Astronomers using ESA’s X-ray observatory, XMM-Newton, have discovered a pair of
X-ray tails, stretching 3 million million kilometres across the sky. They
emanate from the mysterious neutron star known as Geminga. The discovery gives
astronomers new insight into the extraordinary conditions around the neutron star.

A neutron star measures only 20-30 kilometres across and is the dense remnant of
an exploded star. Geminga is one of the closest to Earth, at a distance of about
500 light-years. Most neutron stars emit radio emissions, appearing to pulsate
like a lighthouse, but Geminga is ‘radio-quiet’. It does, however, emit huge
quantities of pulsating gamma rays making it one of the brightest gamma-ray
sources in the sky. Geminga is the only example of a successfully identified
gamma-ray source from which astronomers have gained significant knowledge.

It is 350,000 years old and ploughs through space at 120 kilometres per second.
Its route creates a shockwave that compresses the gas of the interstellar medium
and its naturally embedded magnetic field by a factor of four.

Patrizia Caraveo, Instituto di Astrofisica Spaziale e Fisica Cosmica, Milano,
Italy, and her colleagues (at CESR, France, ESO and MPE, Germany) have
calculated that the tails are produced because highly energetic electrons become
trapped in this enhanced magnetic field. As the electrons spiral inside the
magnetic field, they emit the X-rays seen by XMM-Newton.

The electrons themselves are created close to the neutron star. Geminga’s
breathless rotation rate — once every quarter of a second — creates an
extraordinary environment in which electrons and positrons, their antimatter
counterparts, can be accelerated to extraordinarily high energies. At such
energies, they become powerful high-energy gamma-ray producers. Astronomers had
assumed that all the electrons would be converted into gamma rays. However, the
discovery of the tails proves that some do find escape routes from the maelstrom.

"It is astonishing that such energetic electrons succeed in escaping to create
these tails," says Caraveo, "The tail electrons have an energy very near to the
maximum energy achievable in the environment of Geminga."

The tails themselves are the bright edges of the three-dimensional shockwave
sculpted by Geminga. Such shockwaves are a bit like the wake of a ship
travelling across the ocean. Using a computer model, the team has estimated that
Geminga is travelling almost directly across our line of sight.

Studies of Geminga could not be more important. The majority of known gamma-ray
sources in the Universe have yet to be identified with known classes of
celestial objects. Some astronomers believe that a sizeable fraction of them may
be Geminga-like radio-quiet neutron stars. Certainly, the family of radio-quiet
neutron stars, discovered through their X-ray emission, is continuously growing.
Currently, about a dozen objects are known but only Geminga has a pair of tails!

Note for editors

During the search to track down this elusive celestial object, a co-author on
the paper, Giovanni Bignami, named Geminga almost 30 years ago. He was Principal
Investigator of XMM-Newton’s EPIC camera from 1987 to1997 and is now Director of
the Centre d’Etude Spatiale des Rayonnements (CESR). Geminga was first glimpsed
as a mysterious source of gamma rays, coming from somewhere in the constellation
Gemini by NASA’s SAS-2 spacecraft in 1973.

While searching to pin down its exact location and nature, Bignami named it
Geminga because it was a ‘Gemini gamma-ray source’. As an astronomer in Milan,
Italy, he was also aware that in his native dialect ‘gh’Ë minga’ means ‘it is
not there’, which he found amusing. It was also remarkably apt, for it was not
until 1993 that he succeeded in finally ‘seeing’ and therefore pinpointing
Geminga, using optical wavelengths. While it lacked radio emissions, the
pulsating X-ray and gamma-ray emissions meant Geminga could only be a new class
of radio-quiet neutron star.

The original paper was published yesterday, 24 July 2003, on Science Express, a
feature of Science Online.

For more information, contact:

Dr. Patrizia Caraveo
Instituto di Astrofisica Spaziale e Fisica Cosmica, CNR (Milano, Italy)

For further information, please contact:

ESA Media Relations Service
Tel: +33 (0)
Fax: +33 (0)

Related articles

* ESA’s XMM-Newton gains deep insights into the distant Universe

* ESA’s XMM-Newton makes the first measurement of a dead star’s magnetism

More information

* ESA Science website

* XMM-Newton


[Image 1:]
This image was captured by the EPIC camera on board the satellite. The motion of
Geminga across the sky is indicated, showing that the tails are trailing the
neutron star. The scale bar corresponds to a distance of 1.5 million million
kilometres at the distance of Geminga.

Credits: ESA

[Image 2:]
Computer models of the shockwave created by Geminga show that the best matches
to the data occur if the neutron star is travelling virtually across our line of
sight. These correspond to the inclinations of less than 30 degrees.

Credits: Patrizia Caraveo