University of Arizona astronomers have pinpointed the origin of powerful bursts from nature's most magnetic objects. The bursts are from "magnetars," some of the most enigmatic objects in the universe.
Magnetars are a type of neutron star, which are superdense stars that pack the mass of a sun into a body the size of Manhattan Island. Tiny magnetars possess magnetic fields that are at least 100 trillion times as powerful as Earth's magnetic field. They occasionally produce powerful bursts, hurling high-energy radiation cascading across space. The origin of these energetic eruptions and the strong magnetic fields is a mystery.
Astronomers discovered a magnetar with the NASA's X-Ray Timing Explorer in July 2003, when it brightened by about 100 times its usual faint luminosity. They continued monitoring it regularly with the European Photon Imaging Camera, known as EPIC, on the European Space Agency's XMM-Newton Observatory until March 2006, when the object faded to its pre-outburst brightness.
As the magnetar faded, EPIC recorded changes in the energies of the X-rays released.
Tolga Guver, who is a visiting graduate student at the UA, working with Assistant Professor Feryal Ozel of the UA physics and UA astronomy departments, compared the magnetar's changing X-ray spectrum with predictions from a computer model. They developed the model to describe the physical properties of a magnetar's surface and magnetic field in detail.
Guver, Ozel and their collaborators found that the data was best fitted with a model that traced the outburst to just below the surface of the magnetar and confined it to an area about 3.5 kilometers (about two miles) across.
"This is the first time both the surface emission and its subsequent reprocessing in the magnetosphere have been incorporated into the same computer model," Ozel said.
This is a breakthrough because we can now distinguish between surface and magnetospheric phenomena,'' Guver said.
Determining both the size and the location of the powerful burst is like "performing anatomy on a distant, tiny star,'' Ozel added.
Their model also allowed Guver, Ozel and their colleagues to determine spectroscopically the strength of this object's magnetic field. The magnetar's magnetic field is around 600 trillion times stronger than the Earth's magnetic field.
The scientists say they are encouraged because the measurement is similar to an earlier estimate made based on how fast the source is "spinning down," which is the change in the spin period over time. They said it boosts their confidence that their model is correct.
"It is tremendously exciting to be able to compute exotic quantum phenomena that appear only in these ultrastrong magnetic fields and to see these predictions appear in actual data,'' Ozel added.
The astronomers say that they don't yet understand the mechanism of the outburst, which is probably somehow magnetically triggered.
The researchers say they plan to use their computer model to study more magnetars, using more data from X-ray observatories, in the quest for answers.
They are publishing their results in today's edition (Sept. 20, 2007) of the Astrophysical Journal Letters. The paper's authors are Guver, Ozel, Ersin Gogus of Sabanci University, Istanbul, Turkey, and Chryssa Kouveliotou of the NASA Marshall Space Flight Center, Huntsville, Ala.
SCIENTISTS MEASURE THE MOST POWERFUL MAGNET KNOWN
Scientists have identified the most magnetic object known in the Universe, the result of the first direct measurement of a magnetic field around a peculiar neutron star first observed nearly 25 years ago.
By following the fate of a tiny proton whipping about at near light speed close to the neutron star with NASA's Rossi X-ray Explorer satellite, scientists calculated this star's magnetic field to be up to 10 times more powerful than previously thought -- with a force strong enough to slow a steel locomotive from as far away as the Moon.
This object, named SGR 1806-20, is one of only ten unusual neutron stars classified as magnetars, thousands of times more magnetic than ordinary neutron stars and billions of times more magnetic than the most powerful magnets built on Earth. The strength of its magnetic field is approximately a million billion (10^15) gauss (100 billion tesla), according to a team led by Alaa Ibrahim, a doctoral candidate at George Washington University conducting research at NASA's Goddard Space Flight Center in Greenbelt, Md.
Other magnetars could be just as magnetic, although direct measurements have not yet been made, the team said. The Sun's average magnetic field (or dipole), in comparison, varies between 1 and 5 Gauss. Results are published in two articles in Astrophysical Journal Letters.
"If this magnetar were as close as the Moon, it would rearrange the molecules in our bodies," said Ibrahim. SGR 1806-20, however, is a safe 40,000 light years from Earth. (One light year is about six trillion miles or 9.5 trillion km.) "Although one would not want to get close to such an object, we now have a method of probing from afar to learn about the physics of matter under extreme gravitational and magnetic forces."
A neutron star is a compact sphere approximately 10 miles (16 km) wide, the core remains of a collapsed star once roughly ten time more massive than the Sun. In 1979, scientists observed a huge outburst from a neutron star, which, upon further analysis, marked the discovery of a new class of neutron stars now known as Soft Gamma-ray Repeaters (SGR). Scientists theorized that these objects must be highly magnetic in order to burst with such magnitude, and they coined the term "magnetar".
Scientists have estimated SGR magnetic fields by measuring the spin rate of the star along with the spin-down rate, that is, the rate at which the star's spin is slowing. Two scientists who have led this effort are Dr. Chryssa Kouveliotou of NASA's Marshall Space Flight Center and Dr. Kevin Hurley of the University of California at Berkeley. This is an indirect measure of magnetic field strength, for strong magnetic fields are thought to put the brakes on a spinning neutron star. The long-standing estimate has been over 10^14 Gauss.
Ibrahim's team identified an energy feature in many of the bursts emanating from SGR 1806-20. In analyzing the bursts spectral features, which is a graph showing the energy level emitted by light close to the neutron star surface, the team found a specific energy manifested at 5,000 electron volts.
This energy level, Ibrahim said, corresponds precisely to the energy needed to excite a proton trapped in an immense 10^15 Gauss magnetic field. This fits the magnetar "starquake" model, analogous to an earthquake, in which the surface of the neutron star momentarily cracks open and ejects protons. The quake itself is the source of the bursting seen in magnetars, or SGRs, and the ejected protons get trapped in the star's strong magnetic field loops.
These results on the proton feature meet theoretical predictions made by a number of scientists, including Drs. Silvia Zane of the Mullard Space Science Laboratory in the United Kingdom and Roberto Turolla of the University of Padova, Italy. However, other theorists expected the effect to be very difficult to observe.
Dr. Jean Swank of NASA Goddard, a co-author and the Rossi Explorer Project Scientist, noted that while electron signatures have provided key information about typical neutron stars powered by rotation and gravitation, protons are now revealing their presence in magnetars, providing exciting new information about these mysterious objects.
Co-authors of the Astrophysical Journal Letter reports are Dr. William Parke of the George Washington University in Washington, D.C., and Dr. Samar Safi-Harb of the University of Manitoba, Canada, in addition to Swank, Zane, and Turolla. The Rossi Explorer was launched in December 1995. NASA Goddard manages the day-to-day operation of the satellite and maintains its data archive.