面向伽马射线暴观测的超广角大气切伦科夫望远镜技术

Wide field-of-view (FoV) telescope is essential to observe sporadic and transient source. The ability of a wide FoV is one of the most important features for ground-based observations of sporadic events like gamma-ray bursts (GRBs), highly variable sources like blazars, and the electromagnetic counterparts of gravitational waves (GWs). The highest energy photon of GRB130427 which was recorded by the Large Area Telescope (LAT) instrument onboard FERMI was found to be 95 GeV (~126 GeV after redshift correction). Several methods have been proposed to enlarge the FoV of atmospheric Cherenkov telescopes, for example, MACHETE, GAW, JEM-EUSO, et.al.. We proposed a large dimensional refractive water lens as the light collector for observing the Cherenkov light induced by VHE CRs and rays. We especially aim to detect solitary VHE emission. Supported by grant from the NSFC, a prototype telescope based on water-lens with 0.9 m diameter was manufactured and tested. Using the prototype with a camera with 48 pixels, CR events were observed in coincidence with a scintillator Extensive Air Shower (EAS) array. The prototype has a median angular resolution of 1.0 degree. This project will devote to develop the novel wide FoV of atmospheric Cherenkov telescopes technology furtherly. Main studies are as follows: decrease the energy threshold of the telescope through expand the dimension of the lens; enlarge the FoV; improve the angular resolution and energy resolution; develop the approach of gamma/proton discrimination and the approach of calibration; explore the processing technology of large dimensional lens; research and develop the high sampling rate FEE (Front End Electronics) and DAQ (Data AcQuisition).

爆发源和瞬变源的观测需要超大视场的望远镜.卫星实验已观测到一些伽马射线暴(Gamma Ray Burst，GRB)存在高达~100GeV的高能辐射，但地面实验中一直没有探测到，因此，迫切需要发展地基大口径超广角大气切伦科夫望远镜技术.透镜或Schmidt型望远镜是广角望远镜的主要发展方向，前者如JEM-EUSO，GAW，后者如MACHETE. 在国家自然基金的前期资助下，我们已成功研发了0.9m口径的透镜式超广角大气切伦科夫望远镜，并在西藏羊八井国际宇宙线观测站成功与EAS闪烁体阵列符合探测到了甚高能宇宙线事例. 本项目旨在进一步发展基于透镜的超广角大气切伦科夫望远镜技术，重点工作为降低望远镜工作阈能，扩大视场，提高角分辨和能量分辨率，发展γ/p鉴别技术和标定技术，探索大口径透镜的加工制作工艺，研发适合该型望远镜的电子学和数据获取系统.

甚高能(VHE)伽马射线(GRs)的观测是多波段多信使天文学的重要组成部分，通过对目标源天体的观测，可以研究其物理本质，瞬变源光变也是研究洛伦兹不变性破缺等新物理的最灵敏方法，而且河外源观测是目前研究河外背景光(EBL)最好的手段，进而可以研究早期星系演化和宇宙演化；VHE GRs也是暗物质粒子的间接探测的重要手段。因此VHE GRs观测具有非常丰富和十分重要的物理意义。自本项目立项以来，地面实验以>5σ显著性观测到5个GRB(GRB221009A、GRB190114C、GRB180720B、GRB190829A、GRB201216C)VHE辐射，其中4个为成像大气切伦科夫望远镜阵列观测结果，GRB221009A由地面EAS实验LHAASO观测到。相对空间卫星实验观测，地面实验GRB观测依然处于个位数水平，而且这些观测均是GRB余辉阶段(afterglow phase)的辐射，地面实验尚未探测到GRB瞬时阶段(prompt phase)辐射，而后者具有更重要的物理意义。因此，亟待我们发展建设大视场、低阈能的地面观测设备。.项目组在前期工作基础上与企业合作完成了2m口径半球水透镜研制工作，深入探索了大口径半球水透镜拼接加工技术。分析了0.9m口径望远镜原理样机观测结果，验证了水透镜广角性能。借鉴LHAASO技术，发展了适用于广角大气切伦科夫望远镜的、基于SiPM的照相机技术(包括数据读出、温度补偿板)以及高速电子学和数据获取系统。发展了面向超广角大气切伦科夫望远镜的触发算法和γ/P鉴别技术。提出基于大口径超广角大气切伦科夫望远镜阵列的HADAR(High Altitude Astronomical Radiation Observation，高海拔天体辐射观测实验)计划，拟在极高海拔建设4面大口径(5m)，间距为100m的超广角大气切伦科夫望远镜阵列，预计视场达60^o×60^o，阈能为O(10)GeV。模拟结果表明，当GRB内部吸收能量阈值高于50GeV 时，HADAR具有每年观测到2-3个GRB的能力；预计HADAR阵列每年将以>5σ显著性的水平探测到31个AGN 源(4FGL)和23个河内源(TeVCat)；对Crab Nebula一年的统计显著性达346σ，1TeV以上流强达~1.3%–2.4%水平。