Search

The Tibet-III air shower array and underground water Cherenkov-type muon detector array have been successfully operated since 2014, at an altitude of 4,300m in Tibet, China. The gamma-ray energy and arrival direction is determined by the Tibet AS array, while the MD array enables us to suppress more than 99.9% of background cosmic rays above 100 TeV, by means of counting number of muons in an air shower at 2.4 m underground. We surveyed gamma-ray emissions in the energy region above 10 TeV from the Cygnus region in our Galaxy. Two sources were significantly detected in the directions of the Cygnus OB1 and OB2 associations. One of the gamma-ray emissions had an excess of pre-trial(post-trial) detection significance of 5.3$\sigma$(4.7$\sigma$) in the direction centered on (R.A., Dec) = (308.04$^\circ\pm$ 0.08$^\circ$, 41.4$^\circ\pm$0.06$^\circ$), which was associated with Pulsar PSR J2032+4127. The obtained spectrum from 10 TeV to120 TeV can be expressed by a simple power-law as dF/dE = (4.13 $\pm$ 0.83) $\times$ 10$^{-16}$ (E/40 TeV)$^{3.12\pm 0.12}$ TeV$^{-1}$cm$^{-2}$s$^{-1}$. The other had an excess of pre-trial(post-trial) detection significance of 6.7$\sigma$(6.2$\sigma$) in the direction centered on (R.A., Dec)=(304.99$^{\circ}\pm$ 0.11$^{\circ}$, 36.84$^{\circ}\pm$0.08$^{\circ})$, which was associated 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. The obtained spectrum from 10 TeV to 200 TeV can be expressed by a simple power-law as dF/dE = (10.6 $\pm$ 1.3) $\times$ 10$^{-16}$ (E/40 TeV)$^{2.70\pm 0.13}$ TeV$^{-1}$cm$^{-2}$s$^{-1}$, or including an exponential cutoff as dF/dE = (3.6 $\pm$ 2.0) $\times$ 10$^{-15}$ (E/40 TeV)$^{1.60\pm 0.5}$ exp(E/(44$\pm$ 21TeV))} TeV$^{-1}$cm$^{-2}$s$^{-1}$

Aiming at observing cosmic gamma rays in the energy region beyond several tens of TeV, a water-Cherenkov-type muon detector (MD) array was built under the Tibet air shower (AS) array. The Tibet AS+MD array, which possess high sensitivity to gamma rays beyond 10 TeV, has been operated since 2014. We performed analysis of gamma rays from the Geminga PWN using data collected by the Tibet AS+MD array. We report on the gamma-ray energy spectrum beyond 10 TeV and the morphology of >10 TeV gamma rays.

We accomplished a balloon observation of the high-energy cosmic-ray electrons in 10-1000GeV to reveal the origin and the acceleration mechanism. The observation was carried out for 13 days at an average altitude of 35km by the Polar Patrol Balloon (PPB) around Antarctica in January 2004. The detector is an imaging calorimeter composed of scintillating-fiber belts and plastic scintillation counters sandwiched between lead plates. The geometrical factor is about 600cm^2sr, and the total thickness of lead absorber is 9 radiation lengths. The performance of the detector has been confirmed by a test flight at the Sanriku Balloon Center and by an accelerator beam test using the CERN-SPS (Super Proton Synchrotron at CERN). The new telemetry system using the Iridium satellite, the power system supplied by solar panels and the automatic flight level control operated successfully during the flight. We collected 5.7×10^3 events over 100GeV, and selected the electron candidates by a preliminary data analysis of the shower images. We report here an outline of both detector and observation, and the first result of the electron energy spectrum over 100GeV obtained by an electronic counter.

Results on steady TeV $\gamma$-ray point source search using data taken from the Tibet HD (Feb. 1997 -- Sep. 1999) and Tibet III (Nov. 1999 -- Oct. 2001) arrays are presented. From $0^{\circ}$ to $60^{\circ}$ 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 crosschecking, two independently developed analyses are used in this work.

We present the large-scale sidereal anisotropy ofgalactic cosmic-ray intensity in the multi-TeV region observed with the Tibet-IIIair 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\sim7 $ hours local sidereal time, as well as a deficit around 12 hours local sidereal time. While the amplitude of the excess is not significant when averaged over all declinations, the excess in individual declinaton bands becomes larger and clearer as the viewing direction moves toward the south. The maximum phase of the excess intensity changes from $\sim$7 at the northern hemisphere to $\sim$4 hours 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-hour profiles of the sidereal daily variation in the multi-TeV region with a single air shower experiment., Comment: 13 pages, 3 figures, 1 table. Accepted for publication in ApJL

Several strong TeV gamma-ray flares were detected from Markarian 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 sigma. Markarian 421 was unprecedentedly active at X-ray and TeV gamma-ray energies during this period, and a positive correlation was found between the change of the ASM/RXTE X-ray flux and the Tibet TeV gamma-ray flux. When a power-law energy spectrum for gamma-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., 22 pages, 10 figures, accepted by ApJ

A new EAS hybrid experiment has been designed by constructing a YAC (Yangbajing Air shower Core) detector array inside the existing Tibet-III air shower array. The first step of YAC, called "YAC-I", consists of 16 plastic scintillator units (4 rows times 4 columns) each with an area of 40 cm * 50 cm which is used to check hadronic interaction models used in AS simulations. A Monte Carlo study shows that YAC-I can record high energy electromagnetic component in the core region of air showers induced by primary particles of several tens TeV energies where the primary composition is directly measured by space experiments. It may provide a direct check of the hadronic interaction models currently used in the air shower simulations in the corresponding energy region. In present paper, the method of the observation and the sensitivity of the characteristics of the observed events to the different interaction models are discussed., In Proc. 31st Int. Cosmic Ray Conf. (2009)

We present an updated all-particle energy spectrum of primary cosmic rays in a wide range from 10^14 eV to 10^17 eV using 5.5 times 10^7 events collected in the period from 2000 November through 2004 October by the Tibet-III air-shower array located at 4300 m above sea level (atmospheric depth of 606 g/cm^2). The size spectrum exhibits a sharp knee at a corresponding primary energy around 4 PeV. This work uses increased statistics and new simulation calculations for the analysis. We performed extensive Monte Carlo calculations and discuss the model dependences involved in the final result assuming interaction models of QGSJET01c and SIBYLL2.1 and primary composition models of heavy dominant (HD) and proton dominant (PD) ones. Pure proton and pure iron primary models are also examined as extreme cases. The detector simulation was also made to improve the accuracy of determining the size of the air showers and the energy of the primary particle. We confirmed that the all-particle energy spectra obtained under various plausible model parameters are not significantly different from each other as expected from the characteristics of the experiment at the high altitude, where the air showers of the primary energy around the knee reaches near maximum development and their features are dominated by electromagnetic components leading to the weak dependence on the interaction model or the primary mass. This is the highest-statistical and the best systematics-controlled measurement covering the widest energy range around the knee energy region., Comment: 19 pages, 20 figures, accepted by ApJ

We searched for steady PeV gamma-ray emission from the Monogem ring region with the Tibet air shower array from 1997 February to 2004 October. No evidence for statistically significant gamma-ray signals was found in a region 111$\degr$ $\leq$ R.A. $, Comment: 13 pages 3figures, 1 table

Data from the Tibet-III air shower array (with energies around 3 TeV) and from the Tibet-II array (with energies around 10 TeV) have been searched for diffuse gamma rays from the Galactic plane. These arrays have an angular resolution of about 0.9 degrees. The sky regions searched are the inner Galaxy, 20 degrees, 22 pages, 8 figures, Accepted for publication in ApJ