Astronomers have released pictures of the dramatic embers of the high-temperature fireballs that immediately followed two supernovae.
"This is the first evidence of a new type of supernova remnant - one that was heated right after the explosion," said Hiroya Yamaguchi at the Institute of Physical and Chemical Research in Japan.
A supernova remnant usually cools quickly due to rapid expansion following the explosion. Then, as it sweeps up tenuous interstellar gas over thousands of years, the remnant gradually heats up again.
Using the Suzaku satellite, the team detected unusual features in the X-ray spectrum of IC 443 - the Jellyfish Nebula - which lies about 5,000 light-years away in the constellation Gemini.
Several bumps in the Suzaku spectrum indicate the presence of a large amount of silicon and sulfur atoms from which all electrons have been stripped away, Yamaguchi said. These 'naked' nuclei produce X-rays as they recapture their lost electrons.
But removing all electrons from a silicon atom requires temperatures higher than about 30 million degrees F - hotter still for sulfur atoms. "These ions cannot form in the present-day remnant," Yamaguchi explained. "Instead, we're seeing ions created by the enormous temperatures that immediately followed the supernova."
The team suggests that the supernova occurred in a relatively dense environment. As a massive star ages, it sheds material in the form of stellar wind and creates a cocoon of gas and dust. When the star explodes, the blast wave heats the cocoon to temperatures as high as 100 million degrees F.
Eventually, the shock wave breaks out into interstellar space, where the gas density can be as low as a single atom per cubic centimeter. Once in this low-density environment, the young supernova remnant rapidly expands.
The expansion cools the electrons, but also thins the remnant's gas so much that collisions between particles become rare events. Because an atom may take thousands of years to recapture an electron, the Jellyfish Nebula's hottest ions remain even today.
The research appears in The Astrophysical Journal.