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In the news this (last) month: massive neutron star

Massive neutron star.
Pulses from neutron star (rear) are slowed as they pass near foreground white dwarf. This effect allowed astronomers to measure masses of the system. CREDIT: Bill Saxton, NRAO/AUI/NSF
Left over from the supernova explosions of massive stars, neutron stars are incredibly dense and compact objects, but very little is known about their internal structure. Pulsars, spinning neutron stars with powerful jets of radio emission which act something like cosmic lighthouses, are useful probes of extreme physics such as General Relativity and forms of matter so dense that investigating them in laboratories on the Earth is extremely difficult.

Various models of the internal structure of a neutron star have been proposed, including various exotic forms of matter, but determining which is closest to reality requires knowledge of the distribution of masses and radii, measurements which require careful observations and depend to some extent on the exact models and assumptions used.

Reported in Nature on 28th October, a team of astronomers using the Green Bank radio telescope in the US have discovered a pulsar with a mass twice that of the Sun. These new results have implications not only for our understanding of neutron stars and their formation, but also for our understanding of nuclear physics and matter at very high densities, and suggest that many of the theoretical models of neutron star structure can now be ruled out.

Led by Paul Demorest of the National Radio Astronomy Observatory in the US, the astronomers observed the binary millisecond pulsar J1614-2230, a pulsar orbiting a white dwarf in a binary system which lies almost edge-on to our line of sight. This geometry was vital, allowing them to make use of an effect known as the Shapiro delay, an effect of general relativity. This is the delay of a signal caused as it moves through the gravitational field of the white dwarf companion - a delay which is a maximum when the pulsar lies on the far side of its orbit relative to the Earth. This effect allowed the mass of both the neutron star and its white dwarf companion to be measured precisely.

The neutron star was expected to have a mass of about 1.5 times that of the Sun, but they calculated a mass of 1.97 solar masses for the neutron star, the highest mass to be accurately measured for such an object. Combining this mass measurement with predictions based on various different physical models allows several scenarios to be ruled out, including several exotic states of matter containing subatomic particles such as hyperons or kaons.

The discovery also has implications for other astronomical events. One class of gamma ray burst is thought to be the result of colliding neutron stars. The fact that neutron stars have now been shown to be this massive makes this a viable mechanism for these events.

This blog post is a news story from the Jodcast, aired in the November 2010 edition.

Demorest, P., Pennucci, T., Ransom, S., Roberts, M., & Hessels, J. (2010). A two-solar-mass neutron star measured using Shapiro delay Nature, 467 (7319), 1081-1083 DOI: 10.1038/nature09466

Posted by Megan on Friday 17th Dec 2010 (07:56 UTC) | Add a comment | Permalink


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