Press Releases Archive
18.03.2016
An accelerator of cosmic rays with unprecedented energies at the centre of the Milky Way
Team of scientists identifies the supermassive black hole Sagittarius A* as most probable source
An international team of scientists including members of the University of Tübingen has for the first time identified a source of cosmic rays that accelerates these elementary particles to petaelectron energies. This source of cosmic rays is therefore a hundred times more energetic – in terms of particle energies – than the biggest man-made particle accelerator, the LHC at CERN. It seems that the supermassive black hole at the center of our Milky Way serves as accelerator. The analysis was published on 16th March 2016 in the scientific journal Nature.
For more than ten years the H.E.S.S. observatory in Namibia has been mapping the center 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 reveals now the supermassive black hole Sagittarius A* as most probable source. From the University of Tübingen, Professor Andrea Santangelo and Dr. Gerd Pühlhofer from the Institute of Astronomy and Astrophysics are participating to the collaboration.
The Earth is constantly bombarded by high energy particles of cosmic origin. Those particles are protons, electrons and atomic nuclei and comprise the so-called “cosmic radiation”. Since more than a century, the origin of the cosmic rays remains one of the most enduring mysteries of science. The problem is: 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 randomized by these deflections, making it impossible to directly identify the astrophysical sources responsible for their production. Fortunately for the scientists, cosmic rays interact with light and gas in the neighborhood of their sources and thus produce high energy gamma rays. These gamma rays travel in straight lines to Earth, undeflected by magnetic fields. This gamma radiation can therefore be used to identify the sources of the cosmic rays at the sky.
Sources known so far can produce cosmic rays with energies up to approximately 100 teraelectronvolts (TeV = 1012 eV, corresponding to a trillion of the energy of visible light) in our Galaxy. Theoretical arguments and direct measurements of cosmic rays however indicate that the cosmic ray factories in our Galaxy should be able to provide particles at one petaelectronvolt (PeV = 1000 TeV = 1015 eV) at least. So far, the search for the sources of the highest energy Galactic cosmic rays remained unsuccessful.
Detailed observations of the Galactic center region, made with the H.E.S.S. telescopes over the past ten years, finally provide first answers. Already during the first three years of observations, H.E.S.S. uncovered a very powerful point source of gamma rays in the galactic center 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 clouds’ material. The detection of this diffuse emission with H.E.S.S. strongly indicated the presence of a source of cosmic rays in that region. However, the nature of the source remained a mystery.
Deeper observations obtained with H.E.S.S. between 2004 and 2013 as well as new analysis techniques were now used to considerably refine these results. “For the first time it was possible now with H.E.S.S. to measure simultaneously the spatial distribution and the energies of the cosmic rays” explains Professor Andrea Santangelo, head of the High Energy Astrophysics Section of the Faculty of Science in Tübingen. For the first time it is also possible to pinpoint the source of these particles: “Protons have been accelerated at the center of our Milky way to up to 1 petaelectronvolt” says Dr. Gerd Pühlhofer, who is coordinating the section’s high energy gamma-ray activities. “There has to be an accelerator at this place which has been working for at least a thousand years.” This would be the first “Pevatron” ever observed – in analogy to the “Tevatron”, the first human-built accelerator that reached particle energies of 1 Teraelectronvolt.
The center 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 center of the Galaxy, called Sagittarius A*, is the most plausible source of the PeV protons”, says Felix Aharonian (MPIK, Heidelberg and DIAS, Dublin). The H.E.S.S. measurements however also show that this source alone cannot account for the total flux of cosmic rays detected at Earth. “If, however, Sagittarius A* was more active in the past”, elaborates Christopher van Eldik of the University of Erlangen, vice director of the H.E.S.S. collaboration and speaker of the German groups, the scientists’ argument, “then it could indeed be responsible for the bulk of today’s Galactic cosmic rays.”
Original publication:
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 (online), 16 March 2016 (DOI 10.1038/nature17147); <link http: www.nature.com nature journal vaop ncurrent full nature17147.html>
www.nature.com/nature/journal/vaop/ncurrent/full/nature17147.html
International press release of the H.E.S.S. collaboration:
<link http: www2.cnrs.fr en>
Caption:
A powerful cosmic Pevatron in the centre of the Milky Way – Artistic view of the gamma-ray emission coming from the interaction of relativistic protons, injected by the central supermassive black hole Sagittarius A* (bright source in the center), with the giant clouds of the Central Molecular Zone. The accelerated protons (represented as blue spheres) propagate diffusively until they interact with interstellar protons in the molecular clouds producing neutral pions, which decay almost immediately producing gamma rays (yellow wave). An optical view of the Milky Way is seen in the background.
Image credit: Dr. Mark A. Garlick / H.E.S.S. collaboration
Contact:
University of Tübingen
Faculty of Science
Institut für Astronomie und Astrophysik/Kepler Center for Astro and Particle Physics
Prof. Dr. Andrea Santangelo
Phone +49 7071 29-78128
Santangelo[at]astro.uni-tuebingen.de
Dr. Gerd Pühlhofer
Phone +49 7071 29-74982
Gerd.Puehlhofer[at]astro.uni-tuebingen.de
Institute for Astronomy and Astrophysics Tübingen home page:
<link https: www.uni-tuebingen.de en faculties faculty-of-science departments physics institutes astronomy-astrophysics institute astronomie astronomy-high-enery-astrophysics.html>
Web:
H.E.S.S. home page: <link http: www.mpi-hd.mpg.de hess>www.mpi-hd.mpg.de/HESS
H.E.S.S. instrument: <link http: www.mpi-hd.mpg.de hess pages about>www.mpi-hd.mpg.de/HESS/pages/about/
H.E.S.S. II press release at Tübingen University: <link https: www.uni-tuebingen.de en news press-releases archive archivfullview-press-releases article erstes-licht-des-weltgroessten-tscherenkow-teleskops-hess-ii-tuebinger-forscher-sind-dabe.html>
The H.E.S.S. Telescopes
The collaboration: The High Energy Stereoscopic System (H.E.S.S.) team consists of scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria, Sweden, and the Netherlands. The University of Tübingen is part of the H.E.S.S. collaboration through the High Energy Astrophysics Section of the Institute for Astronomy and Astrophysics Tübingen (IAAT), financially supported by the Federal Ministry for Education and Research.
The instrument: 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 – recently complemented with the huge 28 m H.E.S.S. II telescope – is one of the most sensitive detectors of very high-energy gamma rays. These are absorbed in the atmosphere, where they create a short-lived shower of particles. The H.E.S.S. telescopes detect the faint, short flashes of bluish 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. The H.E.S.S. telescopes have been operating since late 2002. H.E.S.S. has discovered the majority of the known cosmic objects emitting very high-energy gamma rays. A study performed in 2009 listed H.E.S.S. among the top 10 observatories worldwide.