Massively magnetic. Artist’s impression of a magnetar.

Magnetic Family Ties

Don't mess with a magnetar. These compact neutron stars have magnetic fields strong enough to wipe your credit card from a distance halfway to the moon. In the guise of soft gamma repeaters, they crackle with bursts of highly energetic gamma rays and x-rays. Now two teams of astronomers report evidence that magnetars have another trick up their sleeve: They power anomalous x-ray pulsars. The findings might put to rest a long-running debate over the nature of these puzzling objects.

One side of the debate holds that anomalous x-ray pulsars are powered by accretion disks. The idea is analogous to what happens with a regular x-ray pulsar: Gas from a normal star is pulled toward an orbiting neutron star, where it first piles up in a spinning disk and then spirals down onto the neutron star's surface. Anomalous x-ray pulsars lack a partner star, but many astrophysicists still believe they could be powered by similar accretion disks (Science, 5 January 2001, p. 68).

The model got into trouble a few weeks ago, when Brian Kern and Chris Martin of the California Institute of Technology in Pasadena described in Nature optical pulsations from an anomalous x-ray pulsar known as 4U0142+61, in the constellation Cassiopeia. These optical pulsations wouldn't be this strong if the pulsar was surrounded by an accretion disk, says Ferdi Hulleman of Utrecht University in the Netherlands.

A second blow to the accretion disk hypothesis came on 18 June, when a team led by astronomer Victoria Kaspi of McGill University in Montreal, Canada, observed a series of brief x-ray bursts from another anomalous x-ray pulsar, known as 1E 2259+586, also in Cassiopeia. The bursts, observed with NASA's Rossi X-ray Timing Explorer and announced in an electronic circular, are very much like the x-ray bursts that come from soft gamma repeaters, which are known to be magnetars.

One of the original proponents of the accretion disk model for anomalous x-ray pulsars, Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says that the new observations "probably rule out the accretion model." However, he stresses that the magnetar model still has some kinks. As Hulleman points out, the new findings raise the question of whether soft gamma repeaters evolve into the strange pulsars (or the other way around): "We will have to keep a close watch on these enigmatic objects."

Related sites
Background information on soft gamma repeaters and magnetars
Rossi X-ray Timing Explorer home page