Rare class of exotic stars revealed as super-magnets
Posted: September 14, 2002

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.

This is an artist's concept of a magnetar, an intensely magnetic neutron star. The blue lines emanating from the magnetar's pole represent its powerful magnetic field, strong enough to erase a credit card's magnetic strip from 100,000 miles (160,000 kilometers) away. Credit: Robert Mallozzi, University of Alabama, Huntsville
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. "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 Association.

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