LHAASO河外伽玛源的实时监测与预警

At present more than 60 extragalactic sources have been detected. Most of these sources belong to the blazar class of active galactic nuclei (AGNs), mainly as BL Lac objects. The emission from blazars is highly variable and is characterized by a flaring behavior, in which the flux increases dramatically on various time scales, even down to the hour time scale. Many interesting studies in physics can be done on the flaring phenomenon or variability studies. For instance, spectral variations of gamma rays from the sources are used as a tool to understand the physics of the source and the acceleration model of emitted gamma rays. Therefore, studying or monitoring their variability with sufficiently low exposure times is necessary. And detecting and monitoring gamma-ray emissions from flaring sources in the very high energy region is a very important topic in gamma-ray astronomy. On the base of its large field of view, high duty cycle and low threshold energy, LHAASO-WCDA is a good candidate to detect and monitor the flaring phenomena. Based on fast reconstruction and analysis, a real-time monitoring and alerting system for selected VHE extragalactic (and few galactic ) sources will be implemented. Data of any candidate source can be analyzed within short time after it ends the transit in the local sky. A alert will be issued once the monitored sources have flare with intensity higher than certain specific threshold. The corresponding follow-up observation will be enabled to the close experiment and the community, a multi-wavelength and global studying could be implemented.

河外耀变伽玛源的高能辐射现象是高能伽玛天文的研究重点之一，对研究天体演化和与周围物质的相互作用特性具有重要的意义。正在建设中的LHAASO实验具有全天候、大视场、高灵敏度的特点，在河外耀变源的探测和长期监测方面具有得天独厚的优势。本项目的研究内容是基于LHAASO数据建立一套针对河外候选耀变源的在线监测、预警与分析系统，并且实时发布预警信息，由LHAASO主导实现国际上多个不同波段和不同地理位置的实验对河外源耀变现象的及时跟踪观测与分析。基于这一监测系统的建立，我们将开展与其它能段的实验（如Fermi-LAT）的多波段观测，协同位于西半球的同类型的HAWC实验实现对同一耀变现象的全时段观测，并提供条件使得IACT实验能够对转瞬即逝的耀变现象开展深度观测。通过本项目的实施，本领域的实验将会提供足够多的耀变事件样本和多波段、全时段、高精度观测结果，从而为深入、统计研究河源耀源耀变现象奠定基础。

结合实验室小规模的测量结果和大量模拟数据，我们首先研究并且完善了利用小簇射事例进行离线时间标定方法的研究，发表了相应研究结果。其次利用大量的不同原初成分的模拟数据开展了簇射重建方法，区分原初成分的研究。比较了不同的参数的影响，优化了重建参数的设置，最终保障了重建的精度。在此期间为了进一步提高重建的速度也展开了利用GP U的重建算法的研究，目前的研究表明在关键分析步骤中采用GPU的排序算法，可以大幅度的节约重建时间。从2019年4月26日开始LHAASO-WCDA1号探测器开始了科学数据采数，在充分理解实验数据的基础上，进行了gamma/质子区分，获得了预期的伽马事例样本，对TeV标准烛光蟹状星云的观测结果也表明达到预期。在此基础上我们建立和发展了一套对候选时变源的监测系统，对2019年4月-2020年3月的WCDA1号水池的观测表明在2019年7月成功看到一个几天量级的来自Mrk421的耀变现象。为下一阶段，即全阵列，3个水池的同时运行后的监测系统建立了良好的基础。