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.

我们展示了1999年至2003年期间利用西藏三号空气簇射阵列在多太电子伏区域观测到的银河系宇宙线强度的大尺度恒星时各向异性。在本实验中观测到的宇宙线恒星日变化在当地恒星时4 - 7小时左右显示出相对强度的超出，以及在当地恒星时12小时左右出现亏缺。虽然在对所有赤纬取平均时超出的幅度并不显著，但当观测方向向南移动时，单个赤纬带中的超出变得更大且更清晰。超出强度的最大相位从北半球的约7小时变为赤道区域的约4小时。我们还表明，在多太电子伏区域，日变化的一次谐波矢量的幅度和相位都与初级能量显著无关。这是首个通过单次空气簇射实验确定多太电子伏区域恒星日变化完整24小时轮廓的能量和赤纬依赖性的结果。