Andrew Yee[_1_]
February 7th 07, 03:25 AM
Max-Planck-Institut f 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 f Kernphysik
Saupfercheckweg 1
69117 Heidelberg, GERMANY
Tel +49 6221 516 ext 274 & ext 330
Martin Raue
Institut f Experimentalphysik
Universit 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/public/PressRelease/Press_HESSJ1023/ ]
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 f Kernphysik
Saupfercheckweg 1
69117 Heidelberg, GERMANY
Tel +49 6221 516 ext 274 & ext 330
Martin Raue
Institut f Experimentalphysik
Universit 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/public/PressRelease/Press_HESSJ1023/ ]