On the Evidence for Clustering in the Arrival Directions of AGASA's Ultrahigh Energy Cosmic Rays

Previous analyses of cosmic rays above 40 EeV observed by the AGASA experiment have suggested that their arrival directions may be clustered. However, estimates of the chance probability of this clustering signal vary from 10^{-2} to 10^{-6} and beyond. It is essential that the strength of this evidence be well understood in order to compare it with anisotropy studies in other cosmic ray experiments. We apply two methods for extracting a meaningful significance from this data set: one can scan for the cuts which optimize the clustering signal, using simulations to determine the appropriate statistical penalty for the scan. This analysis finds a chance probability of about 0.3%. Alternatively, one can optimize the cuts with a first set of data, and then apply them to the remaining data directly without statistical penalty. One can extend the statistical power of this test by considering cross-correlation between the initial data and the remaining data, as long as the initial clustering signal is not included. While the scan is more useful in general, in the present case only splitting the data set offers an unbiased test of the clustering hypothesis. Using this test we find that the AGASA data is consistent at the 8% level with the null hypothesis of isotropically distributed arrival directions.Comment: 14 pages, 3 figures. Unbiased test expanded to include cross-correlation between initial and later data sets for greater statistical power; minor revisions to discussion. Accepted by Astropart. Phy

Angular Correlation Estimates for Ultrahigh Energy Cosmic Rays

Anisotropy in arrival directions of ultrahigh energy cosmic rays offers the most direct way to search for the sources of these particles. We present estimates of the angular correlation in the HiRes sample of stereo events above 10 EeV, and in the combined sample of HiRes and AGASA events above 40 EeV.Comment: 5 pages, 2 figures; to appear in the proceedings of DPF 2004, Riverside, 26 - 31 Aug. 2004 (Int.J.Mod.Phys.A

Observational Constraints on Multi-messenger Sources of Gravitational Waves and High-energy Neutrinos

It remains an open question to what extent many of the astronomical sources of intense bursts of electromagnetic radiation are also strong emitters of non-photon messengers, in particular gravitational waves (GWs) and high-energy neutrinos (HENs). Such emission would provide unique insights into the physics of the bursts; moreover some suspected classes, e.g. choked gamma-ray bursts, may in fact only be identifiable via these alternative channels. Here we explore the reach of current and planned experiments to address this question. We derive constraints on the rate of GW and HEN bursts per Milky Way equivalent (MWE) galaxy based on independent observations by the initial LIGO and Virgo GW detectors and the partially completed IceCube (40-string) HEN detector. We take into account the blue-luminosity-weighted distribution of nearby galaxies, assuming that source distribution follows the blue-luminosity distribution. We then estimate the reach of joint GW+HEN searches using advanced GW detectors and the completed cubic-km IceCube detector to probe the joint parameter space. We show that searches undertaken by advanced detectors will be capable of detecting, constraining or excluding, several existing models with one year of observation

Search for Cross-Correlations of Ultra--High-Energy Cosmic Rays with BL Lacertae Objects

We present the results of searches for correlation between ultra--high-energy cosmic rays observed in stereo mode by the High Resolution Fly's Eye (HiRes) experiment and objects of the BL Lac subclass of active galaxies. In particular, we discuss an excess of events correlating with confirmed BL Lacs in the Veron 10th Catalog. As described in detail in Abbasi et al. (2005), the significance level of these correlations cannot be reliably estimated due to the a posteriori nature of the search, and the results must be tested independently before any claim can be made. We identify the precise hypotheses that will be tested with independent data.Comment: 4 pages. To be presented at the 2005 ICRC in Pune, India, in Augus

Methods for point source analysis in high energy neutrino telescopes

Neutrino telescopes are moving steadily toward the goal of detecting astrophysical neutrinos from the most powerful galactic and extragalactic sources. Here we describe analysis methods to search for high energy point-like neutrino sources using detectors deep in the ice or sea. We simulate an ideal cubic kilometer detector based on real world performance of existing detectors such as AMANDA, IceCube, and ANTARES. An unbinned likelihood ratio method is applied, making use of the point spread function and energy distribution of simulated neutrino signal events to separate them from the background of atmospheric neutrinos produced by cosmic ray showers. The unbinned point source analyses are shown to perform better than binned searches and, depending on the source spectral index, the use of energy information is shown to improve discovery potential by almost a factor of two.Comment: pdfLaTeX, 16 pages, 12 figures. Submitted to Astroparticle Physic