Sooty neutron star could lie at the heart of Cassiopeia A.
Two astrophysicists believe that they have dispelled the mystery surrounding an object at the centre of a distant supernova remnant.
Around 330 years ago, a massive star in the constellation Cassiopeia exploded. The supernova may have been recorded by John Flamsteed, the English Astronomer Royal, who, at the time, observed a 'star' in the constellation that doesn't correspond to any known on today's charts.
The remains of the supernova, known as Cassiopeia A, have been something of a mystery to astronomers. Supernovae usually leave behind an extremely dense object such as a black hole or neutron star. But for decades no such object was seen at the centre of Cassiopeia A.
In 1999, the Chandra X-ray Observatory finally identified a compact object. However, "what it saw didn't correspond to what astronomers thought a neutron star should look like," says Craig Heinke, an astrophysicist at the University of Alberta in Canada. Neutron stars often have strong, spinning magnetic fields that make them appear to pulse, but the object in the centre of Cassiopeia A burned steadily. Moreover, the energies of X-rays coming from the object didn't match what astronomers expected.
Now Heinke and his colleague Wynn Ho at the University of Southampton, UK, believe they have an explanation for the mysterious object. They propose that Cassiopeia A does contain a neutron star and this star is shrouded in an atmosphere of carbon. Such an atmosphere would make the dead star's remains appear to glow bright blue at optical wavelengths, and would explain the unusual X-ray energies seen by scientists. The work is published in this week's issue of Nature1.
Heinke says that Cassiopeia A's unusual appearance might be due to its youth. Over time, he says, the star might accumulate hydrogen and helium, and develop a detectable spin. This would make it appear more similar to other, older neutron stars.
"I think it's interesting," says Harvey Tananbaum, the director of the Chandra X-ray Center at the Harvard Smithsonian Center for astrophysics in Cambridge, Massachusetts. Tananbaum says that the paper offers no irrefutable evidence that a carbon-covered neutron star sits at the centre of Cassiopeia A, and follow-up observations could yield better clues about the nature of the object. But, he adds, the paper is "maybe the best explanation that's out there".
Ho, W. C. G. & Heinke, C. O. Nature 462, 71-73 (2009).
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Brumfiel, G. Supernova mystery solved?. Nature (2009). https://doi.org/10.1038/news.2009.1063