Starquakes could explain strange behavior of neutron stars

A neutron star expends enormous amounts of energy for what is the burned out shell of a dead sun. The fastest neutron stars turn many times each second for thousands of years in their interstellar graves. Now a University of Chicago scientist and her collaborators have added faulting, starquakes and mountain building to the bizarre list of postmortem behaviors exhibited by these inhabitants of the stellar graveyard.

"Starquakes are different from earthquakes but in some ways they are similar," said Lucia M. Franco, a graduate student in Astronomy & Astrophysics at Chicago. "You're still moving matter and you're still forming mountains." The new starquake theory, motivated by more than three decades of data, appears to explain the mysterious rotational slowdowns that some of the best-observed neutron stars experience.

Franco presented the theory today at the centennial meeting of the American Astronomical Society in Chicago. Her co-authors are Bennett Link, a physics professor at Montana State University in Bozeman, and Richard Epstein, a physicist at Los Alamos National Laboratory in New Mexico. The research is supported by the University of Chicago Center on Astrophysical Thermonuclear Flashes, the National Science Foundation and the National Aeronautics and Space Administration.

A neutron star's theoretical mountains would rise to such tiny heights that a pinhead would dwarf them in comparison. Still, the matter on a neutron star is so tightly compressed and the gravity so incredibly strong that making even a pinhead mountain would require far more energy than that released in a thermonuclear bomb, Franco said.

Neutron stars contain approximately 1.5 times the mass of the sun in a sphere measuring only 10 miles or so in diameter. "I think of a neutron star as the sun compacted to the size of downtown Chicago," she said.

Scientists predicted the existence of neutron stars in the 1930s. After a star exploded in a supernova, scientists speculated, an ultracompressed core of matter made up of all neutrons would be left over. But scientists figured that by the time the supernova cloud of debris had cleared, the neutron star itself would be too small and too cold to detect from Earth.

Then pulsars were discovered in 1967. Pulsars send radio flashes sweeping across the sky like lighthouse beacons. Scientists realized that a beacon rotating as rapidly and as steadily as a pulsar had to come from a neutron star. "Any other star bigger and less compact than a neutron star would break apart trying to spin so fast," Franco explained.

Pulsars steadily flash their radio beacons for years, but occasionally they undergo a sudden increase in rotation speed that astronomers refer to as glitches. One of the best-known pulsars, in the Crab Nebula about 6,000 light years from Earth, has experienced six glitches since its discovery in 1968.

"We know about more than 700 radio pulsars. Of those 700 we know about 20 that glitch, that show these sudden spin-ups," Franco said. "And of these 20, at least three behave like the Crab pulsar, where after a glitch you see them spin down faster than they were spinning down before."

A neutron star's magnetic field plays a key role in the process. It was already known that a neutron star constantly loses rotational energy to its misaligned magnetic field, which then radiates that energy into space. Astronomers see the emitted radio waves as radio pulses.

Franco and her co-authors have modeled stresses in the solid outer crust of a neutron star that would quantitatively explain the more rapid rotational slowdowns that follow glitches. The faster an object such as the Earth or a neutron star rotates, the more stress the sphere builds as it spins down.

"As a neutron star spins down, it becomes less oblate and eventually the brittle crust must break," Franco said. This energy cracks the neutron star at the equator, allowing matter to move from the equator toward the magnetic poles. Little mountains form, throwing the distribution of matter at the surface out of balance, causing the star to wobble.

This wobble increases the difference between the star's rotational axis and the axis of its magnetic field, which in turn increases the star's energy loss.

"If they were perfectly aligned, the magnetic field would not be able to slow down the star at all," Franco said. Energy loss would be maximum, on the other hand, if the axes were perpendicular.

The theory nicely matches the changes in rotational spin-down rates for the three glitching pulsars for which good data are available. Now Franco has put out a call for more and better data on the other glitching pulsars.

"It would be interesting to see if their changes in spin-down rate match what our theory predicts," she said. A pulsar with a slower spin-down rate following a glitch would send the researchers back to the drawing board. Said Franco: "We would have trouble explaining that effect."

A high-resolution (300 dpi) color illustration of a quaking neutron star, as well as a Spanish version of this press release, is available at http://astro.uchicago.edu/home/web/lucia/press/index.html.