Neutron star EXO 0748-676 (blue sphere in Image 1) is part ...



Caption for "Equation of State" animation and images...

Animation 1: Scientists can study a neutron star when explosions on the star's surface fill the region with light and reveal features. This animation shows one such explosion, a thermonuclear X-ray burst.

Images 1-6: These images are frames pulled from the animation. The neutron star in EXO 0748-676 (blue sphere in Image 1) is part of a binary star system, and its neighboring star (yellow-red sphere in Image 1) supplies the fuel for the thermonuclear bursts. During solar outbursts or when the orbit brings the stars closer together, gas from the companion star flows toward the neutron star, attracted by its strong gravity. The flow of gas forms a swirling disk around the neutron star, called an accretion disk (multi-colored swirl around the blue sphere in Image 1).

Thermonuclear bursts arise as gas moving at close to the speed of light crashes onto the neutron star surface. This is shown in Image 2, which is a close-up of the neutron star. The dark blue area is the edge of the neutron star, and the mottled, light blue area is the accretion disk, seen face-on. The gas, pinned to the neutron star by gravity, spreads across the surface. As more and more gas rains down, pressure builds and temperature climbs until there is enough energy for nuclear fusion. This ignites a chain reaction that engulfs the entire neutron star within a second (Images 3 and 4). Let there be light... X-ray light, actually, far more energetic than the visible light our eyes detect. Bursts last for one to two minutes and can occur several times per hour. Images 5 and 6 pull back to show the bursting neutron star with its companion star (red sphere).

Image Credit: NASA, GSFC, Dana Berry

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Images 7-8: What's inside a neutron star? You guessed it: neutrons. But the devil is in the details. The results presented by Strohmayer and Villarreal suggest that the neutron star diameter is about 14 miles and the mass is 1.75 solar masses. Theory states that the neutron star crust is about a mile thick. Under this is likely a superfluid of neutrons. The extreme gravity has compressed protons and electrons into neutrons. Depending on the equation of state (the extent of density and pressure), neutrons could be compressed to liberate quarks. If so, the neutron star would contain a quark-gluon plasma. Gluons are the forces binding quarks within the atomic nucleus. Liberated quarks are thought only to have existed in a fraction of a second after the Big Bang. Strohmayer and Villarreal estimate of the mass-radius ratio of a neutron star has not been thoroughly analyzed by theorists. This ratio determines the density-pressure relationship and thus the state of matter inside the neutron star. The results appear to rule out free quarks (for this star, anyway). The characteristics of the superfluid, however, is a mystery. Note that a so-called quark star can be viewed as a steppingstone between a neutron star and a black hole.

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Animation 2: This is part of the data from the Rossi X-ray Timing Explorer that Villarreal and Strohmayer used to calculate the spin rate of the neutron star in EXO 0748-676. The Rossi Explorer creates spectra, not images. These spectra detail the energy and time characteristics of the X-ray light emitted by hot gas on and around objects such as neutron stars. The data here represent a running average of how strongly the X rays vary over time in each of 38 bursts detected during the past several years. This is called a power spectrum. At first, the data appear to be just noise. Yet as each new burst is added to the running average, the noise shrinks and a distinct, tall line emerges at the 45-hertz mark on the X-axis. (This is on the log-10 format, so "45" is between the 4th and 5th notch after the number 10.) The strong signal at 45 hertz implies that the neutron star is spinning 45 times per second.

Credit: A. Villarreal and T. Strohmayer

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