Session T8: Cosmic Rays

Charged Particle Astronomy with the Pierre Auger Observatory

The Pierre Auger Collaboration has recently discovered a significant correlation between the arrival directions of the highest energy cosmic rays and the positions of relatively nearby active galactic nuclei (AGN). We have demonstrated that the cosmic ray flux at the highest energies is anisotropic with a confidence level greater than 99{\%} using an independent data set with the test parameters specified a priori, chosen from the results of an exploratory scan. We conclude that the observed anisotropy is consistent with the hypothesis that cosmic rays with the highest energies originate from extragalactic sources close enough so that their flux is not significantly attenuated by interactions with the cosmic background radiation (the Greisen-Zatsepin-Kuzmin Effect). Our present data set does not unambiguously identify the sources as nearby AGN, however we demonstrate that the AGN source hypothesis is compatible with the data. In this presentation, I review these findings and discuss the prospect of charged particle astronomy, the identification and measurement of individual cosmic rays sources, within the next several years.   

The Large Scale Anisotropy of TeV Cosmic-Rays as Observed with Milagro

The Milagro observatory is a water Cherenkov detector located in the Jemez mountains outside of Los Alamos, New Mexico. With a high duty cycle and large field-of-view, Milagro has high sensitivity for measuring the large scale cosmic-ray anisotropy at TeV energies. We present a two-dimensional map of the sidereal anisotropy generated by a harmonic analysis of the data collected over a seven year period consisting of more than 160 billion events. We observe an anisotropy with a magnitude on the order of 0.1\% for cosmic rays with a median energy of 3 TeV. The dominant feature is a deficit region of mean depth ($-2.5 \pm 0.046$ stat. $\pm 0.19$ syst.)$\times10^{-3}$ in the direction of the Galactic North Pole with a range in declination of -10 to 45 degrees and 150 to 225 degrees in right ascension. We also present evidence of an increase in the magnitude of this deficit region over time as well as a weakening of the signal for energies above 20 TeV.   

Milagro Observation of a Localized Excess of $\sim10$ TeV Cosmic Rays

A analysis of Milagro data shows two regions in the Northern Sky of excess cosmic rays on an angular scale of $\sim10^\circ$ with greater than $12\sigma$ significance. Diagnostics show that both regions are inconsistent with gamma-ray emission at a confidence level of $>11\sigma$. One of the regions has a different energy spectrum than the cosmic-ray background at a confidence level of $4.6\sigma$, and it is consistent with a hard spectrum with an exponential cutoff, with the most significant excess at $\sim10$ TeV. This is difficult to explain since a 10 TeV proton in a 1 $\mu$G field has a gyroradius of 0.01 pc.   

CREAM-I H and He Spectra and Flight Status of CREAM-III

The Cosmic Ray Energetics and Mass (CREAM) is a balloon-borne experiment to measure the composition and energy spectra of cosmic ray nuclei. During the 2004/05 Antarctic season, CREAM-I had a record-breaking 42 day flight. The CREAM-I instrument has redundant charge identification using a Timing Charge Detector and a Silicon Charge Detector (SCD) and redundant energy measurements using a Transition Radiation Detector and a sampling calorimeter.Preliminary spectra of proton and helium nuclei measured by the calorimeter and SCD during the CREAM-I flight will be discussed. The status of the CREAM-III flight of 2007/08 will also be presented.   

Elemental Abundances in the Cosmic Rays 26$\le $Z$\le $34 -- Evidence For Cosmic-Ray Origin in OB Associations

The TIGER instrument flew on high-altitude balloons over Antarctica for 50 days. The observed elemental abundances with 26$\le $Z$\le $34, when corrected for fragmentation during propagation in the Galaxy and in the atmosphere, show cosmic-ray source abundances that differ from the standard Solar System (SS) abundances. Preferential acceleration of refractory elements does not fully explain these differences. They can be accounted for by a source mixture of $\sim $80{\%} SS and $\sim $20{\%} representative of the ejecta from massive stars, which mixture is then affected by elemental volatility in the acceleration process. This 80/20 mixture has also been shown to account for the isotopic composition of lighter cosmic rays and can be understood as the result of cosmic-ray acceleration in OB associations.   

Elemental abundance measurements of Zn, Ga, and Ge from the Cosmic Ray Isotope Spectrometer (CRIS) experiment on the Advanced Composition Explorer (ACE) satellite

We have measured the elemental abundances of galactic cosmic-ray Zn, Ga, and Ge using the CRIS instrument on the NASA-ACE spacecraft. These ultra-heavy (Z$>$29) nuclei are very rare and require an instrument with a large geometrical factor, such as CRIS possesses, exposed over a long period of time to collect a significant sample. Over the 10+ years since launch in 1997 we have collected $\sim $250 nuclei with Z$>$29.~ Abundances for these elements measured over the energy range of $\sim $150 to 600 MeV/nucleon will be presented and implications for the nature of the cosmic ray source will be discussed.   

A measurement of the atmospheric muon charge ratio between 50 and 400 GeV using the MINOS Near Detector

The torroidally magnetized MINOS Near Detector is located at Fermilab at the end of the NuMI beam line. It is situated at 133.5 meters above sea level with a vertical overburden of 224.6 meters of water equivalent. The detector has collected charge seperated atmospheric muons for a minimum of 37 days in both the forward and reversed B-field directions. Combining equal periods of forward and reversed field data, almost 40 million muons per sample, reduces the systematic effects of geometric acceptance due to the magnetic field and an asymmetric detector. This data set allows an accurate measurement of the atmospheric muon charge ratio between 50 and 400 GeV to be performed.   

Search for Correlations between HiRes Stereo Events and Active Galactic Nuclei

We have searched for correlations between the pointing directions of ultrahigh energy cosmic rays observed by the High Resolution Fly's Eye experiment and active galactic nuclei visible in the northern hemisphere. None other than random correlations have been found. We report our results using search parameters prescribed by the Pierre Auger collaboration which have led them to conclude that a positive correlation exists for sources in the southern hemisphere. We also describe results using two methods for determining the chance probability of correlations: one in which a hypothesis is formed from scanning one half of the data and tested on the second half, and another which involves a scan over the entire data set. The degree of auto-correlation in the data is consistent with isotropy. The largest correlation found occurred with a chance probability of 14\%.   

Observation of the GZK Cutoff by the HiRes Experiment

The High Resolution Fly's Eye (HiRes) Experiment has observed the GZK cutoff. It is seen at $6x10^{19}$ eV, the expected energy for the Greisen Zatsepin Kuz'min threshold. An ``ankle'' structure is seen a decade below the suppression. We present a description of the experiment, data acquisition, analysis and fits to the resulting spectra. The significance of the break in the spectrum is shown to be greater than 5 sigma.   

Milagro and Climax Measurements of the 2005 January 20 GLE Particle Spectrum

Milagro is a ground-based TeV g-ray telescope in the Jemez Mountains near Los Alamos NM. Designed to image TeV g-ray sources, it is also sensitive to energetic solar particles above the local geomagnetic cutoff. It sits relatively close to the Climax neutron monitor in Colorado. Because of their geomagnetic proximity, these two instruments can be jointly used to construct a time-dependent spectrum for GLE events unaffected by particle anisotropies. Modeling of the performance of both instruments to both isotropic and anisotropic particle distributions is completed and will be used to constrain the 2005 January 20 spectrum during the brief event onset as well as the abrupt decay. We present ongoing results of the spectrum and anisotropy of the 2005 January 20 GLE.