An unprecedented measuring campaign has succeeded in precisely defining the place of origin of high-energy gamma radiation in the galaxy Messier 87.

This radiation can only be produced by accelerating elementary particles to very high energies in enormous cosmic objects.

Our neighbouring galaxy Messier 87 (M87) accelerates elementary particles to extremely high energies - millions of times higher than anything possible with the particle accelerator LHC (Large Hadron Collider) at CERN. These particles contribute to the cosmic radiation that can be measured on earth. For the first time, physicists can now locate exactly where the acceleration of the particles takes place, i.e. right next to the black hole in the centre of the galaxy.

Cosmic radiation was discovered more than 98 years ago, but since the particles are deflected in magnetic fields, their origin cannot be measured directly. The fact that M87 accelerates elementary particles was known, because very high-energy photons, so-called gamma rays, are also produced during such acceleration processes.

These gamma rays are not deflected by magnetic fields and therefore reach us on a direct track. This radiation can be detected (see box) using Cherenkov telescopes. However, these telescopes have an angular resolution of only about 0.1 degree, so that it is impossible to pinpoint where exactly in M87 the acceleration takes place.

Telescopes on three continents

In an unusual measuring campaign, the answer to this unsolved issue could now be found. For more than 120 hours, the world's most modern Cherenkov telescopes MAGIC (Major Atmospheric Gamma Imaging Cerenkov) on La Palma, VERITAS in Arizona and H.E.S.S. in Namibia observed the M87 galaxy during a particularly active phase. At the same time, M87 was also observed with the VLBA (Very Long Baseline Array), a combination of several radio telescopes with very high spatial resolution scattered over the whole of the USA.

Normally, radio observations do not allow to draw conclusions on the acceleration of elementary particles, since strong emissions within the radio range can also have many other causes. As the spontaneously developed research collaboration reported in the renowned technical periodical 'Science', however, a breakthrough was attained by combining both measurements.

Simultaneous with the strongest bursts of gamma radiation seen from M87, an extremely high activity was measured in the radio band, and this exclusively in the proximity of the black hole. This indicates that the outbursts of gamma radiation and the radio emission were produced by the same process, and therefore both originate from the proximity of the black hole.

Particle physicist Adrian Biland, coordinator of the ETH group participating in the MAGIC experiment, explains: "For the very first time, we now have a clear reference to where exactly one part of the extragalactic cosmic radiation develops, the origin of which has been an enigma for nearly a hundred years."

The measuring campaign started when MAGIC observed an enormous burst in gamma rays. Biland recounts: "Previously, we had observed M87 time and again without seeing anything particular." Immediately after the burst occurred, the scientists alerted the researchers at the other telescopes who thereupon also directed their devices towards M87. At the three Cherenkov telescopes alone, some 400 scientists were involved, a good part of the world’s gamma ray astrophysics community.

"The Messier 87 galaxy. (Credit: NASA and the Hubble Heritage Team/STScI/AURA)"

Source: ETH Zurich