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Image of the Day: Supernova Cassiopeia A --"A Relativistic Pinball Machine"




This extraordinarily deep Chandra image shows Cassiopeia A (Cas A, for short), the youngest supernova remnant in the Milky Way. New analysis shows that this supernova remnant acts like a relativistic pinball machine by accelerating electrons to enormous energies.

The blue, wispy arcs in the image show where the acceleration is taking place in an expanding shock wave generated by the explosion. The red and green regions show material from the destroyed star that has been heated to millions of degrees by the explosion. Astronomers have used this data to make a map, for the first time, of the acceleration of electrons in a supernova remnant. Their analysis shows that the electrons are being accelerated to almost the maximum theoretical limit in some parts of Cas A.

Protons and ions, which make up the bulk of cosmic rays, are expected to be accelerated in a similar way to the electrons. Therefore, this discovery provides strong evidence that supernova remnants are key sites for energizing cosmic rays.

Charged particles are believed to scatter or bounce off tangled magnetic fields in the shock wave, which act like bumpers in a pinball machine. When the particles cross the shock front they are accelerated, as if they received a kick from a flipper in a pinball machine.



Typically it should take a few hundred scatterings off the shock's magnetic field before the particles cross the shock front. It then takes about 200 crossings of the shock front to accelerate the particles seen in the Chandra data. Image above shows CasA in the infrared via NASA/CXC/SAO.

Scientists estimate it would take about 200 years -- over half the age of the remnant -- to accelerate electrons to cosmic ray energies in the slowest parts of the shocks, but only about 50 years to accelerate the highest energy electrons in the regions of maximum acceleration.

The NRAO radio image of Cas A is shown below.




A new Chandra X-ray study of the remains of Cas A indicates that the supernova that disrupted the massive star may have turned it inside out in the process. Using very long observations of Cassiopeia A a team of scientists has mapped 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.

The predominant concentrations of different elements of Cas A in the image below of the original star are represented by different colors: iron in the core (blue), overlaid by sulfur and silicon (green), then magnesium, neon and oxygen (red). The image uses the same color scheme to show 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 Cas A, and because almost all the oxygen ions have had all their electrons stripped away.




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. Surprisingly, there is no evidence from X-ray (Chandra) or infrared (Spitzer Space Telescope) observations for iron near the center of the remnant, where it was formed. Also, much of the silicon and sulfur, as well as the magnesium, is now found toward the outer edges of the still-expanding debris. The distribution of the elements indicates that a strong instability in the explosion process somehow turned the star inside out.

This latest work, which builds on earlier Chandra observations, represents the most detailed study ever made of X-ray emitting debris in Cas A, or any other supernova remnant resulting from the explosion of a massive star. It is based on a million seconds of Chandra observing time. Tallying up what they see in the Chandra data, astronomers estimate that the total amount of X-ray emitting debris has a mass just over three times that of the Sun. This debris was found to contain about 0.13 times the mass of the Sun in iron, 0.03 in sulfur and only 0.01 in magnesium.

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, or Ti-44. Emission from Ti-44, which is unstable with a half-life of 63 years, has been detected in Cas A with several high-energy observatories including the Compton Gamma Ray Observatory, BeppoSAX, and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

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See for a younger supernova remnant in the Milky Way.

The distribution of the elements indicates that a strong instability in the explosion process somehow turned the star inside out.

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