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

我们试图通过用成像大气切伦科夫望远镜观测TeV伽马射线发射来研究NGC253星系中TeV宇宙射线的空间分布。NGC253位于2.5Mpc附近，以其延伸到约10kpc的大射电晕而闻名，也因其超新星的出现频率很高而闻名。我们用CanGaroo的10米望远镜观测了这个星系，成功地探测到了统计意义为11西格玛的TeV伽马射线。这是首次探测到来自我们银河系以外的正常星系的TeV伽马射线。发射区延伸到比星系的光学图像大得多的距离。这个发射区，即伽马射线晕，是迄今为止探测到的规模最大的区域。最可能的发射机制是超高能宇宙线电子对宇宙微波背景辐射的逆康普顿散射。由于星际物质的数量不足，建立强子模型的可能性不大。我们已经通过数值模拟证实，如果扩散系数比玻姆扩散极限大100倍，电子可以在没有严重同步辐射损失的情况下传播到观察到的发射区。

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.