N. Hotta

The All-Particle Spectrum of Primary Cosmic Rays in the Wide Energy Range from 1014 to 1017 eV Observed with the Tibet-III Air-Shower Array

We present an updated all-particle energy spectrum of primary cosmic rays in a wide range from 10 14 to 10 17 eVusing 5:5 ; 10 7 events collected from 2000 November through 2004 October by the Tibet-III air-shower array located 4300 m abovesealevel(anatmosphericdepthof 606gcm � 2 ).Thesizespectrumexhibitsasharpkneeatacorrespondingprimary energy around 4 PeV. This work uses increased statistics and new simulation calculations for the analysis. We discuss our extensive Monte Carlo calculations and the model dependencies involved in thefinal result, assuming interaction models QGSJET01c and SIBYLL2.1, and heavy dominant (HD) and proton dominant (PD) primary composition models. Pure protonandpureironprimarymodelsarealsoexaminedasextremecases.A detector simulationwasalsoperformedtoimproveouraccuracyindeterminingthesizeof theairshowersandtheenergyof theprimaryparticle.Weconfirmedthatthe all-particle energy spectra obtained under various plausible model parameters are not significantly different from each other, which was the expected result given the characteristics of the experiment at high altitude, where the air showers of the primary energy around the knee reach near-maximum development, with their features dominated by electromagnetic components,leadingtoaweakdependenceontheinteractionmodel or theprimarymass.Thisisthehighest statistical and the best systematics-controlled measurement covering the widest energy range around the knee energy region.

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.

Observations of TeV Gamma-Ray Flares from Markarian 501with the Telescope Array Prototype

We will report the observations of TeV gamma ray flares from Markarian 501 using Telescope Array Prototype. The observation were carried out continuously from the end of March to the end of July in 1997. The energy spectrum, and the time variation of the gamma ray intensities are shown. The intensity has been changed by the order of magnitude in this period and the possible quasi periodic oscillation of 12.7days were discovered.We report the observations of TeV gamma-ray flares from Markarian 501 using the Telescope Array Prototype. The observations were carried out continuously from the end of March to the end of July of 1997. The energy spectrum and the time variation of the gamma-ray intensities are shown. The intensity has been changed by an order of magnitude in this period, and the possible quasi-periodic oscillation of 12.7 days were discovered.

A Northern Sky Survey for Steady Tera-Electron Volt Gamma-Ray Point Sources Using 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 of thundercloud-related gamma rays and neutrons in Tibet

During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet, China, a long-duration count enhancement associated with thunderclouds was detected by a solar-neutron telescope and neutron monitors installed at the Yangbajing Comic Ray Observatory. The event, lasting for similar to 40 min, was observed on July 22, 2010. The solar-neutron telescope detected significant gamma-ray signals with energies >40 MeV in the event. Such a prolonged high-energy event has never been observed in association with thunderclouds, clearly suggesting that electron acceleration lasts for 40 min in thunderclouds. In addition, Monte Carlo simulations showed that >10 MeV gamma rays largely contribute to the neutron monitor signals, while >1 keV neutrons produced via a photonuclear reaction contribute relatively less to the signals. This result suggests that enhancements of neutron monitors during thunderstorms are not necessarily clear evidence for neutron production, as previously thought.

Multi-TeV Gamma-Ray Observation from the Crab Nebula Using the Tibet-III Air Shower Array Finely Tuned by the Cosmic Ray Moon's Shadow

The Tibet-III air shower array, consisting of 533 scintillation detectors, has been operating successfully at Yangbajing in Tibet, China since 1999. Using the data set collected by this array from 1999 November through 2005 November, we obtained the energy spectrum of γ-rays from the Crab Nebula, expressed by a power law as (dJ/dE) = (2.09 ± 0.32) × 10–12(E/3 TeV)–2.96±0.14 cm–2 s–1 TeV–1 in the energy range of 1.7-40 TeV. This result is consistent with other independent γ-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 (MC) 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 of 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 ±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 0011. The deficit rate and the position of the Moons shadow are shown to be very stable within a statistical error of ±6% year by year. This guarantees the long-term stability of pointlike source observation with the Tibet-III array. These systematic errors are adequately taken into account in our study of the Crab Nebula.

On temporal variations of the multi-tev cosmic ray anisotropy using the tibet iii air shower array

We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays (CRs) by the 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 with 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 CRs remains insensitive to solar activities since the observation period covers more than half of the 23rd solar cycle.

Development and performance test of a prototype air shower array for search for gamma ray point sources in the very high energy region

Abstract An air shower array consisting of 16 fast timing detectors and 8 particle density detectors was constructed at Mt Norikura (2780 m above sea level) to see the basic performance and to acquire technical knowledge for an enlarged operation at Tibet in search for extraterrestial point sources emitting very high energy gamma rays (> several times 10 13 eV). In particular, we focus on the angular resolution in determining the arrival direction of air showers. When a lead plate of 5 mm thickness (which is close to the optimum one) is placed on each scintillator, the apparent particle number is increased by a factor of ∼1.9 and the angular resolution is improved by ∼1.5 as compared to the no lead plate case. We also performed a detailed Monte Carlo simulation and critically examined the so-called even-odd method which many authors have been using for estimating the angular resolution. It is pointed out that selecting showers in which the core falls inside the array is crucial for getting the high-precision direction. For such contained showers, the present array gives a median angular resolution of 0.80° for proton-induced showers whose median energy is 200 TeV. In the coming Tibet experiment, a median angular resolution better than 0.65° (E γ /100( TeV )) − 1 2 is expected for gamma rays.

Observation of TeV gamma rays from the fermi bright galactic sources with the tibet air shower array

Using the Tibet-III air shower array, we search for TeV γ-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 seven sources instead of the expected 0.61 sources at a significance of 2σ or more excess. The chance probability from Poisson statistics would be estimated to be 3.8 × 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 × 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 γ-ray excesses. We also find that all seven sources are associated with pulsars, and six of them are coincident with sources detected by the Milagro experiment at a significance of 3σ 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 ≥3σ 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.

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

We show that the large‐scale anisotropy of ∼5 TeV galactic cosmic ray (GCR) intensity observed by Tibet Air Shower experiment can be reproduced by the superposition of a bi‐directional and uni‐directional flows (UDF and BDF) of GCRs. The heliosphere is located inside the local interstellar cloud (LIC) very close to the inner edge of the LIC. If the GCR population is lower inside the LIC than outside, the BDF flow is expected from the parallel diffusion of GCRs into LIC along the local interstellar magnetic field (LISMF) connecting the heliosphere with the region outside the LIC, where the GCR population is higher. A type of the UDF, on the other hand, is expected from the B×∇n drift flux driven by a gradient of GCR density (n) in the LISMF (B). The LISMF orientation deduced from the best‐fit direction of the BDF is almost parallel to the galactic plane and more consistent with the suggestion of Frisch (1996) than that of Lallement et al. (2005). We note that the model, if holds, yields the LISMF polarity ...