A team of scientists using long observations from NASA’s Chandra X-ray Observatory has suggested a massive star that became the Cassiopeia A supernova remnant may have turned inside out as it exploded.
The new X-ray study, published in the Astrophysical Journal, describes the distribution of elements in the supernova remnant in unprecedented detail. This information shows where the different layers of the pre-supernova star are located three hundred years after the explosion, and provides insight into the nature of the supernova.
An artist’s illustration (left) shows a simplified picture of the inner layers of the star that formed Cassiopeia A just before it exploded, with the predominant concentrations of different elements represented by different colors: iron in the core (blue), overlaid by sulfur and silicon (green), then magnesium, neon and oxygen (red).
The image from NASA’s Chandra X-ray Observatory (right) shows the distribution of iron, sulfur and magnesium in the supernova remnant. The data show that the distributions of sulfur and silicon are similar, as are the distributions of magnesium and neon. Oxygen, which according to theoretical models is the most abundant element in the remnant, is difficult to detect because the X-ray emission characteristic of oxygen ions is strongly absorbed by gas in along the line of sight to Cassiopeia A, and because almost all the oxygen ions have had all their electrons stripped away.
The researchers found clumps of almost pure iron, indicating that this material must have been produced by nuclear reactions near the center of the pre-supernova star, where the neutron star was formed. That such pure iron should exist was anticipated because another signature of this type of nuclear reaction is the formation of the radioactive nucleus titanium-44. Emission from titanium-44, which is unstable with a half-life of 63 years, has been detected in Cassiopeia A with several high-energy observatories.
A comparison of the illustration and the Chandra element map shows clearly that most of the iron, which according to theoretical models of the pre-supernova was originally on the inside of the star, is now located near the outer edges of the remnant.
This study represents the most detailed study ever made of X-ray emitting debris in Cassiopeia A, or any other supernova remnant resulting from the explosion of a massive star.