Kunlaya Soiaporn, David Chernoff, Thomas Loredo, David Ruppert, Ira Wasserman
The sources of ultra-high energy cosmic rays (UHECRs) are unknown but are likely nearby galaxies. To assess association of UHECRs and candidate sources, we developed a multilevel Bayesian framework. We demonstrate this framework using simple models similar to those in previous studies, but our results suggest a need for more complex models; these also can be fit within our framework. We use MCMC methods to implement the approach to model data on 69 UHECRs observed by the Pierre Auger Observatory (PAO) during 2004-2009, using a volume-complete catalog of 17 nearby active galactic nuclei (AGN) as candidate sources. The reported PAO data are incomplete; an early portion (Period 1) was used to set an energy cut maximizing anisotropy, and only cosmic rays above that cut are reported. This data-tuning proves problematic for independent analyses. Assuming a common, isotropic UHECR emission rate ("standard candles") and using the untuned data after Period 1, there is no significant evidence for association of UHECRs with nearby AGNs versus an isotropic population of sources. If the association model is adopted, the fraction of UHECRs associated with these AGN is likely nonzero but well below 50%. Relatively small magnetic deflections are favored; models that assign a large fraction of UHECRs to a single nearby source are ruled out unless very large deflections are specified a priori. Including Period 1 data alters the conclusions significantly, and a simulation study suggests that the Period 1 data are anomalous, presumably due to the tuning. Accurate and optimal analysis of future data will likely require more complete disclosure of the data.
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http://arxiv.org/abs/1206.4569
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