Published: 17.08.09
Astrophysics

Galactic particle accelerator located

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. Now the underlying extreme physical processes and inherent implications can be investigated in more detail.

Alexandra von Ascheraden
The MAGIC Telescope in La Palma, involved in the measurements. (Image: Robert Wagner, MPI for Physics)
The MAGIC Telescope in La Palma, involved in the measurements. (Image: Robert Wagner, MPI for Physics) (large view)

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.

In addition to radio and gamma radiation, X-ray emissions from M87 were also observed by satellites during the outburst. The fact that a radiation burst was measured in three different energy regions is so far unique. This measuring campaign enables high-precision tests and even the exclusion of some of the different models for the description of such outbursts. As a result, characteristic physical parameters such as the magnetic field or the Doppler factor of the emission region can be determined with a substantially higher precision.

The Messier 87 galaxy

M87 is a gigantic elliptical so-called radio galaxy, namely a galaxy radiating particularly in the radio frequency range. Its mass exceeds about 3 trillion times the mass of our sun and is situated in the Virgo constellation about 50 million light-years away, i.e. very close to the earth in cosmic terms. A supermassive black hole is located in its centre, with a mass approximately 6 billion times higher than the mass of our sun, that provokes enormous outbursts of energy. Galaxies of this kind are referred to as active galactic nuclei.

The MAGIC telescope

The MAGIC experiment, in which the IPP of ETH Zurich is significantly involved, is a telescope situated on a mountain on the Canary Island of La Palma. With a mirror of 17 metres in diameter, it is the largest Cherenkov telescope in the world. It enables to detect gamma-rays which are absorbed in the terrestrial atmosphere and therefore cannot be recorded directly. This is done as follows: A high-energy gamma particle penetrating into the upper layers of the atmosphere interacts with the atoms of the atmosphere and is thereby converted into an electron and its antiparticle, a positron. The charged electrons and positrons again produce further gammas by means of 'Bremsstrahlung', which for their part again disintegrate into electron-positron pairs. This leads to a kind of snowball effect, producing ever more particles. A so-called air shower develops.
The charged particles in this air shower, moving almost at the speed of light, emit so-called Cherenkov radiation lighting up an area of approximately one hundred metres in diameter for a few billionths of a second. MAGIC with its 934 aluminium mirrors collects a part of this light and can thus detect the extremely weak traces of the Cherenkov flashes. In the focal plane of the telescope, the collected photons are focussed onto an electronic camera, with an exposure time of less than a few billionths of a second. Ultra-rapid optical glass fibres provide for an almost loss-free transmission of the pulses produced in the camera to the researchers' computers.
At present, an almost identical MAGIC II telescope is commissioned on La Palma. The sensitivity of the system will be considerably improved by the stereo observations starting in just a few weeks from now. The long-term goal, however, is the construction of the Cherenkov Telescope Array (CTA), a Pan- European project for the construction of a gamma radiation observatory at least ten times more sensitive than today's devices and covering a far more extended energy range.

Reference work

Acciari VA et al. (The VERITAS, H.E.S.S., MAGIC Collaborations and the VLBA 43 GHz M87 monitoring team). "Radio Imaging of the Very-High-Energy Gamma-Ray emission region in the Central Engine of a Radio Galaxy". Science. 24 July 2009; 325 (5939), 444-448. doi:10.1126/science.1175406

Begelman M. Astronomy: "A Flare for Acceleration". Science. 24. July 2009; 325 (5939), 399-400. doi:10.1126/science.1176908

 
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