Mystery compact object producing high energy radiation (Forwarded)

Particle Physics and Astronomy Research Council
Swindon, U.K.

Contacts:

Media:
Julia Maddock, PPARC Press Office
Tel: 01793 442094

Science:
Dr Guillaume Dubus, Laboratoire Leprince-Ringuet (LLR) CNRS/IN2P3
Ecole Polytechnique
91128 Palaiseau, FRANCE
Phone: +33 1 69 33 31 47 (LLR Paris) or +33 1 44 32 80 75 (IAP Paris)

Dr Mathieu de Naurois, LPNHE (CNRS/IN2P3)
Universities Paris VI & VII
Paris, FRANCE
Phone: +33 1 44 27 23 24 (LPNHE Paris)

Dr Paula Chadwick, University of Durham
Phone: +44 191 334 3560 Fax: +44 191 334 5823

12 July 2005

Mystery compact object producing high energy radiation

In a recent issue of Science Magazine, the High Energy Stereoscopic System
(H.E.S.S.) team of international astrophysicists reports the discovery of
another new type of very high energy (VHE) gamma ray source.

Gamma-rays are produced in extreme cosmic particle accelerators such as
supernova explosions and provide a unique view of the high energy
processes at work in the Milky Way. VHE gamma-ray astronomy is still a
young field and H.E.S.S. is conducting the first sensitive survey at this
energy range, finding previously unknown sources.

The object that is producing the high energy radiation is thought to be a
'microquasar'. These objects consist of two stars in orbit around each
other. One star is an ordinary star, but the other has used up all its
nuclear fuel, leaving behind a compact corpse. Depending on the mass of
the star that produced it, this compact object is either a neutron star or
a black hole, but either way its strong gravitational pull draws in matter
from its companion star. This matter spirals down towards the neutron star
or the black hole, in a similar way to water spiraling down a plughole.

However, sometimes the compact object receives more matter than it can
cope with. The material is then squirted away from the system in a jet of
matter moving at speeds close to that of light, resulting in a
microquasar. Only a few such objects are known to exist in our galaxy and
one of them, an object called LS5039, has now been detected by the
H.E.S.S. team.

In fact, the real nature LS5039 is something of a mystery. It is not clear
what the compact object is. Some of the characteristics suggest it is a
neutron star, some that it is a black hole. Not only that, but the jet
isn't much of a jet; although it is moving at about 20% of the speed of
light, which might seem a lot, in the context of these objects it's
actually quite slow.

Nor is it clear how the gamma rays are being produced. As Dr. Guillaume
Dubus of the Ecole Polytechnique points out "We really shouldn't have
detected this object. Very high energy gamma rays emitted close to the
companion star are more likely to be absorbed, creating a
matter/antimatter cascade, than escape from the system."

Dr Paula Chadwick of the University of Durham adds "It's very exciting to
have added another class of object to the growing catalogue of gamma ray
sources. It's an intriguing object -- it will take more observations to
work out what is going on in there."

The H.E.S.S. array is ideal for finding new VHE gamma ray objects; because
it's wide field of view (ten times the diameter of the Moon) means that it
can survey the sky and discover previously unknown sources.

The results were obtained using the High Energy Stereoscopic System
(H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of
four 13 m diameter telescopes is currently the most sensitive detector of
VHE gamma-rays -- radiation that is a million, million times more
energetic than the visible light. These high energy gamma rays are quite
rare even for relatively strong sources; only about one gamma ray per
month hits a square metre at the top of the Earth's atmosphere. Also,
since they are absorbed in the atmosphere, a direct detection of a
significant number of the rare gamma rays would require a satellite of
huge size. The H.E.S.S. telescopes employ a trick -- they use the
atmosphere as detector medium. When gamma rays are absorbed in the air,
they emit short flashes of blue light, named Cherenkov light, lasting a
few billionths of a second. This light is collected by the H.E.S.S.
telescopes with large mirrors and extremely sensitive cameras and can be
used to create images of astronomical objects as they appear in
gamma-rays.

The H.E.S.S. telescopes represent several years of construction effort by
an international team of more than 100 scientists and engineers from
Germany, France, the UK, Ireland, the Czech Republic, Armenia, South
Africa and the host country Namibia. The instrument was inaugurated in
September 2004 by the Namibian Prime Minister, Theo-Ben Guirrab, and its
first data have already resulted in a number of important discoveries,
including the first astronomical image of a supernova shock wave at the
highest gamma-ray energies.

Notes for Editors

* Collaboration press release
http://www.mpi-hd.mpg.de/hfm/HESS/pu...ssRelease.html
* H.E.S.S. project
http://www.mpi-hd.mpg.de/hfm/HESS/

The H.E.S.S. collaboration

The High Energy Stereoscopic System (H.E.S.S.) team consists of scientists
from Germany, France, the UK, the Czech Republic, Ireland, Armenia, South
Africa and Namibia.

Over the last few years, the H.E.S.S. collaboration has been building a
system of four telescopes in the Khomas Highland region of Namibia, to
study very-high-energy gamma rays from cosmic particle accelerators. The
telescopes, known as Cherenkov telescopes, image the light created when
high-energy cosmic gamma rays are absorbed in the atmosphere, and have
opened a new energy domain for astronomy. The H.E.S.S. telescopes each
feature mirrors of area 107 square metres, and are equipped with highly
sensitive and very fast 960-pixel light detectors in the focal planes.
Construction of the telescope system started in 2001; the fourth telescope
was commissioned in December 2003. Observations were being made even while
the system was being built, first using a single telescope, then with two
and three telescopes. While only the complete four-telescope system
provides the full performance, the first H.E.S.S. telescope alone was
already superior to any of the instruments operated previously in the
southern hemisphere.

About PPARC:
http://www.pparc.ac.uk/Ap/intro.asp

IMAGE CAPTIONS:

[Image 1:
http://www.pparc.ac.uk/Nw/LS5039_Science_NC.jpg (145KB)]
Map of the gamma ray sky in the region of LS5039. The green star shows the
position of LS5039 as measured using radio telescopes and the white
ellipse shows the gamma ray position.

[Image 2:
http://www.pparc.ac.uk/Nw/LS5039survey.jpg (281KB)]
A larger picture of the gamma ray sky as measured with H.E.S.S. LS5039 was
discovered during the first scan of the galactic plane ever made at very
high energies.

[Image 3:
http://www.pparc.ac.uk/Nw/ls5039binsim.jpg (78KB)]
The companion star to the compact object is a massive star that is losing
material from its surface. This matter is then captured by the compact
object's strong gravitational field and spirals down towards the surface.
Some of this material is then ejected in two jets travelling at 20% of the
speed of light. This image was created using software developed by Dr. Rob
Hynes of LSU.

Related Images:

Project images,
http://www.mpi-hd.mpg.de/hfm/HESS/public/hn_images.htm

Further images relating to this specific result are available from the
collaboration press release (see above link).