C. R. Chris Wang

Observation of multi-TeV gamma rays from the Crab Nebula using the Tibet 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.

Measurement of air shower cores to study the cosmic ray composition in the knee energy region

Since 1996, a hybrid experiment consisting of an emulsion chamber and a burst detector array and the Tibet-II air shower array has been operated at Yangbajing (4300 m above sea level) in Tibet. This experiment can detect air-shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. Using the burst event data observed by this experiment, we discuss the primary cosmic ray composition around the knee in comparison with the Monte Carlo simulations. In this paper, we show that all the features of burst events are wholly compatible with the heavy enriched composition in the knee energy region.

Performance of the Tibet II/HD air shower array

Tibet II Air Shower Array consisting of scintillation counters with lattice of 15 m spacing has been operated with very high trigger rate of about 200 Hz. The threshold enegy of this array is estimated to be about 8 TeV for proton induced showers. Tibet High Density (HD) Array with 7.5 m spacing has been operated with the trigger rate of 115 Hz. The Mode energy of this array is estimated to be about 3 TeV for proton showers. Angular resolution of the arrays are estimated to be 0.9 degree above 10 TeV for Tibet II array, and 0.85 degree above TeV for HD array, resepectively. The angular resolution of these arrays and other array performances are examined by observing the Moon shadow resulting from the cosmic ray deficit in the direction of the Moon. Using the deflection of the Moon shadow to the east-west direction, the error of the array can be estimated by observing the displacement of the shadow in the north-south direction, because it is free from the effect of geomagnetic field, especially at Yangbaji...

Sun's shadow by 10 TeV cosmic rays under the influence of solar activity

Abstract We examined the Suns shadow by 10 TeV cosmic rays observed with the Tibet air shower array during the period from 1991 through 1997. The results suggest that there is a causal relation between the shadows movement and the changing inclination of the heliospheric current sheet of the large-scale solar magnetic field. Thus, further observations with higher statistics may provide direct information on the relation between a time variation of the large-scale structure of the solar and interplanetary magnetic fields and the phase of solar activity cycle.

Search for gamma-ray bursts with the Tibet air shower array

We searched for multi-TeV counterparts to the gamma–ray bursts (GRBs) at keV energies detected by BATSE during the period from October 2, 1995 to March 18, 1999 using the data of the Tibet Air Shower Array. In the field of view of the array there were 70 GRBs detected by BATSE in this period. The search was done based on the sky survey within error circles of BATSE bursts and with time scales during 10 to several hundred seconds. No significant signals of counterparts were detected.

Search for multi-TeV gamma-ray emission from nearby SNRs with the Tibet air shower array

The Tibet air-shower array operating at Yangbajing (4300 m above sea level) is sensitive to gamma-ray air showers at energies as low as 3 TeV. The observation of the moon’s shadow has provided a direct check of the angular resolution, the energy estimation and the systematic pointing error of this air-shower array. Using these data, we have searched for multi-TeV gamma-ray emission from 21 SNRs located within 5 kpc distance in the declination band of 0° to +60°. The signal from the Crab Nebula was detected at 5.5 σ level as described another paper. No significant DC excess exceeding 5 σ level was found from any of these SNRs except from the Crab Nebula. The results of nearby eight SNRs piled up gave a flux upper limit slightly lower than the expected values for a simple model of shock acceleration.

Search for Multi-TeV gamma-ray emission from nearby SNRs with the Tibet air shower array The Tibet ASγ Collaboration

The Tibet air-shower array operating at Yangbajing (4300 m above sea level) is sensitive to gamma-ray air showers at energies as low as 3 TeV. The observation of the Moon’s shadow has provided a direct check of the angular resolution, the energy estimation and the systematic pointing error of this air-shower array. Using these data, we have searched for multi-TeV gamma-ray emission from 27 SNRs located within 10 kpc distance in the declination band of −10° to +70°. The signal from the Crab Nebula was detected at 5.5 σ level. The intensity observed is (4.61±0.90)×10−12 cm−2 s−1 TeV−1 at 3 TeV. No significant DC excess was found from any of these SNRs except from the Crab Nebula.