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Sept. 21, 1998: The discovery early this year of the
first magnetar - a highly magnetized star - put the spotlight
on a small class of stars called Anomalous X-ray Pulsars, or
AXPs. While the magnetar discovery involved another small class
of cosmic sources of high-energy radiation, the Soft Gamma Repeaters
(SGR), the magnetar theory holds that these objects may become
AXPs before they fade from the scene altogether.
 In
about a year, Dr. Jan van Paradijs, who won the 1997 Rossi Prize
for identifying the first optical counterpart for a gamma ray
burster, hopes to use the Advanced X-ray Astrophysics Facility
(AXAF) to take a closer look at two AXPs - one third of the known
population.
Above:
A scientist's concept of a magnetar depicts the spinning neutron
star with magnetic field lines rising from its surface and surrounded
by superheated plasma. (Links to 1,024x1,024-pixel,
76KB JPG). Credit: Robert Mallozzi/University of Alabama
at Huntsville and NASA/Marshall Space Flight Center.
"The reason I'm interested in them is that I suspect
they're magnetars," said van Paradijs, an astronomer with
the University of Amsterdam and the University of Alabama in
Huntsville and working at NASA's Marshall Space Flight Center.
Van Paradijs has been allocated 45,000 seconds (12.5 hours)
of observing time with the AXAF CCD Imaging Spectrometer (ACIS),
one of two principal instruments aboard AXAF (the other is a
high-resolution camera). AXAF also carries two spectral gratings
that will spread incoming X-rays in much the same way that a
prism spreads white light into colors.
ACIS actually is a two-in-one
camera designed to make high-resolution images and moderate-resolution
spectra of interesting X-ray sources like galaxies, pulsars,
and supernovae. One CCD - a charge-coupled device, similar to
those in TV camcorders - will produce images covering an area
of sky just 16.9 arc-minutes across (that' a little more than
half the apparent diameter of the Moon). The images will be 2,048
by 2,048 pixels in size, thus making highly detailed images.
The other part of the camera is a spectrometer that will divide
the X-ray spectrum into 8,192 slices - in effect, 8,192 X-ray
"colors" - for precise measurements of a source's energy
in these wavelength bands (slices). This information provides
understanding of the processes involved in the emission. |
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NASA's
NEXT
Great Observatory
 The world's largest and finest X-ray
telescope - the Advanced X-ray Astrophysics Facility (AXAF) -
is scheduled for launch aboard Space Shuttle Columbia in January
1999. With AXAF, astrophysicists at NASA's Marshall Space Flight
Center and around the world will observe energetic bodies ranging
from quasars down to dust clouds in a quest to understand more
of how and why the universe operates.
To help the public understand the purpose and value of AXAF,
we are running a series of stories that describe the science
that AXAF will support, and the investigations that will be carried
out by scientists at NASA/Marshall.
Other stories in the
series:
- How
hot is the Crab?: NASA's
next Great Observatory takes aim at the Crab Nebula pulsar
- Why
did the supernova change colors?
SN 1993J was seen to be one kind of massive explosion, but then
seemed to morph into a distinctly different kind. Scientists
using NASA's Advanced X-ray Astrophysics Facility, launching
in January, 1999, think they can discover why.
- Looking
for Pulsars in the Fast Lane
Scientists are looking for bizarre, short-lived, powerhouse stars
that burst with some of the brightest energy in the universe.
Using AXAF, they hope to find some of the few that may exist.
(this
story)
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Spreading X-rays from a source into its constituent colors
(wavelengths) is what van Paradijs wants to do with two AXPs
- 4U 0142+61 and 1E 2259+586 (the names refer first to the satellites
that detected them, Uhuru and Einstein, and their locations in
the sky).
Pulsars are pulsing neutron stars, discovered by radio astronomers
in 1967 and since observed in virtually every part of the electromagnetic
spectrum. They form when a massive star (about 8 times the mass
of our Sun) runs out of fuel in its core. The gas and radiation
pressures that supported the outer layers of the star disappear,
and gravity drives everything inward. |
 This
compresses the core into a 20 km (12 mi) wide ball of neutrons
crammed cheek-to-jowl, and generates super-intense pressures
that blast the outer layers into outer space. At that density,
a battleship would be stuffed into a pinhead (right). (Black
holes are formed when the star's mass is more than 30 times that
of the Sun.)
The neutron star retains much
of the old star's rotation (angular momentum) and magnetic field.
