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Researchers Close in on X-ray Burst Ignition

You’re sitting on the rocky surface of a distant planet, in your climate controlled space-suit, watching the suns rise. That’s right, two suns, and one of them is a neutron star. Its companion is not dense enough to keep its matter to itself, so there is a trail of stardust linking the two suns.

The more interesting of the two is the accreting neutron star – a neutron star that is gaining matter. This matter forms a ring around the star as it makes its way toward the surface. Near the surface of the neutron star, the hydrogen-rich matter compresses until the nuclei are forced to higher energies. The hydrogen fuses steadily, creating helium and heat in the atmosphere.

Now the conditions are set for a spectacular phenomenon – an X-ray burst. Carbon and oxygen are rapidly converted to heavier elements in the range from nickel to cadmium, releasing extra energy in the form of X-rays. It's a runaway fusion reaction, exploding in the star's atmosphere!

An X-ray burst requires a trigger. The trigger reaction involves a radioactive isotope of oxygen, oxygen-15 (mass number 15), and a helium-4 nucleus (a.k.a “alpha particle”). The oxygen-15 fuses with the alpha particle, resulting in a neon-19 nucleus and a gamma ray.

When gamma rays are emitted in such a reaction, they usually have a range of different energies. The peaks in the energy distribution, or the most common energies, are called resonances.

Twenty years ago, theorists predicted that the gamma ray has only one resonance, and its energy is 4.03 million electronvolts, or 4.03 MeV. Since then, researchers have been struggling to confirm or refute this calculation through experiment.

The trouble is that current facilities can’t make a beam of oxygen-15 that is intense enough to create the trigger reaction in a laboratory. Many scientists tried, but none succeeded.

Researchers at Notre Dame University, led by Michael Weischer, decided to try it backwards. They made neon-19 with extra energy, 4.03 MeV to be exact. The excited neon nucleus would then decay.

Nuclei decay in many ways. They emit protons, neutrons, alpha particles, gamma rays, and X-rays in many combinations. In this case, the scientists wanted the neon-19 to emit an alpha particle, resulting in oxygen-15.

This decay doesn’t happen very often, so sensitive detectors were needed to accurately measure it. They also measured the lifetime of neon-19 in a 4.03 MeV excited state, or how long it remained excited before decaying.

Researchers use these two pieces of information to figure out the reaction rate, how often the alpha particles and oxygen-15 nuclei fuse. These measurements are also helpful in setting tight limits on the accretion rate, and most importantly, the ignition point of the X-ray burst reaction. This point, between steady burning and explosion, is at about 1.9x1028 grams per second.

Want a more formal rundown? Check out the Joint Institute of Nuclear Astrophysics (JINA) "Nugget." Or, if you've got the hook-up, this research is soon to be published in Physical Review Letters.

Illustrations from NASA


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