Identifying the UHECR accelerators using gamma-rays



The origin of the most energetic particles observed in nature, the ultra-high-energy cosmic-rays (UHECRs), is one of the most important open questions in astrophysics. The Pierre Auger Observatory, built to study these particles, has recently been used to make two breakthroughs in this field. Firstly, the detection of a significant anisotropy in the arrival directions of cosmic rays with energies above 6 x 1019 eV, and evidence for a correlation with the locations of active galactic nuclei (AGN) in the nearby universe (distance < 75 Mpc). Secondly, the measurement of a suppression of the cosmic ray flux at energies larger than 4 x 1019 eV. Taken together these measurements strongly suggest that the UHECRs are a) extragalactic in origin, b) have a light mass spectrum, dominated by protons. However, it is not possible at the current time to identify any individual object (or even class of objects) as an UHECR accelerator. The angular resolution of Auger, the expected deflections in the galactic and extragalactic fields, and the small statistics (27 events) make a solid identification using UHECRs alone very unlikely for current instruments. Perhaps the most promising avenue through which to establish an individual astrophysical object as an UHECR source is the detection of characteristic emission in the high or very-high energy gamma-ray do- main. Many predictions exist for GeV-TeV gamma-ray emission, arising both from the propagation of UHECRs from their sources to the Ear th, and inside the accelerator sites themselves. Fortunately, the experimental situation in gamma-ray astronomy has dramatically improved in recent years. A sensitive TeV detector, HESS, is operating in the southern hemisphere, with sky coverage well matched to that of Auger. Additionally, instruments in the nor thern hemisphere VERITAS and MAGIC provide gamma-ray coverage above declination -10 degrees, corresponding to 50% of the Auger sky coverage. GLAST, a wide field of view GeV gamma-ray satellite mission, will be launched on the 16th May. We therefore consider a systematic study of what can be learnt about the UHECR using gamma-rays to be both timely and important. We propose to assemble a team of exper ts, combining knowledge of gamma-ray and UHECR measurements with theoretical modelling of UHECR acceleration and propagation, to study and systematically reassess the complex and inter-related possibilities for gamma-ray production and the detectability of the UHECR signals. The output of the work of this team will take the form of recommendations for the observation strategies of gamma-ray instruments, as well as review papers addressing what can be done with the results of such observations.


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The team work is to be completed between late 2008 and mid 2010. The first meeting is 16-20 February 2009 at ISSI Bern. The second meeting is planned for late 2009 or early 2010.