RARE
CLASS OF EXOTIC STARS REVEALED AS SUPER-MAGNETS
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, Ala.
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. The source is at least 8,000
light years from Earth. (One light year is almost six trillion
miles or 9.6 trillion kilometers.) "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 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.
Back
to Top
|