Scientists Make New Neutron Star Discovery
March 2, 2012

Scientists Make New Neutron Star Discovery

Researchers from several universities have detected all phases of thermonuclear burning in a neutron star for the first time.

The team discovered the "model burster" star located close to the center of the galaxy in the global cluster Terzan 5.

The star is the first of its kind to burst the way that models predict, and it could help explain why a model star like it has gone undetected for so long.

Astrophysicists have been studying neutron stars for decades to try and understand how ultra dense matter behaves.

Researchers from MIT, McGill University, the University of Minnesota and the University of Amsterdam analyzed X-ray observations from NASA's Rossi X-ray Timing Explorer (RXTE) satellite.

Scientists have developed models to predict how a neutron star should burst, based on how much plasma the star is attracting to its surface.

These models predicted that at the highest mass-accretion rates, plasma falls at such a high rate that thermonuclear fusion is stable, and occurs continuously.

Accretion is when white-hot plasma pulled from a neighboring star rains down on the surface of a neutron star, equivalent to the force of 220 pounds of matter slamming into an area the size of a coin every second.

As more plasma falls, it forms a layer of fuel on the neutron star's surface that builds to a certain level, then explodes in a thermonuclear fusion reaction.  The bigger the explosion, the greater the X-ray intensity.

However, recent X-ray observations of about 100 exploding neutron stars have failed to validate models scientists have used.

The researchers found the neutron star in Terzan 5 exhibited X-ray patterns consistent with low mass-accretion rates.  These patterns looked like large spikes in the data, separated by long periods of little activity.

The team discovered evidence for higher mass-accretion rates, where more plasma falls more frequently, but found smaller spikes, spaced closer together.

“We saw exactly the evolution that theory predicts, for the first time,” Deepto Chakrabarty, professor of physics at MIT, and a member of the research team, said in a press release. “But the question is, why didn´t we see that before?”

They found that the neutron star exhibited a much slower rate of rotation at 11 rotations per second, compared to what most neutron stars rotation of 200 to 600 times per second.

Manuel Linares, a postdoc at MIT´s Kavli Institute for Astrophysics and Space Research, said the reason this new star matches models so well is because its rate of rotation is almost negligible, and researchers failed to account for a star's rotation.

He said that "now models will have to incorporate rotation, and will have to explain exactly how that physics works.”

Coleman Miller, professor of astronomy at the University of Maryland, said designing models with rotation in mind is an incredibly data-intensive feat, since thermonuclear fusion occurs so quickly.

“If you´re going to fully model out a burst, you have to resolve microseconds and centimeters,” Miller, who did not take part in the research, said in a press release. “No computer has been designed to do this. So these are interesting, likely suggestions, but it is going to be profoundly difficult to confirm in a definitive way.”

The research will be published in the March 20 issue of The Astrophysical Journal.


Image Caption: Plasma from a neighboring star gets pulled into the orbit of a neutron star, where it slams into the stellar surface, creating thermonuclear explosions. Image: NASA


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