Is the source of ultra-high-energy cosmic rays a gamma-ray burst located in the Milky Way?
2011. 2. 1


[Figure Caption: Depiction of a gamma-ray burst that occurred in the Milky Way. The explosion of a supernova (orange) occurs simultaneously with the appearance of a gamma-ray burst jet (blue), with extremely high energy cosmic rays emitted from the jet. (This illustration is the work of Yasuyuki Kaji, a student majoring in oil painting at Seika University in Kyoto.)]

Cosmic rays have a number of different sources, some of which have been firmly established. However, the source of ultra-high-energy cosmic rays, i.e., those with energies $\sim 10^{20}$eV, is yet unknown. This is regarded as one of the biggest puzzles in modern astrophysics. The distribution of the directions from which ultra-high-energy cosmic rays impinge upon the earth is very nearly isotropic. In particular, it is not larger in the direction of the Milky Way. In addition, it is thought that there is presently no active celestial body in our galaxy that could accelerate particles to such high energies. Based on these observations, it is reasonable to conclude that the source of ultra-high-energy cosmic rays is some fantastically energetic body outside the Milky Way. However, this conclusion has now come into question.

Recently, the research group at the world's largest ultra-high-energy cosmic ray detection facility, located at the Pierre Auger Observatory, made a very interesting announcement concerning the constitution of ultra-high-energy cosmic rays. According to their findings, cosmic rays with energies on the order of $10^{18}$eV consist predominantly of protons, but for larger energies, a significant fraction of the particles are iron nuclei, and this fraction increases with the energy. At energies $\sim 3 \times 10^{19}$eV, the particles are mainly iron. (The particle makeup of ultra-high-energy cosmic rays is determined by observing the development of the particle showers that result when these particles enter the earth's atmosphere, and thus it is determined through a statistical analysis. Due to the inherent statistical uncertainty involved, drawing firm conclusions for higher energy particles is more difficult, and for this reason, information concerning the makeup of cosmic rays released by the Pierre Auger Observatory applies only to particles with energies up to $3 \times 10^{19}$eV. For more energetic particles, no results have yet been released.)

With the announcement of these observations, recently, Calvez, Kusenko and Nagataki proposed a new theory, according to which the source of ultra-high-energy cosmic rays (with energies in the range $10^{18}$eV to $3 \times 10^{19}$eV) lies in the Milky Way, not outside it, as generally believed to this time. Specifically, they pointed out the possibility that the source of such high-energy particles is a gamma-ray burst that appeared as a result of an enormous explosion that took place in our galaxy's past. Such a gamma-ray burst, which is known to have taken place, would indeed have been sufficiently energetic to accelerate particles to the energies of ultra-high-energy cosmic rays (see image). The paper in which this new theory is proposed was published in a 2010 issue of Physical Review Letters and was introduced in the "Research Highlights" section of Nature. The idea proposed in this paper is the following. First, it has been established that there was a gamma-ray burst progenitor existing in the Milky Way that was capable of generating ultra-high-energy cosmic rays (see image). Further, it is known that gamma-ray bursts can indeed accelerate both protons and iron nuclei. However, whereas such protons would leave the galaxy very rapidly, the iron nuclei, with their significantly larger charge, would whirl around the galaxy, being effectively trapped by the magnetic fields existing therein. For this reason, they would remain within the galaxy for a much longer time than the protons. As a result, the relative concentration of iron ultra-high-energy cosmic rays in the Milky Way would have increased over time. This is consistent with the observation of the Pierre Auger Observatory that the earth is now being bombarded by very high energy cosmic rays that are largely composed of iron. In addition, the trapping effect of the iron nuclei would tend to cause their trajectories to meander greatly, and this could account for the observation that they impinge upon the earth from all directions at almost equal rates. Calvez, Kusenko and Nagataki also made a conjecture concerning more energetic cosmic rays, with energies on the order of $10^{20}$eV. In the case of such highly energetic particles, it becomes difficult even within this new scenario to account for the directional isotropy and energy spectrum of the cosmic rays observed on Earth. This leads them to argue that the source of these cosmic rays may not be within the Milky Way and, further, not a gamma-ray progenitor, but some object outside the galaxy, perhaps a neighboring active galactic nucleus. We are awaiting results from the Pierre Auger Observatory and Japan's Telescope Array to provide new information that may help to clarify these points.

This work was reported in the August 20th edition of Physical Review Letters (Phys. Rev. Lett. 105, 091101 (2010)) and featured in Nature Research Highlights (Nature Physics 6, 636 (2010); http://www.nature.com/nphys/journal/v6/n9/full/nphys1786.html) Title: "Role of Galactic Sources and Magnetic Fields in Forming the Observed Energy-Dependent Composition of Ultrahigh-Energy Cosmic Rays" Authors: Antoine Calvez (UCLA), Alexander Kusenko (UCLA/IPMU), Shigehiro Nagatani (Yukawa Inst.)