Astronomers have captured an image of a pulsar flying through space
at 20 times the speed of sound, with radiant twin tails of X-ray
light stretching nearly two billion miles from this tiny, dense
sphere only about 12 miles across.

This first clear image of a pulsar’s X-ray bow shock, from the
European Space Agency’s XMM-Newton observatory, is featured in a
Science magazine cover story on September 5. The object, Geminga, in
the constellation of Gemini, is the closest known pulsar to Earth,
about 500 light years away.

More than a pretty picture, the result provides unique insight into
the contents and density of the interstellar “ocean” Geminga is
plowing through, along with independent estimates of key pulsar
properties: mass, radius, magnetic field strength, and speed at
which such pulsars are “kicked” when born in star explosions.

“We have this spectacular view because Geminga is travelling almost
directly across our line of sight,” said Dr. Patrizia Caraveo of the
Instituto di Astrofisica Spaziale e Fisica Cosmica (IASF-CNR) in
Milan, Italy, lead author on the Science report. “The tails appear
to be the bright edges of a three-dimensional shock wave sculpted by
Geminga, like the wake of a ship travelling across the ocean.”

Caraveo’s co-authors are Giovanni Bignami, Andrea DeLuca, Sandro
Mereghetti, Alberto Pellizzoni, Roberto Mignani, Anatoly Tur, and
Werner Becker. (Respective affiliations are listed below.)

A pulsar is a type of rapidly spinning neutron star that emits steady
pulses of radiation with each rotation, funneled along strong
magnetic field lines, much like a lighthouse beam sweeping across
space. A neutron star is the core remains of an exploded star once
at least eight times as massive as the Sun.

Most pulsars emit radio waves. Yet Geminga is “radio quiet” — a
peculiar object discovered 30 years ago as a “gamma-ray only” source.
(Geminga has since been detected in the X-ray and optical light
wavebands.) Through processes not fully understood, Geminga is the
second-brightest gamma-ray object in the sky. Geminga generates this
highest-energy form of light by accelerating electrons and positrons,
a type of antimatter, to high speeds as it spins like a dynamo four
times per second.

Geminga’s X-ray tails are the latest surprising discovery, Caraveo
said. Moving so swiftly through space, Geminga creates a shock wave
that both compresses the gas in the interstellar medium and boosts
Geminga’s magnetic field by a factor of four. Caraveo’s team
calculated that electrons must get whipped around in the enhanced
magnetic field and then shot out, creating the X rays detected by

Astronomers had assumed that all the electrons would be converted
into gamma rays emitted close to Geminga. The detection of dramatic
X-ray tails show clearly that many electrons are escaping.

Geminga can now serve as an interstellar probe, Caraveo said. The
brightness and thickness of the X-ray tails are a measure of the
density of matter in between stars, the vast regions darkness that
remain largely unmeasured. The intensity and angle of the tails
confirm earlier measurements of Geminga’s proper motion, velocity,
radius, and mass from optical observations.

In addition, this observation of Geminga with an X-ray observatory
may provide clues to the nature of unknown gamma-ray sources,
according to Bignami. Of the 271 higher-energy gamma-ray objects
detected by a NASA telescope called EGRET, 170 remained unidentified
in other wavebands. These unidentified objects about be “gamma-ray
pulsars” like Geminga, whose optical and X-ray light might only be
visible because of its nearness to Earth.

Only about a dozen other radio-quiet pulsars are known, and Geminga
is the only one with tails. Bignami named Geminga for “Gemini
gamma-ray source” in 1973. In his local Milan dialect, the name is a
pun on “gh’ e minga,” which means “it is not there.” Indeed, Geminga
was unidentified in other wavelengths until 1993, twenty years after
its discovery.

Bignami is the director of the Centre d’Etude Spatiale des
Rayonnements (CESR) in Toulouse, France. DeLuca, Mereghetti and
Pellizzoni are researchers at the IASF-CNR. Mignani is with the
European Southern Observatory in Garching, Germany. Tur is with
CESR. And Becker is with the Max-Planck-Institut fuer
extraterrestrische Physik in Garching.

Science magazine’s cover features a swirling pattern of reflected
light from a spare XMM-Newton mirror similar to the one used to
produce the Geminga image, which is featured in the magazine.
High-resolution images of Geminga will be available temporarily at Reporters may contact Drs.
Caraveo and Bignami directly for more information.

Patrizia Caraveo
0039 329 6281486

Giovanni Bignami
0033 61 858 0221