From emission to absorption: the FAST observation of the OH 18-cm lines from the Comet C/2025 A6

Abstract We observed comet C/2025 A6 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) equipped with the ultra-wideband receiver from 2025 October 23 to November 8, and it was the first detection for this comet with FAST. Through trapezoidal fitting of the OH line profiles, the expansion velocities of the water increase from 1.5±0.3 km s -1 at the heliocentric distance of 0.65 AU to 3±0.9 km s -1 at 0.54 AU. Based on these results, we estimated the OH production rates of C/2025 A6 for October 23, October 26, November 4 and November 5 which were (1.0±0.1)×10 29 , (1.2±0.1)×10 29 , (1.4±0.3)×10 29 , and (1.5±0.4)×10 29 s -1 respectively. The results show a significant upward trend.