Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........35b&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0031, Part 0606 134 pages; [Ph.D. dissertation].United States -- California: Un
Physics
Ultrahigh-Energy Cosmic Rays, Cosmic Rays, Auger Detector
Scientific paper
Although the existence of ultra high energy cosmic rays (UHECR) with energies on the order of 10 20 eV, has been shown by past experiments, the source of these particles is not yet understood. Theoretical models motivate the consideration of nearby active galactic nuclei (AGN) as a source candidate. However, AGN have not been declared as the source unambiguously and alternative hypotheses have also been made claiming radio galaxies are a significant source of UHECR. A third source candidate named Centaurus A (Cen A) is also considered.
The focus of this thesis will be to test these hypotheses using the Pierre Auger detector, which observes the air showers generated by the UHECR primary after entering the Earth's atmosphere. The detector utilizes an array of 1600 ground detectors, each consisting of 12 metric tons of water spread over roughly 3000 km 2 . Each tank of water is watched by three 9" photomultipliers that detect Cherenkov radiation emitted by air shower particles passing through the water. The light emitted by atmospheric nitrogen that has been excited by passing air shower particles is also observed in order to obtain a calorimetric measurement of the energy of the UHECR.
The first hypothesis that will be tested concerns the possibility that UHECR with energies larger than 57EeV correlate within 3.2° of AGN within 0.018 redshift. Given that the region 3.2° around these AGN covers 23% of the sky, on average 2.3 events out of 10 are expected to correlate within 3.2° of an AGN under the assumption of isotropy. Given only one out of 10 events in an independent data set are found to correlate, this hypothesis is disfavored.
The second hypothesis that will be tested claims an excess of UHECR are found within 3.5° of radio galaxies within about 70 Mpc. Given that the region 3.5° around radio galaxies covers 10% of the sky, on average one out of 10 even s are expected to correlate within 3.5° of a radio galaxy, under the assumption that the UHECR are isotropic. In fact, no events out of 10 events in an independent data set are found to correlate. Therefore, this hypothesis is also disfavored.
The region 17° around Cen A is shown to have an excess of UHECR with energies larger than 57EeV with a confidence of 2.3x10 -3 . However, Cen A is not shown to be the source unambiguously. Despite this, Cen A is the closest radiogalaxy, at 3.4 Mpc, whose radio lobes extend over about 10° of the sky and is themost likely source of this anisotropy.
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