Study of the origin and propagation of cosmic-rays in a starburst galaxy by observations of TeV gamma-rays

利用成像大气切伦科夫望远镜观测了NGC253星系TeV宇宙线的空间分布。NGC 253位于2.5 Mpc附近，以其巨大的射电晕延伸到1000 kpc而闻名，也以其非常高的超新星发生频率而闻名。我们用CANGAROO的10米望远镜观测了该星系，并成功地探测到了TeV的γ射线，其统计显著性为11 sigma。这是第一次探测到来自我们星系以外的正常星系的TeV伽马射线。发射区被扩展到比星系的光学图像大得多的距离。这个辐射区域，即伽马射线晕，是迄今为止探测到的最大规模的。最可能的发射机制是宇宙微波背景辐射被极高能宇宙线电子的逆康普顿散射。强子模型是不可能的，因为星际物质的数量不足。我们已经证实，通过数值模拟，电子可以传播到所观察到的发射区，而不冷却服务器同步辐射损失，如果扩散系数大于玻姆扩散限制的100倍。

We have tried to study the spatial distribution of TeV cosmic rays in the galaxy NGC 253 by observing TeV gamma-ray emission with Imaging Atmospheric Cerenkov Telescope. NGC 253 is located relatively near at 2.5 Mpc and is well known by its large radio halo extended to 〜10 kpc and is also known by its very high frequency of supernova occurrence. We observed the galaxy with 10 m telescope of CANGAROO and succeeded in detection of TeV gamma-rays with a statistical significance of 11 sigma. This is the first detection of TeV gamma-rays from normal galaxies other than our galaxy. The emission region is extended to distances much larger than the optical image of the galaxy. This emission region, the gamma-ray halo, is of the largest scale so far detected. The most probable emission mechanism is the inverse Compton scattering of the Cosmic Microwave Background Radiation by very high energy cosmic ray electrons. Hadronic model is unlikely due to insufficient amount of the interstellar material. We have confirmed by a numerical simulation that electrons can propagate out to the observed emission region without cooling by server synchrotron losses if the diffusion coefficient is larger by a factor of 100 than the Bohm diffusion limit.