Scientists do not know what constitutes the highest-energy cosmic rays, where they come from or how they get to earth. The highest-energy cosmic rays are so rare that in the last couple of decades only a handful of them have been detected, with less than one particle reaching a square kilometer of the earth in a century. However, given the astounding energies of some of these particles, scientists believe they can only come from the most violent places in the universe. One possible location from where these particles might have arisen is within active galactic nuclei (AGN), which are compact regions at the center of most, if not all, galaxies. These regions are believed to contain supermassive black holes that feed on stars and dust in the galaxy while emitting plasma jets into intergalactic space. Supermassive black holes have masses ranging from a few hundred thousand to 10 billion times that of the sun.
To detect sufficient numbers of highest-energy particles, the Pierre Auger Observatory used 1,600 surface detectors, placed 1.5 kilometers apart, in conjunction with 24 optical telescopes. The entire array of detectors covered 3,000 square kilometers, an area similar to that of Rhode Island, making it the world’s largest cosmic ray receiver. With so few highest-energy particles reaching the earth, direct detections are rare. Instead these particles are usually detected indirectly by sampling the shower of secondary particles produced when these cosmic rays enter the atmosphere. While ground telescopes are used to detect these showers, the optical telescopes view the scintillations produced by nitrogen molecules present in the path of these showers. The ground detectors also allow the direction of the primary cosmic ray to be reconstructed quite precisely, within a degree or so. Unlike the often charged, low-energy cosmic rays that are strongly bent by the earth’s magnetic field, these highest-energy particles suffer much smaller deflections in their direction, making it possible to trace them back to their source.
In the article published in Science, the group observed a strong association between the cosmic-ray directions and nearby AGNs located within 250 million light-years from the earth. Moreover, from their data, they hypothesized that the highest-energy cosmic rays reaching the earth were protons coming from these nearby sources. Alan Watson, spokesperson for the Auger collaboration and astrophysicist at the University of Leeds, U.K., said, “For myself, it’s deeply satisfying to have gotten to the beginning of the end of this whole riddle.”
Science Education Fellow in Astronomy at Amherst College Lara Phillips remarked on the importance of the findings. “The authors are closing in on the source of the highest-energy cosmic-rays,” said Phillips. “And those sources seem to lie in higher density regions in the nearby universe. This is a remarkable step forward in our understanding of the origin of cosmic rays.”
The authors of the paper do, however, caution that their results need to be interpreted judiciously. While their findings don’t prove that AGN produce such cosmic rays, their data suggests that AGNs continue to remain the prime candidates for producing these highest-energy particles. Watson added, “Anything else that’s distributed on the sky in the same way as AGNs could be the source.”
Phillips also emphasized that the findings of the study are preliminary, adding that tracing the cosmic rays to a source or galaxy “may be a bit premature since the uncertainty in the source position is too large to identify the particle source with a given object.” However, more data from Centaurus A, one of the closest AGNs to which two detection events were traced to within 3 degrees, “could help the authors increase the position precision and tamp down the error bars.”
Meanwhile, the Auger team is working on building an array almost three times as large in the Northern Hemisphere to study the entire sky for highest-energy cosmic rays. Yet an important question still remains unanswered: what is the exact mechanism by which these particles are accelerated to such astonishing energies? Future studies will attempt to address this and other remaining mysteries in cosmic ray physics.