"Starquakes" Reveal Clues About Magnetar Composition

At the APS April Meeting in Dallas, Tod Strohmayer of NASA’s Goddard Space Flight Center demonstrated how the detailed trace of x-rays arriving from magnetars can be converted into information about seismic modes shaking the star and how properties of the star’s crust can be deduced from that. Among other findings, Strohmayer and his colleagues have measured the thickness of the crust of a neutron star for the first time.

When massive stars explode at the end of their lives, they can leave behind very dense, spinning neutron stars. Very little is known about their structure, but astronomers believe their cores may contain a state of matter that doesn’t exist anywhere else in the cosmos, at least not since the Big Bang itself. Magnetars are a specific type of neutron star featuring colossal magnetic fields, as high as 10¹⁵ Gauss. These fields might be strong enough to crack the crusts of the stars, and this in turn could prove to be the source of the huge energy bursts–dubbed hyperflares–coming to Earth from these dynamic objects.

One such event in 1998 and another in December 2004 are believed to have dispatched the largest batch of radiation to be detected from outside the solar system. The NASA team used the Rossi X-Ray Timing Explorer to make the measurements of a neutron star named SGR 1806-20, located about 40,000 light years from Earth in the constellation Sagittarius. Vibrations from the explosion revealed details about the star’s composition, much like how the study of seismic waves on Earth can reveal the structure of our planet’s crust and interior.

“We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell,” he said. “The vibrations created in the explosion, although faint, provide very specific clues about what these bizarre objects are made of. Just like a bell, a neutron star’s ring depends on how waves pass through layers of differing density, either slushy or solid.”

Among other data, Strohmayer presented fresh analysis of the 1998 and 2004 events, including the identification of additional vibrational modes. The measurements were confirmed using the Ramaty High Energy Spectroscopic Solar Imager, which also recorded the hyperflare and provided evidence for a high-frequency oscillation at 625 Hz, indicating waves traversing the crust vertically. The abundance of data enabled the researchers to determine the depth of the neutron star’s crust–nearly a mile, assuming a diameter of 12 miles across–by comparing the frequencies from waves traveling around the crust to those traveling radially through it.

According to Strohmayer, starquake seismology is a promising method for determining the properties of neutron stars, and a larger explosion detected in X-rays could reveal the elusive secret of the nature of the matter at such a star’s core. That material may be so dense that a single teaspoon would weigh close to 10 million tons on Earth. Among other exciting possibilities, the core might contain free quarks, which could further advance our scientific understanding of the nature of matter and energy.

“Neutron stars are great laboratories for the study of extreme physics,” said Anna Watts, Strohmayer’s colleague, who is now at the Max Planck Institute for Astrophysics in Garching, Germany. “We’d love to be able to crack one open, but since that’s probably not going to happen, observing the effects of a magnetar hyperflare on a neutron star is perhaps the next best thing.”