Cosmic-ray accelerator in the Galactic Center (Forwarded)

Observatoire de Paris
Paris, France

Contacts:
Catherine Boisson, Observatoire de Paris, LUTH
Tél: 33 1 45 07 74 36
Fax: 33 1 45 07 74 69

Hélène Sol, Observatoire de Paris, LUTH
Tél: 33 1 45 07 74 28
Fax: 33 1 45 07 74 69

8 February 2006

Cosmic-ray accelerator in the Galactic Center

A group of astrophysicists (and among them, researchers from Paris
Observatory and CNRS) has announced the detection with the H.E.S.S.
gamma-ray telescopes (in Namibia) of very-high-energy gamma rays from
huge gas clouds known to pervade the centre of our Galaxy. These gamma
rays are expected to result from the even more energetic cosmic-ray
particles, which permeate our entire Galaxy, crashing into the clouds.
However, precise measurements of the intensity and energies of these
gamma rays, thanks to this most sensitive instrument in the world in
this energy range, further show that in the central region of our Galaxy
these cosmic-ray particles are typically more energetic than those we
measure falling on the Earth's atmosphere. Possible explanations for
this energisation of the cosmic rays near the heart of our Galaxy
include the echo of a Supernova which exploded some hundred centuries
beforehand or a burst of particle acceleration by the supermassive black
hole at the very centre of our galaxy.

In a recent publication in Nature magazine (9 February 2006), the
international H.E.S.S. collaboration report the discovery of gamma-ray
emission from a complex of gas clouds near the centre of our own
Milky-Way Galaxy. These giant clouds of hydrogen gas encompass an amount
of gas equivalent to 50 million times the mass of the sun. With the
highly sensitive H.E.S.S. gamma-ray telescopes, it is possible for the
first time to show that these clouds are glowing in very-high-energy
gamma rays (see Figure 1).

One key issue in our understanding of cosmic rays is their distribution
in space. Do they permeate the entire Galaxy uniformly, or do their
density and distribution in energy vary depending on one's location in
the Galaxy (for example due to the proximity of cosmic particle
accelerators)? We can only take direct measurements of cosmic rays
within our solar system, located about 25,000 light years from the
centre of the Galaxy. However, a subterfuge allows astrophysicists to
investigate cosmic rays elsewhere in the Galaxy: when a cosmic-ray
particle collides with an interstellar gas particle, gamma rays are
produced (cf Figure 2).

The central part of our Galaxy is a complex zoo, containing examples of
every type of exotic object known to astronomers, such as the remnants
of supernova explosions and a super-massive black hole. It also contains
huge quantities of interstellar gas, which tends to clump in clouds. If
gamma rays are detected from the direction of such a gas cloud,
scientists can infer the density of cosmic rays at the location of the
cloud. The intensity and distribution in energy of these gamma rays
reflects that of the cosmic rays.

At low energies, around 100 Million electronVolts (man-made accelerators
reach energies up to 1,000,000 Million electronVolts), this technique
has been used by the EGRET satellite to map cosmic rays in our Galaxy.
At really high energies -- the true domain of cosmic-ray accelerators --
no instrument was so far sensitive enough to "see" interstellar gas
clouds shining in very-high-energy gamma rays. H.E.S.S. has for the
first time demonstrated the presence of cosmic rays in this central
region of our Galaxy.

The surprise from H.E.S.S. data is that the density of cosmic rays
exceeds that in the solar neighbourhood by a significant factor.
Interestingly, this difference increases as we go up in energy, which
implies that the cosmic rays have been recently accelerated. So, these
data hint that the clouds are illuminated by a nearby cosmic-ray
accelerator, which was active over the last ten thousand years.
Candidates for such accelerators are a gigantic stellar explosion which
apparently went off near the heart of our Galaxy in "recent" history
(Chandra press release,
http://chandra.harvard.edu/press/01_releases/press_020101.html),
another possible acceleration site is the super-massive black hole at
the centre of the Galaxy. Jim Hinton (Max-Planck Institute for Nuclear
Physics, Heidelberg, Germany), one of the scientists involved in the
discovery, concludes: "This is only the first step. We are of course
continuing to point our telescopes at the centre of the Galaxy, and will
work hard to pinpoint the exact acceleration site -- I'm sure that there
are further exciting discoveries to come"

See also:

* CNRS Press Release
http://www.insu.cnrs.fr/
* HESS Press Release

http://www.mpi-hd.mpg.de/hfm/HESS/pu...ssRelease.html
* Supernova remnant by HESS (Novembre 2004)
http://www.obspm.fr/actual/nouvelle/nov04/snr.en.shtml

Reference

Aharonian et al (2006),
Discovery of very-high-energy gamma-rays from the Galactic Centre ridge,
Nature, 9 February issue

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/...-residuals.gif (147KB)]
The H.E.S.S. view of the galactic centre region. The top panel shows the
gamma-ray image of the Galactic Centre region taken by H.E.S.S. Two
bright sources dominate the view: HESS J1745-290, a mysterious source
right at the centre of the Galaxy; and, about 1 degree away, the
gamma-ray supernova remnant G 0.9+0.1. The lower panel shows the same
image with the bright sources subtracted. In this image gamma-ray
emission extending along the plane is visible as well as another
mysterious source: HESS J1745-303. The dashed lines show the position of
the Galactic Plane. The white circles show the positions from which the
two sources were removed.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/...ss-cartoon.gif (202KB)]
Interaction between cosmic-rays and nucleons produce gamma-rays.

[Figure 3:
http://www.obspm.fr/actual/nouvelle/feb06/hess-glow.jpg (8KB)]
Gamma-ray map from HESS of the Galactic Center region.

[Figure 4:
http://www.obspm.fr/actual/nouvelle/...hess-photo.gif (195KB)]
View of the 4 telescopes of H.E.S.S. (High Energy Stereoscopic System)
in Namibia, South-West Africa.