Two comets, different radio signatures
Radio observations of two bright comets produced sharply different pictures in their OH lines. C/2023 A3 (Tsuchinshan-ATLAS) showed robust absorption in the 1665 and 1667 MHz main lines during two epochs immediately after its closest approach to Earth. 12P/Pons-Brooks produced a robust absorption detection in both lines at one pre-perihelion epoch, but later observations yielded only tentative signals or upper limits.
The study used FAST centimeter-wave spectroscopy during the comets' 2024 apparitions. It covered 11 observation epochs in all, with five for 12P and six for A3. The measurements centered on the OH lines near 18 centimeters, and the A3 data also included a possible satellite-line signal at 1721 MHz.
A targeted observing run
FAST's UWB receiver covered 500 to 3300 MHz. The 12P observations took place across five days, with roughly one hour of tracking per day, while the A3 observations spanned six days.
The data were calibrated separately in the XX and YY polarizations and placed on an antenna-temperature scale. The analysis corrected for the comet nucleus's Doppler shift, subtracted continuum using source-OFF positions, converted antenna temperature to flux density and fit the line profiles with Gaussian curves. Four-bin boxcar smoothing produced a velocity resolution of 0.76 kilometers per second.
12P's signal changed with time
The clearest 12P result came on April 17, before perihelion, the comet's closest point to the Sun. Both main lines were robustly detected in absorption. The later record was less secure: tentative post-perihelion detections were reported on May 10 and May 13, and on May 13 only the 1665.402 MHz line was tentatively seen, at a signal-to-noise ratio of 2.0.
The line ratios also shifted. The intensity of the 1667 MHz line relative to the 1665 MHz line fell from 2.9 plus or minus 0.3 on April 17 to 1.6 plus or minus 0.2 on May 10. The reported inversion factor changed from -0.164 on April 17 to positive 0.05 by April 24, as the excitation moved from absorption toward emission.
A3's main-line detections were accompanied by a more tentative report. The 1721 MHz satellite-line signal had a signal-to-noise ratio of 2.7 and was treated as a tentative identification in the paper.
The rate estimates rest on a model
To estimate OH production, the analysis combined the integrated area under an OH line with the Earth-comet distance, background temperature, inversion factor, fluorescence-equilibrium fraction and OH lifetime. It assumed a 3 K background and an OH lifetime of 1.1 times 10^5 seconds at 1 AU. The resulting figures are model-derived estimates tied to those inputs.
For 12P on April 17, the reported OH production rate was 4.12 plus or minus 0.19 times 10^29 molecules per second. For A3, the estimates were 7.5 plus or minus 0.3 times 10^28 molecules per second on October 13 and 6.8 plus or minus 0.3 times 10^28 on October 14.
Beam sampling and collisional quenching were included through a correction factor f of 0.3 for 12P and 0.2 for A3. A sensitivity check found that using a characteristic quenching radius of 44,000 kilometers changed QOH by less than 9 percent, while 37,000 kilometers produced a maximum 27 percent difference for the April 17 12P result.
How the authors read the pattern
The authors associate the low OH detection rates with unfavorable heliocentric radial velocity and with the anti-maser, negative-inversion mode of OH excitation. That is an observational interpretation of the pattern rather than a demonstrated causal estimate from the data.
The authors also compare the estimates with published measurements and production-rate trends. They say 12P follows its power-law production trend and is generally typical of Halley-type comets. A3's rates are described as broadly consistent with those of other dynamically new Oort Cloud comets at comparable heliocentric distances.
A narrow result with clear uncertainties
The evidence remains limited to two comets observed at a small number of 2024 epochs. Several signals were low in signal-to-noise or tentative, including the A3 satellite-line feature, and the production estimates depend on beam-size and collisional-quenching corrections. The results therefore provide epoch-specific measurements and comparisons, not a complete picture of comet behavior.
The manuscript is an arXiv preprint, arXiv:2608.25296v1, dated 26 August 2026.
Paper data and sources
Original title: Centimeter-wave OH observations of comets 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS) with FAST
Authors: Long-Fei Chen, Juncen Li, Zhen Wang et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text