Discovery of a new type of very-high-energy gamma ray emitter(Forwarded)

Max-Planck-Institut für Kernphysik
Heidelberg, Germany

Contacts:
Dr. Olaf Reimer
now at Hansen Experimental Physics Labs & Kavli Institute
for Particle Astrophysics and Cosmology
Stanford University
Stanford, CA 94305-4060, USA
Tel +1 650 724-6819

Stefan Hoppe & Prof. Werner Hofmann
Max-Planck-Institut für Kernphysik
Saupfercheckweg 1
69117 Heidelberg, GERMANY
Tel +49 6221 516 ext 274 & ext 330

Martin Raue
Institut für Experimentalphysik
Universität Hamburg
Luruper Chaussee 149
22761 Hamburg, GERMANY
Tel +49 40 8998 2176

EMBARGOED: FOR RELEASE: 10:30 a.m. PST, February 5, 2007

Discovery of a new type of very-high-energy gamma ray emitter

H.E.S.S. detects VHE gamma rays probably associated with the young stellar
cluster Westerlund 2

The H.E.S.S (High Energy Stereoscopic System) collaboration reports the
discovery of a new type of energetic gamma-ray source, probably related to
supersonic winds from massive stars in a young open stellar cluster. Dr.
Olaf Reimer of the Hansen Experimental Research Laboratory and Kavli
Institute for Particle Astrophysics and Cosmology of Stanford University
presents the result at the First GLAST Symposium in Palo Alto, California,
on behalf of a large team of international astrophysicists operating the
H.E.S.S. array of gamma-ray telescopes in Namibia. For the first time,
high energy gamma-ray emission could be convincingly associated with a
stellar cluster characterized by ongoing star formation and presence of
massive stars near the end of their life cycle but prior to their
explosion as supernovae, known as Wolf-Rayet stars. Extreme particle
acceleration connected with stellar winds is becoming a phenomenon more
widely observed at high energies than previously thought, and offers a
hint for cosmic ray particle acceleration in other sources besides the
major candidates, the relics of supernova explosions.

An international team of astrophysicists from the H.E.S.S. collaboration
has announced the discovery of a new type of very-high-energy (VHE) gamma
ray source. Combining data obtained during a systematic survey of the
Galactic Plane and dedicated pointed observations of the telescope array
revealed energetic gamma radiation coincident with the stellar cluster
Westerlund 2, which is embedded in the giant ionized hydrogen cloud RCW49.
The new VHE source, HESS J1023-575, is a first indication of extreme
particle acceleration associated with a young open stellar cluster, an
ensemble of stars which are particularly interesting due to ongoing star
formation and the existence of extremely massive stars, known as
Wolf-Rayet (WR) stars. One of these, WR 20a, a close binary systems of two
WR stars orbiting each other, is the most massive of all
confidently-measured binaries presently known in our Galaxy.

Wolf-Rayet stars (named for their discoverers) are evolved, massive stars
near the end of their stellar live-cycle, when they are rapidly losing
their mass by means of supersonic stellar winds. In the Westerlund 2
cluster, the Wolf-Rayet winds have literally blown bubbles around their
stellar hosts, clearly visible in infrared and radio images of the region.
Integrated over their lifetime, the wind energy output of Wolf-Rayet stars
is not too far from the kinetic energy released in supernova explosions,
and shocked winds a re well suited to accelerate particles to high energy.

The energetic gamma radiation discovered by the H.E.S.S. telescopes,
however, is neither point-like nor centered at the locations of the
Wolf-Rayet stars, but appears extended compared to the point spread
function of the telescopes, on scales beyond the extent of the stellar
cluster, with constant emission over time.

What can we conclude about the origin of these gamma rays? With a
projected angular size of milliarcsecond scale, the WR 20a binary system,
including its colliding wind zone, would appear as a point source for
observations with the H.E.S.S. telescope array. "Unless there are extreme
differences in the spatial extent of the particle distributions producing
radio, X-ray, and VHE gamma-ray emission", says Olaf Reimer, Senior
Research Scientist at Stanford University, "scenarios based on the
colliding stellar winds in the WR 20a binary system face the severe
problem of accounting for a source extension of 0.2 degrees in the VHE
waveband."

