Extreme energy cosmic ray source in galaxy centre

Thursday, 17 March 2016

An international team of astronomers involving University of Adelaide researchers has found the first evidence for an extreme source of cosmic rays ─ at the centre of our Milky Way galaxy.

Using the High Energy Stereoscopic System (HESS) telescopes in Namibia, the research published in the journal Nature today has revealed an exceptionally high energy gamma ray source. These gamma rays act as “a tracer” for cosmic rays and are produced when cosmic rays collide and interact with surrounding gases.

Cosmic rays are extremely high-energy particles such as protons and atomic nuclei that rain down on Earth from outer space, and their origins have been the subject of a 100-year-old debate.

“These results represent the first indications of where the Milky Way's highest energy cosmic rays could come from,” says Associate Professor Gavin Rowell, from the University of Adelaide’s High Energy Astrophysics Group and leader of Australia’s participation in the HESS Collaboration.

“The most plausible 'engine' for this cosmic ray acceleration is the super-massive black hole right at the heart of our galaxy.”

The central region of the Milky Way galaxy harbours a super-massive black hole (with a mass of 4 to 5 million Suns) whose immense gravity dominates the motion of nearby stars and gas. The region is also extremely active in the formation of massive stars, which then lead to the creation of numerous supernova remnants and pulsars. All of this violent activity makes this region one of the brightest objects in the sky for astronomers to study in all forms of light, from radio waves right up to very high-energy gamma rays.

HESS has observed this region intensely since 2004 when it first studied the gamma ray signal from the super-massive black hole region.

“The most striking aspect is that the gamma ray emission properties tell us that they come from a source of cosmic rays with energies reaching 100 times higher than that of the Large Hadron Collider in CERN, Switzerland,” says Associate Professor Rowell.

“The likely culprit accelerating these cosmic rays is the gravitational force and violent activity of the super-massive black hole. This was likely to have been more active in the past and could account for most of our galaxy's highest energy cosmic rays. This research provides firm evidence that the super-massive black hole at the Milky Way's heart plays an important role.”

The HESS team consists of scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, the Netherlands, Austria and Sweden.

HESS in Namibia is a system of four 13-metre diameter telescopes – more recently complemented with the huge 28m HESS II telescope − and is one of the most sensitive detectors of very high-energy gamma rays.

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