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Surprise of the super neutron star
Posted: 28 October 2010

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A heavyweight neutron star has proven to be the most massive of its kind ever found, helping scientists narrow down the range of possibilities for what lurks within its exotic interior. The finding also makes more plausible the idea that short-duration gamma-ray bursts are caused by colliding neutron stars.

This illustration describes how the pulsar’s beams flash close to its companion white dwarf, whose gravity slows the beams. Image: Bill Saxton/NRAO/AUI/NSF.

Neutron stars are the ultra-dense remnants of supernovae – they are the compacted, crushed cores of stars, with the equivalent of the mass of the Sun packed into a sphere no bigger than a few tens of kilometres across. During the supernova, atoms are crushed to such a degree that electrons merge with protons to form neutrons, and within the centre of the neutron star pressures are so extreme that exotic particle physics come into play. There are many theoretical models that describe what may possibly be occurring within the heart of a neutron star, and because the conditions are so extreme they cannot be tested in any laboratory on Earth.

However, natural laboratories in space provide a means to study neutron stars. Using a specially built digital instrument called the Green Bank Ultimate Processing Instrument (GUPPI) on the Green Bank Radio Telescope in the USA, astronomers in the United States and the Netherlands measured an effect first predicted by Albert Einstein that allows a determination of a neutron star’s mass. The object that they studied, PSR J1614-2230, is a rapidly rotating neutron star called a pulsar, which blasts out beams of radio waves from its poles. As the pulsar rotates, we see the beams ‘pulsing’ towards us, in this case 317 rotations per second.

What makes PSR J1614-2230 – which is 3,000 light years away – almost unique is that it is in a binary system with a white dwarf star, which orbits the pulsar every nine days. Thanks to a chance alignment, the white dwarf’s orbit and the direction of the pulsar’s beams are almost edge-on to us, so we see the white dwarf passing directly in front of the pulsar, and the pulsar beams passing very close to the white dwarf. This is where a phenomenon known as the Shapiro delay (named after Harvard Professor Irwin Shapiro, and first predicted in Einstein’s General Theory of Relativity) comes into play. The mass of the white dwarf slows the radio beams as they are deflected by the white dwarf’s gravity, causing a delay in when we receive them. The magnitude of this delay pointed to the mass of the white dwarf, and consequently the mass of the neutron star. They found that the neutron star contains twice as much mass as our Sun, and is the most massive neutron star ever detected.

Its huge mass rules out some theoretical models about how matter behaves under the incredible temperatures and pressures inside a neutron star (temperatures can reach millions of degrees). One model that can now be ruled out postulated the presence of exotic particles called hyperons, which are baryons that contain ‘strange’ quarks but no ‘charm’ or ‘bottom’ quarks (briefly, the building blocks of most particles are things called quarks, which come in six flavours: up, down, top, bottom, strange and charm). Another idea was that condensates of kaons, which contain a strange quark and an up or down antiquark (the antimatter equivalent of a quark), or an strange antiquark and an up or down quark, populate the interiors of neutron stars, but the extreme mass of PSR J1614-2230 rules this out also.

Although the exact nature of the extreme physics occurring within neutron stars is still unknown, the new results help constrain the possibilities. “This measurement tells us that if any quarks are present in a neutron star core, they cannot be ‘free’ but rather must be strongly interacting with each other as they do in normal atomic nuclei,” says Feryal Ozel of the University of Arizona.

Neutron stars – or more specifically colliding neutron stars – have also been blamed for causing short-duration gamma-ray bursts (GRBs) that last for less than a couple of seconds. That neutron stars can be so massive proves that they can provide a big enough gamma-ray punch to explain these GRBs.

The results are published in the 28 October issue of the journal Nature.

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