Unit (IFU). IFUs are able to image an object and take a spectrum of it at the same time. The instrument captures a spectrum at each pixel, allowing observers to see spectroscopic differences across the object. Spectral analysis of the results showed a strong signal due to ionised argon from the centre of the ejected material that surrounds the original site of SN 1987A.

Subsequent observations using Webb’s NIRSpec (Near Infrared Spectrograph) IFU mode, at shorter wavelengths, more heavily ionised chemical species, including five times ionised argon (meaning argon atoms that have lost five of their 18 electrons). Such ions require highly energetic photons to form, and those photons have to come from somewhere. “To create these ions that we observed in the ejecta, it was clear that there had to be a source of high-energy radiation in the centre of the SN 1987A remnant," Fransson said. "In the paper we discuss different possibilities, finding that only a few scenarios are likely, and all of these involve a newly born neutron star.”

More observations of SN 1987A are planned this year, with Webb and ground-based telescopes. The research team hopes ongoing study will provide more clarity about exactly what is happening in the heart of this supernova remnant. These observations will hopefully spur the development of more detailed models, ultimately enabling astronomers to better understand not just SN 1987A, but all core-collapse supernovae.