James Webb Space Telescope Discovery

Webb Finds Evidence for Neutron Star at Heart of Young 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 a type II supernova that 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 thus offered the astronomical community a rare opportunity to study the evolution of a supernova and what was left behind, from the very beginning. SN 1987A was a core-collapse supernova, meaning 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, and whilst indirect evidence for the presence of a neutron star has previously been found, this is the first time that the effects of high energy emission from the young neutron star has 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 for astronomers 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 was observed in 1604. About two hours prior to the visible observation of SN 1987A, three observatories around the world observed a burst of neutrinos lasting only a few seconds. Shortly after, visible light from SN 1987A was observed. The two different observations were linked to the same supernova event, and provided important evidence to inform the theory of how core-collapse supernovae take place. This theory included the supposition that this type of supernova would form a neutron star or a black hole. Astronomers have searched for evidence for one or other of these compact objects at the centre of the expanding remnant material ever since. Indirect evidence for the presence of a neutron star at the centre of the remnant has been found in the past few years, and observations of much older supernova remnants — such as the Crab Nebula — confirm that neutron stars are found in many supernova 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 10-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 JWST, 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 Webb 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 fascinating instruments that are able to image an object and take a spectrum of it at the same time. An IFU forms a spectrum at each pixel, allowing observers to see spectroscopic differences across the object. Analysis of the Doppler shift of each spectrum also permits the evaluation of the velocity at each position. 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 the other IFU, NIRSpec (Near Infrared Spectrograph) at shorter wavelengths, the team found even more heavily ionised chemical species, particularly 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 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 the SN 1987A remnant. These observations will hopefully stimulate the development of more detailed models, ultimately enabling astronomers to better understand not just SN 1987A, but all core-collapse supernovae. Credit: ESA/Webb, NASA, 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). Full image
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