Y. H. Tan

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.

Multi-TeV Gamma-Ray Flares from Markarian 421 in 2000 and 2001 Observed with the Tibet Air Shower Array

Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-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.

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.

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.

Underground water Cherenkov muon detector array with the Tibet air shower array for gamma-ray astronomy in the 100 TeV region

AbstractnWe propose to build a large water-Cherenkov-type muon-detector array (Tibet MD array) around the 37u2009000xa0m2 Tibet air shower array (Tibet AS array) already constructed at 4300xa0m above sea level in Tibet, China. Each muon detector is a waterproof concrete pool, 6xa0m wide × 6xa0m long × 1.5xa0m deep in size, equipped with a 20 inch-in-diameter PMT. The Tibet MD array consists of 240 muon detectors set up 2.5xa0m underground. Its total effective area will be 8640xa0m2 for muon detection. The Tibet MD array will significantly improve gamma-ray sensitivity of the Tibet AS array in the 100xa0TeV region (10–1000xa0TeV) 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 100xa0TeV region by an order of magnitude better than any other previous existing detectors in the world.n

New Estimation of the Spectral Index of High-Energy Cosmic Rays as Determined by the Compton-Getting Anisotropy

The amplitude of the Compton-Getting (CG) anisotropy contains the power-law index of the cosmic-ray energy spectrum. Based on this relation and using the Tibet air shower array data, we measure the cosmic-ray spectral index to be –3.03 ± 0.55stat ± <0.62syst between 6 and 40 TeV, consistent with –2.7 from direct energy spectrum measurements. Potentially, this CG anisotropy analysis can be utilized to confirm the astrophysical origin of the knee against models for nonstandard hadronic interactions in the atmosphere.

The energy spectrum of all-particle cosmic rays around the knee region observed with the Tibet-III air-shower array

We have already reported the first result on the all-particle spectrum around the knee region based on data from 2000 November to 2001 October observed by the Tibet-III air-shower array. In this paper, we present an updated result using data set collected in the period from 2000 November through 2004 October in a wide range over 3 decades between 1014 and 1017 eV, in which the position of the knee is clearly seen at around 4 PeV. The spectral index is 2.68 ± 0.02(stat.) below 1 PeV, while it is 3.12 ± 0.01(stat.) above 4 PeV in the case of QGSJET+HD model, and various systematic errors should be carefully done in the very near future.

Observation of PeV gamma rays from the Monogem ring with the tibet air shower array

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° ≤ R.A. < 114°, 125 ≤ decl. < 155 in the Monogem ring where the MAKET-ANI experiment recently claimed a positive detection of PeV high-energy cosmic radiation, although our flux sensitivity is approximately 10 times better than MAKET-ANIs. We set the most stringent integral flux upper limit at a 99% confidence level of 4.0 × 10-12 cm-2 s-1 sr-1 above 1 PeV on diffuse gamma rays extended in the 3° × 3° region.

Heliospheric signatures seen in the sidereal anisotropy of high-energy galactic cosmic ray intensity

Mapping the global heliosphere by detecting energetic neutral atoms (ENAs), IBEX derived the semi‐direct information on the structure of the large‐scale magnetic field surrounding the heliosphere. Multi‐TeV galactic cosmic rays (GCRs) also sense the global structure of the magnetic field within the whole heliosphere as well as the field surrounding the heliosphere, and can provide an additional tool to explore the global structure of these fields. We analyze, in this context, the multi‐TeV GCR intensity observed with the Tibet Air Shower (AS) experiment, which clearly shows a midscale component structure observed as the excess intensity along Gurnett’s HDP (Hydrogen Deflection Plane), as well as the global component structure. We find that the amplitude of the midscale component is only weakly dependent on GCR energy, while its angular extension along the HDP plane monotonously decreases with increasing energy. On the basis of these results, we discuss the possible origin of the midscale anisotropy.