Fermi Discovers Evidence Of Starquakes In Magnetar Burst Eruptions

Chuck Bednar for redOrbit.com – Your Universe Online
New analysis of high energy blasts from a magnetar, originally observed by NASA’s Fermi Gamma-ray Space Telescope in 2009, has resulted in the discovery of underlying signals related to seismic waves rippling throughout the highly magnetized neutron star, the US space agency revealed on Tuesday.
These signals were originally identified during the fadeout of rare giant flares produced by magnetars, NASA explained. Giant flares have only been observed three times over the past four decades (1979, 1998 and 2004), and signals related to events known as starquakes have only been identified in the two most recent events, they added.
“Fermi’s Gamma-ray Burst Monitor (GBM) has captured the same evidence from smaller and much more frequent eruptions called bursts, opening up the potential for a wealth of new data to help us understand how neutron stars are put together,” Anna Watts, an astrophysicist at the University of Amsterdam and the co-author of a new study about the burst storm, said in a statement. “It turns out that Fermi’s GBM is the perfect tool for this work.”
Neutron stars are basically the compressed core of a massive star that ran out of energy, collapsed beneath its own weight and exploded as a supernova. They are the densest, fastest-spinning and most magnetic objects in the universe than can be directly observed by scientists, NASA noted, and the typical neutron star has mass equal to 500,000 Earths packed into a sphere just 12 miles across (or about the same length as Manhattan Island).
Typical neutron stars have magnetic fields trillions of times stronger than that of our homeworld’s, but the eruptive activity observed from magnetars requires an even more powerful magnetic field – 1,000 times stronger than normal. Only 23 confirmed magnetars have been confirmed to date, NASA officials pointed out.
“Because a neutron star’s solid crust is locked to its intense magnetic field, a disruption of one immediately affects the other,” according to NASA Goddard’s Francis Reddy. “A fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface. Either way, the changes trigger a sudden release of stored energy via powerful bursts that vibrate the crust, a motion that becomes imprinted on the burst’s gamma-ray and X-ray signals.”
A tremendous amount of energy is required to cause convulsions in a neutron star. The closest comparison on Earth would be the 1960 Chilean earthquake, which at 9.5 magnitude is the most powerful tremor ever recorded on the standard seismological scale. Using the same scale, Watts said that a starquake associated with a magnetar’s giant glare would reach magnitude 23.
“The 2009 burst storm came from SGR J1550−5418, an object discovered by NASA’s Einstein Observatory, which operated from 1978 to 1981,” Reddy explained. “Located about 15,000 light-years away in the constellation Norma, the magnetar was quiet until October 2008, when it entered a period of eruptive activity that ended in April 2009.”
“At times, the object produced hundreds of bursts in as little as 20 minutes, and the most intense explosions emitted more total energy than the sun does in 20 years. High-energy instruments on many spacecraft, including NASA’s Swift and Rossi X-ray Timing Explorer, detected hundreds of gamma-ray and X-ray blasts,” the agency added.
Speaking Tuesday at the Fifth Fermi International Symposium in Nagoya, Japan, Watt said that her team’s research examined 263 individual bursts detected by Fermi’s GBM confirming vibrations in the frequency ranges previously seen in giant flares.
“We think these are likely twisting oscillations of the star where the crust and the core, bound by the super-strong magnetic field, are vibrating together,” she explained. “We also found, in a single burst, an oscillation at a frequency never seen before and which we still do not understand.”
“While there are many efforts to describe the interiors of neutron stars, scientists lack enough observational detail to choose between differing models,” NASA added. “Neutron stars reach densities far beyond the reach of laboratories and their interiors may exceed the density of an atomic nucleus by as much as 10 times. Knowing more about how bursts shake up these stars will give theorists an important new window into understanding their internal structure.”



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