On the other hand, at the nominal distance of WR 20a of 8 kpc, this source
extension is equivalent to a diameter of about 28 pc for the emission
region, consistent in size with theoretical predictions of bubbles blown
from massive stars into the interstellar medium. Shocks and turbulent
motion inside a bubble can efficiently transfer energy to cosmic rays,
providing a plausible mechanism for particle acceleration. In size and
location, the gamma-ray source resembles the so-called "blister" as
reported by Whiteoak & Uchida 1997, where the bubble opens up and the wind
expands into the low-density ambient medium. Shock acceleration at the
boundaries of the blister may enable particles to diffusively re-enter
into the dense medium, thereby interacting in hadronic collisions and
producing gamma rays. Similar scenarios were outlined over twenty years
ago for supernova-driven expansion of particles into a low density medium.
If one accepts such a scenario here, it might give the first observational
support of energetic gamma-ray emission due to diffusive shock
acceleration from supersonic winds in a wind-blown bubble created by WR
20a, or by the ensemble of hot and massive stars in Westerlund 2.

Further observations with the H.E.S.S. telescope array, and other
sensitive ground-based gamma-ray telescopes, and the Gamma Ray Large Area
Space Telescope (GLAST) satellite will ultimately clarify whether high
energy gamma-ray emission is a common property to young stellar clusters
or a distinctive feature of Westerlund 2 and its remarkable ensemble of
massive stars. GLAST is due for launch in late 2007.

ACKNOWLEDGMENTS: The support of the Namibian authorities and of the
University of Namibia in facilitating the construction and operation of
H.E.S.S. is gratefully acknowledged, as is the support by the German
Ministry for Education and Research (BMBF), the Max Planck Society, the
French Ministry for Research, the CNRS-IN2P3 and the Astroparticle
Interdisciplinary Programme of the CNRS, the U.K. Particle Physics and
Astronomy Research Council (PPARC), the IPNP of the Charles University,
the South African Department of Science and Technology and National
Research Foundation, and by the University of Namibia. We appreciate the
excellent work of the technical support staff in Berlin, Durham, Hamburg,
Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction
and operation of the equipment.

Notes on H.E.S.S.

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

The detector:
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 13m-diameter telescopes is currently the most sensitive detector of
very high energy gamma rays. These are absorbed in the atmosphere, where
they give a short-lived shower of particles. The H.E.S.S. telescopes
detect the faint, short flashes of blueish light which these particles
emit (named Cherenkov light, lasting a few billionths of a second),
collecting the light with big mirrors which reflect onto extremely
sensitive cameras. Each image gives the position on the sky of a single
gamma-ray photon, and the amount of light collected gives the energy of
the initial gamma ray. Building up the images photon by photon allows
H.E.S.S. to create maps of astronomical objects as they appear in gamma
rays.

The H.E.S.S. telescope array represent a multi-year construction effort by
an international team of more than 100 scientists and engineers. The
instrument was inaugurated in September 2004 by the Namibian Prime
Minister, Theo-Ben Gurirab, 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.

Future plans:
The scientists involved with H.E.S.S. are continuing to upgrade and
improve the system of telescopes. Construction of a central telescope -- a
behemoth 30m in diameter -- is underway, including new partner countries
such as Poland. The improved system, known as H.E.S.S.-II, will be more
sensitive and will cover an increased range of gamma-ray energies, so
enabling the H.E.S.S. team to increase the gamma-ray source catalogue and
to make new discoveries.

SIDEBARS

Gamma rays:
Gamma rays resemble normal light or X-rays, but are much more energetic.
Visible light has an energy of about one electronVolt (1 eV) of energy in
physicist's terms. X-rays are thousands to millions of eV. H.E.S.S.
detects very-high energy gamma-ray photons with an energy of a million
million eVs, or Tera-electronVolt energies. These high energy gamma rays
are quite rare; even for relatively strong astrophysical sources, only
about one gamma ray per month hits a square metre at the top of the
Earth's atmosphere.

Cosmic rays:
High-energy particles from space which continuously bombard the Earth's
atmosphere from all directions. Their energies exceed by far those that
can be reached using man-made particle accelerators. Cosmic rays were
discovered in 1912 by Victor Hess, and while they have been extensively
studied for almost a century, their origin -- often declared as one of the
key themes of astrophysics -- is still not completely understood. One
important early result of the H.E.S.S. experiment was to reveal a
supernova explosion shockwave as a site of intense particle acceleration
(Nature 432, p75)

[NOTE: Images and weblinks supporting this release are available at
http://www.mpi-hd.mpg.de/hfm/HESS/pu...ess_HESSJ1023/ ]