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Correlation of the Highest-Energy Cosmic Rays with Nearby Extragalactic Objects

The Pierre Auger Collaboration

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Cosmic rays are particles and nuclei that bombard the Earth from space in all directions (1). A few have astounding energies—beyond 100 EeV (1 EeV = 1 exa-electron volt = 1018 eV)—orders of magnitude beyond even the future capabilities of any earthly particle accelerator. Such energies are so extreme that they could arise in only the most violent places in the universe. One possible location is within active galactic nuclei (AGN), galaxies hosting central black holes that feed on gas and stars and may eject vast plasma jets into intergalactic space.

As cosmic rays propagate, the highest-energy particles interact strongly with the ubiquitous cosmic background radiation and lose some energy. Thus, they can only travel limited distances and, consequently, their flux is suppressed (the “GZK effect”). So the survival of the highest-energy cosmic rays as they traverse space is in itself a puzzle. Simply stated, we don’t know what they are, where they came from, or how they got here from there.

The highest-energy cosmic rays are so rare that in the last 50 years, only a handful of 100-EeV particles have been detected. The low flux (only a few per km2 on Earth per millennium) renders their direct detection infeasible. Instead, instruments with extremely large collecting areas are deployed and sample the shower of secondary particles produced when the primary cosmic ray collides with Earth’s atmosphere. The Pierre Auger Observatory stretches over 3000 km2 in western Argentina, an area similar to that of Rhode Island. It measures extensive air showers both on the ground with 1600 detectors spaced 1.5 km apart and in the air, viewing the brief flash of nitrogen molecules de-exciting after the shower passes by (the same radiation is seen from a different stimulus and over longer time scales as the Aurora Borealis). The Pierre Auger Observatory uses these two detection techniques routinely at the same time. The size of the data set now exceeds that from all earlier experiments.

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Sky map (2) showing cosmic rays detected by the Pierre Auger Observatory. Low-energy cosmic rays appear to originate from evenly distributed sources (blue dots), but the origins of the highest-energy events (crosses) correlate with the distribution of local matter as represented by nearby active galactic nuclei (red stars). Thus, active galactic nuclei are a likely source of these rare high-energy cosmic rays.

Credit: C. Bickel/Science

The direction of the primary cosmic ray can be reconstructed with good precision—to within 1° or so—by the ground detectors. Most cosmic-ray particles are charged and so their trajectories are bent by the magnetic fields in space. For particles with energies above a few tens of EeV, the deflection is, however, small enough that the prospect of identifying possible sources becomes a reality.

Since 2004, the Auger Observatory has collected a million cosmic-ray events, and about 80 had energies exceeding 40 EeV, the energy at which we expect to begin to see the flux suppression of the GZK effect. First, we examined the data gathered before June 2006. We explored the amount of correlation between the arrival directions and the positions of known AGN by tuning several factors: a cutoff for the maximum distance of an AGN, a cutoff for the minimum energy of cosmic rays, and the angular separation of an event from some AGN.

We found a strong association between the cosmic-ray directions and nearby AGN. Of 15 events with energies greater than about 60 EeV, 12 were located within 3.1° of AGN closer than 75 Mpc from Earth (about 250 million light-years). The likelihood of a random isotropic set of arrival directions conspiring to fool us this much was small. We fixed the values of the correlation parameters and applied them to new data collected after June 2006. Data collected more recently, until August 2007 (see the figure), confirmed the correlation.

Interpretation of these results merits some caution. We used a catalog of AGN that is known to be incomplete, especially in directions in which we peer through the dusty plane of our Galaxy and beyond 300 million light-years away from Earth. (It is notable that most of the few events that do not appear to be near AGN are indeed somewhat near the Galactic plane.) The AGN themselves tend to be distributed among the nearby galaxies, and so based on the statistics of our present data we can only declare that the cosmic-ray sources are correlated with the distribution of nearby matter, including AGN. However, because of the energetic processes within them, AGN have long been considered as likely sources of cosmic rays. Our data suggest that they remain the prime candidates.

Summary References

  1. J. W. Cronin, T. K. Gaisser, S. Swordy, Sci. Am.276, 44 (January 1997).
  2. Equal areas on this plot represent equal exposure on the sky. The declination is on the vertical axis. Declinations 0°, -30°, and -60° are marked (from the top) (the observatory zenith is close to dec = ‒30°). The observatory has more exposure to the AGN indicated by darker stars than those shown in lighter shades of red.

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