From emission to absorption: the FAST observation of the OH 18-cm lines from the Comet C2025/A6

TL;DR

This work reports the first FAST detection of OH 18-cm lines from Comet C/2025 A6, observing a transition from emission to absorption driven by the Swings effect as the heliocentric velocity evolves. By applying Gaussian and trapezoidal fits to the OH line profiles, the authors derive water expansion velocities that rise as $r_h$ decreases, and they estimate OH production rates using beam-size corrections and quenching considerations, finding a general upward trend in $Q_{\mathrm{OH}}$ with decreasing $r_h$. The findings demonstrate how ground-based OH 18-cm observations can trace cometary water production and coma dynamics, and they highlight the importance of quenching corrections and beam effects in deriving robust production rates. The results underscore FAST's utility for monitoring cometary activity and provide a framework for future, more precise determinations of $Q_{\mathrm{OH}}$ and $Q_{\mathrm{H_2O}}$ across different comets.

Abstract

We observed comet C/2025 A6 with FAST telescope equipped with the ultra-wideband receiver from 23$^{\rm rd}$ October to 8$^{\rm th}$ November 2025 and detected the OH 18-cm lines for the first time. The OH lines underwent a reversal from emission to absorption from 23$^{\rm rd}$ October to 5$^{\rm th}$ November, which is mainly caused by variations in the heliocentric velocity. Through trapezoidal fitting of the OH line profiles, we derive expansion velocities of the water that rise as the heliocentric distance decreases. Based on these results, we estimated the OH production rates of C/2025 A6 for 23$^{\rm rd}$ October, 26$^{\rm th}$ October, 4$^{\rm th}$ November, and 5$^{\rm th}$ November and it presents a significant upward trend.

Figures (2)

• Figure 1: Optical image of comet C/2025 A6 obtained on 21$^{\rm st}$ October 2025 (18:35--18:50, UTC+8) with the 70-cm UCASST telescope. The image combines 5-min exposures in each of the $R$, $V$, and $B$ filters and shows a coma with an apparent diameter of $\sim$8--10 arcmin. The telescope is equipped with a 2048$\times$2048 detector with a pixel size of 13.5 $\mu$m, providing a field of view of about 20.8$^{\prime}$$\times$20.8$^{\prime}$, at the time of the observation, 1$^{\prime}$ corresponds to a projected scale of $\sim$25,700 km at the comet.
• Figure 2: The OH 18-cm lines obtained on the individual observing dates. All spectra were processed with a two-channel boxcar smoothing, yielding a final velocity resolution of 0.38 km s$^{-1}$. The OH main lines show emission on 23$^{\rm rd}$ and 26$^{\rm th}$ October, and transition to absorption starting on 3 November. No significant OH signal was detected during 6–8$^{\rm th}$ November. The red vertical line indicates the comet’s line-of-sight radial velocity relative to the observatory, as computed from the JPL Horizons system.