Looking For Pulsars Living In The Fast Lane

September 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.

"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.

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.

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.

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.

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.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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