The supernova remnant known as the Jellyfish Nebula and IC 443 lies 5,000 light years away from Earth in the Gemini constellation (Fig. 1). Left after a stellar explosion, the remnant—hot plasma, surrounded by a cooler shell—is the first of its type to be observed by astronomers1. The finding is based on x-ray data collected aboard the Suzaku satellite.
“The satellite data have enabled us to investigate the explosion mechanisms that led to this supernova, as well as what was happening within the star before it exploded,” explains Hiroya Yamaguchi from the RIKEN Advanced Science Institute, Wako, who led the multi-institutional study.
Light, from long radio waves to x-rays, carries information about the activity in a stellar explosion. Both hot ions and fast moving electrons radiate x-rays in IC 443, which at 4,000 years old is considered a middle-aged remnant. Astronomers estimate the temperature of the ions and electrons in the remnant plasma by measuring the spectrum of these x-rays—that is, how the x-ray intensity varies with energy. The electron and ion temperatures, and any difference between them, yield clues as to how the star exploded and progressed through time.
Yamaguchi and his team noticed a curious discrepancy by analyzing the x-ray spectrum of IC 443: the silicon and sulfur ions, which are estimated to be a searing 14 million degrees Celsius, are nearly twice as hot as the electrons. In fact, the silicon and sulfur ions are so hot that some of them are completely stripped of their electrons.
“This is the first discovery of such spectral features in the x-rays emitted by a supernova remnant,” explains Yamaguchi.
This conclusive evidence for the process that astronomers call ‘overionization’ suggests that when the star that produced IC 443 exploded, a blast wave heated the dense gas around the star to the very high temperatures that stripped the electrons from the silicon and sulfur ions. This was followed by a shock wave that caused the gas to expand and allowed the electrons to cool, but rarefied the ions so much that they could not cool down again.
“Gamma-ray bursts and hypernova—which have energies more than 10 times that of supernova—are known to be some of the most energetic and explosive events in the universe, but the detailed explosion mechanism and nature of their progenitors are still unknown,” says Yamaguchi. “The application of our method will play an important role to solve these issues.”
The corresponding author for this highlight is based at the Cosmic Radiation Laboratory, RIKEN Advanced Science Institute