19/03/2016: A “power house” at the Milky Way centre accelerates PeV protons

An international team of scientists, including David Berge, Jacco Vink, David Salek, Rachel Simoni and Mark Bryan at GRAPPA (University of Amsterdam), has discovered a source accelerating Galactic cosmic rays to energies never observed before in the Milky Way. The researchers suspect that the black hole at the centre of our galaxy can be held responsible. The findings of the scientists, united in the H.E.S.S. collaboration, were published in Nature on 16 March.

For over thirty years a collaboration of 42 institutes in 12 countries, including scientists of the UvA GRAPPA, Anton Pannekoek Institute for Astronomy, and the Institute of Physics, has been mapping the centre of our galaxy in very-high-energy gamma rays. A detailed analysis of the latest data 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 its centre.

Cosmic rays

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”. Since more than a century, the origin of these cosmic rays remains one of the most enduring mysteries of science. The particles, such as protons, electrons and atomic nuclei are electrically charged, and are hence strongly deflected by the interstellar magnetic fields that pervade our galaxy. Fortunately, cosmic rays interact with light and gas in the neighbourhood of their sources and thereby produce gamma rays. These gamma rays travel in straight lines, undeflected by magnetic fields, and can therefore be traced back to their origin.

The source of gamma rays

Researchers of the High Energy Stereoscopic System-consortium (H.E.S.S.-consortium) used their telescopes in Namibia for the measurement. Ten years ago, they had already uncovered a very powerful point source of gamma rays in the galactic centre region, but the nature of the source remained a mystery. Possible objects capable of producing cosmic rays of high energy were supernova remnant, a pulsar wind nebula, and a compact cluster of massive stars.

Deeper observations made it now possible to pinpoint the black hole at the centre of the Milky Way as the source of the particles. This cosmic accelerator is about 100 times as powerful as the LHC at CERN, the largest terrestrial particle accelerator. The black hole is the first discovery of an astrophysical source capable of accelerating protons to energies of about one petaelectronvolt.

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Artist’s impression of the giant molecular clouds surrounding the Galactic Centre, bombarded by very high energy protons accelerated in the vicinity of the central black hole and subsequently shining in gamma rays. © Dr Mark A. Garlick/ HESS Collaboration

The scientists have published their findings on 16 March in the journal Nature. Jacco Vink, David Berge, David Salek, Rachel Simoni and Mark Bryan have contributed to the research. Berge, the coordinator of the galactic science program of H.E.S.S., says: “It is great that we as a team finally found the source of gamma rays in the galactic centre region, after years of measuring and modelling.”

– Publication details: H.E.S.S. collaboration, corresponding authors: F. Aharonian, S. Gabici, E. Moulin and A. Viana; “Acceleration of petaelectronvolt protons in the Galactic Centre” Nature, 16 March 2016.