Diamond Encrusted Dragon's Egg

A long time ago, a massive star about 10,000 light years from Earth went kaboom.
329 years ago, we think, in 1680, the light from the supernova explosion reached Earth and was recorded as a new star by the Flamsteed, then the Astronomer Royal, looking relatively dim as nearby supernove go, due to the layers of dust in the galaxy between us and the site of the explosion.

Now, digging into archival x-ray data, a couple of astronomers may finally have figured out what is going on in Cass A.

It is a ball of ultradense degenerate neutronium, plated with a thin layer of diamond.

A Neutron Star with a Carbon Atmosphere in the Cassiopeia A Supernova Remnant - Ho & Heinke, Nature Nov 5th 2009

The supernova remnant Cassiopeia A is one of the most spectacular in the neighbourhood and has been extensively studied in all bands, particularly with high resolution x-ray imaging using the Chandra observatory.
We think the progenitor was a red supergiant which exploded as a type IIb core collapse supernova, and we can see the heterogenous composition of the shells of hot gas in the remnant, where nuclei fused in the progenitor star and during the explosion, blew off in "onion layers" and the mixed turbulently as the shockwave of the explosion overtook the outer layers.

Cass A on the move - Chandra Press Release 2009

There is an old puzzle with Cass A.
There is, in the x-rays, a clear point source near the center of the supernova remnant.
Its brightness corresponds to about twice the luminosity of the Sun, but radiating mostly in the x-rays.
It is the neutron star remnant of the explosion, a small ball of ultradense, degenerate nuclear matter, the core of the progenitor star, massing about half again as much as the Sun, but compressed to the size of a small city - predicted to be 10-15 km (or 6-10 miles) in radius.

The puzzle is that the spectrum of the neutron star did not match, at all, the predictions.
Most neutron stars have high magnetic fiellds - trillion gauss or more - compared to less than 1 gauss for the Earth. Young neutron stars, less than few million years old, in particular tend to have high magnetic fields.
They are also very hot, with surface temperatures in the millions K.

Spitzer composite image of Cass A - click to embiggen

We expect the bulk of the neutron star to be nuclear matter - almost pure neutronium - but the crust is a layer of neutron rich nuclei, shading to iron, probably, at the boundary.
The atmosphere, we thought would be iron vapour, or H/He vapour, depending on the age, temperature and accretion of gas onto the surface.

Under these assumptions, the Cass A neutron star, was measured to be too small and too hot - it was conjectured this was due to a "hot spot" of only few km radius. But the temperature should rapidly equilibrate across the crust, and it is difficult to sustain such temperature differences.

Cass A - Chandra 5th Anniversary Images

Well, Ho and Heinke have found an atmospheric model that fits the archival Chandra data.
If the atmosphere is carbon - about 1 cm layer of carbon vapour, with one poetically hopes, a thing layer of crystalline carbon deposit on the surface over the iron and neutrons, and if the magnetic field is about 80 billion gauss - hundred times smaller than normally assumed, then the x-ray spectra fit just fine.
The fit also requires that the spectrum observed was "gravitationally redshifted" by about 30% - a general relativistic effect that has been observed before, but is nice to see in the "strong" regime, rather than at the part-per-billion level...

That atmosphere, at 1 cm, is not as thin as it sounds - its about 1/10,000th of the radius of the star - compared to the scale height of the Earth's atmosphere which is about 1/1,000th the radius of the Earth.

Cass A - the movies

Further, this gives a neutron star radius of 12-14 km, and mass of 1.5 - 2 solar masses, or so, indicating a slightly over massive neutron star and a "stiff" equation of state - which implies neutron star masses can be quite large - up to 2 solar masses or so - and that dense nuclear matter has high pressure and cools slowly, compared to the full range of possible models. This is interesting.

The low magnetic field is also puzzling - there are three possibilities:
one is that there is a sub-class of neutron stars that are just born with low magnetic fields, and presumably are not seen as radio pulsars;
or, the magnetic field is there, but buried under the hot crust, and will "bubble out", probably relatively rapidly, increasing in strength maybe by 1-2 orders of magnitude over time scales of 1,000-10,000 years - which might be measurable;
or, the magnetic field "grows", as was conjectured theoretically about 25 years ago, probably through some thermoelectric effect, which would be very cool, and a nice vindication of an old clever idea.

Very cool result.

Now we just have to figure out how the heck pure carbon can end up at the surface without fusing to higher mass elements...

Hmm, does the presence of carbon on the surface increase the probability of life on neutron star's or not?

Cass A - from Chandra - click to embiggen
Note the little dot in the middle. That is the neutron star, glowing hot in x-rays.

Another Cass A view from Chandra.

Now

Cass A - radio false colour composite image - click to embiggen
Note the absence of a bright point source near the center - the neutron star is radio quiet,