79 research outputs found
Observation of Multi-Tev Gamma Rays from the Crab Nebula Using the Tibet Air Shower Array
The Tibet experiment, operating at Yangbajing (4,300 m above sea level), is
the lowest energy air shower array and the new high density array constructed
in 1996 has sensitivity to -ray air showers at energies as low as 3
TeV. With this new array, the Crab Nebula was observed in multi-TeV
-rays and a signal was detected at the 5.5 level. We also
obtained the energy spectrum of -rays in the energy region above 3 TeV
which partially overlaps those observed with imaging atmospheric Cherenkov
telescopes. This is the first observation of -ray signals from point
sources with a conventional air shower array using scintillation detectors.Comment: 9 pages, 4 figures, Accepted for publication in ApJ Letter
Detection of Multi-TeV Gamma Rays from Markarian 501 during an Unforeseen Flaring State in 1997 with the Tibet Air Shower Array
In 1997, the BL Lac Object Mrk 501 entered a very active phase and was the
brightest source in the sky at TeV energies, showing strong and frequent
flaring. Using the data obtained with a high density air shower array that has
been operating successfully at Yangbajing in Tibet since 1996, we searched for
gamma-ray signals from this source during the period from February through
August in 1997. Our observation detected multi-TeV -ray signals at the
3.7-Sigma level during this period. The most rapid increase of the excess
counts was observed between April 7 and June 16 and the statistical
significance of the excess counts in this period was 4.7-Sigma. Among several
observations of flaring TeV gamma-rays from Mrk 501 in 1997, this is the only
observation using a conventional air shower array. We present the energy
spectrum of gamma-rays which will be worthy to compare with those obtained by
imaging atmospheric Cerenkov telescopes.Comment: 9 pages, 7 figures, To appear in Ap
Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array
Since 1996, a hybrid experiment consisting of the emulsion chamber and burst
detector array and the Tibet-II air-shower array has been operated at
Yangbajing (4300 m above sea level, 606 g/cm^2) in Tibet. This experiment can
detect air-shower cores, called as burst events, accompanied by air showers in
excess of about 100 TeV. We observed about 4300 burst events accompanied by air
showers during 690 days of operation and selected 820 proton-induced events
with its primary energy above 200 TeV using a neural network method. Using this
data set, we obtained the energy spectrum of primary protons in the energy
range from 200 to 1000 TeV. The differential energy spectrum obtained in this
energy region can be fitted by a power law with the index of -2.97 0.06,
which is steeper than that obtained by direct measurements at lower energies.
We also obtained the energy spectrum of helium nuclei at particle energies
around 1000 TeV.Comment: 25 pages, 22 figures, Accepted for publication in Phys. Rev.
Observation of Multi-Tev Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array
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 <= l <= 55 degrees, and outer Galaxy, 140 degrees <= l <= 225
degrees, and |b| <= 2 degrees or <= 5 degrees. No significant Galactic plane
gamma-ray excess was observed. The 99% confidence level upper limits for
gamma-ray intensity obtained are (for |b| <= 2 degrees) 1.1 times 10^{-15}
cm^{-2}s^{-1}sr^{-1}MeV^{-1} at 3 TeV and 4.1 times 10^{-17}
cm^{-2}s^{-1}sr^{-1}MeV^{-1} at 10 TeV for the inner Galaxy, and 3.6 times
10^{-16} cm^{-2}s^{-1}sr^{-1}MeV^{-1} at 3 TeV and 1.3 times 10^{-17}
cm^{-2}s^{-1}sr^{-1}MeV^{-1} at 10 TeV for the outer Galaxy, assuming a
differential spectral index of 2.4. The upper limits are significant in the
multi-TeV region when compared to those from Cherenkov telescopes in the lower
energy region and other air shower arrays in the higher energy region; however,
the results are not sufficient to rule out the inverse Compton model with a
source electron spectral index of 2.0.Comment: 22 pages, 8 figures, Accepted for publication in Ap
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