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.
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