Author: Gan Hengqian

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

As Prof. Nan Rendong, the founder of FAST, once wrote in a poem, "Dawodang always brings us discoveries, and also surprises." FAST, the world's largest single-aperture radio telescope, recently brought us new surprises in the observation of comets. Comets have long been regarded as 'frozen time capsules' from the early solar system, preserving key information about the primordial environment of the solar system in their material composition. The observation and research on the comet compositions enable a more precise reconstruction of the formation and evolutionary history of the solar system.

Researchers speculate that the 18 cm OH emission or absorption spectral lines could be observed when a comet approaches the Sun, because hydroxyl (OH) is generated from the sublimation and photolysis of its interior water ice. A collaborative team led by Jiang Dongyue from Guizhou University, Qian Lei from the National Astronomical Observatories, and researchers from institutions such as Beijing Normal University and Peking University used FAST to conduct several observations of the long-period comet C/2025 A6 (Lemmon) during its perihelion passage. They successfully detected the OH spectral lines from a comet with FAST for the first time. Through meticulously designed observation plans and detailed data analysis, the research team confirmed from multiple perspectives that the received OH spectral line signals originated from the comet. They also precisely calculated key parameters such as the OH production rate, opening up new research directions for FAST in cometary studies as well as the research on the solar system primordial environment, formation and evolution.

The core equipment for this comet OH spectral line observation is the FAST ultra-wideband receiver, developed in 2022 by the FAST team in collaboration with leading domestic receiver research teams. The receiver's RF front-end covers 500–3300 MHz, achieving a relative bandwidth of 6.6:1, which surpasses that of other similar international instruments. The digital back-end is divided into four sub-bands, covering frequencies of 500–1000 MHz, 900–1800 MHz, 1700–2600 MHz, and 2500–3300 MHz. The research team successfully obtained hyperfine transition spectral lines of OH (1665, 1667 MHz) using the second sub-band (900–1800 MHz) of this receiver. This achievement once again demonstrates FAST's exceptionally high observational sensitivity in this frequency band, while also fully showcasing the superior stability of the ultra-wideband receiver system and its solid foundation for observations. We have every reason to believe that more surprises from Dawodang are yet to come.