September 12, 2012
New Clues Into Kepler Supernova Explosion Reveal Power, Distance
April Flowers for redOrbit.com - Your Universe Online
A bright new star outshone even Jupiter in 1604 and then it dimmed over several weeks. Johannes Kepler and other sky watchers witnessed this event, and centuries later, the debris left from this stellar explosion is known at the Kepler supernova remnant.
Astronomers have studied the Kepler supernova remnant for a long time trying to determine exactly what happened when the star exploded. New analysis of a long observation by NASA's Chandra X-ray Observatory is providing new clues.
Chandra is a telescope designed specially to detect X-ray emissions from very hot regions of the Universe, such as exploded stars, clusters of galaxies, and the matter around black holes. Chandra orbits above Earth to mitigate the X-ray absorption of the atmosphere.
The analysis suggests that the supernova explosion was more powerful and much farther away than was previously thought.
The image is a composite of Chandra data obtained over a 8 day observation. The X-rays are shown in five colors from lower to higher energies: red, yellow, green, blue and purple. These various X-ray slices were then combined with an optical image from the Digitized Sky Survey (light yellow and blue) showing stars in the field.
The Digitized Sky Survey is a digital version of several photographic atlases of the night sky. It is also an ongoing project to produce even more digital versions of photographic astronomical datasets.
Scientists determined from previous analysis of the image that the explosion that created Kepler was what is called a "Type Ia" supernova. A Type Ia occurs when a white dwarf star gains mass, either by pulling gas off a companion star or merging with another white dwarf, until it becomes unstable and is destroyed by a thermonuclear explosion. Kepler's debris field, unlike other Type Ia supernova remnants, is being strongly shaped by what it is running into. Most Type Ia remnants are very symmetrical, but the Kepler remnant is asymmetrical with a bright arc of X-ray emission in its northern region. This indicates the expanding ball of debris from the supernova explosion is plowing into the gas and dust around the now-dead star.
The study, published in The Astrophysical Journal, suggests that the bright X-ray arc can be explained in two ways. In one model, the pre-supernova star and its companion were moving through interstellar gas and losing mass at a significant rate via a wind. This created a bow shock wave similar to that of a boat moving through water. The second model suggests that the X-ray arc is caused by debris from the supernova expanding into a an interstellar cloud of gradually increasing density.
The first model requires that Kepler supernova remnant is located at a distance of more than 23,000 light years. In the second model, the gas into which the remnant is expanding has a higher density than average, and the distance of the remnant from the earth is between 16,000 — 20,000 light years. Both models give greater distances than the commonly used value of 13,000 light years.
In either model, the X-ray spectrum--that is, the amount of X-rays produced at different energies--reveals the presence of a large amount of iron, and indicates an explosion more energetic than the average Type Ia supernova. Additionally, to explain the observed X-ray spectrum in this model, a small cavity must have been cleared out around the star before it exploded. Such a cavity, which would have a diameter less than a tenth that of the remnant's current size, might have been produced by a fast, dense outflow from the surface of the white dwarf before it exploded, as predicted by some models of Type Ia supernovas.
Evidence for an unusually powerful Type Ia supernova has previously been observed in another remnant with Chandra and an optical telescope. These results were independently verified by subsequent observations of light from the original supernova explosion that bounced off gas clouds, a phenomenon called light echoes. This other remnant is located in the Large Magellanic Cloud, a small galaxy about 160,000 light years from Earth, making it much farther away than Kepler and therefore more difficult to study.