Gamma-Ray Observation of the Cygnus Region in the 100-TeV Energy Region

We report observations of gamma-ray emissions with energies in the 100-TeV energy region from the Cygnus region in our Galaxy. Two sources are significantly detected in the directions of the Cygnus OB1 and OB2 associations. Based on their positional coincidences, we associate one with a pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN G75.2+0.1, with the pulsar moving away from its original birthplace situated around the centroid of the observed gamma-ray emission. This work would stimulate further studies of particle acceleration mechanisms at these gamma-ray sources.

First Detection of sub-PeV Diffuse Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays beyond PeV Energies

We report, for the first time, the long-awaited detection of diffuse gamma rays with energies between 100 TeV and 1 PeV in the Galactic disk. Particularly, all gamma rays above 398 TeV are observed apart from known TeV gamma-ray sources and compatible with expectations from the hadronic emission scenario in which gamma rays originate from the decay of π^{0}'s produced through the interaction of protons with the interstellar medium in the Galaxy. This is strong evidence that cosmic rays are accelerated beyond PeV energies in our Galaxy and spread over the Galactic disk.

First Detection of Photons with Energy beyond 100 TeV from an Astrophysical Source

We report on the highest energy photons from the Crab Nebula observed by the Tibet air shower array with the underground water-Cherenkov-type muon detector array. Based on the criterion of a muon number measured in an air shower, we successfully suppress 99.92% of the cosmic-ray background events with energies E>100  TeV. As a result, we observed 24 photonlike events with E>100  TeV against 5.5 background events, which corresponds to a 5.6σ statistical significance. This is the first detection of photons with E>100  TeV from an astrophysical source.

The cosmic ray energy spectrum measured with the new Tibet hybrid experiment

We have upgraded the new Tibet ASgamma experiment in China since 2014 to measure the chemical composition of cosmic rays around the knee. This hybrid experiment consist of an air-shower-core detector array (YAC-II) to detect high energy electromagnetic component, the Tibet air-shower array (Tibet-III) and a large underground water-Cherenkov muon-detector array (MD). We have carried out a detailed air-shower Monte Carlo (MC) simulation to study the performance of the hybrid detectors by using CORSIKA (version 7.5000), which includes EPOS-LHC, QGSJETII-04, SIBYLL2.1 and SIBYLL2.3 hadronic interaction models. The preliminary results of the interaction model checking above 50 TeV energy region are reported in this paper, and the primary proton and helium spectra in the energy range 50 TeV to 1015 eV was derived from YAC-I data and is smoothly connected with direct observation data at lower energies and also with our previously reported works at higher energies within statistical errors. The knee of the (P+He) spectra is located around 400 TeV. The interaction model dependence in deriving the primary (P+He) spectra is found to be small (less than 25% in absolute intensity, 10% in position of the knee), and the composition model dependence is less than 10% in absolute intensity.

Test of the hadronic interaction models SIBYLL2.3, EPOS-LHC and QGSJETII- 04 with Tibet EAS core data

A hybrid experiment has been started by the ASγ experiment at Yangbajing (4300m a.s.l.) in Tibet since May 2009, that consists of a high-energy air-shower-core array (YAC-I) and a high-density air-shower array (Tibet-III). In this paper, we report our results to check the hadronic interaction models SIBYLL2.3, SIBYLL2.1, EPOS-LHC and QGSJETII-04 in the multi-tens TeV energy region using YAC-I+Tibet-III experimental data from May 2009 through January 2010. The effective live time is calculated as 106.05 days. The results show that the description of transverse momentum, inelastic cross-section and inelasticity for the 4 hadronic interaction models is consistent with YAC-I experimental data within 15% systematic errors range in the forward region below 100 TeV. Among them, the EPOS-LHC model is the best hadronic interaction model. Furthermore, we find that the H4a composition model is the best one below the 100 TeV energy region.

On the Solar Cycle Variation of the Solar Diurnal Anisotropy of Multi-TeV Cosmic-ray Intensity Observed with the Tibet Air Shower Array

We analyze the temporal variation of the solar diurnal anisotropy of the multi-TeV cosmic-ray intensity observed with the Tibet air shower array from 2000 to 2009, covering the maximum and minimum of the 23rd solar cycle. We comfirm that a remarkable additional anisotropy component is superposed on the Compton-Getting anisotropy at 4.0 TeV, while its amplitude decreases at higher energy regions. In constrast to the additional anisotropy reported by the Matsushiro experiment at 0.6 TeV, we find the residual component measured by Tibet at multi-TeV energies is consistent with being stable, with a fairly constant amplitude of 0.041% ± 0.003% and a phase at around 07.17 ± 00.16 local solar time at 4.0 TeV. This suggests the additional anisotropy observed by the Tibet experiment could result from mechanisms unrelated to solar activities.

Evaluation of the Interplanetary Magnetic Field Strength Using the Cosmic-Ray Shadow of the Sun

We analyze the Sun's shadow observed with the Tibet-III air shower array and find that the shadow's center deviates northward (southward) from the optical solar disk center in the "away" ("toward") interplanetary magnetic field (IMF) sector. By comparing with numerical simulations based on the solar magnetic field model, we find that the average IMF strength in the away (toward) sector is 1.54±0.21_{stat}±0.20_{syst} (1.62±0.15_{stat}±0.22_{syst}) times larger than the model prediction. These demonstrate that the observed Sun's shadow is a useful tool for the quantitative evaluation of the average solar magnetic field.

Probe of the solar magnetic field using the "cosmic-ray shadow" of the sun

We report on a clear solar-cycle variation of the Sun’s shadow in the 10 TeV cosmic-ray flux observed by the Tibet air shower array during a full solar cycle from 1996 to 2009. In order to clarify the physical implications of the observed solar cycle variation, we develop numerical simulations of the Sun’s shadow, using the potential field source surface model and the current sheet source surface (CSSS) model for the coronal magnetic field. We find that the intensity deficit in the simulated Sun’s shadow is very sensitive to the coronal magnetic field structure, and the observed variation of the Sun’s shadow is better reproduced by the CSSS model. This is the first successful attempt to evaluate the coronal magnetic field models by using the Sun’s shadow observed in the TeV cosmic-ray flux.

Anisotropy and Corotation of Galactic Cosmic Rays

The intensity of Galactic cosmic rays is nearly isotropic because of the influence of magnetic fields in the Milky Way. Here, we present two-dimensional high-precision anisotropy measurement for energies from a few to several hundred teraelectronvolts (TeV), using the large data sample of the Tibet Air Shower Arrays. Besides revealing finer details of the known anisotropies, a new component of Galactic cosmic ray anisotropy in sidereal time is uncovered around the Cygnus region direction. For cosmic-ray energies up to a few hundred TeV, all components of anisotropies fade away, showing a corotation of Galactic cosmic rays with the local Galactic magnetic environment. These results have broad implications for a comprehensive understanding of cosmic rays, supernovae, magnetic fields, and heliospheric and Galactic dynamic environments.

Observation by an air-shower array in Tibet of the multi-TeV cosmic-ray anisotropy due to terrestrial orbital motion around the Sun

We report on the solar diurnal variation of the galactic cosmic-ray intensity observed by the Tibet III air shower array during the period from 1999 to 2003. In the higher-energy event samples (12 and 6.2 TeV), the variations are fairly consistent with the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun, while the variation in the lower-energy event sample (4.0 TeV) is inconsistent with this anisotropy. This suggests an additional anisotropy superposed at the multi-TeV energies, e.g., the solar modulation effect. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

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.