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05/31/2002 07:00:00

Astronomers discover the strongest known magnet in the universe

Astrophysicists at the California Institute of Technology, using the Palomar 200-inch telescope, have uncovered evidence that a special type of pulsar has the strongest magnetic field in the universe.

Reporting in the May 30 issue of the journal Nature, Caltech graduate student Brian Kern and his advisor Chris Martin report on the nature of pulses emanating from a faint object in the constellation Cassiopeia. Using a specially designed camera and the Palomar 200-inch telescope, the team discovered that a quarter of the visible light from the pulsar known as 4U0142+61 is pulsed, while only 3 percent of the X rays emanating from the object are pulsed, meaning that the pulsar must be an object known as a magnetar.

"We were amazed to see how strongly the object pulsed in optical light compared with X rays," said Martin, who is a professor of physics at Caltech. "The light had to be coming from a strong, rotating magnetic field rather than a disk of infalling gas."

To explain the precise chain of reasoning that led the team to their conclusion, a certain amount of explanation of the nature of stars and pulsars is in order. Normal stars are powered by nuclear fusion in their hot cores. When a massive star exhausts its nuclear fuel, its core collapses, causing a titanic "supernova" explosion.

The collapsing core forms a "neutron star" which is as dense as an atomic nucleus and the size of Los Angeles. The very weak magnetism of the original star is greatly amplified (a billion- to a trillion-fold) during the collapse. The slow rotation of the original star grows as well, just as an ice skater spins much faster when her arms are drawn in.

The combination of a strong magnetic field and rapid spin often produces a "pulsar," an object that rotates its beam of light just like a lighthouse, but usually in the radio band of the electromagnetic spectrum. Pulsars have been discovered that rotate almost one thousand times every second. In conventional pulsars that have been studied since their discovery in the 1960s, the source of the energy that produces this pulsing light is the rotation itself.

In the last decade, a new type of pulsar has been discovered that is very different from the conventional radio pulsar. This type of object, dubbed an "anomalous X-ray pulsar," has a very lazy rotation (one every 6 to 12 seconds) and pulses in the X- ray frequencies but is invisible in radio waves. However, the X-ray power is hundreds of times the power provided by their slow rotation. Their source of energy is unknown, and therefore "anomalous." One of the brightest of these pulsars is 4U0142+61, named for its sky coordinates and detection by the Uhuru X-ray mission in the 1970s.

Two sources of energy for the X rays are possible. In the first model, bits of gas blown off in the supernova explosion fall back onto the resulting neutron star, whose magnetic field is no stronger than an ordinary pulsar's. As the gas slowly falls (accretes) onto the surface, it becomes hot and emits X rays.

A second model, proposed by Robert Duncan (University of Texas) and Christopher Thompson (Canadian Institute for Theoretical Astrophysics), holds that anomalous X-ray pulsars are magnetars, or neutron stars with ultra-strong magnetic fields. The magnetic field is so strong that it can power the neutron star by itself, generating X rays and optical light. Magnetic fields power solar flares in our own sun, but with only a tiny fraction of the power of nuclear fusion. Magnetars would be the only objects in the universe powered mainly by magnetism.

"Scientists would be thrilled to investigate these enormous magnetic fields, if they exist," says Kern. "Identifying 4U0142+61 as a magnetar is the essential first step in these studies."

The missing observational clue to distinguish between these very different power sources was provided by a novel camera designed to look at optical light coming from very faint pulsars. While most of the light appears in X-ray frequencies, anomalous X-ray pulsars emit a small amount of optical light. In pulsars powered by disks of gas, optical pulsations would be a diluted byproduct of X-ray pulsations, which are weak in this pulsar. A magnetar, on the other hand, would be expected to pulse as much or more in optical light as in X ray frequencies.

The problem is that the optical light from the object is extremely faint, about the brightness of a candle sitting on the moon. Astronomical cameras designed to look at very faint stars and galaxies must take very long exposures, as long as many hours, in order to detect the faint light, even with a 200-inch telescope. But in order to detect pulsations that repeat every eight seconds, the rotation period of 4U0142+61, exposure times must be very short, less than a second.

Martin and Kern invented a camera to solve this problem. The camera takes 10 separate pictures of the sky during a single rotation of the pulsar, each picture for less than one second. The camera then shuffles the pictures back to their starting point, and re-exposes the same 10 pictures for the next pulsar rotation. This exposure cycle is repeated hundreds of times before the camera data is recorded. The final image shows the pulsar at 10 different points in its repetitive cycle. During the cycle, part of the image is bright while part is dim. The large optical pulsations seen in 4U0142+61 show that it must be a magnetar.

How strong is the magnetic field of this magnetar? It is as much as a quadrillion times the strength of the earth's magnetic field, and ten billion times as strong as the strongest laboratory magnet ever made. A toy bar magnet placed near the pulsar would feel a force of a trillion pounds pulling its ends into alignment with the pulsar's magnetic poles.

A magnetar would be an unsafe place for humans to go. Because the pulsar acts as a colossal electromagnetic generator, a person in a spacecraft floating above the pulsar as it rotated would feel 100 trillion volts between his head and feet.

The magnetism is so strong that it has bizarre effects even on a perfect vacuum, polarizing the light traveling through it. Kern and Martin hope to measure this polarization with their camera in the near future in order to measure directly the effects of this ultra-strong magnetism, and to study the behavior of matter in extreme conditions that will never be reproduced in the laboratory.

Additional information available at http://www.astro.caltech.edu/palomar/

Written by Robert Tindol