Protons with Horsepowers:
On the Traces of the Universe's Highest Energy Particles

An experienced observer gazing at the night-sky for decades, could probably witness with naked eyes something that only powerful electron microscopes are able to reveal: a single proton in action. With some care, he could observe the characteristic flashes caused by the most energetic particles in the Universe as they hit the Earth's atmosphere. The observer would need to be patient, as these particles are very rare. Computer simulations undertaken at the Max-Planck-Institute for Astrophysics in Garching have tested several scenarios proposed for the mysterious origin of these remarkable particles. The results: The trajectories of the exotic particles can be reproduced without exotic explanations. In addition, the particles probably come from sources that are distributed in a similar fashion to cosmic matter in extragalactic space. Thus they could originate from neighbouring galaxies.

Fig. 1: Distribution of matter and galaxies in the simulation (red means high density). The assumed position of our Milky Way is marked with a star. Distances are in Mpc = roughly 3 million lightyears.

Fig. 2: As in Fig. 1 but here the simulated magnetic field strength in the neighbourhood of the Milky Way (red means strong magnetic fields).

Fig. 3: The arrival directions of ultrahigh-energy particles in the simulation (bright points) in sky projection. The nearby Virgo galaxy cluster (red circle) is clearly visible as an excess of arrival events since a more than hundred times enhanced number of arrival events is displayed.

Ultrahigh-energy particles are the most energetic particles known to exist in the Universe. Each of them owns an energy of up to 10 to 21 electron-Volts. This is nearly the energy of a horse's kick, concentrated in a single proton. Particles with such an enormous energy cannot be confined even within the vast extent of our own Milky Way galaxy leading to the conclusion that they must have an extragalactic origin.

Whenever an ultrahigh-energy particle hits the Earth's atmosphere, the atmosphere's atoms burst in such collisions. The debris of the collision penetrates deeper into the atmosphere and causes a fluorescencing flash of light. During the last decades less than a hundred of such events were recorded with specialised telescopes which, by looking at the light flashes they produce, reconstruct the particles energy and arrival direction. However, the light flashes do not provide information about the particles' journey to Earth. In fact, the particle arrival direction does not necessarily say where the particle is coming from, because the particle direction of motion is deflected by the presence of extended extragalactic magnetic fields. Strength, orientation, and length-scale of the magnetic fields outside our Milky Way are unknown. It is also unknown from where the particles originate: radio galaxies, giant black holes, mysterious gamma-ray bursts, or from completely unknown processes?

Günter Sigl, Francesco Miniati and Torsten Enßlin simulated possible extragalactic journeys of these protons and tried to infer which ones are more likely by making a statistical comparison of the result of different scenarios with observational data. More specifically, they investigated whether the particles originate in a small number of powerful sources (i.e. each producing many particles) or, conversely, in a large number of weak sources; whether the sources are distributed as cosmic matter in the neighbourhood of the Milky Way or are homogeneous in space; what kind of magnetic fields (strong or weak) and thereby of deflections of the particle trajectories should be expected.

The conclusion is that in a statistical sense the observational data favour a scenario in which the sources of ultrahigh-energy particle are neither few and strong nor numerous and weak, but moderate both in number density and power of particle production. They are placed with an average separation of 100 million lightyears from each other, but their distribution is not homogeneous. They are roughly distributed as the galaxies in the neighbourhood of the Milky Way. According to the best-fit scenario, the magnetic fields are strong in the vicinity of their sources, and weak but non-negligible within the huge space in between the sources and the Milky Way. Therefore, most of the particles should have been deflected by several ten degrees. However, a significant fraction should have taken a relatively direct route and the directions of the atmospheric lightflashes should in those cases be roughly the initial directions of the particles and point back to their sources. Thus, as soon a sufficient high number of detections will be reached, an excess of events in the direction of the most important sources should be observable.

This scenario implies that many of the ultrahigh-energy particles may originate in the galaxies of the nearby Virgo galaxy cluster. However, due to the presence of strong magnetic fields within the Virgo cluster it should be difficult to identify their source locations exactly within the cluster.

In conclusion, the results of the simulation favour conservative theories about the origin of the particles, which suggest a connection between the galaxy distribution and the source locations. They also fit well to our ideas about extragalactic magnetic fields. Future observations with telescopes (e.g. the linkPfeilExtern.gifPierre Auger or the linkPfeilExtern.gifEUSO project) currently under development will examine many thousands of lightflashes from ultrahigh-energy particles and thereby allow us to follow the traces of these particles towards their sources more closely.

Torsten Enßlin, Francesco Miniati, Günter Sigl


linkPfeilExtern.gif"Ultra-High Energy Cosmic Rays in a Structured and Magnetized Universe",
Günter Sigl, Francesco Miniati, Torsten A. Enßlin,
Physical Review D 68, 043002

linkPfeilExtern.gif"Signatures of Magnetized Large Scale Structure in Ultra-High Energy Cosmic Rays",
Günter Sigl, Francesco Miniati, Torsten A. Enßlin,
submitted, astro-ph/0309695

linkPfeilExtern.gif"Ultra-High Energy Cosmic Ray Probes of Large Scale Structure and Magnetic Fields",
Günter Sigl, Francesco Miniati, Torsten A. Enßlin,
submitted, astro-ph/0401084