NuSTAR Bringing the High Energy Universe into Focus

In late May of 2023, the light from a red supergiant star that exploded in the galaxy M101 arrived at the Earth. This event, known as SN 2023ixf, is the closest supernova discovered in nearly a decade and is the closest supernova to come from the collapse of a massive star since the famous SN1987A, which exploded in one of the Milky Way's satellite galaxies and was first observed in February 1987. NuSTAR observations of this more recent supernova have enabled astronomers a unique view into the explosion and provide insights into the last years of a massive star’s life.

Unlike many supernovae, we know the time that SN 2023ixf exploded with an accuracy of only a few minutes, thanks to observations by amateur astronomers. M101 is a face-on spiral galaxy (earning the moniker the “Pinwheel Galaxy”) and is a common target for astrophotographers and astronomers on the ground. The supernova was discovered by an amateur astronomer and supernova hunter in Japan, Koichi Itagaki, and word quickly spread around the globe. Other amateur astronomers inspected their exposures from M101, resulting in the precise identification of the time that light from the supernova first arrived at the Earth. The announcement of this supernova also immediately spurred follow-up observations by professional astronomers, including radio telescopes, the Hubble Space Telescope, as well as X-ray observatories like NuSTAR.

The fact that the supernova was discovered early enables an entirely unique view of the early days of the supernova and the last years of the dying star. Massive stars lose their upper atmosphere in a “stellar wind” of material as they age. In the weeks and months after the star explodes, the shock wave from the fast-moving explosion plows into the tenuous, slower moving stellar wind, providing a detailed view of how much material the star was losing. In the first few days after the explosion, the shock wave encounters material lost by the host star only a year or two before the explosion.

The alert that a new supernova had been found arrived at the NuSTAR Science Operations Center late on a Friday afternoon, just hours after the supernova had been discovered. Thanks to the hard work of the NuSTAR operations team, the telescope was able to observe M101 starting early Monday morning, roughly four days after the supernova was discovered. For comparison, it was 17 days before NuSTAR observed the previous record holder, SN 2017eaw in NGC 6946 (AKA the “Fireworks Galaxy”). NASA’s fastest-slewing X-ray telescope, the Neil Gehrels Swift Observatory which primarily observes low-energy X-rays and high-energy gamma-rays, had already started observing M101 but had not detected any X-ray emission. In the high-energy X-rays observed by NuSTAR, the supernova already shone brightly.

In a report published in the Astronomical Journal in early June, NuSTAR scientist Dr. Brian Grefenstette and collaborators describe these first NuSTAR observations of the supernova explosion. X-rays are caused by a shock wave plowing into material lost by the star in the years before it died. As that circumstellar material spreads out over time, it becomes less dense (and therefore more transparent to low energy X-rays). Close in to the star, the wind material is dense enough to absorb the low energy X-ray light, while the high energy X-rays can penetrate this material and be observed by NuSTAR. In the observation taken 4 days after the explosion, absorption from this gas and dust was clearly seen in the NuSTAR data. However, when NuSTAR returned to look at the supernova again roughly a week later, the absorption was gone (and the soft X-ray signal had started to be observed). This changing absorption can be explained by the shock wave traveling out into the less-dense circumstellar material. By comparing NuSTAR data from the two time periods, astronomers were able to estimate the amount of material lost by the progenitor star.

The shock speed can also be estimated from the temperature of the hot gas observed by NuSTAR, which turned out to be roughly 5000 km per sec (equivalent to 11 million miles per hour). This fast-moving shock rapidly catches up with the material in the star’s wind, which travels out at a more pedestrian 10 km per sec (equivalent to 22,000 mph). Four days after the explosion, the shock had traveled roughly 38 times the distance from the Earth to the Sun (i.e., 38 astronomical units, or 38 AU), roughly equivalent to the distance from the Sun to Pluto. At that distance, the supernova ejecta was encountering material lost by the progenitor star only three and a half years before it exploded. Eleven days after the explosion, the shock had traveled out to 96 AU and was catching up with stellar wind released a decade before the star exploded.

These observations are only the start of our understanding of SN2023ixf. A special session at the American Astronomical Society Winter Meeting held in New Orleans in January 2024 brought together astronomers who have studied this supernova (and its progenitor star) to try to understand the mystery of how stars explode. The rich set of observational data, including additional NuSTAR observations at later times, will truly allow a detailed exploration of how this “Supernova of the Decade” exploded.