Scientists have found that a rare and enigmatic class of neutron
stars, of which only five are known, are actually magnetars —
exotic stars with magnetic fields trillions of times stronger
than the Sun’s or Earth’s, so powerful that they could strip a
credit card clean 100,000 miles (about 160,000 kilometers) away.

These neutron stars, called Anomalous X-ray Pulsars (AXP), had
defied physical explanation since the first such object was
discovered in 1982. The newly exposed AXP-magnetar relationship
is featured in the September 12 issue of Nature, based on data
obtained with NASA’s Rossi X-ray Timing Explorer spacecraft.

The finding, by a team led by Prof. Victoria Kaspi of the
McGill University Department of Physics in Montreal, Canada,
essentially doubles the number of known magnetars.

“People have suspected for years that these bizarre AXPs might
be magnetars,” said Kaspi, “but at last we have definitive
proof. We finally caught one bursting, just like a magnetar.”

Joining Kaspi on this observation is Fotis Gavriil, lead author
on the Nature article and a graduate student in the Physics
Department of McGill University, and Peter Woods of the
National Space Science and Technology Center in Huntsville,

A neutron star is an ultra-dense sphere approximately 10 miles
(16 kilometers) wide, the core remains of a collapsed star once
roughly ten time more massive than the Sun. Neutron stars that
emit steady pulses of X-ray radiation with each rotation are
called X-ray pulsars.

AXPs have been labeled “anomalous” because scientists have
been unable to determine their energy source. Other types of
neutron stars shine by virtue of either gravitational or
rotational energy, options that are simply not available to
AXPs. Mysteriously, AXPs are also extremely dim or completely
undetectable in any other region of the electromagnetic
spectrum except X rays.

The AXPs’ source of X rays, this latest Rossi Explorer
observation demonstrates, is magnetic energy.

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). These stars enter into periods of outburst
when they flare with low-energy, or “soft,” gamma-ray light
(also called high-energy, or “hard,” X rays).

By the mid-1990s, scientists had evidence that SGRs had
magnetic fields a thousand times stronger that ordinary
neutron stars, measuring 10^14 to 10^15 Gauss. (That’s about
a hundred trillion refrigerator magnets at 10 Gauss each;
the Sun’s magnetic field is about 5 Gauss.) SGRs were then
labeled as magnetars, which were theorized objects not yet
observed by that point. Only five SGR-magnetars are known.

AXPs have not been known to burst, like SGRs … until now.
Kaspi’s team has observed the first burst from an AXP,
linking the two types of exotic sources. AXPs, in fact, may
be younger versions of SGRs.

The team spotted the bursting from a source named AXP
1E 1048-5937, in the constellation Carina. “It is the
combination of similarities in spin parameters, spectra, and
especially now the bursting that provide a compelling link
between AXPs, SGRs, and magnetars,” said Gavriil.

(In 1996, theorists Christopher Thompson and Robert Duncan
predicted that magnetar-like bursts might one day be seen
from an AXP, contrary to all other models of the day.)

Kaspi said the team also detected a spectral feature in one of
the AXP bursts at the energy level of 14 kilo-electron volts
(keV). If this spectral feature is from a fast-flying proton
buzzing around the neutron star’s surface, this would imply
that the AXP has a magnetic field of a million billion (10^15)
Gauss. However, celestial magnetic fields are difficult to
measure precisely, and this technique remains unproven.

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

For an artist’s conception of a magnetar and more information,
link to: