The High Energy Stereoscopic System (HESS) – an array of Cherenkov telescopes in Namibia – has detected gamma-ray emission from the central region of the Milky Way at energies never reached before. The likely source of this diffuse emission is the supermassive black hole at the centre of our Galaxy, which would have accelerated protons to peta-electron-volt (PeV) energies.

The Earth is constantly bombarded by high-energy particles (protons, electrons and atomic nuclei). Being electrically charged, these cosmic rays are randomly deflected by the turbulent magnetic field pervading our Galaxy. This makes it impossible to directly identify their source, and led to a century-long mystery as to their origin (CERN Courier July/August 2012 p14). A way to overcome this limitation is to look at gamma rays produced by the interaction of cosmic rays with light and gas in the neighbourhood of their source. 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 photodetectors, and fast electronics, more than 100 sources of very-high-energy gamma rays have been identified over the past three decades. HESS is the only state-of-the-art array of Cherenkov telescopes that is located in the southern hemisphere – a perfect viewpoint for the centre of the Milky Way (CERN Courier January/February 2005 p30).

Earlier observations have shown that cosmic rays with energies up to approximately 100 tera-electron-volts (TeV) are produced by supernova remnants and pulsar-wind nebulae. Although theoretical arguments and direct measurements of cosmic rays suggest a galactic origin of particles up to PeV energies, the search for such a "Pevatron" accelerator has been unsuccessful, so far.

The HESS collaboration has now found evidence that there is a "Pevatron" in the central 33 light-years of the Galaxy. This result, published in Nature, is based on deep observations – obtained between 2004 and 2013 – of the surrounding giant molecular cloud extending approximately 500 light-years. The production of PeV protons is deduced from the obtained spectrum of gamma rays, which is a power law extending to multi-TeV energies without showing a high-energy cut-off. The spatial localisation comes from the observation that the cosmic-ray density decreases with a 1/r relation, where r is the distance from the galactic centre. The 1/r profile indicates a quasi-continuous central injection of protons during at least about 1000 years.

Given these properties, the most plausible source of PeV protons is Sagittarius A*, the supermassive black hole at the centre of our Galaxy. According to the authors, the acceleration could originate in the accretion flow in the immediate vicinity of the black hole or further away, where a fraction of the material falling towards the black hole is ejected back into the environment. However, to account for the bulk of PeV cosmic rays detected on Earth, the currently quiet supermassive black hole would have had to be much more active in the past million years. If true, this finding would dramatically influence the century-old debate concerning the origin of these enigmatic particles.