In the News this month: mystery at the centre of Cas A
The Cassiopeia A supernova remnant CREDIT: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech/Steward/O.Krause et al
When massive stars explode as supernovae, they leave behind a dense, compact object: either a neutron star or a black hole depending on the mass of the original star. They also produce an expanding shell of debris known as a supernova remnant. Many of these shells are known in the Milky Way, but compact objects are not detected in all of them. One object in particular, the remnant known as Cassiopeia A has been expanding since its progenitor star exploded about 330 years ago, but for a long time no compact object was detected, despite many searches. Then, in 1999, observations from the Chandra X-ray Observatory showed X-ray emission coming from the centre of the remnant. But, the emission characteristics of the object did not match what astronomers expected to see from a neutron star or black hole, so its nature remained uncertain. Now, two researchers think they know why.
In a paper published in the journal Nature on November 5th, the astrophysicists suggest that Cas A does contain a central compact neutron star, but that it may be surrounded by an unusual atmosphere of carbon. The Cassiopeia A remnant is located at a distance of 3.4 kiloparsecs from the Earth and is one of the youngest known in the Milky Way, the explosion of the star which created the remnant may have been witnessed by the Astronomer Royal at the time, John Flamsteed, in 1680. Wynne Ho from the University of Southampton and Craig Heinke of the University of Alberta, created models of the emission that would be expected from various different types of neutron star atmospheres, and compared their models to spectra of Cas A from archive Chandra observations of the central compact object.
What their models showed was that if the neutron star has a normal hydrogen or helium atmosphere, the size of the emission region would be only 4 or 5 kilometres in diameter, much smaller than the size of this type of star predicted by standard models. This would suggest that the emission would be coming not from the entire surface, but from hotspots, however such hotspots would be hard to produce, and difficult to maintain at a constant temperature. If the star had a carbon atmosphere however, the predicted radius of the emission region is 12 to 15 kilometres, closely matching the predictions for the radius of a normal neutron star.
So why is the compact object in Cas A so unusual? The authors suggest that this could be due to its young age. They think that the neutron star could accumulate an atmosphere of hydrogen and helium over time, and develop a detectable spin, making it appear more like other, well-studied, neutron stars. While this is a plausible model for the compact object in Cas A, further observations are still required in order to prove it is the correct explanation.
Ho, W., & Heinke, C. (2009). A neutron star with a carbon atmosphere in the Cassiopeia A supernova remnant Nature, 462 (7269), 71-73 DOI: 10.1038/nature08525