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Register a new userMulti-Tev Gamma-Ray Observation from the Crab Nebula Using the Tibet-III Air Shower Array Finely Tuned by the Cosmic-Ray Moons Shadow
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The Tibet-III air shower array, consisting of 533 scintillation detectors, has been operating successfully at Yangbajing in Tibet, China since 1999. Using the dataset collected by this array from 1999 November through 2005 November, we obtained the energy spectrum of $gamma$-rays from the Crab Nebula, expressed by a power law as $(dJ/dE) = (2.09pm0.32)times10^{-12} (E/{rm 3 TeV})^{-2.96pm0.14} {rm cm}^{-2} {rm s}^{-1} {rm TeV}^{-1}$ in the energy range of 1.7 to 40 TeV. This result is consistent with other independent $gamma$-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 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 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 $pm$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 $0fdg011$. The deficit rate and position of the moons shadow are shown to be very stable within a statistical error of $pm$6% year by year. This guarantees the long-term stability of point-like source observation with the Tibet-III array. These systematic errors are adequately taken into account in our study of the Crab Nebula.
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We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by 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 in 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 cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle.
Using the Tibet-III air shower array, we search for TeV gamma-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 7 sources instead of the expected 0.61 sources at a significance of 2 sigma or more excess. The chance probability from Poisson statistics would be estimated to be 3.8 x 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 x 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 gamma-ray excesses. We also find that all 7 sources are associated with pulsars, and 6 of them are coincident with sources detected by the Milagro experiment at a significance of 3 sigma 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 >=3sigma 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.
We report the analysis of the $10-1000$ TeV large-scale sidereal anisotropy of Galactic cosmic rays (GCRs) with the data collected by the Tibet Air Shower Array from October, 1995 to February, 2010. In this analysis, we improve the energy estimate and extend the declination range down to $-30^{circ}$. We find that the anisotropy maps above 100 TeV are distinct from that at multi-TeV band. The so-called tail-in and loss-cone features identified at low energies get less significant and a new component appears at $sim100$ TeV. The spatial distribution of the GCR intensity with an excess (7.2$sigma$ pre-trial, 5.2$sigma$ post-trial) and a deficit ($-5.8sigma$ pre-trial) are observed in the 300 TeV anisotropy map, in a good agreement with IceCubes results at 400 TeV. Combining the Tibet results in the northern sky with IceCubes results in the southern sky, we establish a full-sky picture of the anisotropy in hundreds of TeV band. We further find that the amplitude of the first order anisotropy increases sharply above $sim100$ TeV, indicating a new component of the anisotropy. All these results may shed new light on understanding the origin and propagation of GCRs.
Using the data taken from Tibet II High Density (HD) Array (1997 February-1999 September) and Tibet-III array (1999 November-2005 November), our previous northern sky survey for TeV $gamma-$ray point sources has now been updated by a factor of 2.8 improved statistics. From $0.0^{circ}$ to $60.0^{circ}$ in declination (Dec) range, no new TeV $gamma-$ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV $gamma-$ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.
We propose to build a large water-Cherenkov-type muon-detector array (Tibet MD array) around the 37,000 m$^{2}$ Tibet air shower array (Tibet AS array) already constructed at 4,300 m above sea level in Tibet, China. Each muon detector is a waterproof concrete pool, 6 m wide $times$ 6 m long $times$ 1.5 m deep in size, equipped with a 20 inch-in-diameter PMT. The Tibet MD array consists of 240 muon detectors set up 2.5 m underground. Its total effective area will be 8,640 m$^{2}$ for muon detection. The Tibet MD array will significantly improve gamma-ray sensitivity of the Tibet AS array in the 100 TeV region (10-1000 TeV) by means of gamma/hadron separation based on counting the number of muons accompanying an air shower. The Tibet AS+MD array will have the sensitivity to gamma rays in the 100 TeV region by an order of magnitude better than any other previous existing detectors in the world.
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