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Press release | Matter ＆ the Universe | Astrophysics
For more than ten years the
H.E.S.S. observatory in Namibia, run by an international collaboration of 42
institutions in 12 countries, has been mapping the centre of our galaxy in
very-high-energy gamma rays. These gamma rays are produced by cosmic rays from
the innermost region of the Galaxy. A detailed analysis of the latest H.E.S.S.
data, published on 16th March 2016 in Nature, reveals for the first
time a source of this cosmic radiation at energies never observed before in the
Milky Way: the supermassive black hole at the centre of the Galaxy, likely to
accelerate cosmic rays to energies 100 times larger than those achieved at the
largest terrestrial particle accelerator, the LHC at CERN.
The Earth is constantly
bombarded by high energy particles (protons, electrons and atomic nuclei) of
cosmic origin, particles that comprise the so-called “cosmic radiation”. These
“cosmic rays” are electrically charged, and are hence strongly deflected by the
interstellar magnetic fields that pervade our galaxy. Their path through the
cosmos is randomised by these deflections, making it impossible to directly
identify the astrophysical sources responsible for their production. Thus, for
more than a century, the origin of the cosmic rays remains one of the most
enduring mysteries of science.
Fortunately, cosmic rays interact with light and
gas in the neighbourhood of their sources, producing gamma rays. These gamma
rays travel in straight lines, undeflected by magnetic fields, and can
therefore be traced back to their origin. When a very-high-energy gamma ray
reaches the Earth, it interacts with a molecule in the upper atmosphere, producing
a shower of secondary particles that emit a short pulse of “Cherenkov light”.
By detecting these flashes of light using telescopes equipped with large
mirrors, sensitive photo-detectors, and fast electronics, more than 100 sources
of very-high-energy gamma rays have been identified over the past three
decades. The H.E.S.S. (High Energy Stereoscopic System) observatory in Namibia
represents the latest generation of such telescope arrays. It is operated by
scientists from 42 institutions in 12 countries, with major contributions by
MPIK Heidelberg, Germany, CEA and CNRS, France.
Today we know that cosmic rays with energies up
to approximately 100 teraelectronvolts (TeV)1 are produced in our galaxy, by
objects such as supernova remnants and pulsar wind nebulae. Theoretical
arguments and direct measurements of cosmic rays reaching the Earth
indicate, however, that the cosmic-ray factories in our galaxy should be able
to provide particles up to one petaelectronvolt (PeV)2 at least. While many multi-TeV accelerators
have been discovered in recent years, so far the search for the sources of the
highest energy Galactic cosmic rays has, so far, been unsuccessful.
of the Galactic centre region, made by H.E.S.S. over the past ten years, and
published today in the journal Nature, finally provide direct indications for
such PeV cosmic-ray acceleration. During the first three years of observations,
H.E.S.S. uncovered a very powerful point source of gamma rays in the
galactic-centre region, as well as diffuse gamma-ray emission from the giant
molecular clouds that surround it in a region approximately 500 light years
across. These molecular clouds are bombarded by cosmic rays moving at close to
the speed of light, which produce gamma rays through their interactions with
the matter in the clouds. A remarkably good spatial coincidence between the
observed gamma rays and the density of material in the clouds indicated the
presence of one or more accelerators of cosmic rays in that region. However, the
nature of the source remained a mystery.
Deeper observations obtained by H.E.S.S. between
2004 and 2013 shed new light on the processes powering the cosmic rays in this
region. According to Aion Viana (MPIK, Heidelberg), “the unprecedented amount
of data and progress made in analysis methodologies enables us to measure
simultaneously the spatial distribution and the energy of the cosmic rays.”
With these unique measurements, H.E.S.S. scientists are for the first time able
to pinpoint the source of these particles: “Somewhere within the central 33
light years of the Milky Way there is an astrophysical source capable of
accelerating protons to energies of about one petaelectronvolt, continuously
for at least 1,000 years”, says Emmanuel Moulin (CEA, Saclay). In analogy to
the “Tevatron”, the first human-built accelerator that reached energies of 1
TeV, this new class of cosmic accelerator has been dubbed a “Pevatron”. “With
H.E.S.S. we are now able to trace the propagation of PeV protons in the central
region of the Galaxy”, adds Stefano Gabici (CNRS, Paris).
The centre of our galaxy is home to many objects
capable of producing cosmic rays of high energy, including, in particular, a
supernova remnant, a pulsar wind nebula, and a compact cluster of massive stars.
However, “the supermassive black hole located at the centre of the Galaxy,
called Sgr A*, is the most plausible source of the PeV protons”, says Felix
Aharonian (MPIK, Heidelberg and DIAS, Dublin), adding that, “several possible
acceleration regions can be considered, either in the immediate vicinity of the
black hole, or further away, where a fraction of the material falling into the
black hole is ejected back into the environment, thereby initiating the
acceleration of particles”.
The H.E.S.S. measurement
of the gamma-ray emission can be used to infer the spectrum of the protons that
have been accelerated by the central black hole – revealing that Sgr A* is very
likely accelerating protons to PeV energies. Currently, these protons cannot
account for the total flux of cosmic rays detected at the Earth. “If, however,
our central black hole was more active in the past”, the scientists argue,
“then it could indeed be responsible for the bulk of the Galactic cosmic rays
that are observed today at the Earth”. If true, this would dramatically
influence the century old debate concerning the origin of these enigmatic
Acceleration of Petaelectronvolt protons
in the Galactic Centre, H.E.S.S. collaboration, corresponding authors: F. Aharonian,
S. Gabici, E. Moulin and A. Viana, Nature 16th March 2016
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.