搜寻地外文明（SETI）是探索宇宙中智慧生命的前沿领域，其核心目的在于追踪并 分析可能源自其他星系的技术信号，试图解答古老且根深蒂固的问题：在这浩渺无垠的宇宙中，我们是唯一的存在吗？假设地外文明（ETI）拥有和我们人类同等甚至更高等的 射电技术，那么和地球上的工程信号类似的射电信号有可能由其他文明传播，而且射电 波能以最快的速度跨越遥远的星际距离传递信息并同时节省能量。所以，射电天文学一 直在寻找地外文明中发挥着突出的作用，国际上大多数的SETI项目都是使用射电望远镜

进行搜寻。

中国天眼500米口径球面射电望远镜（FAST）是世界上最大的单口径射电望远镜，其 极高的灵敏度对SETI观测有着重要的意义，并且搜寻地外文明也是FAST的五大科学目标 之一。射电 SETI 观测可以通过两种方式开展：巡天观测和目标观测。2019 年，FAST以 共时巡天观测的方式开展了首次 SETI 巡天观测，并探测到了两组候选信号。SETI目标观 测可以根据科学研究和发现（例如已知的系外行星）来选择目标，以增加检测到信号的 可能性。但是在此之前FAST还没有开展过SETI目标观测，本文针对FAST开展了SETI目标 观测的研究，主要内容如下：

我们使用FAST进行了第一次SETI目标观测，并为FAST设计了一种名为多波束重合匹 配(MBCM)观测策略。针对33个已知的系外行星系统，我们分别在两个正交的极化偏振方 向上搜索1.05-1.45 GHz频率范围内的ETI技术印迹。我们在Kepler-438观测数据中搜索到 了1140.604 MHz频率附近的一个特殊的信号，这个信号最初引起了我们极大的兴趣，因 为它的许多特征与假设的ETI技术特征大体一致。然而，通过分析极化特征等证据排除了 这个信号地外起源的可能性。FAST第一次SETI目标观测达到了前所未有的灵敏度，能够 探测到的最小等效各向同性辐射功率(EIRP)达到1.48 × 10^9 W。

巴纳德星是距离太阳第二近的恒星系统，也是FAST可观测天空中距离太阳最近的恒 星，这使得FAST望远镜探测到巴纳德星假想无线电发射机所需的最小等效各向性辐射 功率仅EIRPmin = 4.36 × 10^8 W。因此我们第三章在介绍了FAST对巴纳德星(GJ 699)进行 了一系列观测。在FAST望远镜上应用多波束符合匹配策略，在1.05-1.45 GHz频率范围内 搜ETI技术印迹，记录两个正交的线性极化方向。尽管在搜索之后我们没有在这一系列观测中发现射电技术印迹的证据，但我们基于我们的观测策略对巴纳德星系统发出的假想 的地外文明信号做出了预测。

地球上的天文学家可以利用凌日法探测到遥远的系外行星，位于地球凌日带 （rETZ）的地外文明理论上也可以观测到我们的地球，这些文明可能已经知道我们存 在并试图与我们沟通。因此地球凌日带的SETI观测可以为我们提供一个可能与我们试图 沟通的外星文明的有利位置。我们选择了9个35 pc以内的地球凌日带恒星使用FAST进行 了SETI目标观测，虽然在1.05-1.45 GHz频率范围内没有找到地外技术印迹的证据。但是我们优化了观测策略，每组都观测三个及以上的源，这样通过比较同一次观测的几个源 可以避免NBS 210629事件类似的干扰。

虽然以上几次观测并未发现任何表明地外技术印迹存在的证据，但是我们的研究 为FAST地外文明搜寻开拓了新的方法，为未来的观测提供了宝贵的经验。

The Search for Extraterrestrial Intelligence (SETI) is a scientific effort to detect and study potential signals from intelligent civilizations beyond Earth. SETI's significance lies in its potential to answer one of the most profound questions in human history: ”Are we alone in the universe?” Assuming that an extraterrestrial civilization (ETI) has radio technology equal to or even superior to ours, it is possible that radio signals similar to those engineered on Earth could be transmitted by other civilizations, and that radio waves could transmit information across vast interstellar distances at the fastest speeds while conserving energy. Therefore, radio astronomy has always played a prominent role in the search for extraterrestrial intelligence, and most international SETI projects use radio telescopes to search.

Five-hundred-meter Aperture Spherical Radio Telescope (FAST) is the world’s largest single-aperture radio telescope. Its extremely high sensitivity is of great significance for SETI observation, and the search for extraterrestrial civilization is also one of FAST’s five scientific goals. Radio SETI observations can be carried out in two ways: sky survey observations and target observations. In 2019, FAST conducted the first SETI survey in the form of synchronic sky survey observations, and detected two sets of candidate signals. SETI target observations can select targets based on scientific research and discoveries, such as known exoplanets, to increase the likelihood of a signal being detected. However, FAST has not carried out SETI target observation before. This paper carries out research on SETI target observation for FAST, and the main contents are as follows:

We used FAST for the first SETI target observation and designed an observation strategy called Multi-beam coincidence Matching (MBCM) for FAST. For 33 known exoplanet systems, we search for ETI narrow-band drift signals in the 1.05-1.45 GHz frequency range in two orthogonal polarized polarization directions. We searched for a particular signal in the Kepler-438 observations near the 1140.604 MHz frequency, which initially attracted great interest because many of its features were broadly consistent with those of the assumed ETI technology. However, the possibility of extraterrestrial origin of this signal is ruled out by analyzing the evidence such as polarization signature. FAST’s first SETI target observation achieved unprecedented sensitivity, with a minimum equivalent isotropic radiated power (EIRP) of 1.48 × 10^9 W.

Barnard’s Star is the second closest star system to the Sun and the closest star in FAST's observable sky, making the minimum equivalent anisotropic radiation power required for FAST to detect Barnard's hypothetical radio transmitter EIRPmin = 4.36 × 10^8 W. Therefore, in Chapter 3, we introduce a series of observations of Barnard's star (GJ 699) by FAST. The multi-beam matching strategy is applied on the FAST telescope to search for narrowband signals (∼Hz) in the frequency range of 1.05-1.45 GHz, and record two orthogonal linear polarization directions. Although we found no evidence of radio imprinting in this series of observations after searching, we made predictions about hypothetical extraterrestrial signals emanating from Barnard's star system based on our observation strategy.

Astronomers on Earth can detect distant exoplanets using the transit method, and our planet can theoretically be observed by extraterrestrial civilizations located in the Earth Transit Zone (rETZ) that may already know we exist and are trying to communicate with us. So SETI observations of Earth's transit belt could give us a vantage point from which we might be able to communicate with alien civilizations. We selected nine stars within 35 pc of Earth's transit belt for SETI target observations using FAST, although no evidence of extraterrestrial technical imprinting was found in the 1.05-1.45 GHz frequency range. However, we have optimized the observation strategy to observe two or more sources in each group, so that interference such as the NBS 210629 event can be avoided by comparing several sources in the same observation.

Although these observations did not uncover any evidence of extraterrestrial technological signatures, our research has pioneered new methods for FAST's search for extraterrestrial civilizations, offering invaluable insights for future observations.