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Register a new userOn Temporal Variations of the Multi-TeV Cosmic Ray Anisotropy using the Tibet III Air Shower Array
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We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by Tibet Air Shower Array, with the data taken from 1999 November to 2008 December. To explore temporal variations of the anisotropy, the data set is divided into nine intervals, each in a time span of about one year. The sidereal anisotropy of magnitude about 0.1% appears fairly stable from year to year over the entire observation period of nine years. This indicates that the anisotropy of TeV Galactic cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle.
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We report the analysis of the $10-1000$ TeV large-scale sidereal anisotropy of Galactic cosmic rays (GCRs) with the data collected by the Tibet Air Shower Array from October, 1995 to February, 2010. In this analysis, we improve the energy estimate and extend the declination range down to $-30^{circ}$. We find that the anisotropy maps above 100 TeV are distinct from that at multi-TeV band. The so-called tail-in and loss-cone features identified at low energies get less significant and a new component appears at $sim100$ TeV. The spatial distribution of the GCR intensity with an excess (7.2$sigma$ pre-trial, 5.2$sigma$ post-trial) and a deficit ($-5.8sigma$ pre-trial) are observed in the 300 TeV anisotropy map, in a good agreement with IceCubes results at 400 TeV. Combining the Tibet results in the northern sky with IceCubes results in the southern sky, we establish a full-sky picture of the anisotropy in hundreds of TeV band. We further find that the amplitude of the first order anisotropy increases sharply above $sim100$ TeV, indicating a new component of the anisotropy. All these results may shed new light on understanding the origin and propagation of GCRs.
The Tibet-III air shower array, consisting of 533 scintillation detectors, has been operating successfully at Yangbajing in Tibet, China since 1999. Using the dataset collected by this array from 1999 November through 2005 November, we obtained the energy spectrum of $gamma$-rays from the Crab Nebula, expressed by a power law as $(dJ/dE) = (2.09pm0.32)times10^{-12} (E/{rm 3 TeV})^{-2.96pm0.14} {rm cm}^{-2} {rm s}^{-1} {rm TeV}^{-1}$ in the energy range of 1.7 to 40 TeV. This result is consistent with other independent $gamma$-ray observations by imaging air Cherenkov telescopes. In this paper, we carefully checked and tuned the performance of the Tibet-III array using data on the moons shadow in comparison with a detailed Monte Carlo simulation. The shadow is shifted to the west of the moons apparent position as an effect of the geomagnetic field, although the extent of this displacement depends on the primary energy positively charged cosmic rays. This finding enables us to estimate the systematic error in determining the primary energy from its shower size. This error is estimated to be less than $pm$12% in our experiment. This energy scale estimation is the first attempt among cosmic-ray experiments at ground level. The systematic pointing error is also estimated to be smaller than $0fdg011$. The deficit rate and position of the moons shadow are shown to be very stable within a statistical error of $pm$6% year by year. This guarantees the long-term stability of point-like source observation with the Tibet-III array. These systematic errors are adequately taken into account in our study of the Crab Nebula.
We report on the observation of anisotropy in the arrival direction distribution of cosmic rays at PeV energies. The analysis is based on data taken between 2009 and 2012 with the IceTop air shower array at the South Pole. IceTop, an integral part of the IceCube detector, is sensitive to cosmic rays between 100 TeV and 1 EeV. With the current size of the IceTop data set, searches for anisotropy at the 10^-3 level can, for the first time, be extended to PeV energies. We divide the data set into two parts with median energies of 400 TeV and 2 PeV, respectively. In the low energy band, we observe a strong deficit with an angular size of about 30 degrees and an amplitude of (-1.58 +/- 0.46 (stat) +/- 0.52 (sys)) x 10^(-3) at a location consistent with previous observations of cosmic rays with the IceCube neutrino detector. The study of the high energy band shows that the anisotropy persists to PeV energies and increases in amplitude to (-3.11 +/- 0.38 (stat) +/- 0.96 (sys)) x 10^(-3).
The Stockholm Educational Air Shower Array (SEASA) project has established a network of GPS time-synchronised scintillator detector stations at high-schools in the Stockholm region. The primary aim of this project is outreach. A part of the network comprises a dense cluster of detector stations located at AlbaNova University Centre. This cluster is being used to study the cosmic ray anisotropy around the knee. Each station consists of three scintillator detectors in a triangular geometry which allows multiple timing measurements as the shower front sweeps over the station. The timing resolution of the system has been determined and the angular resolution has been studied using Monte Carlo simulations and is compared to data. The potential of this system to study small and large scale cosmic ray anisotropies is discussed.
Using the Tibet-III air shower array, we search for TeV gamma-rays from 27 potential Galactic sources in the early list of bright sources obtained by the Fermi Large Area Telescope at energies above 100 MeV. Among them, we observe 7 sources instead of the expected 0.61 sources at a significance of 2 sigma or more excess. The chance probability from Poisson statistics would be estimated to be 3.8 x 10^-6. If the excess distribution observed by the Tibet-III array has a density gradient toward the Galactic plane, the expected number of sources may be enhanced in chance association. Then, the chance probability rises slightly, to 1.2 x 10^-5, based on a simple Monte Carlo simulation. These low chance probabilities clearly show that the Fermi bright Galactic sources have statistically significant correlations with TeV gamma-ray excesses. We also find that all 7 sources are associated with pulsars, and 6 of them are coincident with sources detected by the Milagro experiment at a significance of 3 sigma or more at the representative energy of 35 TeV. The significance maps observed by the Tibet-III air shower array around the Fermi sources, which are coincident with the Milagro >=3sigma sources, are consistent with the Milagro observations. This is the first result of the northern sky survey of the Fermi bright Galactic sources in the TeV region.
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