The mystery of the most energetic cosmic rays deepens

 Recently, the Telescope Array (TA) Collaboration reported on the observation of a cosmic ray with enormous energy: 244 exa-electron volts (EeV, 2.44*10²⁰ eV), or roughly 40 Joules.  This is one of a handful of events seen in this energy range - with an energy roughly comparable to a well hit tennis ball.   Although this is not the most energetic particle ever seen, it is pretty close.  

Fig. 1. A skymap, showing the best-estimate directions for the event, after correcting for expected deflection in galactic and extra-galactic magnetic fields.   The labelled circles are the best estimates for different assumptions about nuclear composition, from protons (p) through iron (Fe) for two different models of the galactic magnetic fields - PT2011 and JF2012 which predicts larger deflections .  The active galaxy PKS1717+177 is a flaring source, but it is 600 Mpc away - probably too far to be the source of this event.  Also shown is a broad hotspot previously seen by TA at lower energies, along  with the galactic plane (the solid line), with the galactic center also indicated.

 

 The intriguing thing about this event is its direction.  The arrival direction was measured to better than 1 degree.  Fig. 1 (above) shows the predicted arrival direction.   At these energies, cosmic rays are not expected to bend very much in galactic and intergalactic magnetic fields.   The TA Collaboration calculated the expected bending for different hypotheses about the nuclear species of the incident particle, from protons to iron, and accounted for that bending.  Then, they looked in that general direction, and found nothing that seemed likely to be able to accelerate particles to those energies. Because 250 EeV particles can interact with the cosmic microwave background radiation and lose energy, their range is limited to 10-30 megaparsecs (Mpc), depending on nuclear species.  The active galaxy PKS1717+177 was considered as a source, but, at 600 Mpc distance, is too far to be a likely source.

This is a finite volume for a possible source, and it was devoid of 'interesting' objects.  'Interesting objects' include active galactic nuclei (galaxies with supermassive black holes at their center, with significant accretion which leads to a relativistic jet) and other sites that may contain the ingredients for a powerful particle accelerator.

There are a couple of possible explanations for the lack of an apparent source - all interesting.  It may be that we need to expand our definition of what is 'interesting' here - the accelerating sites are something that we have not thought of.  Or, maybe, the accelerators do not leave obvious other traces, or are distributed in space.  Or, possibly, the galactic and/or intergalactic magnetic fields are significantly larger than we expect.   

As the name implies, the Telescope Array is an array of surface scintillator detectors (to detect charged particles in cosmic ray air showers) and fluorescence detector telescopes to detect the fluorescence from nitrogen in the air as these charged particles propagate through the atmosphere.

The paper was published in Science, but is also freely available on the arXiv.