Kazuo Kasahara

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.

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 γ-ray signals from this source during the period in 1997 from February through August. Our observation detected multi-TeV γ-ray signals at the 3.7 σ level during this period. The most rapid increase in 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 σ. Among several observations of flaring TeV γ-rays from Mrk 501 in 1997, this is the only observation using a conventional air shower array. We present the energy spectrum of γ-rays, which will be worth comparing with those obtained by imaging atmospheric Cerenkov telescopes.

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.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 09. The sky regions searched are the inner Galaxy, 20? ? l ? 55?, and outer Galaxy, 140? ? l ? 225?, and |b| ? 2? or ? 5?. No significant Galactic-plane gamma-ray excess was observed. The 99% confidence level upper limits for gamma-ray intensity obtained are (for |b| ? 2?) 1.1 ? 10-15 cm-2 s-1 sr-1 MeV-1 at 3 TeV and 4.1 ? 10-17 cm-2 s-1 sr-1 MeV-1 at 10 TeV for the inner Galaxy, and 3.6 ? 10-16 cm-2 s-1 sr-1 MeV-1 at 3 TeV and 1.3 ? 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 Cerenkov 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.

Observation by an air-shower array in tibet of the multi-tev cosmic-ray anisotropy due to terrestrial orbital motion around the sun

We report on the solar diurnal variation of the galactic cosmic-ray intensity observed by the Tibet III air shower array during the period from 1999 to 2003. In the higher-energy event samples (12 and 6.2 TeV), the variations are fairly consistent with the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun, while the variation in the lower-energy event sample (4.0 TeV) is inconsistent with this anisotropy. This suggests an additional anisotropy superposed at the multi-TeV energies, e.g., the solar modulation effect. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

A Study of the Shadowing of Galactic Cosmic Rays by the Sun in a Quiet Phase of Solar Activity with the Tibet Air Shower Array

We have shown that the Suns shadow by high-energy cosmic rays moves year by year and its behavior is correlated with a time variation of the large-scale structure of the solar and interplanetary magnetic fields. The solar activity was near minimum in the period from 1996 through 1997. Using the data obtained with the Tibet air shower array, we examined the shadowing of cosmic rays by the Sun in this quiet phase of the solar cycle and found that the Suns shadow was just in the apparent direction of the Sun, though it was observed at the position considerably away from the Sun to the southwest in the period between 1990 and 1993. It is known that the magnetic pole of the equivalent solar dipole was reversed during the previous active phase, and near solar minimum the dipole was aligned with the rotating axis, preserving its N-pole on the north pole side of the Sun. This causes the solar magnetic field to shift the Suns shadow to the east. Thus, the observed results suggest that the shift of the Suns shadow due to the solar magnetic field was pushed back by the effect of the geomagnetic field, since the geomagnetic field always makes the shadow shift to the west. We discuss the Suns shadow observed during the period near solar minimum in 1996-1997 and compare it with the simulation results.