SCIENTISTS MEASURE THE MOST POWERFUL MAGNET KNOWN: November

by Goddard Space Flight Center

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.