Everything Science Forum

Scientists using NASA's Rossi X-ray Timing Explorer have estimated the depth of the crust on a neutron star, the densest object known in the universe. The crust, they say, is close to a mile deep and so tightly packed that a teaspoon of this material would weigh about 10 million tons on Earth.

The measurement, the first of its kind, came courtesy of a massive explosion on a neutron star in December 2004. Vibrations from the explosion revealed details about the star's composition. The technique is analogous to seismology, the study of seismic waves from earthquakes and explosions that reveal the structure of the Earth's crust and interior.

This new seismology technique provides a way to probe a neutron star's interior, a place of great mystery and speculation. Pressure and density are so intense here that the core might harbor exotic particles thought to have existed only at the moment of the Big Bang.

Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., presents this result in a press conference this week at the April meeting of the American Physical Society in Dallas. His colleague, Anna Watts, formerly at Goddard, is now at the Max Planck Institute for Astrophysics in Germany.

"We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell," said Strohmayer. "The vibrations created in the explosion, although faint, provide very specific clues about what makes up these bizarre objects. A neutron star's ring depends on how waves pass through layers of differing density, either slushy or solid."
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Strohmayer and Watts examined a neutron star named SGR 1806-20, about 40,000 light years from Earth in the constellation Sagittarius.  . . . .

On December 27, 2004, the surface of SGR 1806-20 experienced an unprecedented explosion. As reported by NASA and the National Science Foundation in early 2005, this was the brightest X-ray flash ever seen from beyond our solar system. The explosion, called a hyperflare, was caused by a sudden change in the star's powerful magnetic field that cracked the crust, likely producing a massive starquake. The event was detected by several space observatories, including the Rossi Explorer, which observed the X-ray light emitted.

Strohmayer and Watts think that the oscillations are evidence of global torsional vibrations within the star's crust. These vibrations, like waves moving through a rope, are analogous to the S-waves observed during terrestrial earthquakes. Their study, building on observations of vibrations from this source by GianLuca Israel of Italy's National Institute of Astrophysics, found several new frequencies during the hyperflare.

Watts and Strohmayer subsequently confirmed their measurements using NASA's Ramaty High Energy Spectroscopic Solar Imager, a solar observatory that also recorded the hyperflare. And they found the evidence for a high-frequency oscillation at 625 Hz, indicative of waves traversing the crust vertically.

Because the gravitational field of a neutron star is much stronger than that of a white dwarf, the accretion under degenerate conditions leads to much higher temperatures than in the nova outburst. This in turn tends to produce X-rays rather than visible light in the thermonuclear runaway on the surface of the neutron star.

SGR 1900+14 - the strongest known magnet in the galaxy - is believed to be a city-sized [~ several km], spinning, super-magnetic neutron star, or Magnetar. How strong is a Magnetar's magnetic field? The Earth's magnetic field which deflects compass needles is measured to be about 1 Gauss, the strongest fields sustainable (steady-state) in Earth-based laboratories are about 100,000 Gauss, yet the Magnetar's monster magnetic field is estimated to be 1,000,000,000,000,000 Gauss. A magnet this strong, located at about half the distance to the Moon would easily erase your credit cards and suck pens out of your pocket. From a distance of about 20,000 light-years, SGR 1900+14 recently generated a powerful flash of gamma-rays detected by many spacecraft.