Evidence for a thin carbon atmosphere on a neutron star at the center of Cas A has been found. The properties of the carbon atmosphere are remarkable. It is only about four inches thick, has a density similar to diamond and a pressure more than ten times that found at the center of the Earth. As with the Earth’s atmosphere, the extent of an atmosphere on a neutron star is proportional to the atmospheric temperature and inversely proportional to the surface gravity. The temperature is estimated to be almost two million degrees, much hotter than the Earth’s atmosphere. However, the surface gravity on Cas A is 100 billion times stronger than on Earth, resulting in an incredibly thin atmosphere.
Evidence for a thin veil of carbon has been found on the neutron star in the Cassiopeia A supernova remnant. This discovery, made with NASA’s Chandra X-ray Observatory, resolves a ten-year mystery surrounding this object.
“The compact star at the center of this famous supernova remnant has been an enigma since its discovery,” said Wynn Ho of the University of Southampton and lead author of a paper that appears in the latest issue of Nature. “Now we finally understand that it can be produced by a hot neutron star with a carbon atmosphere.”
By analyzing Chandra’s X-ray spectrum – akin to a fingerprint of energy – and applying it to theoretical models, Ho and his colleague Craig Heinke, from the University of Alberta, determined that the neutron star in Cassiopeia A, or Cas A for short, has an ultra-thin coating of carbon. This is the first time the composition of an atmosphere of an isolated neutron star has been confirmed.
The Chandra “First Light” image of Cas A in 1999 revealed a previously undetected point-like source of X-rays at the center. This object was presumed to be a neutron star, the typical remnant of an exploded star, but researchers were unable to understand its properties. Defying astronomers’ expectations, this object did not show any X-ray or radio pulsations or any signs of radio pulsar activity.
By applying a model of a neutron star with a carbon atmosphere to this object, Ho and Heinke found that the region emitting X-rays would uniformly cover a typical neutron star. This would explain the lack of X-ray pulsations because — like a lightbulb that shines consistently in all directions — this neutron star would be unlikely to display any changes in its intensity as it rotates.
Scientists previously have used a neutron star model with a hydrogen atmosphere giving a much smaller emission area, corresponding to a hot spot on a typical neutron star, which should produce X-ray pulsations as it rotates. Interpreting the hydrogen atmosphere model without pulsations would require a tiny size, consistent only with exotic stars made of strange quark matter.
“Our carbon veil solves one of the big questions about the neutron star in Cas A,” said Craig Heinke. “People have been willing to consider some weird explanations, so it’s a relief to discover a less peculiar solution.”