Neutron Star Explosion Detected Deep Within the Milky Way--Physics of Extreme Matter Revealed for 1st Time
For the first time, researchers at MIT and elsewhere have detected all phases of thermonuclear burning in a neutron star. The star, located close to the center of a galaxy in the globular cluster Terzan 5 (image above), is a “model burster,” says Manuel Linares, at MIT’s Kavli Institute for Astrophysics and Space Research.
Terzan 5 (more details below) is located deep within our galaxy closely above the galactic plain about 20,000 light-years distance to the Earth. It has the highest density of stars of all globular clusters and contains the largest number of millisecond radio pulsars. The latter are rapidly rotating neutron stars which are thought to be part of close binary systems.
“These are extreme laboratories,” Linares says. “We can study fundamental physics by looking at what happens on and around the surface of neutron stars.”
Neutron stars typically arise from the collapse of massive stars. These stellar remnants are made almost entirely of neutrons, and are incredibly dense — about the mass of the sun, but squeezed into a sphere just a few miles wide. For the past three decades, astrophysicists have studied neutron stars to understand how ultradense matter behaves.
In particular, researchers have focused on the extremely volatile surfaces of neutron stars. In a process called accretion, white-hot plasma pulled from a neighboring star rains down on the surface of a neutron star with incredible force — equivalent to 100 kilograms (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. This explosion can be detected as X-rays in space: The bigger the explosion, the greater the X-ray intensity, which can be measured as a spike in satellite data.
Researchers have developed models to predict how a neutron star should burst, based on how much plasma the star is attracting to its surface. For example, as more and more plasma falls on a neutron star, explosions should occur more frequently, resulting in more X-ray spikes. Models have predicted that at the highest mass-accretion rates, plasma falls at such a high rate that thermonuclear fusion is stable, and occurs continuously, without giant explosions.
However, in the last several decades, X-ray observations from nearly 100 exploding neutron stars have failed to validate these theoretical predictions.
“Since the late 1970s, we mostly saw bursts at low mass-accretion rates, and few or no bursts at all at high mass-accretion rates,” Linares says. “It should be happening, but for three decades, we didn’t see it. That’s the puzzle.”
In late 2010, the RXTE satellite detected X-ray spikes from a binary star system — two stars bound by gravity and orbiting close to each other — in Terzan 5. Linares and his colleagues obtained data from the satellite and analyzed the data for characteristic spikes.
The team found the system’s neutron star indeed exhibited X-ray patterns consistent with low mass-accretion rates, in which plasma fell to the surface slowly. These patterns looked like large spikes in the data, separated by long periods of little activity.
To their surprise, the researchers found evidence for higher mass-accretion rates, where more plasma falls more frequently — but in these cases, the X-ray data showed smaller spikes, spaced much closer together. Even higher still, the data seemed to even out, looking more like an oscillating wave. Linares interpreted this last observation as a sign of marginally stable burning: a stage where a neutron star attracts plasma to its surface at such a high rate that nuclear fusion reactions take place evenly throughout the plasma layer, without exhibiting large explosions or spikes.
“We saw exactly the evolution that theory predicts, for the first time,” says Deepto Chakrabarty, professor of physics at MIT, and a member of the research team. “But the question is, why didn’t we see that before?”
The team soon identified a possible explanation by comparing the neutron star with others that have been studied in the past. The one big difference they found was that the neutron star in question exhibited a much slower rate of rotation. While most neutron stars rotate a dizzying 200 to 600 times per second, this new star rotated much more slowly, at 11 rotations per second.
The group reasoned that in predicting bursting behavior, existing models have failed to account for a star’s period of rotation. The reason this new star matches models so well, Linares says, is because its rate of rotation is almost negligible.
It’s still unclear exactly how rotation affects thermonuclear burning, although Linares has a hunch: Rotation can cause friction between layers of plasma and a neutron star’s surface. This friction can release heat, which in turn can affect the rate of nuclear burning.
“That’s something that we need to look into,” Linares says. “And 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, agrees that rotation may be the most significant factor that models have overlooked. However, he says designing models with rotation in mind is an incredibly data-intensive feat, since thermonuclear fusion often occurs incredibly quickly, in tiny pockets of a neutron star.
“If you’re going to fully model out a burst, you have to resolve microseconds and centimeters,” says Miller, who did not take part in the research. “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.”
Terzan 5 is a massive blob of over a million tightly packed stars, with up to 10,000 per cubic light-year. (Out with us it's less 0.02 over the same volume.) Examining their output with the wonderfully named Very Large Telescope (which combines four eight-meter apertures into a singe instrument effectively two hundred meters across) scientists have spotted distinct stellar signatures of both old and young stars.
This composition indicates that Terzan 5 evolved its stellar populations as a dwarf galaxy, and must once have been much larger than it is now. As old stars explode in supernovae they spread newly created material huge distances across space - so subsequently formed stars begin with more of these distinctive elements. But the blasts are so gigantic they would throw this starborne material right out of the current globular cluster - indicating Terzan 5 and others like it were once small galaxies in their own right.
Obscured behind galactic dust clouds the faint cluster was discovered in 1968 by Agop Terzan on photographic plates of the Haute-Provence Observatory in France. About 150 known globular clusters, concentrated spherical collection of very old stars, orbit the centre of our galaxy in form of a spherical swarm as part of the galactic halo.
Terzan 5 gained particular attention in 2009 when it turned out that is has two star populations of different age (12 and 6 billion years, respectively). Due to these unique properties Terzan 5 is assumed to be the remnant of a dwarf galaxy which has been captured by our galaxy.
Researchers of the Max Planck Institute for Nuclear Physics in Heidelberg and 33 other institutions within the H.E.S.S. collaboration reported the discovery of a source (HESS J1747 – 248) of very-high-energy gamma-rays from the direction of Terzan 5. The location in the close vicinity to the cluster suggests that the source is a so far unknown part of Terzan 5. The probability of a chance coincidence with an unrelated gamma emission (derived from the abundance of known sources) is less than 1/10,000.
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