S. Ayabe

Large-scale sidereal anisotropy of Galactic cosmic-ray intensity observed by the Tibet air shower array

We present the large-scale sidereal anisotropy of Galactic cosmic-ray intensity in the multi-TeV region observed with the Tibet-III air shower array during the period from 1999 through 2003. The sidereal daily variation of cosmic rays observed in this experiment shows an excess of relative intensity around 4-7 hr local sidereal time as well as a deficit around 12 hr local sidereal time. While the amplitude of the excess is not significant when averaged over all declinations, the excess in individual declination bands becomes larger and clearer as the viewing direction moves toward the south. The maximum phase of the excess intensity changes from ~7 hr at the Northern Hemisphere to ~4 hr at the equatorial region. We also show that both the amplitude and the phase of the first harmonic vector of the daily variation are remarkably independent of primary energy in the multi-TeV region. This is the first result determining the energy and declination dependences of the full 24 hr profiles of the sidereal daily variation in the multi-TeV region with a single air shower experiment.

Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array

Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606

A Northern Sky Survey for Steady Tera-Electron Volt Gamma-Ray Point Sources Using the Tibet Air Shower Array

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Multi-TeV Gamma-Ray Flares from Markarian 421 in 2000 and 2001 Observed with the Tibet Air Shower Array

in Tibet. This experiment can detect air-shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons in the energy range from 200 to 1000 TeV. The differential energy spectrum obtained in this energy region can be fitted by a power law with the index of

Detection of Multi-TeV Gamma Rays from Markarian 501 during an Unforeseen Flaring State in 1997 with the Tibet Air Shower Array

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Observation of multi-TeV gamma rays from the Crab Nebula using the Tibet air shower array

which is steeper than that obtained by direct measurements at lower energies. We also obtained the energy spectrum of helium nuclei at particle energies around 1000 TeV.

Observation of Multi-TeV Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array

Results of a steady TeV γ-ray point-source search using data taken from the Tibet HD (1997 February-1999 September) and Tibet III (1999 November-2001 October) arrays are presented. From 0° to 60° in declination, significant excesses from the well-known steady source Crab Nebula and the high state of the flare-type source Markarian 421 are observed. Because the levels of significance from other positions are not sufficiently high, 90% confidence level upper limits on the flux are set assuming different power-law spectra. To allow cross-checking, two independently developed analyses are used in this work.

Observation by an air-shower array in tibet of the multi-tev cosmic-ray anisotropy due to terrestrial orbital motion around the sun

Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.

A Study of the Shadowing of Galactic Cosmic Rays by the Sun in a Quiet Phase of Solar Activity with the Tibet Air Shower Array

In 1997, the BL Lac object Mrk 501 entered a very active phase and was the brightest source in the sky at TeV energies, showing strong and frequent flaring. Using the data obtained with a high-density air shower array that has been operating successfully at Yangbajing in Tibet since 1996, we searched for γ-ray signals from this source during the period in 1997 from February through August. Our observation detected multi-TeV γ-ray signals at the 3.7 σ level during this period. The most rapid increase in the excess counts was observed between April 7 and June 16, and the statistical significance of the excess counts in this period was 4.7 σ. Among several observations of flaring TeV γ-rays from Mrk 501 in 1997, this is the only observation using a conventional air shower array. We present the energy spectrum of γ-rays, which will be worth comparing with those obtained by imaging atmospheric Cerenkov telescopes.

Implication of the sidereal anisotropy of ∼5 TeV cosmic ray intensity observed with the Tibet III air shower array

The Tibet experiment, operating at Yangbajing (4300 m above sea level), is the lowest energy air shower array, and the new high-density array constructed in 1996 is sensitive to gamma-ray air showers at energies as low as 3 TeV. With this new array, the Crab Nebula was observed in multi-TeV gamma-rays and a signal was detected at the 5.5 sigma level. We also obtained the energy spectrum of gamma-rays in the energy region above 3 TeV which partially overlaps those observed with imaging atmospheric Cerenkov telescopes. The Crab spectrum observed in this energy region can be represented by the power-law fit dJ&parl0;E&parr0;&solm0;dE=&parl0;4.61+/-0.90&parr0;x10-12&parl0;E&solm0;3 TeV&parr0;-2.62+/-0.17 cm-2 s-1 TeV-1. This is the first observation of gamma-ray signals from point sources with a conventional air shower array using scintillation detectors.