Magnetars, The Most Magnetic Stars Known, More Common than Previously Thought

Observations of explosions from an ultra-powerful magnetic neutron star
playing hide-and-seek with astronomers suggest that these exotic objects
called magnetars — capable of stripping a credit card clean 100,000 miles
away — are far more common than previously thought.

Scientists from the United States and Canada present this result today at
the meeting of the American Astronomical Society in Atlanta. The work is
based on observations with the European Space Agency’s XMM-Newton
observatory and NASA’s Rossi X-ray Timing Explorer.

“We only know of about ten magnetars in the Milky Way galaxy,” said the
investigation’s leader, Dr. Peter Woods of NASA’s Marshall Space Flight
Center in Huntsville, Ala., based at the National Space Science and
Technology in Huntsville. “If the antics of the magnetar we are studying now
are typical, turning on and off but never getting exceptionally bright, then
there very well could be hundreds more out there.”

Wood’s colleagues are: Dr. Vicky Kaspi and Mr. Fotis Gavriil of McGill
University in Montreal; Dr. Christopher Thompson of the Canadian Institute
for Theoretical Astrophysics; Drs. Herman Marshall, Deepto Chakrabarty and
Kathy Flanagan at the Massachusetts Institute of Technology; and Drs. Jeremy
Heyl and Lars Hernquist at the Harvard-Smithsonian Center for Astrophysics.

The source in question is a magnetar “candidate” named 1E 2259+586 in the
constellation Cassiopeia, approximately 18,000 light years from Earth. A
magnetar is a special neutron star. A neutron star is a compact sphere
approximately 15 kilometers (10 miles) in diameter, the core remains of a
collapsed star roughly ten times more massive than the Sun. Magnetars, for
reasons poorly understood, have magnetic fields a thousand times stronger
than ordinary neutron stars, measuring 1014 to 1015 Gauss (or about a
hundred-trillion refrigerator magnets; the Sun’s magnetic field is about 5
Gauss.)

Not all scientists are convinced that neutron stars can be so magnetic. As
such, magnetar candidates are often referred to in the scientific literature
as either Soft Gamma-ray Repeaters (SRGs) or Anomalous X-ray Pulsars (AXPs),
depending on their bursting characteristics. Members of this observation
team helped established the connection between SRGs and AXPs in 2002. The
source 1E 2259 is sometimes called an AXP.

For all their power, magnetars are not always majestic beacons. The
opportunity to study them comes when they erupt for hours to months, without
warning, emitting visible light and other wavelengths before growing dim
once more. Magnetar 1E 2259 suddenly began bursting in June 2002.
Scientists collected data on over 80 bursts recorded within a 4-hour window.
No other bursts have been detected since.

These same changes in emissions happened 12 years ago and remained a mystery
until this study. “Knowing what we know now, we realize that the earlier
burst activity was too dim to observe,” said Woods.

The cumulative properties of the outburst in 1E 2259+586 led the team to
make several conclusions: First, the star suffered some major event lasting
several days with two distinct components, one on the surface of the star
(perhaps a fracture in the crust) and the other beneath the surface.

According to Kaspi, “The changes in persistent emission properties suggest
that the star underwent a plastic deformation of the crust that
simultaneously impacted the superfluid interior and the magnetosphere.” (A
neutron star’s interior is thought to be a superfluid of neutrons. The
magnetosphere refers to the region in which the neutron star’s magnetic
field controls the behavior of the charged particles.)

The emission after the bursting was similar to that of an SGR, further
blurring the distinction between these two exotic species, Kaspi said.
Also, from the changes in emission, the scientists could infer previous
burst active episodes from this and other magnetar candidates.

“This sort of behavior could be happening all the time in other sources like
it throughout the Galaxy and we would never know it because our gamma-ray
‘eyes’ are not sensitive enough,” said Woods.

Thus, the non-detection of such outbursts by telescopes scanning the entire
sky for X-ray and gamma-ray sources suggests that the number of magnetar
candidates in our Galaxy is larger than previously thought but that they are
in a prolonged dim phase. The team plans to calculate this number. Helping
them will be the NASA Swift Gamma-Ray Burst Explorer, scheduled for launch
in mid-2004. Swift will be about 20 times more sensitive to magnetar bursts
than anything that has flown before. “If there is a big population of these
objects out there, Swift should find them,” Woods said.

“Magnetars are not just the most magnetic stars known but they are stars not
powered by a conventional mechanism such as nuclear fusion, rotation or
accretion,” Kaspi said. “Magnetars represent a new way for a star to shine,
which makes this a fascinating field.”

ESA’s XMM-Newton was launched in December 1999. NASA helped fund mission
development and supports guest observer time. The Rossi Explorer was
launched in December 1995. NASA’s Goddard Space Flight Center in Greenbelt,
Md., manages the day-to-day operation of the satellite and maintains its
data archive.

Peter Woods joins the National Space Science and Technology Center through
the Universities Space Research Association. Fotis Gavriil is a graduate
student in the Physics Department of McGill University.