In a First, Neutron Star Bursts as Models Predicted

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Astronomers using NASA’s Rossi X-ray Timing Explorer (RXTE) satellite have detected what they say is the first neutron star to burst the way that models predict.

“Since the late 1970s, we mostly saw burst at low mass-accretion rates, and little or no bursts at high-mass accretion rates,” Massachusetts Institute of Technology (MIT) researcher Manuel Linares said in a press release previewing a paper he co-wrote for the March 20 issue of The Astrophysical Journal. “It should be happening, but for three decades, we didn’t see it. That’s the puzzle.”

While researchers have developed models — based on how much plasma the star is attracting to its surface — for predicting how a neutron star bursts, X-ray observations from nearly 100 exploding neutron stars taken over the last several decades have failed to validate these theoretical predictions.

But in late 2010, the RXTE satellite detected X-ray spikes from a binary star system in Terzan 5. Linares and his colleagues obtained data from the satellite and found evidence for higher mass-accretion rates, where more plasma falls more frequently.

“We saw exactly the evolution that theory predicts, for the first time,” MIT astrophysics professor Deepto Chakrabarty said in a statement. “But the question is, why didn’t we see it before?”

One possible explanation, according to the researchers, is that this particular neutron star exhibited a much slower rate of rotation than previously observed neutron stars — 11 rotations per second compared with the 200 to 600 times per second common to neutron stars.

Linares said it is still unclear how rotation affects thermonuclear burning, but his hunch is that rotation causes friction between layers of plasma and a neutron star’s surface. This friction releases heat, which in turn affects the rate of nuclear burning.

“That’s something we need to look into,” Linares said. “And now models will have to incorporate rotation, and will have to explain exactly how that physics works.”