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 “neutronThe 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

Related Links

* Palomar Observatory