If conditions are right, the neutron star becomes a magnetar
with a magnetic field of a quadrillion gauss (Earth has a puny
field of 1 gauss). The magnetic field is so intense that it can
wrinkle the neutron star crust. When the wrinkles collapse, the
energy is transmitted into the surrounding plasma (ionized gas)
and becomes a blast of soft gamma radiation eventually detected
by satellites orbiting Earth.
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Short life is the price of being a magnetar. The SGR phase
lasts only 10,000 years, and may be followed by the AXP phase
for another 10,000 years or so. The implication of such short
lives is that SGRs and AXPs are quite common, but that at any
instant only a few will be young enough to be active. The galaxy
may actually be populated with tens of millions of dead magnetars
that have flashed (on astronomical timescales) through the SGR
and AXP phases.
Until recently, astronomers did not link the SGR and AXP classes.
Indeed, the AXP class was formed because a handful of X-ray pulsars
did not fit into the categories that the other X-ray pulsars
filled.
"It's a combination of things," van Paradijs said.
"They have have an awkward spectrum and they also have a
very limited period range which says there's something very special
going on." X-ray pulsars have periods ranging from less
than a tenth of a second to thousands of seconds. Generally,
slower pulsars are older ones.
But the six pulsars with awkward
spectra all have periods between 6 and 12 seconds, and some of
them are associated with relatively recent supernova remnants.
"We know six AXPs that are different from the bulk of
the X-ray pulsars," van Paradijs said. " In terms of
colors, the X-ray colors of the anomalous pulsars were very red
compared to what you might call the normal blue pulsars. Their
pulse periods were close together. All of them are 6 to 12 seconds,
which is very different from what you find with normal X-ray
pulsars, which have pulse periods as short as less than a tenth
of a second and as long as half an hour."
All these factors, van Paradijs believes, add up to a strong
magnetic field that is aging the pulsar faster than normal. |
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See what a spinning,
bursting magnetar might look like!
Click your network speed for
most appropriate movie:
- 28.8Kb or don't know
- 56Kb
- 300Kb or greater
Credit Dr. Robert Mallozzi /UAH
and NASA/Marshall Space Flight Center
Requires Realvideo player (version
5). Click to download the RealVideo
Player (from RealNetworks, Inc.
Animated Gif movies available (shows short
segment of burst animation)
- 117KB
- 346KB
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Neutron stars are the only stars with a solid
surface, a crystalline lattice of neutrons about 200 m (660 ft)
thick (left, 1296x781-pixel, 229K JPG). They also have strong
magnetic fields that rotate with the star (right; 774x576-pixel,
91K JPG). Since the 1970s, astronomers have observed signs
of starquakes when the crust on radio pulsars apparently cracked
and shifted, and slightly changed the pulsar's rotation. Magnetars,
though, are believed to have truly massive starquakes that emit
bursts of soft gamma radiation and slow the star even more. Credit:
NASA/Marshall Space Flight Center. |
 The magnetic field
whipping through the gases blown off by the supernova can form
a plerion (a supernova remnant powered by a rotating neutron
star) making the clouds of charged particles glow. Many supernovas
form beautiful nebulas, or gas clouds, that glow from the heat
of atoms colliding as they rush away from the explosion, like
beautiful bubbles in space. Some have obscuring clouds of dust
and gas which, when illuminated by hot stars, produce beautiful
effects - like the magnificent sunsets seen when clouds on the
western horizon play tricks with the sun's light.
Right: The Crab
Nebula, while not a magnetar, is a plerion. The lumps and bands
in the expanding gas cloud glow from the energy of the magnetic
field in the pulsar. This composite image places detailed Hubble
Space Telescope view of the center within a larger view taken
by the 5-meter (200-inch) Hale Observatory on Mount Palomar.
Van Paradijs expects the AXPs to be surrounded by plerions.
They will appear to be lumpy, as compared to the smooth bubble
of a shockwave nebula, and emit synchrotron radiation, so named
for the type of particle accelerator where scientists first observed
it on Earth.
"With AXAF, we hope to get evidence that they are indeed
magnetars," van Paradijs said. "With the X-ray spectroscopy
part of ACIS, we hope to see evidence of matter being ejected,
to see glowing wisps of materials across a few arc-seconds of
space."
If AXAF's data prove van Paradijs right, he will be able to
cement the connection between SGRs and AXPs, and help place them
under the heading of magnetar. Unfortunately, it is not likely
that he will be able to hunt for what comes after the AXP phase.
After 20,000 years, magnetars are believed to fade away from
notice. The neutron star is still there spinning, but the magnetic
dynamo that made all the uproar is expended, and the magnetar
husk spends the rest of eternity cold, dark, and unnoticed. |
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